U.S. patent application number 13/024528 was filed with the patent office on 2011-08-11 for multicylinder internal combustion engine, inter-cylinder air/fuel ratio imbalance determination apparatus, and method therefor.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Yusuke FUJITSU, Takahiro NISHIGAKI, Hitoki SUGIMOTO.
Application Number | 20110192146 13/024528 |
Document ID | / |
Family ID | 44352597 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110192146 |
Kind Code |
A1 |
SUGIMOTO; Hitoki ; et
al. |
August 11, 2011 |
MULTICYLINDER INTERNAL COMBUSTION ENGINE, INTER-CYLINDER AIR/FUEL
RATIO IMBALANCE DETERMINATION APPARATUS, AND METHOD THEREFOR
Abstract
A multicylinder engine includes: combustion chambers; fuel
injection valves corresponding to the individual combustion
chambers; an exhaust control catalyst; an upstream-side air/fuel
ratio sensor disposed upstream of the exhaust control catalyst; and
a downstream-side A/F sensor disposed downstream of the catalyst. A
first abnormality determination-purpose rich A/F control of
controlling the A/F of the mixture formed in each combustion
chamber to an A/F richer than stoichiometric is executed when it
needs to be determined whether the downstream-side A/F sensor is
abnormal. An inter-cylinder A/F imbalance determination of
estimating the A/F of the mixture in each combustion chamber based
on the output of the upstream-side A/F sensor is executed when the
first abnormality determination-purpose rich A/F control is being
executed, and it is determined whether there is a difference
between the estimated A/Fs of the mixtures.
Inventors: |
SUGIMOTO; Hitoki;
(Toyota-shi, JP) ; NISHIGAKI; Takahiro;
(Nagoya-shi, JP) ; FUJITSU; Yusuke; (Anjo-shi,
JP) |
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
DENSO CORPORATION
Kariya-City
JP
TOYOTA JIDOSHA KABUSHIKI KAISHA
Toyota-shi
JP
DENSO CORPORATION
Kariya-City
JP
|
Family ID: |
44352597 |
Appl. No.: |
13/024528 |
Filed: |
February 10, 2011 |
Current U.S.
Class: |
60/276 |
Current CPC
Class: |
F02D 41/1495 20130101;
F02D 41/1458 20130101; F02D 41/1441 20130101; F02D 41/222 20130101;
F02D 41/405 20130101; F02D 41/0085 20130101 |
Class at
Publication: |
60/276 |
International
Class: |
F01N 11/00 20060101
F01N011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2010 |
JP |
2010-027674 |
Claims
1. A multicylinder internal combustion engine comprising: a
plurality of combustion chambers; fuel injection valves disposed
corresponding to the individual combustion chambers; an exhaust
control catalyst disposed in an exhaust passageway so as to remove
a specific component of exhaust gas discharged from the combustion
chambers; an upstream-side air/fuel ratio sensor disposed in the
exhaust passageway upstream of the exhaust control catalyst so as
to detect air/fuel ratio of the exhaust gas discharged from the
combustion chambers; a downstream-side air/fuel ratio sensor
disposed in the exhaust passageway downstream of the exhaust
control catalyst so as to detect the air/fuel ratio of the exhaust
gas that flows out from the exhaust control catalyst; a control
device that executes a first abnormality determination-purpose rich
air/fuel ratio control of controlling the air/fuel ratio of a
mixture formed in each combustion chamber to an air/fuel ratio
richer than a stoichiometric air/fuel ratio when it needs to be
determined whether the downstream-side air/fuel ratio sensor is
abnormal; and a determination device that executes an
inter-cylinder air/fuel ratio imbalance determination of estimating
the air/fuel ratio of the mixture formed in each combustion chamber
based on an output of the upstream-side air/fuel ratio sensor when
the first abnormality determination-purpose rich air/fuel ratio
control is being executed, and of determining whether there is a
difference between the air/fuel ratios of the mixtures that are
estimated.
2. The multicylinder internal combustion engine according to claim
1, wherein: the control device executes a second abnormality
determination-purpose rich air/fuel ratio control of controlling
the air/fuel ratio of the mixture formed in each combustion chamber
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
when it needs to be determined whether the upstream-side air/fuel
ratio sensor is abnormal; and the determination device executes the
inter-cylinder air/fuel ratio imbalance determination when the
second abnormality determination-purpose rich air/fuel ratio
control is being executed.
3. The multicylinder internal combustion engine according to claim
2, wherein the determination device executes the inter-cylinder
air/fuel ratio imbalance determination if it is determined that the
upstream-side air/fuel ratio sensor is not abnormal when the second
abnormality determination-purpose rich air/fuel ratio control is
being executed.
4. The multicylinder internal combustion engine according to claim
1, wherein the control device executes an engine start-time rich
air/fuel ratio control of controlling the air/fuel ratio of the
mixture formed in each combustion chamber to an air/fuel ratio
richer than the stoichiometric air/fuel ratio when operation of the
multicylinder internal combustion engine is started, a post-fuel
injection stop rich air/fuel ratio control of controlling the
air/fuel ratio of the mixture formed in each combustion chamber to
an air/fuel ratio richer than the stoichiometric air/fuel ratio
when injection of fuel from the fuel injection valves is re-started
after the injection of the fuel from the fuel injection valves is
stopped, or an exhaust control catalyst-purpose rich air/fuel ratio
control of controlling the air/fuel ratio of the mixture formed in
each combustion chamber to an air/fuel ratio richer than the
stoichiometric air/fuel ratio when temperature of the exhaust
control catalyst is higher than a predetermined permissible
upper-limit temperature; and the determination device executes the
inter-cylinder air/fuel ratio imbalance determination when the
engine start-time rich air/fuel ratio control is being executed, or
when the post-fuel injection stop rich air/fuel ratio control is
being executed, or when the exhaust control catalyst-purpose rich
air/fuel ratio control is being executed.
5. A multicylinder internal combustion engine comprising: a
plurality of combustion chambers; fuel injection valves disposed
corresponding to the individual combustion chambers; an exhaust
control catalyst disposed in an exhaust passageway so as to remove
a specific component of exhaust gas discharged from the combustion
chambers; an upstream-side air/fuel ratio sensor disposed in the
exhaust passageway upstream of the exhaust control catalyst so as
to detect air/fuel ratio of the exhaust gas discharged from the
combustion chambers; a control device that executes a second
abnormality determination-purpose rich air/fuel ratio control of
controlling the air/fuel ratio of a mixture formed in each
combustion chamber to an air/fuel ratio richer than a
stoichiometric air/fuel ratio when it needs to be determined
whether the upstream-side air/fuel ratio sensor is abnormal; and a
determination device that executes an inter-cylinder air/fuel ratio
imbalance determination of estimating the air/fuel ratio of the
mixture formed in each combustion chamber based on an output of the
upstream-side air/fuel ratio sensor when the second abnormality
determination-purpose rich air/fuel ratio control is being
executed, and of determining whether there is a difference between
the air/fuel ratios of the mixtures that are estimated.
6. The multicylinder internal combustion engine according to claim
5, wherein the determination device executes the inter-cylinder
air/fuel ratio imbalance determination if it is determined that the
upstream-side air/fuel ratio sensor is not abnormal when the second
abnormality determination-purpose rich air/fuel ratio control is
being executed.
7. The multicylinder internal combustion engine according to claim
6, wherein: the control device executes an engine start-time rich
air/fuel ratio control of controlling the air/fuel ratio of the
mixture formed in each combustion chamber to an air/fuel ratio
richer than the stoichiometric air/fuel ratio when operation of the
multicylinder internal combustion engine is started, a post-fuel
injection stop rich air/fuel ratio control of controlling the
air/fuel ratio of the mixture formed in each combustion chamber to
an air/fuel ratio richer than the stoichiometric air/fuel ratio
when injection of fuel from the fuel injection valves is re-started
after the injection of the fuel from the fuel injection valves is
stopped, or an exhaust control catalyst-purpose rich air/fuel ratio
control of controlling the air/fuel ratio of the mixture formed in
each combustion chamber to an air/fuel ratio richer than the
stoichiometric air/fuel ratio when temperature of the exhaust
control catalyst is higher than a predetermined permissible
upper-limit temperature; and the determination device executes the
inter-cylinder air/fuel ratio imbalance determination when the
engine start-time rich air/fuel ratio control is being executed, or
when the post-fuel injection stop rich air/fuel ratio control is
being executed, or when the exhaust control catalyst-purpose rich
air/fuel ratio control is being executed.
8. A multicylinder internal combustion engine comprising: a
plurality of combustion chambers; fuel injection valves disposed
corresponding to the individual combustion chambers; an exhaust
control catalyst disposed in an exhaust passageway so as to remove
a specific component of exhaust gas discharged from the combustion
chambers; an upstream-side air/fuel ratio sensor disposed in the
exhaust passageway upstream of the exhaust control catalyst so as
to detect air/fuel ratio of the exhaust gas discharged from the
combustion chambers; a control device that controls the air/fuel
ratio of a mixture formed in each of the combustion chambers to an
air/fuel ratio richer than a stoichiometric air/fuel ratio for a
purpose other than a purpose of determining whether there is a
difference in the air/fuel ratio of the mixture between the
combustion chambers; and a determination device that determines
whether there is a difference in the air/fuel ratio of the mixture
between the combustion chambers based on an output of the
upstream-side air/fuel ratio sensor when the air/fuel ratio of the
mixture is being controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio for the purpose other than the
purpose of determining whether there is a difference between in the
air/fuel ratio of the mixture between the combustion chambers.
9. An inter-cylinder air/fuel ratio imbalance determination
apparatus comprising: an upstream-side air/fuel ratio sensor
disposed in an exhaust passageway upstream of an exhaust control
catalyst so as to detect air/fuel ratio of exhaust gas discharged
from a plurality of combustion chambers of a multicylinder internal
combustion engine, the exhaust control catalyst being disposed in
the exhaust passageway so as to remove a specific component of the
exhaust gas discharged from the combustion chambers; a control
device that controls the air/fuel ratio of a mixture in each of the
plurality of combustion chambers to an air/fuel ratio richer than a
stoichiometric air/fuel ratio for a purpose other than a purpose of
determining whether there is a difference in the air/fuel ratio of
the mixture between the combustion chambers; and a determination
device that determines whether there is a difference in the
air/fuel ratio of the mixture between the combustion chambers based
on an output of the upstream-side air/fuel ratio sensor when the
air/fuel ratio of the mixture is being controlled to the air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
other than the purpose of determining whether there is a difference
in the air/fuel ratio of the mixture between the combustion
chambers.
10. An inter-cylinder air/fuel ratio imbalance determination method
comprising: determining whether a first condition for determining
whether there is a difference in air/fuel ratio between a plurality
of combustion chambers of a multicylinder internal combustion
engine is satisfied; determining whether a second condition for
controlling the air/fuel ratio of a mixture formed in each
combustion chamber to an air/fuel ratio richer than a
stoichiometric air/fuel ratio for a purpose other than a purpose of
determining whether there is a difference in the air/fuel ratio
between the combustion chambers; detecting the air/fuel ratios of
exhaust gas discharged from the plurality of combustion chambers of
the multicylinder internal combustion engine; and controlling the
air/fuel ratio of the mixture in each of the combustion chambers to
the air/fuel ratio richer than the stoichiometric air/fuel ratio
when the first condition and the second condition are satisfied,
and then determining whether there is a difference in the air/fuel
ratio of the mixture between the combustion chambers based on the
detected air/fuel ratios.
Description
[0001] The disclosure of Japanese Patent Application No. 2010-27674
filed on Feb. 10, 2010, including the specification, drawings and
abstract is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a multicylinder internal combustion
engine, and to a determination apparatus and a determination method
that determine the presence or absence of inter-cylinder air/fuel
ratio imbalance.
[0004] 2. Description of the Related Art
[0005] In a multicylinder internal combustion engine in which fuel
injection valves are disposed corresponding to each one of a
plurality of combustion chambers so that fuel is injected from the
fuel injection valves into the corresponding combustion chambers,
it is preferable that the amounts of fuel injected from the fuel
injection valves be all controlled to equal and optimum amounts, in
order to minimize the exhaust emissions discharged from the
combustion chambers. That is, in the multicylinder internal
combustion engine, for example, in the case where the amount of
fuel to be injected from the fuel injection valves is set at an
amount that will minimize the exhaust emissions discharged from the
combustion chambers (i.e., an optimum amount), there is a
possibility of the exhaust emissions from the combustion chambers
increasing unexpectedly if the amount of fuel injected from any one
of the fuel injection valves is not controlled to the foregoing
optimum amount.
[0006] Therefore, in the foregoing multicylinder internal
combustion engine, if there occurs an event in which the amount of
fuel injected from any one of the fuel injection valves is not
controlled to the optimum amount, that is, in which there is a
difference among the air/fuel ratios of the mixtures formed in the
combustion chambers, that is, a difference in the air/fuel ratio of
the mixture among the combustion chambers, it is very important to
know about that event, from the viewpoint of minimizing the exhaust
emissions discharged from the combustion chambers.
[0007] An apparatus for detecting the event in which there is a
difference in the air/fuel ratio of the mixture among the
combustion chambers (hereinafter, referred to as "inter-cylinder
air/fuel ratio imbalance") is disclosed in U.S. Pat. No.
7,152,594.
[0008] In the apparatus disclosed in U.S. Pat. No. 7,152,594, an
air/fuel ratio sensor that detects the air/fuel ratio of exhaust
gas by detecting the oxygen concentration in the exhaust gas is
disposed in the exhaust passageway. This apparatus compares the
frequency of changes in the output value of the air/fuel ratio
sensor with a predetermined reference value, and determines that an
inter-cylinder air/fuel ratio imbalance is present if the frequency
of changes in the output value of the air/fuel ratio sensor has
deviated from a predetermined reference value concerned therewith.
Alternatively, the apparatus compares the length of the signal
trace of the output of the air/fuel ratio sensor with a
predetermined reference value, and determines that an
inter-cylinder air/fuel ratio imbalance is present if the length of
the signal trace of the output of the air/fuel ratio sensor has
deviated from a predetermined reference value concerned
therewith.
[0009] By the way, in the multicylinder internal combustion engine
disclosed in U.S. Pat. No. 7,152,594, the output characteristic of
the air/fuel ratio sensor varies depending on the air/fuel ratio of
the mixture formed in each combustion chamber (hereinafter, simply
referred to as "air/fuel ratio of the mixture"). That is, the
output characteristic of the air/fuel ratio sensor varies among the
case in which the air/fuel ratio of the mixture is controlled to
the stoichiometric air/fuel ratio or substantially to the
stoichiometric air/fuel ratio, the case where the air/fuel ratio of
the mixture is controlled to an air/fuel ratio that is leaner than
the stoichiometric air/fuel ratio, and the case where the air/fuel
ratio of the mixture is controlled to an air/fuel ratio that is
richer than the stoichiometric air/fuel ratio. Therefore, in each
of these cases, the accuracy of the determination regarding the
inter-cylinder air/fuel ratio imbalance varies.
SUMMARY OF THE INVENTION
[0010] The invention provides an apparatus and a method that
accurately determines the inter-cylinder air/fuel ratio imbalance
in an multicylinder internal combustion engine.
[0011] A first aspect of the invention relates to a multicylinder
internal combustion engine which includes: a plurality of
combustion chambers; fuel injection valves disposed corresponding
to the individual combustion chambers; an exhaust control catalyst
disposed in an exhaust passageway so as to remove a specific
component of exhaust gas discharged from the combustion chambers;
an upstream-side air/fuel ratio sensor disposed in the exhaust
passageway upstream of the exhaust control catalyst so as to detect
air/fuel ratio of the exhaust gas discharged from the combustion
chambers; and a downstream-side air/fuel ratio sensor disposed in
the exhaust passageway downstream of the exhaust control catalyst
so as to detect the air/fuel ratio of the exhaust gas that flows
out from the exhaust control catalyst; and a control device that
executes a first abnormality determination-purpose rich air/fuel
ratio control of controlling the air/fuel ratio of a mixture formed
in each combustion chamber to an air/fuel ratio richer than a
stoichiometric air/fuel ratio when it needs to be determined
whether the downstream-side air/fuel ratio sensor is abnormal. The
multicylinder internal combustion engine further includes a
determination device that executes an inter-cylinder air/fuel ratio
imbalance determination of estimating the air/fuel ratio of the
mixture formed in each combustion chamber based on an output of the
upstream-side air/fuel ratio sensor when the first abnormality
determination-purpose rich air/fuel ratio control is being
executed, and of determining whether there is a difference between
the air/fuel ratios of the mixtures that are estimated.
[0012] In the case where the air/fuel ratio of the mixture formed
in each of the combustion chambers is estimated on the basis of the
output value of the upstream-side air/fuel ratio sensor disposed in
the exhaust passageway in order to detect the air/fuel ratio of
exhaust gas discharged from the combustion chambers, the accuracy
of the estimation is higher when the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio than when the air/fuel ratio of the
mixture is being controlled to the stoichiometric air/fuel ratio or
to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.
Therefore, in the case where it is determined whether there is a
difference in the air/fuel ratio of the mixture between the
combustion chambers through the use of the air/fuel ratios of the
mixtures formed in the combustion chambers which are estimated on
the basis of output values of the upstream-side air/fuel ratio
sensor, the accuracy of the determination is also higher when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio than when the
air/fuel ratio of the mixture is being controlled to the
stoichiometric air/fuel ratio or being controlled to an air/fuel
ratio leaner than the stoichiometric air/fuel ratio.
[0013] According to the first aspect of the invention, when the
air/fuel ratio of the mixture formed in each combustion chamber is
being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio in order to determine whether the
downstream-side air/fuel ratio sensor has abnormality, the air/fuel
ratio of the mixture formed in each combustion chamber is estimated
on the basis of the output value of the upstream-side air/fuel
ratio sensor. On the basis of the estimated air/fuel ratios of the
mixtures, it is determined whether there is a difference in the
air/fuel ratio of the mixture among the combustion chambers.
Therefore, the accuracy of the determination is high.
[0014] Furthermore, according to the first aspect of the invention,
it is determined whether there is a difference in the air/fuel
ratio of the mixture among the combustion chambers when the
air/fuel ratios of the mixtures formed in the combustion chambers
are being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio in order to determine whether the
downstream-side air/fuel ratio sensor has abnormality. Therefore,
an increased accuracy of the determination as to whether there is a
difference in the air/fuel ratio of the mixture among the
combustion chambers is achieved, and decline of the fuel economy of
the internal combustion engine can be restrained.
[0015] In the multicylinder internal combustion engine of the first
aspect, the control device may execute a second abnormality
determination-purpose rich air/fuel ratio control of controlling
the air/fuel ratio of the mixture formed in each combustion chamber
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
when it needs to be determined whether the upstream-side air/fuel
ratio sensor is abnormal, and the determination device may execute
the inter-cylinder air/fuel ratio imbalance determination when the
second abnormality determination-purpose rich air/fuel ratio
control is being executed.
[0016] With this construction, since the inter-cylinder air/fuel
ratio imbalance determination is executed also when the air/fuel
ratio of the mixture is being controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio in order to determine
whether the upstream-side air/fuel ratio sensor is abnormal, the
frequency of occasions to execute the inter-cylinder air/fuel ratio
imbalance determination increases.
[0017] Besides, in the foregoing multicylinder internal combustion
engine, the determination device may execute the inter-cylinder
air/fuel ratio imbalance determination if it is determined that the
upstream-side air/fuel ratio sensor is not abnormal when the second
abnormality determination-purpose rich air/fuel ratio control is
being executed.
[0018] With this construction, the inter-cylinder air/fuel ratio
imbalance determination is executed on the basis of output values
of the upstream-side air/fuel ratio sensor that is normal, so that
the accuracy of the inter-cylinder air/fuel ratio imbalance
determination is high.
[0019] A second aspect of the invention relates to a multicylinder
internal combustion engine comprising: a plurality of combustion
chambers; fuel injection valves disposed corresponding to the
individual combustion chambers; an exhaust control catalyst
disposed in an exhaust passageway so as to remove a specific
component of exhaust gas discharged from the combustion chambers;
an upstream-side air/fuel ratio sensor disposed in the exhaust
passageway upstream of the exhaust control catalyst so as to detect
air/fuel ratio of the exhaust gas discharged from the combustion
chambers; a control device that executes a second abnormality
determination-purpose rich air/fuel ratio control of controlling
the air/fuel ratio of a mixture formed in each combustion chamber
to an air/fuel ratio richer than a stoichiometric air/fuel ratio
when it needs to be determined whether the upstream-side air/fuel
ratio sensor is abnormal; and a determination device that executes
an inter-cylinder air/fuel ratio imbalance determination of
estimating the air/fuel ratio of the mixture formed in each
combustion chamber based on an output of the upstream-side air/fuel
ratio sensor when the second abnormality determination-purpose rich
air/fuel ratio control is being executed, and of determining
whether there is a difference between the air/fuel ratios of the
mixtures that are estimated.
[0020] In the case where the air/fuel ratio of the mixture formed
in each of the combustion chambers is estimated on the basis of the
output value of the upstream-side air/fuel ratio sensor disposed in
the exhaust passageway in order to detect the air/fuel ratio of
exhaust gas discharged from the combustion chambers, the accuracy
of the estimation is higher when the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio than when the air/fuel ratio of the
mixture is being controlled to the stoichiometric air/fuel ratio or
to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.
Therefore, in the case where it is determined whether there is a
difference in the air/fuel ratio of the mixture between the
combustion chambers through the use of the air/fuel ratios of the
mixtures formed in the combustion chambers which are estimated on
the basis of output values of the upstream-side air/fuel ratio
sensor, the accuracy of the determination is also higher when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio than when the
air/fuel ratio of the mixture is being controlled to the
stoichiometric air/fuel ratio or to an air/fuel ratio leaner than
the stoichiometric air/fuel ratio.
[0021] According to the second aspect of the invention, when the
air/fuel ratio of the mixture formed in each combustion chamber is
being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio in order to determine whether the
upstream-side air/fuel ratio sensor has abnormality, the air/fuel
ratio of the mixture formed in each combustion chamber is estimated
on the basis of the output value of the upstream-side air/fuel
ratio sensor. On the basis of the estimated air/fuel ratios of the
mixtures, it is determined whether there is a difference in the
air/fuel ratio of the mixture among the combustion chambers.
Therefore, the accuracy of the determination is high.
[0022] Furthermore, according to the second aspect of the
invention, it is determined whether there is a difference in the
air/fuel ratio of the mixture among the combustion chambers when
the air/fuel ratios of the mixtures formed in the combustion
chambers are being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio in order to determine whether the
upstream-side air/fuel ratio sensor has abnormality. Therefore,
according to the invention, it is possible to increase the accuracy
of the determination as to whether there is a difference in the
air/fuel ratio of the mixture among the combustion chambers while
restraining decline in the fuel economy of the internal combustion
engine.
[0023] In the multicylinder internal combustion engine of the
second aspect, the determination device may execute the
inter-cylinder air/fuel ratio imbalance determination if it is
determined that the upstream-side air/fuel ratio sensor is not
abnormal when the second abnormality determination-purpose rich
air/fuel ratio control is being executed.
[0024] With this construction, the inter-cylinder air/fuel ratio
imbalance determination is executed on the basis of output values
of the upstream-side air/fuel ratio sensor that is normal, so that
the accuracy of the inter-cylinder air/fuel ratio imbalance
determination is high.
[0025] In the foregoing multicylinder internal combustion engine,
the control device may execute at least one of: (1) an engine
start-time rich air/fuel ratio control of controlling the air/fuel
ratio of the mixture formed in each combustion chamber to an
air/fuel ratio richer than the stoichiometric air/fuel ratio when
operation of the multicylinder internal combustion engine is
started; (2) a post-fuel injection stop rich air/fuel ratio control
of controlling the air/fuel ratio of the mixture formed in each
combustion chamber to an air/fuel ratio richer than the
stoichiometric air/fuel ratio when injection of fuel from the fuel
injection valves is re-started after the injection of the fuel from
the fuel injection valves is stopped; and (3) an exhaust control
catalyst-purpose rich air/fuel ratio control of controlling the
air/fuel ratio of the mixture formed in each combustion chamber to
an air/fuel ratio richer than the stoichiometric air/fuel ratio
when temperature of the exhaust control catalyst is higher than a
predetermined permissible upper-limit temperature, and the
determination device may execute the inter-cylinder air/fuel ratio
imbalance determination when the engine start-time rich air/fuel
ratio control, the post-fuel injection stop rich air/fuel ratio
control or the exhaust control catalyst-purpose rich air/fuel ratio
control is being executed.
[0026] With this construction, since the inter-cylinder air/fuel
ratio imbalance determination is executed when operation of the
multicylinder internal combustion engine is started, or when the
injection of fuel from the fuel injection valves is re-started
after the injection of fuel from the fuel injection valves is
stopped, or when the temperature of the exhaust control catalyst is
higher than the predetermined permissible upper-limit temperature,
the frequency of occasions to execute the inter-cylinder air/fuel
ratio imbalance determination increases.
[0027] A third aspect of the invention relates to a multicylinder
internal combustion engine that includes: a plurality of combustion
chambers; fuel injection valves disposed corresponding to the
individual combustion chambers; an exhaust control catalyst
disposed in an exhaust passageway so as to remove a specific
component of exhaust gas discharged from the combustion chambers;
and an upstream-side air/fuel ratio sensor disposed in the exhaust
passageway upstream of the exhaust control catalyst so as to detect
air/fuel ratio of the exhaust gas discharged from the combustion
chambers. This multicylinder internal combustion engine further
includes: a control device that controls the air/fuel ratio of a
mixture formed in each of the combustion chambers to an air/fuel
ratio richer than a stoichiometric air/fuel ratio for a purpose
other than a purpose of determining whether there is a difference
in the air/fuel ratio of the mixture between the combustion
chambers; and a determination device that determines whether there
is a difference in the air/fuel ratio of the mixture between the
combustion chambers based on an output of the upstream-side
air/fuel ratio sensor when the air/fuel ratio of the mixture is
being controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio for the purpose other than the
purpose of determining whether there is a difference in the
air/fuel ratio of the mixture between the combustion chambers.
[0028] A fourth aspect of the invention relates to an
inter-cylinder air/fuel ratio imbalance determination apparatus.
This inter-cylinder air/fuel ratio imbalance determination
apparatus includes an upstream-side air/fuel ratio sensor disposed
in an exhaust passageway upstream of an exhaust control catalyst so
as to detect air/fuel ratio of exhaust gas discharged from a
plurality of combustion chambers of a multicylinder internal
combustion engine. The exhaust control catalyst is disposed in the
exhaust passageway so as to remove a specific component of the
exhaust gas discharged from the combustion chambers. The
inter-cylinder air/fuel ratio imbalance determination apparatus
further includes: a control device that controls the air/fuel ratio
of a mixture in each of the plurality of combustion chambers to an
air/fuel ratio richer than a stoichiometric air/fuel ratio for a
purpose other than a purpose of determining whether there is a
difference in the air/fuel ratio of the mixture between the
combustion chambers; and a determination device that determines
whether there is a difference in the air/fuel ratio of the mixture
between the combustion chambers based on an output of the
upstream-side air/fuel ratio sensor when the air/fuel ratio of the
mixture is being controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio for the purpose other than the
purpose of determining whether there is a difference in the
air/fuel ratio of the mixture between the combustion chambers.
[0029] A fifth aspect of the invention relates to an inter-cylinder
air/fuel ratio imbalance determination method that includes:
determining whether a first condition for determining whether there
is a difference in air/fuel ratio between a plurality of combustion
chambers of a multicylinder internal combustion engine is
satisfied; determining whether a second condition for controlling
the air/fuel ratio of a mixture formed in each combustion chamber
to an air/fuel ratio richer than a stoichiometric air/fuel ratio
for a purpose other than a purpose of determining whether there is
a difference in the air/fuel ratio between the combustion chambers;
detecting the air/fuel ratios of exhaust gas discharged from the
plurality of combustion chambers of the multicylinder internal
combustion engine; and controlling the air/fuel ratio of the
mixture in each of the combustion chambers to the air/fuel ratio
richer than the stoichiometric air/fuel ratio when the first
condition and the second condition are satisfied, and then
determining whether there is a difference in the air/fuel ratio of
the mixture between the combustion chambers based on the detected
air/fuel ratios.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The features, advantages, and technical and industrial
significance of this invention will be described below with
reference to the accompanying drawings, in which like numerals
denote like elements, and wherein:
[0031] FIG. 1 is an overall diagram of a spark ignition type
multicylinder internal combustion engine to which an inter-cylinder
air/fuel ratio imbalance determination apparatus in accordance with
an embodiment of the invention is applied;
[0032] FIG. 2 is a diagram showing the exhaust gas purification
performances of an upstream-side catalyst and a downstream-side
catalyst;
[0033] FIG. 3A is a diagram showing an output characteristic of an
upstream-side air/fuel ratio sensor, and FIG. 3B is a diagram
showing an output characteristic of a downstream-side air/fuel
ratio sensor;
[0034] FIG. 4 is a diagram showing a map for use for determining
the target air/fuel ratio;
[0035] FIG. 5 is a diagram showing an example of a flowchart for
calculating a duration of injection of fuel from the fuel injection
valve;
[0036] FIG. 6 is a diagram showing an example of a flowchart for
calculating an air/fuel ratio correction coefficient;
[0037] FIG. 7 is a diagram showing an example of a flowchart for
calculating a stepwise increase value and a stepwise reduction
value;
[0038] FIG. 8A is a diagram showing transition of the output value
of the upstream-side air/fuel ratio sensor when all the fuel
injection valves are normal while the air/fuel ratio of the mixture
is being controlled to the stoichiometric air/fuel ratio, and FIG.
8B is a diagram showing transition of the output value of the
upstream-side air/fuel ratio sensor when the fuel injection valve
that corresponds to the first cylinder #1 has an abnormality of
injecting an amount of fuel that is larger than a command value of
fuel injection amount and the other fuel injection valves are
normal while the air/fuel ratio of the mixture is being controlled
to the stoichiometric air/fuel ratio, and FIG. 8C is a diagram
showing transition of the output value of the upstream-side
air/fuel ratio sensor when the fuel injection valve that
corresponds to the first cylinder #1 has an abnormality of
injecting an amount of fuel that is less than the command value of
fuel injection amount and the other fuel injection valves are
normal while the air/fuel ratio of the mixture is being controlled
to the stoichiometric air/fuel ratio;
[0039] FIG. 9A is a diagram showing transition of the output value
of the upstream-side air/fuel ratio sensor when all the fuel
injection valve are normal while the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio, and FIG. 9B is a diagram
showing transition of the output value of the upstream-side
air/fuel ratio sensor when the fuel injection valve that
corresponds to the first cylinder #1 has an abnormality of
injecting an amount of fuel that is larger than a command value of
fuel injection amount and the other fuel injection valves are
normal while the air/fuel ratio of the mixture is being controlled
to the air/fuel ratio richer than the stoichiometric air/fuel
ratio, and FIG. 9C is a diagram showing transition of the output
value of the upstream-side air/fuel ratio sensor when the fuel
injection valve that corresponds to the first cylinder #1 has an
abnormality of injecting an amount of fuel that is less than the
command value of fuel injection amount and the other fuel injection
valves are normal while the air/fuel ratio of the mixture is being
controlled to the air/fuel ratio richer than the stoichiometric
air/fuel ratio;
[0040] FIG. 10 is a schematic partial perspective view showing a
portion of the upstream-side air/fuel ratio sensor;
[0041] FIG. 11 is a partial sectional view showing a portion of the
upstream-side air/fuel ratio sensor;
[0042] FIG. 12A is a sectional view showing a construction of an
air/fuel ratio detection element of the upstream-side air/fuel
ratio sensor, and FIG. 12B is a sectional view showing a state of
the air/fuel ratio detection element when exhaust gas whose
air/fuel ratio is leaner than the stoichiometric air/fuel ratio
comes to the air/fuel ratio detection element of the upstream-side
air/fuel ratio sensor, and FIG. 12C is a state of the air/fuel
ratio detection element when exhaust gas whose air/fuel ratio is
richer than the stoichiometric air/fuel ratio comes to the air/fuel
ratio detection element of the upstream-side air/fuel ratio
sensor;
[0043] FIG. 13 is a diagram showing a relation between the air/fuel
ratio of exhaust gas that comes to the upstream-side air/fuel ratio
sensor and the limiting current value that the upstream-side
air/fuel ratio sensor outputs;
[0044] FIG. 14 is a diagram showing a flowchart of an example of a
routine of executing the determination of the presence or absence
of the state of inter-cylinder air/fuel ratio imbalance in
accordance with a first embodiment;
[0045] FIG. 15 is a diagram showing a flowchart of an example of a
routine of executing the setting of an inter-cylinder air/fuel
ratio imbalance determination execution flag that shows whether to
execute the determination of the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance in accordance with the
first embodiment;
[0046] FIG. 16 is a diagram showing a flowchart showing an example
of a routine of executing the setting of the inter-cylinder
air/fuel ratio imbalance determination execution flag that shows
whether to execute the determination of the presence or absence of
the state of inter-cylinder air/fuel ratio imbalance in accordance
with a second embodiment;
[0047] FIG. 17 is a diagram showing a flowchart of an example of a
routine of executing the setting of the inter-cylinder air/fuel
ratio imbalance determination execution flag that shows whether to
execute the determination of the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance in accordance with a
third embodiment;
[0048] FIG. 18 is a diagram showing a flowchart of an example of a
routine of executing the setting of the inter-cylinder air/fuel
ratio imbalance determination execution flag that shows whether to
execute the determination of the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance in accordance with a
fourth embodiment;
[0049] FIG. 19 is a diagram showing a flowchart of an example of a
routine of executing the setting of the inter-cylinder air/fuel
ratio imbalance determination execution flag that shows whether to
execute the determination of the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance in accordance with a
fifth embodiment;
[0050] FIG. 20 is a diagram showing a flowchart of an example of a
routine of executing the setting of the inter-cylinder air/fuel
ratio imbalance determination execution flag that shows whether to
execute the determination of the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance in accordance with a
sixth embodiment;
[0051] FIG. 21 is a sectional view showing a construction of an
air/fuel ratio detection element that is provided in an
upstream-side air/fuel ratio sensor and that has a catalyst;
and
[0052] FIG. 22 is a diagram showing a construction of a hybrid
system.
DETAILED DESCRIPTION OF EMBODIMENTS
[0053] FIG. 1 is an overall diagram of a spark ignition type
multicylinder internal combustion engine (hereinafter, simply
referred to as "internal combustion engine") to which an
inter-cylinder air/fuel ratio imbalance determination apparatus in
accordance with an embodiment of the invention is applied. The
spark ignition type multicylinder internal combustion engine
described below is a so-called four-stroke internal combustion
engine that cyclically undergoes four strokes, that is, the intake
stroke, the compression stroke, the expansion stroke and the
exhaust stroke.
[0054] The internal combustion engine 10 has a body (hereinafter,
referred to as "engine body") 20. The engine body 20 has a cylinder
block and a cylinder head. Besides, the engine body 20 has four
combustion chambers 21 each of which is defined by an internal wall
surface of a cylinder bore formed in the cylinder block, a top
surface of a piston disposed in the cylinder bore, and a lower wall
surface of the cylinder head.
[0055] FIG. 1 shows a combustion chamber (hereinafter, also
referred to as "first cylinder #1") 21 that is shown at the lowest
position, and a combustion chamber (hereinafter, also referred to
as "second cylinder #2") 21 that is shown immediately above the
first cylinder #1, and a combustion chamber (hereinafter, also
referred to as "third cylinder #3") 21 that is shown immediately
above the second cylinder #2, and a combustion chamber
(hereinafter, also referred to as "fourth cylinder #4") 21 that is
shown immediately above the third cylinder #3.
[0056] Besides, the cylinder head is provided with intake ports 22
that communicate with the combustion chambers 21. Through the
intake ports 22, air is taken into the combustion chambers 21. The
intake ports 22 are opened and closed by intake valves (not shown).
The cylinder head is also provided with exhaust ports 23 that
communicate with the combustion chambers 21. Exhaust gas is
discharged from the combustion chambers 21 into the exhaust ports
23. The exhaust ports 23 are opened and closed by exhaust valves
(not shown).
[0057] Besides, in the cylinder head, ignition plugs 24 are
disposed corresponding to the individual combustion chambers 21.
Each ignition plug 24 is disposed in the cylinder head so as to be
exposed in a corresponding one of the combustion chambers 21 and
therefore be able to ignite a mixture of fuel and air, which is
formed in the corresponding combustion chamber 21. Furthermore, in
the cylinder head, fuel injection valves 25 are disposed
corresponding to the intake ports 22. The fuel injection valves 25
are disposed in the cylinder head so as to be exposed in the intake
ports 22 and therefore be able to inject fuel into the intake ports
22.
[0058] An intake manifold 31 is connected to the intake ports 22.
The intake manifold 31 has branch portions each of which is
connected to a corresponding one of the intake ports 22, and a
surge tank portion where the branch portions meet. Besides, an
intake pipe 32 is connected to the surge tank portion of the intake
manifold 31. In this embodiment, the intake ports 22, the intake
manifold 31 and the intake pipe 32 form an intake passageway 30. An
air filter 33 is disposed in the intake pipe 32. A throttle valve
34 is pivotably disposed in the intake pipe 32 between the air
filter 33 and the intake manifold 31. An actuator 34a that drives
the throttle valve 34 is connected to the throttle valve 34. The
throttle 34 is pivoted by the actuator 34a so as to change the flow
path area in the intake pipe 31, whereby the amount of air taken
into the combustion chambers 21 is controlled.
[0059] On the other hand, the exhaust ports 23 are connected to an
exhaust manifold 41. The exhaust manifold 41 has branch portions
41a each of which is connected to a corresponding one of the
exhaust ports 23, and an exhaust confluence portion 41b where the
branch portions 41a meet. An exhaust pipe 42 is connected to the
exhaust confluence portion 41b of the exhaust manifold 41. In this
embodiment, the exhaust ports 23, the exhaust manifold 41 and the
exhaust pipe 42 form an exhaust passageway 40. Besides, an exhaust
gas control catalyst 43 that substantially removes specific
components from the exhaust gas (hereinafter, referred to as
"upstream-side catalyst 43") is disposed in the exhaust pipe 42.
The exhaust pipe 42 downstream of the upstream-side catalyst 43 is
provided with an exhaust gas control catalyst 44 that also
substantially removes specific components from exhaust gas
(hereinafter, referred to as "downstream-side catalyst 44").
[0060] The upstream-side catalyst 43 is a so-called three-way
catalyst that is able to simultaneously remove nitrogen oxides
(hereinafter, termed NOx), carbon monoxide (hereinafter, termed CO)
and hydrocarbons (hereinafter, termed HCs) from exhaust gas at high
removal rates when the temperature of the catalyst is higher than a
certain temperature (i.e., a so-called activation temperature) and
the air/fuel ratio of exhaust gas that flows into the catalyst is
within a range X in the vicinity of the stoichiometric air/fuel
ratio, as shown in FIG. 2. On the other hand, the upstream-side
catalyst 43 has a capability of storing oxygen from exhaust gas
when the air/fuel ratio of the exhaust gas that flows into the
catalyst is leaner than the stoichiometric air/fuel ratio, and of
releasing oxygen stored in the catalyst when the air/fuel ratio of
the exhaust gas that flows into the catalyst is richer than the
stoichiometric air/fuel ratio (hereinafter, this capability will be
referred to as "oxygen storage/release capability"). Therefore, as
long as this oxygen storage/release capability functions normally,
an internal atmosphere in the upstream-catalyst 43 is maintained
substantially in the vicinity of the stoichiometric air/fuel ratio
regardless of whether the air/fuel ratio of the exhaust gas that
flows into the upstream-side catalyst 43 is leaner or richer than
the stoichiometric air/fuel ratio. Therefore, the upstream-side
catalyst 43 simultaneously removes NOx, CO and HCs from exhaust gas
at high removal rates.
[0061] The downstream-side catalyst 44 is also a so-called
three-way catalyst, and is able to simultaneously remove NOx, CO
and HCs at high removal rates and also has the oxygen
storage/release capability, similarly to the upstream-side catalyst
43.
[0062] The intake pipe 32 is also provided with an air flow meter
51 that detects the amount of air that flows in the intake pipe 32,
that is, the amount of air that is taken into the combustion
chambers 21 (hereinafter, this amount of air will be referred to as
"intake gas amount").
[0063] Besides, a crank position sensor 53 that detects the
rotation phase of a crankshaft (not shown) is disposed on the
engine body 20. The crank position sensor 53 outputs a short pulse
every time the crank shaft rotates 10.degree., and outputs a long
pulse every time the crankshaft rotates 360.degree.. The rotation
speed of the crankshaft, that is, the engine rotation speed, is
calculated on the basis these pulses. Besides, an accelerator
operation amount sensor 57 detects the amount of depression of an
accelerator pedal AP.
[0064] The exhaust pipe 42 (the exhaust confluence portion 41b)
upstream of the upstream-side catalyst 43 is provided with an
air/fuel ratio sensor (hereinafter, referred to as "upstream-side
air/fuel ratio sensor") 55 that detects the air/fuel ratio of
exhaust gas. Furthermore, the exhaust pipe 42 downstream of the
upstream-side catalyst 43 but upstream of the downstream-side
catalyst 44 is provided with an air/fuel ratio sensor (hereinafter,
referred to as "downstream-side air/fuel ratio sensor") 56 that
detects the air/fuel ratio of exhaust gas similarly to the
upstream-side air/fuel ratio sensor 43.
[0065] The upstream-side air/fuel ratio sensor 55 is a so-called
limiting-current type oxygen concentration sensor that outputs an
output value I that is smaller the richer the detected air/fuel
ratio of exhaust gas and that is greater the leaner the detected
air/fuel ratio, as shown in FIG. 3A.
[0066] On the other hand, the downstream-side air/fuel ratio sensor
56 is a so-called electromotive force type oxygen concentration
sensor that, as show in FIG. 3B, outputs a relatively large
constant output value Vg when the detected air/fuel ratio of
exhaust gas is richer than the stoichiometric air/fuel ratio, and
that outputs a relatively small constant output value Vs when the
detected air/fuel ratio of exhaust gas is leaner than the
stoichiometric air/fuel ratio. Furthermore, the downstream-side
air/fuel ratio sensor 56 outputs an intermediate output value Vm
between the relatively large constant output value Vg and the
relatively small constant output value Vs when the detected
air/fuel ratio of exhaust gas is substantially equal to the
stoichiometric air/fuel ratio.
[0067] An electric control unit (ECU) 60 shown in FIG. 1 includes a
microcomputer having a CPU (microprocessor) 61, a ROM (read-only
memory) 62, a RAM (random access memory) 63, a backup RAM 64, an
interface 65 that includes an AD converter. These components are
interconnected by a bidirectional bus. The interface 65 is
connected to the ignition plugs 24, the fuel injection valves 25,
and the actuator 34a of the throttle valve 34. Besides, the air
flow meter 51, the crank position sensor 53, the upstream-side
air/fuel ratio sensor 55, the downstream-side air/fuel ratio 56,
and the accelerator operation amount sensor 57 are also connected
to the interface 65.
[0068] By the way, in this embodiment, the air/fuel ratio TA/F that
is set as a target of the air/fuel ratio of a mixture formed in
each of the combustion chambers 21 according to the state of
operation of the internal combustion engine, particularly the
engine rotation speed and the engine load (hereinafter, the mixture
formed in the combustion chambers will be referred to simply as
"mixture"), which will hereinafter be referred to simply as "target
air/fuel ratio", is stored beforehand in the electronic control
unit 60 in the form of a map of a function between the engine
rotation speed N and the engine load L, as shown in FIG. 4. Then,
during operation of the internal combustion engine (hereinafter,
referred to as "during operation of the engine"), the target
air/fuel ratio TA/F commensurate with the engine rotation speed N
and the engine load L is retrieved, and the amount of fuel injected
from each fuel injection valve 25 (hereinafter, referred to as
"fuel injection amount") is controlled according to the intake gas
amount detected by the air flow meter 51 so that the air/fuel ratio
of the mixture becomes equal to the target air/fuel ratio.
Incidentally, the intake gas amount is controlled by controlling
the degree of opening of the throttle valve 34 according to the
output demanded of the internal combustion engine.
[0069] The control of the fuel injection amount performed when the
target air/fuel ratio is the stoichiometric air/fuel ratio and the
air/fuel ratio of the mixture is controlled to the stoichiometric
air/fuel ratio will be described below.
[0070] When it is detected by the upstream-side air/fuel ratio
sensor 55 that the air/fuel ratio of exhaust gas is leaner than the
stoichiometric air/fuel ratio, it means that the air/fuel ratio of
the mixture is leaner than the stoichiometric air/fuel ratio. At
this time, according to this embodiment, the fuel injection amount
is gradually increased so that the air/fuel ratio of the mixture
approaches the stoichiometric air/fuel ratio. On the other hand,
when it is detected by the upstream-side air/fuel ratio sensor 55
that the air/fuel ratio of exhaust gas is richer than the
stoichiometric air/fuel ratio, it means that the air/fuel ratio of
the mixture is richer than the stoichiometric air/fuel ratio. At
this time, according to this embodiment, the fuel injection amount
is gradually decreased so that the air/fuel ratio of the mixture
approaches the stoichiometric air/fuel ratio. By controlling the
fuel injection amount in this manner, the air/fuel ratio of the
mixture is controlled to the stoichiometric air/fuel ratio as a
whole.
[0071] By the way, when the fuel injection amount is controlled as
described above, the air/fuel ratio of the mixture changes about
the stoichiometric air/fuel ratio, that is, becomes richer than the
stoichiometric air/fuel ratio at some times, and becomes leaner
than the stoichiometric air/fuel ratio at some other times. In
other words, the air/fuel ratio of the mixture oscillates about the
stoichiometric air/fuel ratio. From the viewpoint of controlling
the air/fuel ratio of the mixture to the stoichiometric air/fuel
ratio, it is desirable that the amplitude of the oscillation of the
air/fuel ratio of the mixture about the stoichiometric air/fuel
ratio be small. Specifically, when the air/fuel ratio of the
mixture is leaner than the stoichiometric air/fuel ratio, it is
desirable to change the air/fuel ratio of the mixture closer to the
stoichiometric air/fuel ratio as quickly as possible, and when the
air/fuel ratio of the mixture is richer than the stoichiometric
air/fuel ratio, it is desirable to change the air/fuel ratio of the
mixture closer to the stoichiometric air/fuel ratio as quickly as
possible.
[0072] Therefore, in this embodiment, when it is detected by the
upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of
the mixture has switched from an air/fuel ratio that is leaner than
the stoichiometric air/fuel ratio to an air/fuel ratio that is
richer than the stoichiometric air/fuel ratio, the fuel injection
amount is reduced by a relatively large amount in a stepwise
manner. According to this operation, when the air/fuel ratio of the
mixture has switched from an air/fuel ratio that is leaner than the
stoichiometric air/fuel ratio to an air/fuel ratio that is richer
than the stoichiometric air/fuel ratio, the air/fuel ratio of the
mixture is changed closer to the stoichiometric air/fuel ratio by a
relatively large amount of change. On the other hand, when it is
detected by the upstream-side air/fuel ratio sensor 55 that the
air/fuel ratio of the mixture has switched from an air/fuel ratio
that is richer than the stoichiometric air/fuel ratio to an
air/fuel ratio that is leaner than the stoichiometric air/fuel
ratio, the fuel injection amount is increased by a relatively large
amount in a stepwise manner. According to this operation, when the
air/fuel ratio of the mixture has switched from an air/fuel ratio
that is richer than the stoichiometric air/fuel ratio to an
air/fuel ratio that is leaner than the stoichiometric air/fuel
ratio, the air/fuel ratio of the mixture is changed closer to the
stoichiometric air/fuel ratio by a relatively large amount of
change. Thus, the amplitude of oscillation of the air/fuel ratio of
the mixture about the stoichiometric air/fuel ratio becomes
smaller.
[0073] By the way, in order to change the air/fuel ratio of the
mixture closer to the stoichiometric air/fuel ratio more quickly,
it is desirable that the amount by which the fuel injection amount
is reduced in the stepwise manner (hereinafter, the amount will be
referred to as "stepwise reduction value") when the air/fuel ratio
of the mixture has switched from the lean side of the
stoichiometric air/fuel ratio to the rich side thereof be made
larger the larger the difference between the air/fuel ratio of the
mixture and the stoichiometric air/fuel ratio which is found when
air/fuel ratio of the mixture has switched from the lean side of
the stoichiometric air/fuel ratio to the rich side thereof, and it
is desirable that the amount by which the fuel injection amount is
increased in the stepwise manner (hereinafter, the amount will be
referred to as "stepwise increase value") when the air/fuel ratio
of the mixture has switched from the rich side of the
stoichiometric air/fuel ratio to the lean side thereof be made
larger the larger the difference between the air/fuel ratio of the
mixture and the stoichiometric air/fuel ratio which is found when
the air/fuel ratio of the mixture has switched from the rich side
to the lean side of the stoichiometric air/fuel ratio.
[0074] Therefore, in this embodiment, the stepwise reduction value
and the stepwise increase value are controlled as follows.
[0075] That is, it can be said that the longer the period during
which the air/fuel ratio detected by the downstream-side air/fuel
ratio sensor 56 remains on the lean side of the stoichiometric
air/fuel ratio (hereinafter, referred to as "lean period"), the
greater the extent by which the air/fuel ratio of the mixture is
leaner than the stoichiometric air/fuel ratio. Specifically, the
air/fuel ratio of the exhaust gas that flows out of the
upstream-side catalyst 43 is considered to be equal to the
stoichiometric air/fuel ratio due to the oxygen storage/release
capability of the upstream-side catalyst 43. However, there occurs
a case where the lean period is long despite the oxygen
storage/release capability of the upstream-side catalyst 43. That
case can be said to be the case where a large amount of oxygen that
cannot be stored by the upstream-side catalyst 43 is flowing into
the upstream-side catalyst 43, that is, the case where the air/fuel
ratio of the mixture is leaner by a great extent than the
stoichiometric air/fuel ratio. Therefore, in this embodiment, when
it is detected by the upstream-side air/fuel ratio sensor 55 that
the air/fuel ratio of the mixture has switched from the rich side
to the lean side of the stoichiometric air/fuel ratio, the stepwise
increase value is made larger the longer the lean period.
[0076] On the other hand, it can be said that the longer the period
during which the air/fuel ratio on the rich side of the
stoichiometric air/fuel ratio is detected by the downstream-side
air/fuel ratio sensor 56 (hereinafter, referred to as "rich
period"), the greater the extent by which the air/fuel ratio of the
mixture is richer than the stoichiometric air/fuel ratio.
Specifically, the air/fuel ratio of the exhaust gas that flows out
of the upstream-side catalyst 43 is considered to be equal to the
stoichiometric air/fuel ratio due to the oxygen storage/release
capability of the upstream-side catalyst 43. However, there occurs
a case where the rich period is long despite the oxygen
storage/release capability of the upstream-side catalyst 43. That
case can be said to be the case where the amount of oxygen flowing
into the upstream-side catalyst 43 is so small that the entire
amount of oxygen stored in the upstream-side catalyst 43 is
released, that is, the case where the air/fuel ratio of the mixture
is richer than the stoichiometric air/fuel ratio by a great extent.
Therefore, in this embodiment, when it is detected by the
upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of
the mixture has switched from the lean side to the rich side of the
stoichiometric air/fuel ratio, the stepwise reduction value is made
larger the longer the rich period.
[0077] By controlling the fuel injection amount in this manner, the
air/fuel ratio of the mixture is accurately controlled to the
stoichiometric air/fuel ratio as a whole.
[0078] Next, an example of a flowchart for executing a control of
the fuel injection amount in accordance with this embodiment will
be described. As the flowchart for executing the control of the
fuel injection amount in accordance with this embodiment,
flowcharts shown in FIGS. 5 to 7 can be used.
[0079] FIG. 5 shows a flowchart for calculating the time during
which fuel is injected from each fuel injection valve 25. Upon
start of the routine shown in FIG. 5, firstly the proportion Ga/N
of the intake gas amount Ga to the engine rotation speed N is
calculated in step 10. Next in step 11, a value Ga/N.alpha.
obtained by multiplying the proportion Ga/N calculated in step 10
by a constant .alpha. is input as a basic fuel injection duration
TAUP. Next in step 12, a value TAUPFAF.beta..gamma. obtained by
multiplying the basic fuel injection duration TAUP calculated in
step 11 by an air/fuel ratio correction coefficient FAF (that is a
coefficient calculated by a routine shown in FIG. 6, and will be
described in detail later) as well as a constant .beta. and .alpha.
constant .gamma. that are determined according to the state of
operation of the internal combustion engine is input as a fuel
injection duration TAU. After that, the routine ends. In this
embodiment, fuel is injected from the fuel injection valves 25 for
the fuel injection duration TAU calculated in step 12.
[0080] FIG. 6 is a flowchart for calculating the air/fuel ratio
correction coefficient FAF that is for use in step 12 in FIG. 5.
When the routine shown in FIG. 6 starts, firstly in step 20 it is
determined whether the air/fuel ratio A/F detected by the
upstream-side air/fuel ratio sensor 55 is greater than the
stoichiometric air/fuel ratio A/Fst (A/F>A/Fst), that is,
whether the air/fuel ratio of the exhaust gas discharged from the
combustion chamber 21 is leaner than the stoichiometric air/fuel
ratio. If it is determined that A/F>A/Fst, the routine proceeds
to step 21 and subsequent steps. On the other hand, if it is
determined that A/FsA/Fst, the routine proceeds to step 25 and
subsequent steps.
[0081] If it is determined in step 20 that A/F>A/Fst, that is,
if it is determined that the air/fuel ratio of the exhaust gas
discharged from the combustion chamber 21 is leaner than the
stoichiometric air/fuel ratio, it is then determined in step 21
whether the air/fuel ratio of the exhaust gas detected by the
upstream-side air/fuel ratio sensor 55 has just switched from the
rich side to the lean side of the stoichiometric air/fuel ratio. If
it is determined in step 21 that the air/fuel ratio of the exhaust
gas has just switched from the rich side to the lean side of the
stoichiometric air/fuel ratio, the routine proceeds to step 22, in
which a value FAF+RSR obtained by adding a stepwise increase value
RSR (that is a value calculated by the routine shown in FIG. 7, and
will be described in detail later) to the air/fuel ratio correction
coefficient FAF calculated in the previous execution of the routine
of FIG. 6 is set as a new air/fuel ratio correction coefficient
FAF. Next in step 23, guarding is performed so that the air/fuel
ratio correction coefficient FAF becomes a value within a
permissible range. After that, the routine ends. On the other hand,
if in step 21 it is determined that the air/fuel ratio of exhaust
gas has not just switched from the rich side to the lean side of
the stoichiometric air/fuel ratio, the routine proceeds to step 24,
in which a value FAF+KIR obtained by adding a constant value KIR to
the air/fuel ratio correction coefficient FAF calculated in the
previous execution of the routine of FIG. 6 is set as a new
air/fuel ratio correction coefficient FAF. Next in step 23,
guarding is performed so that the air/fuel ratio correction
coefficient FAF calculated in step 24 becomes a value within the
permissible range. After that, the routine ends.
[0082] If in step 20 it is determined that A/FsA/Fst, that is, it
is determined that the air/fuel ratio of the exhaust gas discharged
from the combustion chamber 21 is richer than the stoichiometric
air/fuel ratio, the routine proceeds to step 25 as mentioned above.
In step 25, it is determined whether the air/fuel ratio of the
exhaust gas detected by the upstream-side air/fuel ratio sensor 55
has just switched from the lean side to the rich side of the
stoichiometric air/fuel ratio. If it is determined in step 25 that
the air/fuel ratio of the exhaust gas has just switched from the
lean side to the rich side of the stoichiometric air/fuel ratio,
the routine proceeds to step 26, in which a value FAF-RSL obtained
by subtracting a stepwise reduction value RSL (that is a value
calculated in the routine shown in FIG. 7, and will be described in
detail later) from the air/fuel ratio correction coefficient FAF
calculated in the previous execution of the routine of FIG. 6 is
set as a new air/fuel ratio correction coefficient FAF. Next in
step 23, guarding is performed so that the air/fuel ratio
correction coefficient FAF calculated in step 26 becomes a value
within the permissible range. After that, the routine ends. On the
other hand, if in step 25 it is determined that the air/fuel ratio
of the exhaust gas has not just switched from the lean side to the
rich side of the stoichiometric air/fuel ratio, the routine
proceeds to step 27, in which a value obtained by subtracting a
constant value KIL from the air/fuel ratio correction coefficient
FAF calculated in the previous execution of the routine of FIG. 6
is set as a new air/fuel ratio correction coefficient. Next in step
23, guarding is performed so that the air/fuel ratio correction
coefficient FAF calculated in step 27 becomes a value within the
permissible range. After that, the routine ends.
[0083] FIG. 7 is a flowchart for calculating the stepwise increase
value RSR for use in step 22 in FIG. 6 and the stepwise reduction
value RSL for use in step 26 in FIG. 6. When the routine shown in
FIG. 7 starts, it is firstly determined in step 30 whether the
air/fuel ratio A/F of the exhaust gas detected by the
downstream-side air/fuel ratio sensor 56 is greater than the
stoichiometric air/fuel ratio A/Fst (A/F>A/Fst), that is,
whether the air/fuel ratio of the exhaust gas flowing out of the
upstream-side catalyst 43 is leaner than the stoichiometric
air/fuel ratio. If it is determined in step 40 that A/F>A/Fst,
the routine proceeds to step 31. On the other hand, if it is
determined that A/F.ltoreq.A/Fst, the routine proceeds to step
34.
[0084] If in step 30 it is determined that A/F>A/Fst, that is,
if it is determined that the air/fuel ratio of the exhaust gas
flowing out of the upstream-side catalyst 43 is leaner than the
stoichiometric air/fuel ratio, then in step 31a value RSR+.DELTA.RS
obtained by adding a predetermined amount .DELTA.RS to the stepwise
increase value RSR calculated in the previous execution of the
routine of FIG. 7 is set as a new stepwise increase value RSR. Next
in step 32, guarding is performed so that the stepwise increase
value RSR calculated in step 31 becomes a value within a
permissible value. Next in step 33, a value obtained by subtracting
the stepwise increase value RSR guarded in step 32 from a constant
R is set as a new stepwise reduction value RSL. After that, the
routine ends.
[0085] On the other hand, if in step 30 it is determined that
A/F.ltoreq.A/Fst, that is, if it is determined that the air/fuel
ratio of the exhaust gas flowing out of the upstream-side catalyst
43 is richer than the stoichiometric air/fuel ratio, the routine
proceeds to step 34 as mentioned above. In step 34, a value
RSR-.DELTA.RS obtained by subtracting the predetermined value
.DELTA.RS from the stepwise increase value RSR calculated in the
previous execution of the routine of FIG. 7 is set as a new
stepwise increase value RSR. Next in step 32, guarding is performed
so that the stepwise increase value RSR calculated in step 34
becomes a value within the permissible range. Next in step 33, a
value obtained by subtracting the stepwise increase value RSR
guarded in step 32 from the constant value R is set as a new
stepwise increase value RSL. After that, the routine ends.
[0086] The internal combustion engine 10 has four fuel injection
valves 25. If, of the fuel injection valves 25, for example, one
fuel injection valve 25, has a fault, the following phenomenon
occurs.
[0087] In the embodiment, the amount of fuel injected from each
fuel injection valve 25 is controlled so that the air/fuel ratio of
the mixture becomes equal to the target air/fuel ratio, on the
basis of the air/fuel ratios of the exhaust gas detected by the
air/fuel ratio sensors 55 and 56. That is, if it is determined that
the air/fuel ratio of the mixture is leaner than the stoichiometric
air/fuel ratio on the basis of the air/fuel ratios of the exhaust
gas detected by the air/fuel ratio sensors 55 and 56, the fuel
injection amount of each fuel injection valve 25 is increased. If
it is determined that the air/fuel ratio of the mixture is richer
than the stoichiometric air/fuel ratio on the basis of the air/fuel
ratios of the exhaust gas detected by air/fuel ratio sensors 55 and
56, the fuel injection amount of each fuel injection valve 25 is
reduced. In other words, in this embodiment, the air/fuel ratio
sensors 55 and 56 are provided not separately for each combustion
chamber 21, but commonly for all the combustion chambers 25.
Therefore, when it is determined that the air/fuel ratio of the
mixture is leaner than the stoichiometric air/fuel ratio, it means
that it is determined that, in all the combustion chambers 21, the
air/fuel ratio of the mixture is leaner than the stoichiometric
air/fuel ratio. Likewise, when it is determined that the air/fuel
ratio of the mixture is richer than the stoichiometric air/fuel
ratio, it means that it is determined that, in all the combustion
chambers 21, the air/fuel ratio of the mixture is richer than the
stoichiometric air/fuel ratio. Therefore, if it is determined that
the air/fuel ratio of the mixture is leaner than the stoichiometric
air/fuel ratio, the fuel injection amount is increased with respect
to all the fuel injection valves 25. Likewise, if it is determined
that the air/fuel ratio of the mixture is richer than the
stoichiometric air/fuel ratio, the fuel injection amount is reduced
with respect to all the fuel injection valves 25.
[0088] For example, in the case where one of the fuel injection
valves 25 has a fault of injecting an amount of fuel that is larger
than the amount commanded by the electronic control unit 60
(hereinafter, referred to as "command value of fuel injection
amount") when the electronic control unit 60 outputs a command to
each fuel injection valve 25 such that all the fuel injection
valves 25 inject equal amounts of fuel (hereinafter, the fuel
injection valve with this fault will be referred to as "abnormal
fuel injection valve"), the air/fuel ratio of the mixture formed in
the combustion chamber 21 that corresponds to the abnormal fuel
injection valve 25 becomes richer than the stoichiometric air/fuel
ratio when the other fuel injection valves (hereinafter, referred
to as "normal fuel injection valves") 25 inject the command value
of injection amount of fuel and the air/fuel ratio of the mixture
formed in each of the corresponding combustion chambers 21 is equal
to the stoichiometric air/fuel ratio. Therefore, at this time, the
emission level of the exhaust gas discharged from the combustion
chamber 21 that corresponds to the abnormal fuel injection valve 25
deteriorates.
[0089] Then, when the exhaust gas discharged from the combustion
chamber 21 that corresponds to the abnormal fuel injection valve 25
reaches the upstream-side air/fuel ratio sensor 55, it is then
determined that the air/fuel ratio of the mixture is richer than
the stoichiometric air/fuel ratio, so that the fuel injection
amount is reduced with respect to all the fuel injection valves 25.
Hence, the air/fuel ratio of the mixture formed in each of the
combustion chambers 21 that correspond to the normal fuel injection
valves 25 becomes leaner than the stoichiometric air/fuel ratio.
Therefore, at this time, the emission level of the exhaust gas
discharged from the combustion chambers 21 that correspond to the
normal fuel injection valves 25 also deteriorates.
[0090] Of course, although the air/fuel ratio of the mixture formed
in the combustion chamber 21 that corresponds to the abnormal fuel
injection valve 25 occasionally becomes richer than the
stoichiometric air/fuel ratio and the air/fuel ratio of the mixture
formed in each of the combustion chambers 21 that correspond to the
normal fuel injection valves 25 occasionally becomes leaner than
the stoichiometric air/fuel ratio, the air/fuel ratio control of
this embodiment controls the fuel injection amount of the fuel
injection valves 25 so that the air/fuel ratio of the mixture
formed in the combustion chambers 21 will become equal to the
stoichiometric air/fuel ratio. Therefore, according to this
embodiment, it can be said that the air/fuel ratio of the mixture
is controlled to the stoichiometric air/fuel ratio as a whole.
However, although the air/fuel ratio of the mixture can be said to
be controlled to the stoichiometric air/fuel ratio as a whole, it
is true, in view of the air/fuel ratio of the mixture formed in
each one of the combustion chambers 21, that the air/fuel ratio of
the mixture formed in a combustion chamber 21 occasionally becomes
considerably richer than the stoichiometric air/fuel ratio and the
air/fuel ratio of the mixture formed in another combustion chamber
21 occasionally becomes considerably leaner than the stoichiometric
air/fuel ratio during execution of the air/fuel ratio control of
this embodiment, and therefore that the emission level of the
exhaust gas discharged from each combustion chamber 21 deteriorates
during the execution of the control.
[0091] On another hand, in the case where one of the fuel injection
valves 25 has a fault of injecting an amount of fuel that is
smaller than the amount commanded by the electronic control unit
60, that is, the command value of fuel injection amount, when the
electronic control unit 60 outputs to each fuel injection valve 25
a command such that all the fuel injection valves 25 inject equal
amounts of fuel (hereinafter, the fuel injection valve with this
fault will be also referred to as "abnormal fuel injection valve"),
the air/fuel ratio of the mixture formed in the combustion chamber
21 that corresponds to the abnormal fuel injection valve 25 becomes
leaner than the stoichiometric air/fuel ratio even when the normal
fuel injection valves 25 inject the command value of injection
amount of fuel and the air/fuel ratio of the mixture formed in the
corresponding combustion chambers 21 is equal to the stoichiometric
air/fuel ratio. Therefore, at this time, the emission level of the
exhaust gas discharged from the combustion chamber 21 that
corresponds to the abnormal fuel injection valve 25
deteriorates.
[0092] Then, as the exhaust gas discharged from the combustion
chamber 21 that corresponds to the abnormal fuel injection valve 25
reaches the upstream-side air/fuel ratio sensor 55, it is
determined that the air/fuel ratio of the mixture is leaner than
the stoichiometric air/fuel ratio, and therefore the fuel injection
amount is increased with respect to all the fuel injection valves
25. Hence, the air/fuel ratio of the mixture formed in each of the
combustion chambers 21 that correspond to the normal fuel injection
valves 25 becomes richer than the stoichiometric air/fuel ratio.
Therefore, at this time, the emission level of the exhaust gas
discharged from the combustion chambers that correspond to the
normal fuel injection valves 25 also deteriorates.
[0093] Of course, although the air/fuel ratio of the mixture formed
in the combustion chamber 21 that corresponds to the abnormal fuel
injection valve 25 occasionally becomes leaner than the
stoichiometric air/fuel ratio and the air/fuel ratio of the mixture
formed in each of the combustion chambers 21 that correspond to the
normal fuel injection valves 25 occasionally becomes richer than
the stoichiometric air/fuel ratio, the air/fuel ratio control of
this embodiment controls the fuel injection amount of the fuel
injection valves 25 so that the air/fuel ratio of the mixture
formed in the combustion chambers 21 will become equal to the
stoichiometric air/fuel ratio. Therefore, it can be said that the
air/fuel ratio of the mixture is controlled to the stoichiometric
air/fuel ratio as a whole. However, although the air/fuel ratio of
the mixture can be said to be controlled to the stoichiometric
air/fuel ratio as a whole, it is true, in view of the air/fuel
ratio of the mixture formed in each one of the combustion chambers
21, that the air/fuel ratio of the mixture formed in a combustion
chamber 21 occasionally becomes considerably leaner than the
stoichiometric air/fuel ratio and the air/fuel ratio of the mixture
formed in another combustion chamber 21 occasionally becomes
considerably richer than the stoichiometric air/fuel ratio during
execution of the air/fuel ratio control of this embodiment, and
therefore that the emission level of the exhaust gas discharged
from each combustion chamber 21 deteriorates during the execution
of the control.
[0094] Thus, both in the case where a fuel injection valve 25 has a
fault of injecting an amount of fuel larger than the command value
of fuel injection amount and in the case where a fuel injection
valve 25 has a fault of injecting an amount of fuel smaller than
the command value of fuel injection amount, the emission level of
the exhaust gas discharged from the combustion chambers 21
deteriorates.
[0095] Considering the foregoing circumstances, it is very
important to know whether there is a state in which a certain fuel
injection valve 25 has a fault such that the fuel injection valve
25 injects an amount of fuel larger than the command value of fuel
injection amount or such that the fuel injection valve 25 injects
an amount of fuel smaller than the command value of fuel injection
amount, that is, whether there is a state in which there is a
difference in the air/fuel ratio of the mixture among the
combustion chambers (hereinafter, referred to as "state of
inter-cylinder air/fuel ratio imbalance"), in order to know the
state of emission of exhaust gas and take a countermeasure for
eliminating or reducing the deterioration of the emission level of
exhaust gas.
[0096] Therefore, in this embodiment, it is determined whether the
state of inter-cylinder air/fuel ratio imbalance is present, that
is, the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance, on the basis of a finding as described below.
[0097] That is, the internal combustion engine 10 is designed so
that the combustion chambers 21 of the cylinders undergo the
exhaust stroke at timings spaced from each there by 180.degree. in
the crank angle, that is, the rotation angle of the crankshaft,
sequentially in the order of the first cylinder #1, the fourth
cylinder #4, the third cylinder #3 and the second cylinder #2.
Therefore, exhaust gas is discharged sequentially from the
combustion chambers 21 at intervals of 180.degree. in the crank
angle, and the exhaust gases from the combustion chambers 21
sequentially reach the upstream-side air/fuel ratio sensor 55.
Therefore, generally the upstream-side air/fuel ratio sensor 55
sequentially detects the air/fuel ratio of the exhaust gas
discharged from the first cylinder #1, the air/fuel ratio of the
exhaust gas discharged from the fourth cylinder #4, the air/fuel
ratio of the exhaust gas discharged from the third cylinder #3, and
then the air/fuel ratio of the exhaust gas discharged from the
second cylinder #2.
[0098] In the case where all the fuel injection valves 25 are
normal, the output value that the upstream-side air/fuel ratio
sensor 55 outputs corresponding to the air/fuel ratio of the
exhaust gas that reaches the upstream-side air/fuel ratio sensor 55
(hereinafter, the output value will be referred to as
"upstream-side air/fuel ratio sensor output value") changes as
shown in FIG. 8A. That is, as described above, according to the
air/fuel ratio control of this embodiment, in the case where the
air/fuel ratio of the exhaust gas formed in each combustion chamber
21 is to be controlled to the stoichiometric air/fuel ratio, the
air/fuel ratio of the mixture formed in the combustion chamber 21
is made richer than the stoichiometric air/fuel ratio at some times
and is made leaner than the stoichiometric air/fuel ratio at some
other times so that the air/fuel ratio is controlled to the
stoichiometric air/fuel ratio as a whole. Specifically, the
following arrangement is provided. When it is detected by the
upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of
the mixture is leaner than the stoichiometric air/fuel ratio, the
value of increase for the fuel injection amount of the fuel
injection valves 25 is set so that the air/fuel ratio of the
mixture reaches the stoichiometric air/fuel ratio as quickly as
possible. When it is detected by the upstream-side air/fuel ratio
sensor 55 that the air/fuel ratio of the mixture is richer than the
stoichiometric air/fuel ratio, the value of reduction for the fuel
injection amount of the fuel injection valves 25 is set so that the
air/fuel ratio of the mixture reaches the stoichiometric air/fuel
ratio as quickly as possible. Therefore, if all the fuel injection
valves 25 are normal, the upstream-side air/fuel ratio sensor
output value repeatedly increases and decreases in relatively small
ranges on both side of an upstream-side air/fuel ratio sensor
output value that corresponds to the stoichiometric air/fuel ratio
as shown in FIG. 8A.
[0099] On another hand, in the case where the fuel injection valve
25 corresponding to the first cylinder #1 has a fault of injecting
an amount of fuel that is larger than the command value of fuel
injection amount while the fuel injection valves 25 corresponding
to the other cylinders #2 to #4 are normal, the upstream-side
air/fuel ratio sensor output value changes as shown in FIG. 8B.
That is, since the air/fuel ratio of the mixture formed in the
first cylinder #1 corresponding to the abnormal fuel injection
valve 25 is considerably richer than the stoichiometric air/fuel
ratio, the air/fuel ratio of the exhaust gas discharged from the
first cylinder #1 is also considerably richer than the
stoichiometric air/fuel ratio. Therefore, when the exhaust gas
discharged from the first cylinder #1 reaches the upstream-side
air/fuel ratio sensor 55, the upstream-side air/fuel ratio sensor
output value rapidly lessens toward an output value that
corresponds to the air/fuel ratio of the exhaust gas discharged
from the first cylinder #1, that is, an air/fuel ratio that is
considerably richer than the stoichiometric air/fuel ratio. Then,
according to the air/fuel ratio control of this control, when the
upstream-side air/fuel ratio sensor output value reaches an output
value that corresponds to the air/fuel ratio that is considerably
richer than the stoichiometric air/fuel ratio, that is, when the
upstream-side air/fuel ratio sensor 55 detects the air/fuel ratio
that is considerably richer than the stoichiometric air/fuel ratio,
the fuel injection amounts of all the fuel injection valves 25 are
considerably reduced, so that the air/fuel ratio of the mixture
formed in each of the fourth cylinder #4, the third cylinder #3 and
the second cylinder #2 changes to an air/fuel ratio that is
considerably leaner than the stoichiometric air/fuel ratio.
Therefore, when the exhaust gases discharged from the fourth
cylinder #4, the third cylinder #3 and the second cylinder #2 reach
the upstream-side air/fuel ratio sensor 55, the upstream-side
air/fuel ratio sensor output value rapidly increases toward an
output value that corresponds to the air/fuel ratio of the exhaust
gas discharged from the cylinders #4, #3 and #2, that is, the
air/fuel ratio that is considerably leaner than the stoichiometric
air/fuel ratio. Then, according to the air/fuel ratio control of
this embodiment, when the upstream-side air/fuel ratio sensor
output value reaches an output value that corresponds to the
air/fuel ratio that is leaner than the stoichiometric air/fuel
ratio, that is, when the upstream-side air/fuel ratio sensor 55
detects the air/fuel ratio that is leaner than the stoichiometric
air/fuel ratio, the fuel injection amounts of all the fuel
injection valves 25 are increased, so that the air/fuel ratio of
the mixture formed in the first cylinder #1 corresponding to the
abnormal fuel injection valve 25 changes again to an air/fuel ratio
that is considerably richer than the stoichiometric air/fuel ratio.
Therefore, in the case where a certain fuel injection valve 25 has
a fault of injecting an amount of fuel that is larger than the
command value of fuel injection amount, the upstream-side air/fuel
ratio sensor output value repeatedly increases and decreases in
relatively large ranges on both side of the output value that
corresponds to the stoichiometric air/fuel ratio as shown in FIG.
8B.
[0100] On still another hand, in the case where the fuel injection
valve 25 corresponding to the first cylinder #1 has a fault of
injecting an amount of fuel that is smaller than the command value
of fuel injection amount and where the fuel injection valves 25
corresponding to the other cylinder #2 to #4 are normal, the
upstream-side air/fuel ratio sensor output value changes as shown
in FIG. 8C. That is, since the air/fuel ratio of the mixture formed
in the first cylinder #1 corresponding to the abnormal fuel
injection valve 25 is considerably leaner than the stoichiometric
air/fuel ratio, the air/fuel ratio of the exhaust gas discharged
from the first cylinder #1 is also considerably leaner than the
stoichiometric air/fuel ratio. Therefore, when the exhaust gas
discharged from the first cylinder #1 reaches the upstream-side
air/fuel ratio sensor 55, the upstream-side air/fuel ratio sensor
output value rapidly increases toward an output value that
corresponds to the air/fuel ratio of the exhaust gas discharged
from the first cylinder #1, that is, an air/fuel ratio that is
considerably leaner than the stoichiometric air/fuel ratio. Then,
according to the air/fuel ratio control of this embodiment, when
the upstream-side air/fuel ratio sensor output value reaches the
output value that corresponds to the air/fuel ratio that is
considerably leaner than the stoichiometric air/fuel ratio, that
is, when the upstream-side air/fuel ratio sensor 55 detects an
air/fuel ratio that is considerably leaner than the stoichiometric
air/fuel ratio, the fuel injection amounts of all the fuel
injection valves 25 are considerably increased, so that the
air/fuel ratio of the mixture formed in each of the fourth cylinder
#4, the third cylinder #3 and the second cylinder #2 changes to an
air/fuel ratio that is considerably richer than the stoichiometric
air/fuel ratio. Therefore, when the exhaust gases discharged from
the fourth cylinder #4, the third cylinder #3 and the second
cylinder #2 reach the upstream-side air/fuel ratio sensor 55, the
upstream-side air/fuel ratio sensor output value rapidly lessens
toward an output value that corresponds to the air/fuel ratio of
the exhaust gas discharged from the cylinders #4 to #2, that is, an
air/fuel ratio that is considerably richer than the stoichiometric
air/fuel ratio. Then, according to the air/fuel ratio control of
this embodiment, when the upstream-side air/fuel ratio sensor
output value reaches an output value that corresponds to the
air/fuel ratio that is richer than the stoichiometric air/fuel
ratio, that is, when the upstream-side air/fuel ratio sensor 55
detects the air/fuel ratio that is richer than the stoichiometric
air/fuel ratio, the fuel injection amounts of all the fuel
injection valves 25 are reduced, so that the air/fuel ratio of the
mixture formed in the first cylinder #1 changes again to an
air/fuel ratio that is considerably leaner than the stoichiometric
air/fuel ratio. Therefore, in the case where a certain fuel
injection valve 25 has a fault of injecting an amount of fuel that
is larger than the command value of fuel injection amount, the
upstream-side air/fuel ratio sensor output value repeatedly
increases and decreases in relatively large ranges on both side of
the output value that corresponds to the stoichiometric air/fuel
ratio, as shown in FIG. 8C.
[0101] Thus, the transition of the upstream-side air/fuel ratio
sensor output value occurring when a certain fuel injection valve
25 has abnormality is greatly different from the transition of the
upstream-side air/fuel ratio sensor output value occurring when all
the fuel injection valves 25 are normal.
[0102] In particular, in the case where all the fuel injection
valves 25 are normal, the average slope of a line that the
upstream-side air/fuel ratio sensor output value follows
(hereinafter, the average slope will be referred to simply as
"slope") is a slope .alpha.1 that is relatively small in absolute
value when the upstream-side air/fuel ratio sensor output value
lessens as the air/fuel ratio of the exhaust gas that reaches the
upstream-side air/fuel ratio sensor 55 changes to the richer side,
as shown in FIG. 8A. On the other hand, when the upstream-side
air/fuel ratio sensor output value increases as the air/fuel ratio
of the exhaust gas that reaches the upstream-side air/fuel ratio
sensor 55 changes to the leaner side, the average slope of the line
that the upstream-side air/fuel ratio sensor output value follows
(hereinafter, this average slope will also be referred to simply as
"slope") is a relatively small slope .alpha.2. In this case, the
absolute value of the slope .alpha.1 and the absolute value of the
slope .alpha.2 are substantially equal.
[0103] In the case where a certain fuel injection valve 25 has a
fault of injecting an amount of fuel that is larger than the
command value of fuel injection amount, the average slope of a line
that the upstream-side air/fuel ratio sensor output value follows
is a slope .alpha.3 that is relatively large in absolute value when
the upstream-side air/fuel ratio sensor output value lessens as the
air/fuel ratio of the exhaust gas that reaches the upstream-side
air/fuel ratio sensor 55 changes to the richer side, as shown in
FIG. 8B. On the other hand, when the upstream-side air/fuel ratio
sensor output value increases as the air/fuel ratio of the exhaust
gas that reaches the upstream-side air/fuel ratio sensor 55 changes
to the leaner side, the average slope of the line that the
upstream-side air/fuel ratio sensor output value follows is a
relatively large slope .alpha.4. In this case, the absolute value
of the slope .alpha.3 of the line that the upstream-side air/fuel
ratio sensor output value follows when the upstream-side air/fuel
ratio sensor output value lessens is slightly larger than the
absolute value of the slope .alpha.4 of the line that the
upstream-side air/fuel ratio sensor output value follows when the
upstream-side air/fuel ratio sensor output value increases.
[0104] In the case where a certain fuel injection valve 25 has a
fault of injecting an amount of fuel that is smaller than the
command value of fuel injection amount, the average slope of a line
that the upstream-side air/fuel ratio sensor output value follows
is a relatively large slope .alpha.5 when the upstream-side
air/fuel ratio sensor output value increases as the air/fuel ratio
of the exhaust gas that reaches the upstream-side air/fuel ratio
sensor 55 changes to the leaner side, as shown in FIG. 8C. On the
other hand, when the upstream-side air/fuel ratio sensor output
value lessens as the air/fuel ratio of the exhaust gas that reaches
the upstream-side air/fuel ratio sensor 55 changes to the richer
side, the average slope of the line that the upstream-side air/fuel
ratio sensor output value follows is a slope .alpha.6 that is
relatively large in absolute value. In this case, the absolute
value of the slope .alpha.5 of the line that the upstream-side
air/fuel ratio sensor output value follows when the upstream-side
air/fuel ratio sensor output value increases is slightly larger
than the absolute value of the slope .alpha.6 of the line that the
upstream-side air/fuel ratio sensor output value follows when the
upstream-side air/fuel ratio sensor output value lessens.
[0105] Thus, the absolute value of the slope of the line that the
upstream-side air/fuel ratio sensor output value follows assumes
different characteristic values in the case where all the fuel
injection valves 25 are normal, the case where a certain fuel
injection valve 25 has an abnormality of injecting an amount of
fuel that is larger than the command value of fuel injection
amount, and the case where a certain fuel injection valve 25 has an
abnormality of injecting an amount of fuel that is smaller than the
command value of fuel injection amount fuel. Therefore, by using
the absolute value of the slope, it is possible to determine the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance. Specifically, the absolute value of the slope of the
line that the upstream-side air/fuel ratio sensor output value
follows when a certain fuel injection valve 25 has abnormality is
basically larger than the absolute value of the slope of the line
that the upstream-side air/fuel ratio sensor output value follows
when all the fuel injection valves 25 are normal. Therefore, if the
absolute value of a slope that can be assumed by the line that
upstream-side air/fuel ratio sensor output value follows in the
case where all the fuel injection valves 25 are normal is set as a
threshold value, or if a value that is larger than that absolute
value of the slope is set as a threshold value, it can be
determined that the state of inter-cylinder air/fuel ratio
imbalance is present when the absolute value of the slope of the
line that the upstream-side air/fuel ratio sensor output value
follows is larger than the threshold value during operation of the
engine.
[0106] By the way, the determination of the presence or absence of
the state of inter-cylinder air/fuel ratio imbalance described
above with reference to FIG. 8A to FIG. 8C is performed in the case
where the air/fuel ratio of the mixture is to be controlled to the
stoichiometric air/fuel ratio (i.e., the case where the air/fuel
ratio of the exhaust gas that comes to the upstream-side air/fuel
ratio sensor 55 is the stoichiometric air/fuel ratio). However,
this determination of the presence or absence of the state of
air/fuel ratio imbalance can also be applied to the case where the
air/fuel ratio of the mixture is to be controlled to an air/fuel
ratio that is richer than the stoichiometric air/fuel ratio (i.e.,
the case where the air/fuel ratio of the exhaust gas that comes to
the upstream-side air/fuel ratio sensor 55 is richer than the
stoichiometric air/fuel ratio).
[0107] That is, in the case where all the fuel injection valves 25
are normal when the air/fuel ratio of the mixture is controlled to
an air/fuel ratio that is richer than the stoichiometric air/fuel
ratio, the upstream-side air/fuel ratio sensor output value (i.e.,
the output value of the upstream-side air/fuel ratio sensor 55)
changes as shown in FIG. 9A, corresponding to the air/fuel ratio of
the exhaust gas that reaches the upstream-side air/fuel ratio
sensor 55. Specifically, as for the transition of the upstream-side
air/fuel ratio sensor output value at this time, the output value
repeatedly increases and decreases in relatively small ranges, and
the entire curve indicating the transition of the output value is
positioned in the richer air/fuel ratio side in comparison with the
transition of the upstream-side air/fuel ratio sensor output value
shown in FIG. 8A. However, when the upstream-side air/fuel ratio
sensor output value lessens as the air/fuel ratio of the exhaust
gas that reaches the upstream-side air/fuel ratio sensor 55 changes
to the richer side, an average slope of a line shown in FIG. 9A
that the upstream-side air/fuel ratio sensor output value follows
is the slope .alpha.1 relatively small in absolute value, as in the
case of the average slope of the line shown in FIG. 8A that the
upstream-side air/fuel ratio sensor output value follows. Likewise,
when the upstream-side air/fuel ratio sensor output value increases
as the air/fuel ratio of the exhaust gas that reaches the
upstream-side air/fuel ratio sensor 55 changes to the leaner side,
an average slope of the line shown in FIG. 9A that the
upstream-side air/fuel ratio sensor output value follows is the
relatively small slope .alpha.2, as in the case of the average
slope of the line shown in FIG. 8A that the upstream-side air/fuel
ratio sensor output value follows.
[0108] Furthermore, in the case where the fuel injection valve 25
corresponding to the first cylinder #1 has a fault of injecting an
amount of fuel that is larger than the command value of fuel
injection amount (i.e., the amount of fuel that the electronic
control unit 60 commands each fuel injection valve 25 to inject)
and the fuel injection valves 25 corresponding to the other
cylinders #2 to #4 are normal when the air/fuel ratio of the
mixture is controlled to an air/fuel ratio that is richer than the
stoichiometric air/fuel ratio, the upstream-side air/fuel ratio
sensor output value changes as shown in FIG. 9B. As for the
transition of the upstream-side air/fuel ratio sensor output value
at this time, the output value repeatedly increases and decreases
in relatively large ranges, and the entire curve indicating the
transition of the output value is positioned in the richer air/fuel
ratio side in comparison with the transition of the upstream-side
air/fuel ratio sensor output value shown in FIG. 8B. When the
upstream-side air/fuel ratio sensor output value lessens as the
air/fuel ratio of the exhaust gas that reaches the upstream-side
air/fuel ratio sensor 55 changes to the richer side, an average
slope of a line shown in FIG. 9B that the upstream-side air/fuel
ratio sensor output value follows is the slope .alpha.3 relatively
large in absolute value, as in the case of the average slope of the
line shown in FIG. 8B that the upstream-side air/fuel ratio sensor
output value follows. Likewise, an average slope of the line that
the upstream-side air/fuel ratio sensor output value follows as the
air/fuel ratio of the exhaust gas that reaches the upstream-side
air/fuel ratio sensor 55 changes to the leaner side is the
relatively large slope .alpha.4, as in the case of the average
slope of the line that the upstream-side air/fuel ratio sensor
output shown in FIG. 8B follows.
[0109] Furthermore, in the case where the fuel injection valve 25
corresponding to the first cylinder #1 has a fault of injecting an
amount of fuel that is smaller than the command value of fuel
injection amount and the fuel injection valves 25 corresponding to
the other cylinders #2 to #4 are normal when the air/fuel ratio of
the mixture is controlled to an air/fuel ratio that is richer than
the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio
sensor output value changes as shown in FIG. 9C. As for the
transition of the upstream-side air/fuel ratio sensor output value,
the output value repeatedly increases and decreases in relatively
large ranges, and the entire curve indicating the transition of the
output value is positioned in the richer air/fuel ratio side, in
comparison with the transition of the upstream-side air/fuel ratio
sensor output value shown in FIG. 8C. When the upstream-side
air/fuel ratio sensor output value increases as the air/fuel ratio
of the exhaust gas that reaches the upstream-side air/fuel ratio
sensor 55 changes to the leaner side, an average slope of a line
shown in FIG. 9C that the upstream-side air/fuel ratio sensor
output value follows is the relatively large slope .alpha.5, as in
the case of the average slope of the line shown in FIG. 8C that the
upstream-side air/fuel ratio sensor output value follows. Likewise,
when the upstream-side air/fuel ratio sensor output value lessens
as the air/fuel ratio of the exhaust gas that reaches the
upstream-side air/fuel ratio sensor 55 changes to the richer side,
an average slope of the line shown in FIG. 9C that the
upstream-side air/fuel ratio sensor output value follows is the
slope .alpha.6 relatively large in absolute value, as in the case
of the average slope of the line shown in FIG. 8C that the
upstream-side air/fuel ratio sensor output value follows.
[0110] Thus, when the air/fuel ratio of the mixture is controlled
to an air/fuel ratio that is richer than the stoichiometric
air/fuel ratio, too, the absolute value of the slope of the line
that the upstream-side air/fuel ratio sensor output value follows
in the case where a certain fuel injection valve 25 has abnormality
(i.e., where the state of inter-cylinder air/fuel ratio imbalance
is present) is larger than the absolute value of the slope of the
line that the upstream-side air/fuel ratio sensor output value
follows in the case where all the fuel injection valves 25 are
normal (i.e., where the state of inter-cylinder air/fuel ratio
imbalance is not present), similarly to when the air/fuel ratio of
the mixture is controlled to the stoichiometric air/fuel ratio.
Therefore, if the absolute value of the slope that can be assumed
by the line that the upstream-side air/fuel ratio sensor output
value follows in the case where all the fuel injection valves 25
are normal when the air/fuel ratio of the mixture is controlled to
an air/fuel ratio that is richer than the stoichiometric air/fuel
ratio is set beforehand as a threshold value, or if a value that is
larger than the absolute value of the slope is set beforehand as a
threshold value, it can be determined that the state of
inter-cylinder air/fuel ratio imbalance is present when the
absolute value of the slope of the line that the upstream-side
air/fuel ratio sensor output value follows when the air/fuel ratio
of the mixture is controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio is larger than the threshold
value.
[0111] By the way, the determination of the presence or absence of
the state of inter-cylinder air/fuel ratio imbalance described
above with reference to FIG. 8A to FIG. 8C is performed in the case
where the air/fuel ratio of the mixture is to be controlled to the
stoichiometric air/fuel ratio (i.e., the case where the air/fuel
ratio of the exhaust gas that comes to the upstream-side air/fuel
ratio sensor 55 is the stoichiometric air/fuel ratio). However,
this determination of the presence or absence of the state of
air/fuel ratio imbalance can also be applied to the case where the
air/fuel ratio of the mixture is to be controlled to an air/fuel
ratio leaner than the stoichiometric air/fuel ratio (i.e., the case
where the air/fuel ratio of the exhaust gas that comes to the
upstream-side air/fuel ratio sensor 55 is leaner than the
stoichiometric air/fuel ratio), just as the determination of the
presence or absence of the state of air/fuel ratio imbalance can be
applied to the case where the air/fuel ratio of the mixture is to
be controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio.
[0112] Thus, the foregoing determination of the presence or absence
of the state of inter-cylinder air/fuel ratio imbalance can be
performed in any one of the case where the air/fuel ratio of the
mixture is controlled to the stoichiometric air/fuel ratio, the
case where the air/fuel ratio of the mixture is controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio, and
the case where the air/fuel ratio of the mixture is controlled to
an air/fuel ratio leaner than the stoichiometric air/fuel
ratio.
[0113] However, in the case where the air/fuel ratio of the mixture
is controlled to the stoichiometric air/fuel ratio and the case
where the air/fuel ratio of the mixture is controlled to an
air/fuel ratio leaner than the stoichiometric air/fuel ratio, the
accuracy of the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance based on the
foregoing idea is lower than in the case where the air/fuel ratio
of the mixture is controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio. A reason why the accuracy of the
determination is low when the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined in the case
where the air/fuel ratio of the mixture is controlled to the
stoichiometric air/fuel ratio and the case where the air/fuel ratio
of the mixture is controlled to an air/fuel ratio leaner than the
stoichiometric air/fuel ratio will be explained. Besides, a reason
why the determination accuracy is high when the presence or absence
of the state of inter-cylinder air/fuel ratio imbalance is
determined in the case where the air/fuel ratio of the mixture is
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio will also be explained.
[0114] Firstly, a reason why the determination accuracy is low when
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is determined in the case where the air/fuel ratio
of the mixture is controlled to the stoichiometric air/fuel ratio
will be explained.
[0115] As described above, the upstream-side air/fuel ratio sensor
55 outputs an output value that follows the characteristic shown in
FIG. 3A, according to the air/fuel ratio of the exhaust gas that
comes to the sensor 55. A mechanism by which the upstream-side
air/fuel ratio sensor 55 outputs the output value in this manner is
as follows.
[0116] That is, the upstream-side air/fuel ratio sensor 55, as
shown in FIG. 10 and FIG. 11, has an air/fuel ratio detection
element 55a, an outer protection cover 55b, and an inner protection
cover 55c. The protection covers 55b and 55c house, inside thereof,
the air/fuel ratio detection element 55a so as to cover the
air/fuel ratio detection element 55a. Besides, the protection
covers 55b and 55c have inflow holes 55b1 and 55c1 that allow the
exhaust gas that reaches the upstream-side air/fuel ratio sensor 55
to flow from the exhaust pipe 42 into an internal space inside the
protection covers 55b and 55c and reach the air/fuel ratio
detection element 55a, and outflow holes 55b2 and 55c2 that allow
the exhaust gas that flows into the internal space to flow out into
the exhaust pipe 42.
[0117] The upstream-side air/fuel ratio sensor 55 is disposed on
the exhaust pipe 42 so that the protection covers 55b and 55c are
exposed to an internal space of the exhaust pipe 42. Therefore,
exhaust gas EX that flows in the exhaust pipe 42 flows into a space
between the outer protection cover 55b and the inner protection
cover 55c through the inflow hole 55b1 of the outer protection
cover 55b, as shown by an arrow Ar1 in FIG. 10 and FIG. 11. Next,
the exhaust gas flows into an internal space of the inner
protection cover 55c through the inflow hole 55c1 of the inner
protection cover 55c, and reaches the air/fuel ratio detection
element 55a, as shown by an arrow Ar2. After that, the exhaust gas
flows out into the exhaust pipe 42 through the outflow hole 55c2 of
the inner protection cover 55c and the outflow hole 55b2 of the
outer protection cover 55b, as shown by an arrow Ar3. Since the
exhaust gas having reached the upstream-side air/fuel ratio sensor
55 flows in the upstream-side air/fuel ratio sensor 55 in this
manner, the exhaust gas having reached the upstream-side air/fuel
ratio sensor 55 is drawn into the inflow hole 55b1 of the outer
protection cover 55b, due to the flow of exhaust gas moving in the
vicinity of the outflow hole 55b2 of the outer protection cover
55b.
[0118] The air/fuel ratio detection element 55a, as shown in FIG.
12A, has a solid electrolyte layer 551, an exhaust gas-side
electrode layer 552, an atmosphere-side electrode layer 553, a
diffusion resistance layer (or diffusion rate-determining layer)
554, an exhaust gas-side wall 555, an atmosphere-side wall 556, and
a heater 557.
[0119] The solid electrolyte layer 551 is a sintered body of an
oxygen ion-conductive oxide, for example, a stabilized zirconia
element in which CaO as a stabilizer is dissolved in ZrO.sub.2
(zirconia) in a solid form. The solid electrolyte layer 551 exerts
an oxygen cell characteristic and an oxygen pump characteristic
(that will be described later) when its temperature is higher than
a certain temperature (i.e., higher than a so-called activation
temperature).
[0120] Besides, the exhaust gas-side electrode layer 552 includes a
noble metal that has high catalytic activity, for example, Pt
(platinum). The exhaust gas-side electrode layer 552 is disposed on
a surface of the solid electrolyte layer 551. Besides, the exhaust
gas-side electrode layer 552 is formed, for example, by chemical
plating, so as to have sufficient permeability, that is, so as to
be porous.
[0121] On the other hand, the atmosphere-side electrode layer 553
includes a noble metal that has high catalytic activity, for
example, Pt (platinum). Besides, the atmosphere-side electrode
layer 553 is disposed on a surface of the solid electrolyte layer
551 that is opposite the surface thereof on which the electrode
layer 552 is disposed. That is, the solid electrolyte layer 551 is
sandwiched between the exhaust gas-side electrode layer 552 and the
atmosphere-side electrode layer 553. Besides, the atmosphere-side
electrode layer 553 is formed, for example, by chemical plating, so
as to have sufficient permeability, that is, so as to be
porous.
[0122] An electric power source 560 is connected to the exhaust
gas-side electrode layer 552 and the atmosphere-side electrode
layer 553. Voltage is applied from the electric power source 560 to
the exhaust gas-side electrode layer 552 and the atmosphere-side
electrode layer 553 so that the electric potential of the
atmosphere-side electrode layer 553 is higher than that of the
exhaust gas-side electrode layer 552.
[0123] The diffusion resistance layer 554 is made of a porous
ceramic material that is a heat-resistant inorganic substance. The
diffusion resistance layer 554 is disposed, for example, by a
plasma spraying method, so as to cover the surfaces of the exhaust
gas-side electrode layer 552 except the surface thereof that is in
contact with the adjacent surface of the solid electrolyte layer
551.
[0124] The exhaust gas-side wall 555 is made of an alumina ceramics
that is dense and impermeable to exhaust gas. Besides, the exhaust
gas-side wall 555 is disposed so as to cover the diffusion
resistance layer 554 except portions thereof (particularly, corner
portions of the diffusion resistance layer 554). Specifically, the
exhaust gas-side wall 555 has through holes 558 each of which
exposes a portion of the diffusion resistance layer 554 to the
outside.
[0125] The atmosphere-side wall 556 is made of an alumina ceramics
that is dense and impermeable to exhaust gas. Besides, the
atmosphere-side wall 556 is disposed so that the atmosphere-side
wall 556 partially defines, on its inner side, a space 559 that
surrounds the atmosphere-side electrode layer 553 (hereinafter,
this space will be referred to as "atmospheric chamber").
Atmospheric air is introduced into the atmospheric chamber 559.
[0126] The heater 557 is buried in the atmosphere-side wall 556.
When supplied with electric power, the heater 557 generates heat,
so that the solid electrolyte layer 551, the exhaust gas-side
electrode layer 552 and the atmosphere-side electrode layer 553 are
heated.
[0127] The air/fuel ratio detection element 55a functions as shown
in FIG. 12B when an exhaust gas whose air/fuel ratio is leaner than
the stoichiometric air/fuel ratio comes to the air/fuel ratio
detection element 55a. That is, since the concentration of oxygen
in exhaust gas around the exhaust gas-side electrode layer 552 is
relatively high, oxygen moves from the exhaust gas-side electrode
layer 552 to the atmosphere-side electrode layer 553 through the
solid electrolyte layer 551. Specifically, exhaust gas that reaches
the air/fuel ratio detection element 55a flows into the diffusion
resistance layer 554 through the through holes 558. Then, when the
exhaust gas reaches the exhaust gas-side electrode layer 552
through the diffusion resistance layer 554, oxygen in the exhaust
gas is ionized by the exhaust gas-side electrode layer 552. The
ionized oxygen, that is, oxygen ions, reaches the atmosphere-side
electrode layer 553 through the solid electrolyte layer 551. After
reaching the atmosphere-side electrode layer 553, the oxygen ions
lose electrons to the atmosphere-side electrode layer 553 and
become oxygen, which in turn flows into the atmospheric chamber
559. Through this operation, current I flows from the positive
electrode to the negative electrode of the electric power source
560. The magnitude of the current I that flows in this manner is a
constant value that is proportional to the concentration of oxygen
in the exhaust gas that reaches the exhaust gas-side electrode
layer 552 if the voltage of the electric power source 560 is set at
a predetermined value Vp as shown in FIG. 13. In this embodiment,
when the voltage of the electric power source 560 is set at the
predetermined value Vp and the air/fuel ratio of the exhaust gas
that comes to the upstream-side air/fuel ratio sensor 55 is leaner
than the stoichiometric air/fuel ratio, the air/fuel ratio of the
exhaust gas is known on the basis of the current I (i.e., the
limiting current Ip) that flows from the positive electrode toward
the negative electrode of the electric power source 560.
[0128] On the other hand, when an exhaust gas whose air/fuel ratio
is richer than the stoichiometric air/fuel ratio comes to the
air/fuel ratio detection element 55a, the air/fuel ratio detection
element 55a functions as shown in FIG. 12C. That is, since the
concentration of oxygen in the exhaust gas around the exhaust
gas-side electrode layer 552 is relatively low, oxygen moves from
the atmosphere-side electrode layer 553 to the exhaust gas-side
electrode layer 552 through the solid electrolyte layer 551.
Specifically, oxygen in the atmospheric air introduced in the
atmospheric chamber 559 is ionized by the atmosphere-side electrode
layer 553. The ionized oxygen, that is, oxygen ions, reaches the
exhaust gas-side electrode layer 553 through the solid electrolyte
layer 551. After reaching the exhaust gas-side electrode layer 552,
the oxygen ions lose electrons to the exhaust gas-side electrode
layer 552 and oxidize unburnt materials in the exhaust gas around
the exhaust gas-side electrode layer 552, for example, hydrocarbons
(HCs), carbon monoxide (CO) and hydrogen (H.sub.2). Through this
operation, current I flows from the negative electrode to the
positive electrode of the electric power source 560. The magnitude
of the current I that flows in this manner is also a constant value
that is proportional to the concentration of oxygen in the exhaust
gas around the exhaust gas-side electrode layer 552 if the voltage
of the electric power source 560 is set at the predetermined value
Vp as shown in FIG. 13. In this embodiment, when the voltage of the
electric power source 560 is set at the predetermined value Vp and
the air/fuel ratio of the exhaust gas that comes to the
upstream-side air/fuel ratio sensor 55 is richer than the
stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust
gas is known on the basis of the current I (i.e., the limiting
current Ip) that flows from the negative electrode to the positive
electrode of the electric power source 560.
[0129] As described above, the upstream-side air/fuel ratio sensor
55 outputs an output value commensurate with the air/fuel ratio of
the exhaust gas that comes to the sensor 55. That is, when the
air/fuel ratio of the exhaust gas that comes to the upstream-side
air/fuel ratio sensor 55 is leaner than the stoichiometric air/fuel
ratio, the upstream-side air/fuel ratio sensor 55 outputs a
positive value of current that is greater the greater the degree of
leanness of the air/fuel ratio of the exhaust gas that comes to the
sensor 55. When the air/fuel ratio of the exhaust gas that comes to
the sensor 55 is richer than the stoichiometric air/fuel ratio, the
upstream-side air/fuel ratio sensor 55 outputs a negative value of
current whose absolute value is greater the greater the degree of
richness of the air/fuel ratio of the exhaust gas that comes to the
sensor 55. Therefore, when the air/fuel ratio of the exhaust gas
that flows into the upstream-side air/fuel ratio sensor 55 is the
stoichiometric air/fuel ratio, the value of the current that the
upstream-side air/fuel ratio sensor 55 outputs is zero.
[0130] Therefore, in the case where the air/fuel ratio of the
exhaust gas that comes to the upstream-side air/fuel ratio sensor
55 changes from an air/fuel ratio leaner than the stoichiometric
air/fuel ratio to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, the output value of the upstream-side air/fuel
ratio sensor 55 changes from a positive value of current to a
negative value of current. In this case, oxygen ions flowing from
the exhaust gas-side electrode layer 552 to the atmosphere-side
electrode layer 553 of the upstream-side air/fuel ratio sensor 55
through the solid electrolyte layer 551 come to flow from the
atmosphere-side electrode layer 553 to the exhaust gas-side
electrode layer 552 through the solid electrolyte layer 551. That
is, the flow direction of oxygen ions flowing in the solid
electrolyte layer 551 reverses. However, when the air/fuel ratio of
the exhaust gas that comes to the upstream-side air/fuel ratio
sensor 55 changes from an air/fuel ratio leaner than the
stoichiometric air/fuel ratio to an air/fuel ratio richer than the
stoichiometric air/fuel ratio, the oxygen ions having been flowing
in the solid electrolyte layer 551 from the exhaust gas-side
electrode layer 552 toward the atmosphere-side electrode layer 553
need to changes the flow direction thereof. Therefore, when the
air/fuel ratio of the exhaust gas that comes to the upstream-side
air/fuel ratio sensor 55 changes from the lean side of the
stoichiometric air/fuel ratio to the rich side of the
stoichiometric air/fuel ratio, the direction of flow of oxygen ions
in the solid electrolyte layer 551 does not instantly reverse.
[0131] On the other hand, in the case where the air/fuel ratio of
the exhaust gas that comes to the upstream-side air/fuel ratio
sensor 55 changes from an air/fuel ratio richer than the
stoichiometric air/fuel ratio to an air/fuel ratio leaner than the
stoichiometric air/fuel ratio, the output value of the
upstream-side air/fuel ratio sensor 55 changes from a negative
value of current to a positive value of current. In this case,
oxygen ions having been flowing from the atmosphere-side electrode
layer 553 to the exhaust gas-side electrode layer 552 of the
upstream-side air/fuel ratio sensor 55 through the solid
electrolyte layer 551 come to flow from the exhaust gas-side
electrode layer 552 to the atmosphere-side electrode layer 553
through the solid electrolyte layer 551. That is, the flow
direction of oxygen ions flowing in the solid electrolyte layer 551
reverses. However, when the air/fuel ratio of the exhaust gas that
comes to the upstream-side air/fuel ratio sensor 55 changes from an
air/fuel ratio richer than the stoichiometric air/fuel ratio to an
air/fuel ratio leaner than the stoichiometric air/fuel ratio, the
oxygen ions having been flowing in the solid electrolyte layer 551
from the atmosphere-side electrode layer 553 toward the exhaust
gas-side electrode layer 552 need to change the flow direction
thereof. Therefore, when the air/fuel ratio of the exhaust gas that
comes to the upstream-side air/fuel ratio sensor 55 changes form an
air/fuel ratio richer than the stoichiometric air/fuel ratio to an
air/fuel ratio leaner than the stoichiometric air/fuel ratio, the
flow direction of oxygen ions in the solid electrolyte layer 551
does not instantly reverse.
[0132] It is to be noted herein that when the control of bringing
the air/fuel ratio of the mixture to the stoichiometric air/fuel
ratio is being performed, the air/fuel ratio of the exhaust gas
that comes to the upstream-side air/fuel ratio sensor 55 repeatedly
changes between the lean side of the stoichiometric air/fuel ratio
and the rich side of the stoichiometric air/fuel ratio. Therefore,
in this case, in order for the upstream-side air/fuel ratio sensor
55 to output an output value that very accurately shows the
air/fuel ratio of the exhaust gas that comes to the upstream-side
air/fuel ratio sensor 55, the flow direction of oxygen ions flowing
in the solid electrolyte layer 551 of the upstream-side air/fuel
ratio sensor 55 needs to instantly reverse when the air/fuel ratio
of the exhaust gas that comes to the upstream-side air/fuel ratio
sensor 55 changes between the lean side of the stoichiometric
air/fuel ratio and the rich side of the stoichiometric air/fuel
ratio, that is, from the lean side to the rich side or the other
way around. However, as described above, the flow direction of
oxygen flowing in the solid electrolyte layer 551 does not
instantly reverse. Therefore, when the air/fuel ratio of the
mixture is controlled to the stoichiometric air/fuel ratio, the
output value of the upstream-side air/fuel ratio sensor 55 does not
very accurately show the air/fuel ratio of the exhaust gas that
comes to the upstream-side air/fuel ratio sensor 55 (i.e., the
air/fuel ratio of the mixture).
[0133] In order to accurately determine the presence or absence of
the state of inter-cylinder air/fuel ratio imbalance on the basis
of the output value of the upstream-side air/fuel ratio sensor 55,
it is desired that the output value of the upstream-side air/fuel
ratio sensor 55 very accurately show the air/fuel ratio of the
exhaust gas that comes to the upstream-side air/fuel ratio sensor
55 (i.e., the air/fuel ratio of the mixture). In other words, when
the control of bringing the air/fuel ratio of the mixture to the
stoichiometric air/fuel ratio is being performed, the output value
of the upstream-side air/fuel ratio sensor 55 does not very
accurately show the air/fuel ratio of the exhaust gas that comes to
the upstream-side air/fuel ratio sensor 55, and therefore it is
impossible to accurately determine the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance on the basis of
the output value of the upstream-side air/fuel ratio sensor 55.
[0134] For the foregoing reason, the accuracy of the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance becomes low if the determination is performed while
the control of bringing the air/fuel ratio of the mixture to the
stoichiometric air/fuel ratio is being performed.
[0135] Next explained will be a reason why the accuracy of the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance becomes low if the
determination is performed while the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio leaner than the
stoichiometric air/fuel ratio.
[0136] When the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio leaner than the stoichiometric air/fuel ratio,
the combustion in each combustion chamber 21 tends to be unstable,
and in some cases, misfire occurs in a combustion chamber 21. In
such a case, the air/fuel ratio of the exhaust gas discharged from
each combustion chamber 21 does not reflect the air/fuel ratio of
the mixture that is to be achieved by the control.
[0137] As stated above, in order to accurately determine the
presence or absence the state of inter-cylinder air/fuel ratio
imbalance on the basis of the output value of the upstream-side
air/fuel ratio sensor 55, it is desired that the output value of
the upstream-side air/fuel ratio sensor 55 very accurately show the
air/fuel ratio of the mixture. However, when the air/fuel ratio of
the mixture is being controlled to an air/fuel ratio leaner than
the stoichiometric air/fuel ratio, the air/fuel ratio of the
exhaust gas discharged from each combustion chamber 21 does not
reflect the air/fuel ratio of the mixture that is to be achieved by
the control as mentioned above, and therefore it is not possible to
accurately determine the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance on the basis of the output
value of the upstream-side air/fuel ratio sensor 55.
[0138] For the foregoing reason, the accuracy of the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance becomes low if the determination is performed when
the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio leaner than the stoichiometric air/fuel ratio.
[0139] Next explained will be a reason why the accuracy of the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance becomes high if the
determination is performed while the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio.
[0140] When the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio,
the air/fuel ratio of the exhaust gas that comes to the
upstream-side air/fuel ratio sensor 55 is also richer than the
stoichiometric air/fuel ratio. Therefore, in this case, the
upstream-side air/fuel ratio sensor 55 outputs an output value that
accurately shows the air/fuel ratio of the exhaust gas that comes
to the sensor 55, without a need for the reversal of the flow
direction of oxygen ions flowing in the diffusion resistance layer
551 of the upstream-side air/fuel ratio sensor 55. That is, when
the air/fuel ratio of the mixture is controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio, the output
value of the upstream-side air/fuel ratio sensor 55 very accurately
shows the air/fuel ratio of the exhaust gas that comes to the
sensor 55 (i.e., the air/fuel ratio of the mixture), and therefore
it is possible to accurately determine the presence or absence of
the state of inter-cylinder air/fuel ratio imbalance on the basis
of the output value of the upstream-side air/fuel ratio sensor
55.
[0141] Besides, when the air/fuel ratio of the mixture is being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, the combustion in each combustion chamber 21 is
stable, and therefore the air/fuel ratio of the exhaust gas
discharged from each combustion chamber 21 reflects the air/fuel
ratio of the mixture that is to be achieved by the control.
Therefore, this means that when the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio, the upstream-side air/fuel ratio
sensor 55 very accurately shows the air/fuel ratio of the mixture,
and therefore that it is possible to accurately determine the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance on the basis of the output value of the upstream-side
air/fuel ratio sensor 55.
[0142] For the foregoing reason, the accuracy of the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance becomes high if the determination is performed
while the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio.
[0143] By the way, as stated above, the accuracy of the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is high if the
determination is performed while the air/fuel ratio of the mixture
is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio. Therefore, when the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance
needs to be determined, the determination of the presence or
absence thereof can be accurately performed by performing the
determination while forcing the air/fuel ratio of the mixture to be
an air/fuel ratio richer than the stoichiometric air/fuel
ratio.
[0144] However, if the air/fuel ratio of the mixture is brought to
an air/fuel ratio richer than the stoichiometric air/fuel ratio
only for the purpose of determining the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance, the fuel economy
of the internal combustion engine 10 deteriorates. Sometimes, the
internal combustion engine 10 causes the air/fuel ratio of the
mixture to be richer than the stoichiometric air/fuel ratio for the
purpose of determining the presence or absence of abnormality of
the output of the downstream-side air/fuel ratio sensor 56. The
determination of the presence or absence of abnormality of the
output of the downstream-side air/fuel ratio sensor 56 is performed
as follows.
[0145] After the air/fuel ratio of the mixture is controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio,
exhaust gas whose air/fuel ratio is richer than the stoichiometric
air/fuel ratio is discharged from each combustion chamber 21. Then,
exhaust gas whose air/fuel ratio is richer than the stoichiometric
air/fuel ratio flows into the upstream-side catalyst 43. At this
time, since the upstream-side catalyst 43 has an oxygen
storage/release capability, the oxygen stored in the upstream-side
catalyst 43 is let out from the upstream-side catalyst 43, so that
exhaust gas of the stoichiometric air/fuel ratio flows out from the
upstream-side catalyst 43. Then, when all the oxygen stored in the
upstream-side catalyst 43 is used as exhaust gas whose air/fuel
ratio is richer than the stoichiometric air/fuel ratio continues to
flow into the upstream-side catalyst 43, exhaust gas whose air/fuel
ratio is richer than the stoichiometric air/fuel ratio begins to
flow out from the upstream-side catalyst 43. At this time, if the
downstream-side air/fuel ratio sensor 56 does not have abnormality
regarding its output, that is, if the downstream-side air/fuel
ratio sensor 56 is normal, the output value of the downstream-side
air/fuel ratio sensor 56 (hereinafter, referred to as
"downstream-side air/fuel ratio sensor output value") accurately
corresponds to the air/fuel ratio of the exhaust gas that flows out
from the upstream-side catalyst 43, that is, the air/fuel ratio of
the mixture that is being controlled to the rich side of the
stoichiometric air/fuel ratio. Therefore, if the downstream-side
air/fuel ratio sensor output value is an output value that
accurately corresponds to the air/fuel ratio of the mixture when
exhaust gas whose air/fuel ratio is richer than the stoichiometric
air/fuel ratio is flowing out from the upstream-side catalyst 43,
it can be said that there is no abnormality of the output of the
downstream-side air/fuel ratio sensor 56 and therefore the
downstream-side air/fuel ratio sensor 56 is normal. On the other
hand, if the downstream-side air/fuel ratio sensor output value is
an output value that does not correspond to the air/fuel ratio of
the mixture when exhaust gas whose air/fuel ratio is richer than
the stoichiometric air/fuel ratio is flowing out from the
upstream-side catalyst 43, it can be said that there is abnormality
of the output of the downstream-side air/fuel ratio sensor 56.
[0146] Therefore, in the internal combustion engine 10, when the
presence or absence of abnormality of the output of the
downstream-side air/fuel ratio sensor 56 needs to be determined,
the air/fuel ratio of the mixture is controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio over a
predetermined period of time. The predetermined period of time
herein is set to a time that is sufficiently long for the oxygen
stored in the upstream-side catalyst 43 to run out if exhaust gas
whose air/fuel ratio is richer than the stoichiometric air/fuel
ratio continues to flow into the upstream-side catalyst 43. Then,
when the foregoing predetermined period elapses after the air/fuel
ratio of the mixture begins to be controlled to the air/fuel ratio
richer than the stoichiometric air/fuel ratio, it is determined
whether the downstream-side air/fuel ratio sensor output value
shows an air/fuel ratio of exhaust gas that corresponds to the
air/fuel ratio of the mixture that is being controlled to the
air/fuel ratio richer than the stoichiometric air/fuel ratio. Then,
if it is determined that the downstream-side air/fuel ratio sensor
output value at this time shows an air/fuel ratio of exhaust gas
that corresponds to the air/fuel ratio of the mixture that is being
controlled to the air/fuel ratio richer than the stoichiometric
air/fuel ratio, it is then determined that there is no abnormality
of the output of the downstream-side air/fuel ratio sensor 56 and
therefore the downstream-side air/fuel ratio sensor 56 is normal.
On the other hand, if it is determined that the downstream-side
air/fuel ratio sensor output value at this time does not show an
air/fuel ratio of exhaust gas that corresponds to the air/fuel
ratio of the mixture that is being controlled to the air/fuel ratio
richer than the stoichiometric air/fuel ratio, it is then
determined that there is abnormality of the output of the
downstream-side air/fuel ratio sensor 56.
[0147] In this embodiment (hereinafter, referred to as "first
embodiment"), the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of abnormality of the output
of the downstream-side air/fuel ratio sensor 56.
[0148] According to this determination process, since the air/fuel
ratio of the mixture is not controlled to an air/fuel ratio richer
than the stoichiometric air/fuel ratio only for the purpose of
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance, the fuel economy of the internal
combustion engine 10 improves in comparison with the case where the
air/fuel ratio is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio only for the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0149] Next, an example of a routine of executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in accordance with the first embodiment will be
described. The determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance in the first
embodiment utilizes flowcharts shown in FIG. 14 and FIG. 15. The
routine shown in FIG. 14 and the routine shown in FIG. 15 are each
executed at every predetermined time interval.
[0150] When the routine shown in FIG. 14 starts, it is firstly
determined in step 100 whether a flag F1 that shows whether
execution of the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance is permitted
(hereinafter, referred to as "inter-cylinder air/fuel ratio
imbalance determination execution flag F1") has been set (F1=1). If
the inter-cylinder air/fuel ratio imbalance determination execution
flag F1 has been set (F1=1), it means that execution of the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is permitted. If the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1 has been reset (F1=0), it means that execution of the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not permitted. Besides,
the inter-cylinder air/fuel ratio imbalance determination execution
flag F1 is set, for example, in accordance with the routine shown
in FIG. 15.
[0151] If in step 100 it is determined that F1=1, it means that
execution of the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance is permitted.
Therefore, in order to execute the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance,
the routine proceeds to step 101. On the other hand, if in step 100
it is determined that F1=0, it means that execution of the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not permitted, and
therefore the routine immediately ends.
[0152] In step 101, to which the routine proceeds after in step 100
it is determined that F1=1, the absolute value |.DELTA.A/F| of the
rate of change in the air/fuel ratio of the exhaust gas that comes
to the upstream-side air/fuel ratio sensor 55 is calculated on the
basis of the output value of the upstream-side air/fuel ratio
sensor 55. Next, in step 102, a counter value C that shows the
number of times that the absolute value |.DELTA.A/F| of the rate of
change in the air/fuel ratio of the exhaust gas has been calculated
in step 101 is incremented by one.
[0153] Next in step 103, it is determined whether the counter value
C incremented by one in step 102 has become equal to a
predetermined threshold value Cth (C=Cth). If it is determined that
C=Cth, the routine proceeds to step 104. On the other hand, if it
is determined that C.noteq.Cth, the routine returns to step
100.
[0154] Incidentally, step 102 and step 103 are provided for
acquiring an increased number of values of the rate of change in
the air/fuel ratio of exhaust gas calculated in step 101 in order
to heighten the accuracy of the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance.
Therefore, steps 102 and 103 may be omitted in the case where high
accuracy of the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance is achieved even
if the number of values of the rate of change in the air/fuel ratio
of exhaust gas calculated in step 101 is one.
[0155] When the routine returns to step S100 after it is determined
in step S103 that C.noteq.Cth, it is determined again whether the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1 has been set (F1=1). If in step S101 it is determined that
F1=1, the routine proceeds to step 101 and the following steps. On
the other hand, if it is determined that F1=0, the routine
ends.
[0156] In step 104, to which the routine proceeds after in step 103
it is determined that C=Cth, an average value .DELTA.A/Fave of the
absolute values of the rate of change in the air/fuel ratio of
exhaust gas calculated by a plurality of executions of step 101 is
calculated.
[0157] Next in step 105, it is determined whether the average value
.DELTA.A/Fave calculated in step 104 is greater than a
predetermined threshold .DELTA.A/Faveth
(.DELTA.A/Fave>.DELTA.A/Faveth). If it is determined that
.DELTA.A/Fave>.DELTA.A/Faveth, it means that the state of
inter-cylinder air/fuel ratio imbalance is present. Then, the
routine proceeds to step 106, in which an alarm that shows that the
state of inter-cylinder air/fuel ratio imbalance is present is
activated.
[0158] Then, because the determination of the presence or absence
of the state of inter-cylinder air/fuel ratio imbalance has now
ended, in step 107 the absolute values |.DELTA.A/F| of the rate of
change in the air/fuel ratio of exhaust gas calculated in step 101
are cleared. Subsequently in step 108, the counter value C
incremented in step 102 is cleared. After that, the routine
ends.
[0159] On another hand, if in step 105 it is determined that
.DELTA.A/Fave.ltoreq..DELTA.A/Faveth, that it, if it is determined
that the average value .DELTA.A/Fave calculated in step 104 is less
than or equal to the predetermined threshold value .DELTA.A/Faveth,
it means that the state of inter-cylinder air/fuel ratio imbalance
is not present, and therefore the routine immediately ends.
[0160] Next described will be the routine shown by the flowchart in
FIG. 15 that is an example of the routine of executing the setting
of the inter-cylinder air/fuel ratio imbalance determination
execution flag in the first embodiment which is utilized in step
100 in the routine shown in FIG. 14.
[0161] When the routine shown in FIG. 15 starts, firstly in step
200 it is determined whether a flag F2 that shows whether a
condition that is a prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is satisfied (hereinafter, this flag will be
referred to as "inter-cylinder air/fuel ratio imbalance
determination prerequisite condition flag F2") has been set (F2=1).
If the inter-cylinder air/fuel ratio imbalance determination
prerequisite condition flag F2 has been set (F2=1), it means that
the condition as the prerequisite for executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied. On the other hand, if the
inter-cylinder air/fuel ratio imbalance determination prerequisite
condition flag F2 has been reset (F2=0), it means that the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has not been satisfied.
[0162] If in step 200 it is determined that F2=1, it means that the
condition as the prerequisite for executing determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance has been satisfied, and then routine proceeds to step
201. On the other hand, if it is determined that F2=0, it means
that the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied, and
then the routine proceeds to step 203.
[0163] Incidentally, the condition as the prerequisite for
executing the determination of the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance is a condition that the
temperature of a coolant for cooling the internal combustion engine
10 (this temperature represents the temperature of the internal
combustion engine 10) is higher than or equal to a predetermined
temperature (e.g., 75.degree. C.), and that the engine rotation
speed is within a predetermined range (e.g., of 1200 rpm to 2000
rpm), and that the intake gas amount is within a predetermined
range (e.g., of 10 g/sec to 20 g/sec), and that the temperature of
the upstream-side air/fuel ratio sensor 55 is higher than or equal
to its activation temperature, and that the atmospheric temperature
is higher than or equal to a predetermined value (e.g., 75
kPa).
[0164] In step 201, to which routine proceeds after step 200 it is
determined that F2=1, it is determined whether a flag F3 that shows
whether the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio for
the purpose of determining the presence or absence of abnormality
of the output of the downstream-side air/fuel ratio sensor 56
(hereinafter, this flag will be referred to as "first abnormality
determination-purpose rich air/fuel ratio control flag F3") has
been set (F3=1). If the first abnormality determination-purpose
rich air/fuel ratio control flag F3 has been set (F3=1), it means
that the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio for
the purpose of determining the presence or absence of abnormality
of the output of the downstream-side air/fuel ratio sensor 56. If
the first abnormality determination-purpose rich air/fuel ratio
control flag F3 has been reset (F3=0), it means that the air/fuel
ratio of the mixture is not being controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
determining the presence or absence of abnormality of the output of
the downstream-side air/fuel ratio sensor 56.
[0165] If in step 201 it is determined that F3=1, it means that the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of abnormality of the output
of the downstream-side air/fuel ratio sensor 56. Then, the routine
proceeds to step S202, in which "1" is input to the inter-cylinder
air/fuel ratio imbalance determination execution flag F1. After
that, the routine ends. That is, when the routine proceeds to step
201 is when in step 200 it is determined that the inter-cylinder
air/fuel ratio imbalance determination prerequisite condition flag
F2 has been set (F2=1) Therefore, when it is determined in step 201
that the first abnormality determination-purpose rich air/fuel
ratio control flag F3 has been set (F3=1), the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
been satisfied, and the air/fuel ratio of the mixture is being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, so that the present state is a state in which the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance can be accurately determined. Then, the routine proceeds
to step 202, in which "1" is input to the inter-cylinder air/fuel
ratio imbalance determination execution flag F1. In this case, in
step 100 in the routine in FIG. 14, it is determined that F1=1, so
that the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is performed.
[0166] On the other hand, if in step 201 it is determined that
F3=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of determining the presence or
absence of abnormality of the output of the downstream-side
air/fuel ratio sensor 56. Then, the routine proceeds to step 203,
in which "0" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. After that, the routine
ends. That is, when the routine proceeds to step 201, it has been
determined in step 200 that F2=1, which means that the condition as
the prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
been satisfied. However, when in step 201 it is determined that
F3=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, and therefore that the present state is a state in
which the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance cannot be accurately determined. Then, the
routine proceeds to step 203, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. In this case, in step 100 in the routine shown in FIG. 14,
it is determined that F1=0, so that the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance is not performed.
[0167] Incidentally, if in step 200 it is determined that F2.0, the
routine proceeds to step 203, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. Then, the routine ends. Specifically, when in step 200 it
is determined that F2=0, it means that the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
not been satisfied. Then, the routine proceeds to step 203, in
which "0" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. In this case, in step 100 in the
routine shown in FIG. 14, it is determined that F1=0, and therefore
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0168] By the way, in the internal combustion engine 10, sometimes
the air/fuel ratio of the mixture is controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of degradation of the oxygen
storage/release capability of the upstream-side catalyst 43. The
determination of the presence or absence of the oxygen
storage/release capability of the upstream-side catalyst 43 is
performed as follows.
[0169] That is, when the air/fuel ratio of the mixture is
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, exhaust gas whose air/fuel ratio is richer than the
stoichiometric air/fuel ratio is discharged from each combustion
chamber 21. Then, the exhaust gas whose air/fuel ratio is richer
than the stoichiometric air/fuel ratio flows into the upstream-side
catalyst 43. Since the upstream-side catalyst 43 has the oxygen
storage/release capability, the oxygen stored in the upstream-side
catalyst 43 is discharged from the upstream-side catalyst 43, so
that exhaust gas of the stoichiometric air/fuel ratio flows out
from the upstream-side catalyst 43. Then, when exhaust gas whose
air/fuel ratio is richer than the stoichiometric air/fuel ratio
continues to flow into the upstream-side catalyst 43 and therefore
all the oxygen stored in upstream-side catalyst 43 is used, exhaust
gas whose air/fuel ratio is richer than the stoichiometric air/fuel
ratio begins to flow out from the upstream-side catalyst 43. Then,
when it is detected by the downstream-side air/fuel ratio sensor 56
that exhaust gas whose air/fuel ratio is richer than the
stoichiometric air/fuel ratio has begun to flow out from the
upstream-side catalyst 43, the air/fuel ratio of the mixture is
brought to an air/fuel ratio leaner than the stoichiometric
air/fuel ratio. As a result, exhaust gas whose air/fuel ratio is
leaner than the stoichiometric air/fuel ratio is discharged from
each combustion chamber 21. As a result, exhaust gas whose air/fuel
ratio is leaner than the stoichiometric air/fuel ratio flows into
the upstream-side catalyst 43. It is to be noted herein that since
the upstream-side catalyst 43 has the oxygen storage/release
capability, oxygen in the exhaust gas flowing into the
upstream-side catalyst 43 is stored into the upstream-side catalyst
43, so that exhaust gas of the stoichiometric air/fuel ratio flows
out from the upstream-side catalyst 43. Then, when exhaust gas
whose air/fuel ratio is leaner than the stoichiometric air/fuel
ratio continues to flow into the upstream-side catalyst 43 and
therefore the amount of oxygen stored in the upstream-side catalyst
43 reaches a maximum amount of oxygen that the upstream-side,
catalyst 43 can store, the upstream-side catalyst 43 can store no
more oxygen from exhaust gas, so that exhaust gas whose air/fuel
ratio is leaner than the stoichiometric air/fuel ratio flows out
from the upstream-side catalyst 43. Therefore, it is possible to
know the amount of oxygen stored in the upstream-side catalyst 43,
that is, the maximum amount of oxygen that the upstream-side
catalyst 43 is able to store (hereinafter, referred to as "maximum
storable oxygen amount"), on the basis of the amount of time that
elapses from when exhaust gas whose air/fuel ratio is leaner than
the stoichiometric air/fuel ratio begins to flow into the
upstream-side catalyst 43 following the commencement of the control
of the air/fuel ratio of the mixture to an air/fuel ratio leaner
than the stoichiometric air/fuel ratio to when it is detected by
the downstream-side air/fuel ratio sensor 56 that exhaust gas whose
air/fuel ratio is leaner than the stoichiometric air/fuel ratio has
begun to flow out from the upstream-side catalyst 43, and on the
basis of the degree of leanness of the air/fuel ratio of the
mixture obtained when the air/fuel ratio is controlled to an
air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then,
if the maximum storable amount of oxygen is greater than a
predetermined threshold value, it can be said that the oxygen
storage/release capability of the upstream-side catalyst 43 has not
degraded and therefore the upstream-side catalyst 43 is normal. On
the other hand, if the maximum storable amount of oxygen is less
than or equal to the predetermined threshold value, it can be said
that the oxygen storage/release capability of the upstream-side
catalyst 43 has degraded.
[0170] Therefore, in the internal combustion engine 10, when the
presence or absence of degradation of the oxygen storage/release
capability of the upstream-side catalyst 43 needs to be determined,
the air/fuel ratio of the mixture is firstly controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio. Then,
after it is detected by the downstream-side air/fuel ratio sensor
56 that exhaust gas whose air/fuel ratio is richer than the
stoichiometric air/fuel ratio has begun to flow out from the
upstream-side catalyst 43, the air/fuel ratio of the mixture is
controlled to an air/fuel ratio leaner than the stoichiometric
air/fuel ratio. Then, after it is detected by the downstream-side
air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is
leaner than the stoichiometric air/fuel ratio has begun to flow out
from the upstream-side catalyst 43, the amount of oxygen stored in
the upstream-side catalyst 43, that is, the maximum amount of
oxygen than the upstream-side catalyst 43 is able to store, that
is, the maximum storable amount of oxygen, is calculated on the
basis of the amount of time that elapses from when exhaust gas
whose air/fuel ratio is leaner than the stoichiometric air/fuel
ratio begins to flow into the upstream-side catalyst 43 following
the commencement of the control of the air/fuel ratio of the
mixture to an air/fuel ratio leaner than the stoichiometric
air/fuel ratio to when it is detected by the downstream-side
air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is
leaner than the stoichiometric air/fuel ratio has begun to flow out
from the upstream-side catalyst 43, and on the basis of the degree
of leanness of the air/fuel ratio of the mixture obtained when the
air/fuel ratio is controlled to the air/fuel ratio leaner than the
stoichiometric air/fuel ratio. Then, it is determined whether the
calculated maximum storable amount of oxygen is greater than the
predetermined threshold value. The predetermined threshold value is
set at such a maximum storable amount of oxygen that it can be said
that the oxygen storage/release capability of the upstream-side
catalyst 43 has not degraded. Then, if it is determined that the
calculated maximum storable amount of oxygen is greater than the
predetermined threshold value, it is determined that the oxygen
storage/release capability of the upstream-side catalyst 43 has not
degraded. On the other hand, if it is determined that the
calculated maximum storable amount of oxygen is less than or equal
to the predetermined threshold value, it is determined that the
oxygen storage/release capability of the upstream-side catalyst 43
has degraded.
[0171] Then, in this embodiment (hereinafter, referred to as
"second embodiment"), the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of degradation of the
oxygen/release capability of the upstream-side catalyst 43.
[0172] According to this determination process, since the air/fuel
ratio of the mixture is not controlled to an air/fuel ratio richer
than the stoichiometric air/fuel ratio only for the purpose of
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance, the fuel economy of the internal
combustion engine 10 improves in comparison with the case where the
air/fuel ratio is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio only for the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0173] Next, an example of a routine of executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in accordance with the second embodiment will be
described. The determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance in the second
embodiment utilizes the flowchart shown in FIG. 14 and a flowchart
shown in FIG. 16. The routine shown in FIG. 14 and the routine
shown in FIG. 16 are each executed at every predetermined time
interval. Incidentally, since the routine shown in FIG. 14 has
already been described above, the description of the routine shown
in FIG. 14 will be omitted below.
[0174] When the routine shown in FIG. 16 starts, firstly in step
300 it is determined whether the flag F2 that shows whether the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is satisfied has been set (F2=1). It is to be noted
herein that the flag F2 is the same as the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 used
in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 has
been set (F2=1), it means that the condition as the prerequisite
for executing the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance has been
satisfied. If the inter-cylinder air/fuel ratio imbalance
determination prerequisite condition flag F2 has been reset (F2=0),
it means that the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied.
[0175] If in step 300 it is determined that F2=1, it means that the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied, and the routine proceeds to
step 301. On the other hand, if in step 300 it is determined that
F2=0, it means that the condition as the prerequisite for executing
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied, and
the routine proceeds to step 303.
[0176] In step 301, to which the routine proceeds after in step 300
it is determined that F2=1, it is determined whether a flag F4 that
shows whether the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
for the purpose of determining the presence or absence of
degradation of the oxygen storage/release capability of the
upstream-side catalyst 43 (hereinafter, referred to as
"upstream-side catalyst degradation determination-purposed rich
air/fuel ratio control flag F4") has been set (F4=1). It is to be
noted herein that when the upstream-side catalyst degradation
determination-purposed rich air/fuel ratio control flag F4 has been
set (F4=1), the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
for the purpose of determining the presence or absence of
degradation of the oxygen/release capability of the upstream-side
catalyst 43. When the upstream-side catalyst degradation
determination-purposed rich air/fuel ratio control flag F4 has been
reset (F4=0), it means that the air/fuel ratio of the mixture is
not being controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio for the purpose of determining the
presence or absence of degradation of the oxygen/release capability
of the upstream-side catalyst 43.
[0177] If in step 301 it is determined that F4=1, it means that the
air/fuel ratio of the mixture is being controlled to the air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of degradation of the oxygen
storage/release capability of the upstream-side catalyst 43, and
then the routine proceeds to step 302. In step 302, "1" is input to
the inter-cylinder air/fuel ratio imbalance determination execution
flag F1. After that, the routine ends. That is, when the routine
proceeds to step 301 is when in step 300 it is determined that the
inter-cylinder air/fuel ratio imbalance determination prerequisite
condition flag F2 has been set (F2=1). Therefore, when in step 301
it is determined that F4=1, the condition as the prerequisite for
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance has been satisfied and the air/fuel ratio
of the mixture is being controlled to an air/fuel ratio richer than
the stoichiometric air/fuel ratio, so that the present state is a
state in which the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance can be accurately
determined. Then, the routine proceeds to step 302, in which "1" is
input to the inter-cylinder air/fuel ratio imbalance determination
execution flag F1. In this case, in step 100 in the routine shown
in FIG. 14, it is determined that F1=1, so that the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is performed.
[0178] On the other hand, if in step 301 it is determined that
F4=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of determining the presence or
absence of degradation of the oxygen/release capability of the
upstream-side catalyst 43, and then the routine proceeds to step
303. In step 303, "0" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. After that, the routine
ends. That is, when the routine proceeds to step 301, it has been
determined in step 300 that F2=1, and therefore the condition as
the prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
been satisfied. However, when in step 301 it is determined that
F4=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, and therefore that the present state is a state in
which the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance cannot be accurately determined. Then, the
routine proceeds to step 303, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. In this case, in step 100 in the routine shown in FIG. 14,
it is determined that F1=0, so that the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance is not performed.
[0179] Incidentally, if in step 300 it is determined that F2=0, the
routine proceeds to step 303, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. Then, the routine ends. That is, when in step 300 it is
determined that F2=0, it means that the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
not been satisfied. Then, the routine proceeds to step 303, in
which "0" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. In this case, in step 100 in the
routine shown in FIG. 14, it is determined that F1=0, so that the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0180] By the way, in the internal combustion engine 10, sometimes
the air/fuel ratio of the mixture is set to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
determining the presence or absence of abnormality in the response
of the upstream-side air/fuel ratio sensor 55. The determination of
the presence or absence of abnormality in the response of the
upstream-side air/fuel ratio sensor 55 as follows.
[0181] That is, if the air/fuel ratio of the mixture is controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio,
exhaust gas whose air/fuel ratio is richer than the stoichiometric
air/fuel ratio is discharged from each combustion chamber 21. As a
result, exhaust gas whose air/fuel ratio is richer than the
stoichiometric air/fuel ratio comes to the upstream-side air/fuel
ratio sensor 55. On the other hand, if the air/fuel ratio of the
mixture is controlled to an air/fuel ratio leaner than the
stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is
leaner than the stoichiometric air/fuel ratio is discharged from
each combustion chamber 21. As a result, exhaust gas whose air/fuel
ratio is leaner than the stoichiometric air/fuel ratio comes to the
upstream-side air/fuel ratio sensor 55. Therefore, if the air/fuel
ratio of the mixture is changed from an air/fuel ratio richer than
the stoichiometric air/fuel ratio to an air/fuel ratio leaner than
the stoichiometric air/fuel ratio, the air/fuel ratio of the
exhaust gas that comes to the upstream-side air/fuel ratio sensor
55 also changes from an air/fuel ratio richer than the
stoichiometric air/fuel ratio to an air/fuel ratio leaner than the
stoichiometric air/fuel ratio. In this case, the upstream-side
air/fuel ratio sensor output value (i.e., the output value of the
upstream-side air/fuel ratio sensor 55) increases from a negative
value that corresponds to the air/fuel ratio richer than the
stoichiometric air/fuel ratio toward a positive value that
corresponds to the air/fuel ratio leaner than the stoichiometric
air/fuel ratio. If at this time the upstream-side air/fuel ratio
sensor 55 is normal, the upstream-side air/fuel ratio sensor output
value increases at a relatively large rate of increase. Therefore,
if the rate of increase in the upstream-side air/fuel ratio sensor
output value is larger than a predetermined threshold value when
the air/fuel ratio of exhaust gas coming to the upstream-side
air/fuel ratio sensor 55 changes from an air/fuel ratio richer than
the stoichiometric air/fuel ratio to an air/fuel ratio leaner than
the stoichiometric air/fuel ratio, it can be said that abnormality
in the response of the upstream-side air/fuel ratio sensor 55 is
not present and the upstream-side air/fuel ratio sensor 55 is
normal. On the other hand, if the rate of increase in the
upstream-side air/fuel ratio sensor output value is less than or
equal to the predetermined threshold value when the air/fuel ratio
of exhaust gas coming to the upstream-side air/fuel ratio sensor 55
changes from an air/fuel ratio richer than the stoichiometric
air/fuel ratio to an air/fuel ratio leaner than the stoichiometric
air/fuel ratio, it can be said that abnormality in the response of
the upstream-side air/fuel ratio sensor 55 is present.
[0182] Therefore, in the internal combustion engine 10, when the
presence or absence of abnormality in the response of the
upstream-side air/fuel ratio sensor 55 needs to be determined, the
air/fuel ratio of the mixture is controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for a predetermined
period of time before the air/fuel ratio of the mixture is
controlled to an air/fuel ratio leaner than the stoichiometric
air/fuel ratio. Then, when the air/fuel ratio of the mixture has
changed from the air/fuel ratio richer than the stoichiometric
air/fuel ratio to the air/fuel ratio richer than the stoichiometric
air/fuel ratio and therefore the air/fuel ratio of exhaust gas
coming to the upstream-side air/fuel ratio sensor 55 has changed
from an air/fuel ratio richer than the stoichiometric air/fuel
ratio to an air/fuel ratio leaner than the stoichiometric air/fuel
ratio, it is determined whether the rate of increase in the
upstream-side air/fuel ratio sensor output value is greater than
the predetermined threshold value. Incidentally, the predetermined
threshold value is set at such a rate of increase in the
upstream-side air/fuel ratio sensor output value that it can be
said that abnormality in the response of the upstream-side air/fuel
ratio sensor 55 is not present. Then, when it is determined that
the rate of increase in the upstream-side air/fuel ratio sensor
output value is greater than the predetermined rate of increase, it
is determined that abnormality in the response of the upstream-side
air/fuel ratio sensor 55 is not present and therefore the
upstream-side air/fuel ratio sensor 55 is normal. On the other
hand, when it is determined that the rate of increase in the
upstream-side air/fuel ratio sensor output value is less than the
predetermined threshold value, it is determined that abnormality in
the response of the upstream-side air/fuel ratio sensor 55 is
present.
[0183] Then, in this embodiment (hereinafter, referred to as "third
embodiment"), the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of abnormality in the
response of the upstream-side air/fuel ratio sensor 55 as described
above.
[0184] According to this determination process, since the air/fuel
ratio of the mixture is not controlled to an air/fuel ratio richer
than the stoichiometric air/fuel ratio only for the purpose of
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance, the fuel economy of the internal
combustion engine 10 improves in comparison with the case where the
air/fuel ratio is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio only for the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0185] Incidentally, in the internal combustion engine 10, the
presence or absence of abnormality in the response of the
upstream-side air/fuel ratio sensor 55 may be performed in the
following manner. That is, when the presence or absence of
abnormality in the response of the upstream-side air/fuel ratio
sensor 55 is to be determined, an air/fuel ratio control process of
controlling the air/fuel ratio of the mixture to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for a predetermined
period of time and then controlling the air/fuel ratio of the
mixture to an air/fuel ratio leaner than the stoichiometric
air/fuel ratio is executed a plurality of times, and then, as a
final control step, the air/fuel ratio of the mixture is controlled
to the air/fuel ratio richer than the stoichiometric air/fuel
ratio. While this control operation is performed, the rate of
increase in the upstream-side air/fuel ratio sensor output value is
found every time the air/fuel ratio of the exhaust gas that comes
to the upstream-side air/fuel ratio sensor 55 changes from an
air/fuel ratio richer than the stoichiometric air/fuel ratio to an
air/fuel ratio leaner than the stoichiometric air/fuel ratio or the
other way around. If an average value of the rates of increase thus
found (or the smallest rate of the thus-found rates of increase) is
greater than the predetermined threshold value, it is determined
that abnormality in the response of the upstream-side air/fuel
ratio sensor 55 is not present and therefore that the upstream-side
air/fuel ratio sensor 55 is normal. On the other hand, if the
average of the thus-found rates of increase (or the smallest rate
of the rates of increase) is less than or equal to the
predetermined threshold value, it is determined that abnormality in
the response of the upstream-side air/fuel ratio sensor 55 is
present.
[0186] In this case, only in the case where it is determined that
abnormality in the response of the upstream-side air/fuel ratio
sensor 55 is not present and therefore that the upstream-side
air/fuel ratio sensor 55 is normal, the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance may be determined
during the final control step in which the air/fuel ratio of the
mixture is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio.
[0187] According to this determination process, since the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
is determined on the basis of output values of the normal
upstream-side air/fuel ratio sensor 55, the accuracy of the
determination can be said to be high.
[0188] Next, an example of a routine of executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in accordance with the third embodiment will be
described. The determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance in the third
embodiment utilizes the flowchart shown in FIG. 14 and a flowchart
shown in FIG. 17. The routines shown by the flowcharts in FIG. 14
and FIG. 17 are each executed at every predetermined time interval.
Since the routine shown in FIG. 14 has already been described
above, description of the routine in FIG. 14 will be omitted
below.
[0189] When the routine shown in FIG. 17 starts, firstly in step
400 it is determined whether the flag F2 that shows whether the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is satisfied has been set (F2=1). It is to be noted
herein that the flag F2 is the same as the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 used
in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 has
been set (F2=1), it means that the condition as the prerequisite
for executing the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance has been
satisfied. If the inter-cylinder air/fuel ratio imbalance
determination prerequisite condition flag F2 has been reset (F2=0),
it means that the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied.
[0190] If in step 400 it is determined that F2=1, it means that the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied, and the routine proceeds to
step 401. On the other hand, if in step 400 it is determined that
F2=0, it means that the condition as the prerequisite for executing
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied, and
the routine proceeds to step 403.
[0191] In step 401, to which the routine proceeds after in step 400
it is determined that F2=1, it is determined whether a flag F5 that
shows whether the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
for the purpose of determining the presence or absence of
abnormality in the response of the upstream-side air/fuel ratio
sensor 55 (hereinafter, referred to as "second abnormality
determination-purpose rich air/fuel ratio control flag F5") has
been set (F5=1). It is to be noted herein that if the second
abnormality determination-purpose rich air/fuel ratio control flag
F5 has been set (F5=1), it means that the air/fuel ratio of the
mixture is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio for the purpose of determining the
presence or absence of abnormality in the response of the
upstream-side air/fuel ratio sensor 55, and that if the second
abnormality determination-purpose rich air/fuel ratio control flag
F5 has been reset (F5=0), it means that the air/fuel ratio of the
mixture is not being controlled to the air/fuel ratio richer than
the stoichiometric air/fuel ratio for the purpose of determining
the presence or absence of abnormality in the response of the
upstream-side air/fuel ratio sensor 55.
[0192] If in step 401 it is determined that F5=1, it means that the
air/fuel ratio of the mixture is being controlled to the air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of determining the presence or absence of abnormality in the
response of the upstream-side air/fuel ratio sensor 55, and then
the routine proceeds to step 402. In step 402, "1" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. After that, the routine ends. That is, because when the
routine proceeds to step 401 is when in step 400 it is determined
that the inter-cylinder air/fuel ratio imbalance determination
prerequisite condition flag F2 has been set (F2=1), the
determination of F5=1 in step 401 means that the condition as the
prerequisite for determining the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance has been satisfied and
the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio, and
therefore that the present state is a state in which the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
can be accurately determined. Then, the routine proceeds to step
402, in which "1" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. In this case, in step
100 in the routine shown in FIG. 14, it is determined that F1=1, so
that the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is performed.
[0193] On the other hand, if in step 401 it is determined that
F5=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of determining the presence or
absence of abnormality in the response of the upstream-side
air/fuel ratio sensor 55, and then the routine proceeds to step
403. In step 403, "0" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. After that, the routine
ends. That is, when the routine proceeds to step 401, it has been
determined in step 400 that F2=1, and therefore the condition as
the prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
been satisfied. However, when in step 401 it is determined that
F5=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, and therefore that the present state is a state in
which the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance cannot be accurately determined. Then, the
routine proceeds to step 403, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. In this case, in step 100 in the routine shown in FIG. 14,
it is determined that F1=0, so that the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance is not performed.
[0194] Incidentally, if in step 400 it is determined that F2=0, the
routine proceeds to step 403, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. Then, the routine ends. That is, when in step 400 it is
determined that F2=0, it means that the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
not been satisfied. Then, the routine proceeds to step 403, in
which "0" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. In this case, in step 100 in the
routine shown in FIG. 14, it is determined that F1=0, so that the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0195] By the way, in the internal combustion engine 10, sometimes
the air/fuel ratio of the mixture is set to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
quickly raising the temperature of the internal combustion engine
10 when the internal combustion engine 10 is started (i.e., for the
purpose of so-called warmup the engine 10).
[0196] That is, immediately after the internal combustion engine 10
is started, the temperature of the internal combustion engine 10 is
relative low, so that fuel supplied into each combustion chamber 21
does not readily burn. Therefore, when the internal combustion
engine 10 is started, the air/fuel ratio of the mixture is set to
an air/fuel ratio richer than the stoichiometric air/fuel ratio
over a certain period of time. Due to this control, since the
amount of fuel supplied into each combustion chamber 21 is
increased and therefore the amount of heat generated by the
combustion of fuel in each combustion chamber 21 increases, the
temperature of the internal combustion engine 10 rises relatively
quickly, so that fuel favorably burns in each combustion chamber 21
even if the air/fuel ratio of the mixture is controlled to the
stoichiometric air/fuel ratio, or controlled to an air/fuel ratio
leaner than the stoichiometric air/fuel ratio.
[0197] In this embodiment (hereinafter, referred to as "fourth
embodiment"), the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of quickly raising the temperature of the internal combustion
engine 10 when the internal combustion engine 10 is started.
[0198] According to this determination process, since the air/fuel
ratio of the mixture is not controlled to an air/fuel ratio richer
than the stoichiometric air/fuel ratio only for the purpose of
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance, the fuel economy of the internal
combustion engine 10 improves in comparison with the case where the
air/fuel ratio is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio only for the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0199] Next, an example of a routine of executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in accordance with the fourth embodiment will be
described. The determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance in the fourth
embodiment utilizes the flowchart shown in FIG. 14 and a flowchart
shown in FIG. 18. The routines shown by the flowcharts in FIG. 14
and FIG. 18 are executed when the internal combustion engine 10 is
started. Since the routine shown in FIG. 14 has already been
described above, description of the routine in FIG. 14 will be
omitted below.
[0200] When the routine shown in FIG. 18 starts, firstly in step
500 it is determined whether the flag F2 that shows whether the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is satisfied has been set (F2=1). It is to be noted
herein that the flag F2 is the same as the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 used
in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 has
been set (F2=1), it means that the condition as the prerequisite
for executing the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance has been
satisfied. If the inter-cylinder air/fuel ratio imbalance
determination prerequisite condition flag F2 has been reset (F2=0),
it means that the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied.
[0201] If in step 500 it is determined that F2=1, it means that the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied, and the routine proceeds to
step 501. On the other hand, if in step 500 it is determined that
F2=0, it means that the condition as the prerequisite for executing
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied, and
the routine proceeds to step 503.
[0202] In step 501, to which the routine proceeds after in step 500
it is determined that F2=1, it is determined whether a flag F6 that
shows whether the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
for the purpose of quickly raising the temperature of the internal
combustion engine 10 when the internal combustion engine 10 is
started (hereinafter, referred to as "engine warmup-purpose rich
air/fuel ratio control flag F6") has been set (F6=1). It is to be
noted herein that if the engine warmup-purpose rich air/fuel ratio
control flag F6 has been set (F6=1), it means that the air/fuel
ratio of the mixture is being controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
quickly raising the temperature of the internal combustion engine
10, and that if the engine warmup-purpose rich air/fuel ratio
control flag F6 has been reset (F6=0), it means that the air/fuel
ratio of the mixture is not being controlled to the air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
quickly raising the temperature of the internal combustion engine
10.
[0203] If in step 501 it is determined that F6=1, it means that the
air/fuel ratio of the mixture is being controlled to the air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of quickly raising the temperature of the internal combustion
engine 10, and then the routine proceeds to step 502. In step 502,
"1" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. After that, the routine ends. That
is, because when the routine proceeds to step 501 is when in step
500 it is determined that the inter-cylinder air/fuel ratio
imbalance determination prerequisite condition flag F2 has been set
(F2=1), the determination of F6=1 in step 501 means that the
condition as the prerequisite for determining the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
been satisfied and the air/fuel ratio of the mixture is being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, and therefore that the present state is a state in
which the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance can be accurately determined. Then, the
routine proceeds to step 502, in which "1" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. In this case, in step 100 in the routine shown in FIG. 14,
it is determined that F1=1, so that the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance is performed.
[0204] On the other hand, if in step 501 it is determined that
F6=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of quickly raising the temperature
of the internal combustion engine 10, and then the routine proceeds
to step 503. In step 503, "0" is input to the inter-cylinder
air/fuel ratio imbalance determination execution flag F1. After
that, the routine ends. That is, when the routine proceeds to step
501, it has been determined in step 500 that F2=1, and therefore
the condition as the prerequisite for executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied. However, when in step 501 it is
determined that F6=0, it means that the air/fuel ratio of the
mixture is not being controlled to an air/fuel ratio richer than
the stoichiometric air/fuel ratio, and therefore that the present
state is a state in which the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance cannot be accurately
determined. Then, the routine proceeds to step 503, in which "0" is
input to the inter-cylinder air/fuel ratio imbalance determination
execution flag F1. In this case, in step 100 in the routine shown
in FIG. 14, it is determined that F1=0, so that the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is not performed.
[0205] Incidentally, if in step 500 it is determined that F2=0, the
routine proceeds to step 503, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. Then, the routine ends. That is, when in step 500 it is
determined that F2=0, it means that the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
not been satisfied. Then, the routine proceeds to step 503, in
which "0" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. In this case, in step 100 in the
routine shown in FIG. 14, it is determined that F1=0, so that the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0206] By the way, in the internal combustion engine 10, when the
output demanded of the internal combustion engine 10 is very small
and, in particular, zero, a control of stopping the injection of
fuel from the fuel injection valves 25 into the combustion chambers
21 (i.e., a so-called fuel-cut control) is executed. Then, in the
internal combustion engine 10, the fuel-cut control is stopped when
the output demanded of the internal combustion engine 10 becomes
relatively large.
[0207] When the fuel-cut control is being executed, the air/fuel
ratio of the mixture is an air/fuel ratio that is much leaner than
the stoichiometric air/fuel ratio. Therefore, exhaust gas whose
air/fuel ratio is much leaner than the stoichiometric air/fuel
ratio is discharged from each combustion chamber 21, so that the
exhaust gas whose air/fuel ratio is much leaner than the
stoichiometric air/fuel ratio flows into the upstream-side catalyst
43. That is, during execution of the fuel-cut control, since
exhaust gas whose air/fuel ratio is much leaner than the
stoichiometric air/fuel ratio flows into the upstream-side catalyst
43, it is highly likely that the upstream-side catalyst 43 will
store a large amount of oxygen from the exhaust gas flowing into
the catalyst 43 and the amount of oxygen stored in the
upstream-side catalyst 43 will reach a maximum storable oxygen
amount (i.e., a maximum amount of oxygen that the upstream-side
catalyst 43 is able to store). In this case, if exhaust gas whose
air/fuel ratio is leaner than the stoichiometric air/fuel ratio
flows into the upstream-side catalyst 43 after the fuel-cut control
is stopped, the upstream-side catalyst 43 cannot store oxygen from
the exhaust gas that flows into the catalyst 43. That is, the
upstream-side catalyst 43 cannot exert its oxygen storage/release
capability. On the other hand, if exhaust gas whose air/fuel ratio
is richer than the stoichiometric air/fuel ratio flows into the
upstream-side catalyst 43, the upstream-side catalyst 43 releases
oxygen that has been stored therein, as described above. Therefore,
when the amount of oxygen stored in the upstream-side catalyst 43
has reached the maximum storable oxygen amount, it is appropriate
to send exhaust gas of a rich air/fuel ratio into the upstream-side
catalyst 43 so that the upstream-side catalyst 43 releases oxygen
that has been stored therein. After that, if exhaust gas whose
air/fuel ratio is leaner than the stoichiometric air/fuel ratio
flows into the upstream-side catalyst 43, the upstream-side
catalyst 43 can store oxygen from the exhaust gas.
[0208] Therefore, in the internal combustion engine 10, immediately
after the fuel-cut control is stopped, the air/fuel ratio of the
mixture is set to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of causing the upstream-side
catalyst 43 to release oxygen that has been stored in the
upstream-side catalyst 43.
[0209] That is, in the embodiment (hereinafter, referred to as
"fifth embodiment"), the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of causing the upstream-side catalyst 43 to release stored oxygen
after the fuel-cut control is stopped as described above.
[0210] According to this determination process, since the air/fuel
ratio of the mixture is not controlled to an air/fuel ratio richer
than the stoichiometric air/fuel ratio only for the purpose of
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance, the fuel economy of the internal
combustion engine 10 improves in comparison with the case where the
air/fuel ratio is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio only for the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0211] Next, an example of a routine of executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in accordance with the fifth embodiment will be
described. The determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance in the fifth
embodiment utilizes the flowchart shown in FIG. 14 and a flowchart
shown in FIG. 19. The routines shown by the flowcharts in FIG. 14
and FIG. 19 are executed when the fuel-cut control is stopped.
Since the routine shown in FIG. 14 has already been described
above, description of the routine in FIG. 14 will be omitted
below.
[0212] When the routine shown in FIG. 19 starts, firstly in step
600 it is determined whether the flag F2 that shows whether the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is satisfied has been set (F2=1). It is to be noted
herein that the flag F2 is the same as the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 used
in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 has
been set (F2=1), it means that the condition as the prerequisite
for executing the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance has been
satisfied. If the inter-cylinder air/fuel ratio imbalance
determination prerequisite condition flag F2 has been reset (F2=0),
it means that the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied.
[0213] If in step 600 it is determined that F2=1, it means that the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied, and the routine proceeds to
step 601. On the other hand, if in step 600 it is determined that
F2=0, it means that the condition as the prerequisite for executing
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied, and
the routine proceeds to step 603.
[0214] In step 601, to which the routine proceeds after in step 600
it is determined that F2=1, it is determined whether a flag F7 that
shows whether the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
for the purpose of causing the upstream-side catalyst 43 to release
stored oxygen after the fuel-cut control is stopped (hereinafter,
referred to as "stored oxygen release-purpose rich air/fuel ratio
control flag F7") has been set (F7=1). It is to be noted herein
that if the stored oxygen release-purpose rich air/fuel ratio
control flag F7 has been set (F7=1), it means that the air/fuel
ratio of the mixture is being controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
causing the upstream-side catalyst 43 to release oxygen stored
therein, and that if stored oxygen release-purpose rich air/fuel
ratio control flag F7 has been reset (F7=0), it means that the
air/fuel ratio of the mixture is not being controlled to the
air/fuel ratio richer than the stoichiometric air/fuel ratio for
the purpose of causing the upstream-side catalyst 43 to release
oxygen stored therein.
[0215] If in step 601 it is determined that F7=1, it means that the
air/fuel ratio of the mixture is being controlled to the air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of causing the upstream-side catalyst 43 to release oxygen stored
therein, and then the routine proceeds to step 602. In step 602,
"1" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. After that, the routine ends. That
is, because when the routine proceeds to step 601 is when in step
600 it is determined that the inter-cylinder air/fuel ratio
imbalance determination prerequisite condition flag F2 has been set
(F2=1), the determination of F7=1 in step 601 means that the
condition as the prerequisite for determining the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
been satisfied and the air/fuel ratio of the mixture is being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio, and therefore that the present state is a state in
which the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance can be accurately determined. Then, the
routine proceeds to step 602, in which "1" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. In this case, in step 100 in the routine shown in FIG. 14,
it is determined that F1=1, so that the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance is performed.
[0216] On the other hand, if in step 601 it is determined that
F7=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of causing the upstream-side
catalyst 43 to release oxygen stored therein, and then the routine
proceeds to step 603. In step 603, "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. After that, the routine ends. That is, when the routine
proceeds to step 601, it has been determined in step 600 that F2=1,
and therefore the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has been satisfied.
However, when in step 601 it is determined that F7=0, it means that
the air/fuel ratio of the mixture is not being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio, and
therefore that the present state is a state in which the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
cannot be accurately determined. Then, the routine proceeds to step
603, in which "0" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. In this case, in step
100 in the routine shown in FIG. 14, it is determined that F1=0, so
that the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0217] Incidentally, if in step 600 it is determined that F2=0, the
routine proceeds to step 603, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. Then, the routine ends. That is, when in step 600 it is
determined that F2=0, it means that the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
not been satisfied. Then, the routine proceeds to step 603, in
which "0" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. In this case, in step 100 in the
routine shown in FIG. 14, it is determined that F1=0, so that the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0218] By the way, when the output demanded of the internal
combustion engine 10 is very large, the temperature of exhaust gas
discharged from each combustion chamber 21 becomes very high. If
such a very high temperature exhaust gas continues to flow into the
upstream-side catalyst 43, there is possibility of the temperature
of the upstream-side catalyst 43 becoming very high and therefore
the upstream-side catalyst 43 undergoing thermal degradation. On
the other hand, if an exhaust gas whose air/fuel ratio is richer
than the stoichiometric air/fuel ratio flows into the upstream-side
catalyst 43, vaporization of fuel in the exhaust gas on the
upstream-side catalyst 43 removes heat from the upstream-side
catalyst 43 and therefore the temperature of the upstream-side
catalyst 43 declines.
[0219] Therefore, in the internal combustion engine 10, when the
output demanded of the internal combustion engine 10 is very large,
sometimes the air/fuel ratio of the mixture is controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio for
the purpose of lowering the temperature of the upstream-side
catalyst 43.
[0220] In this embodiment (hereinafter, referred to as "sixth
embodiment"), the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of lowering the temperature of the upstream-side catalyst 43 when
the output demanded of the internal combustion engine 10 is very
large.
[0221] According to this determination process, since the air/fuel
ratio of the mixture is not controlled to an air/fuel ratio richer
than the stoichiometric air/fuel ratio only for the purpose of
determining the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance, the fuel economy of the internal
combustion engine 10 improves in comparison with the case where the
air/fuel ratio is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio only for the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0222] Next, an example of a routine of executing the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in accordance with the sixth embodiment will be
described. The determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance in the sixth
embodiment utilizes the flowchart shown in FIG. 14 and a flowchart
shown in FIG. 20. The routines shown by the flowcharts in FIG. 14
and FIG. 20 are executed when the fuel-cut control is stopped.
Since the routine shown in FIG. 14 has already been described
above, description of the routine in FIG. 14 will be omitted
below.
[0223] When the routine shown in FIG. 20 starts, firstly in step
700 it is determined whether the flag F2 that shows whether the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance is satisfied has been set (F2=1). It is to be noted
herein that the flag F2 is the same as the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 used
in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel
ratio imbalance determination prerequisite condition flag F2 has
been set (F2=1), it means that the condition as the prerequisite
for executing the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance has been
satisfied. If the inter-cylinder air/fuel ratio imbalance
determination prerequisite condition flag F2 has been reset (F2=0),
it means that the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied.
[0224] If in step 700 it is determined that F2=1, it means that the
condition as the prerequisite for executing the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance has been satisfied, and the routine proceeds to
step 701. On the other hand, if in step 700 it is determined that
F2=0, it means that the condition as the prerequisite for executing
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has not been satisfied, and
the routine proceeds to step 703.
[0225] In step 701, to which the routine proceeds after in step 700
it is determined that F2=1, it is determined whether a flag F8 that
shows whether the air/fuel ratio of the mixture is being controlled
to an air/fuel ratio richer than the stoichiometric air/fuel ratio
for the purpose of lowering the temperature of the upstream-side
catalyst 43 when the output demanded of the internal combustion
engine 10 is very large (hereinafter, referred to as "catalyst
temperature-lowering purpose rich air/fuel ratio control flag F8")
has been set (F8=1). It is to be noted herein that if the catalyst
temperature-lowering purpose rich air/fuel ratio control flag F8
has been set (F8=1), it means that the air/fuel ratio of the
mixture is being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio for the purpose of lowering the
temperature of the upstream-side catalyst 43. Besides, if catalyst
temperature-lowering purpose rich air/fuel ratio control flag F8
has been reset (F8=0), it means that the air/fuel ratio of the
mixture is not being controlled to the air/fuel ratio richer than
the stoichiometric air/fuel ratio for the purpose of lowering the
temperature of the upstream-side catalyst 43.
[0226] If in step 701 it is determined that F8=1, it means that the
air/fuel ratio of the mixture is being controlled to the air/fuel
ratio richer than the stoichiometric air/fuel ratio for the purpose
of lowering the temperature of the upstream-side catalyst 43, and
then the routine proceeds to step 702. In step 702, "1" is input to
the inter-cylinder air/fuel ratio imbalance determination execution
flag F1. After that, the routine ends. That is, because when the
routine proceeds to step 701 is when in step 700 it is determined
that the inter-cylinder air/fuel ratio imbalance determination
prerequisite condition flag F2 has been set (F2=1), the
determination of F8=1 in step 701 means that the condition as the
prerequisite for determining the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance has been satisfied and
the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio, and
therefore that the present state is a state in which the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
can be accurately determined. Then, the routine proceeds to step
702, in which "1" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. In this case, in step
100 in the routine shown in FIG. 14, it is determined that F1=1, so
that the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is performed.
[0227] On the other hand, if in step 701 it is determined that
F8=0, it means that the air/fuel ratio of the mixture is not being
controlled to an air/fuel ratio richer than the stoichiometric
air/fuel ratio for the purpose of purpose of lowering the
temperature of the upstream-side catalyst 43, and then the routine
proceeds to step 703. In step 703, "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. After that, the routine ends. That is, when the routine
proceeds to step 701, it has been determined in step 700 that F2=1,
and therefore the condition as the prerequisite for executing the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance has been satisfied.
However, when in step 701 it is determined that F8=0, it means that
the air/fuel ratio of the mixture is not being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio, and
therefore that the present state is a state in which the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
cannot be accurately determined. Then, the routine proceeds to step
703, in which "0" is input to the inter-cylinder air/fuel ratio
imbalance determination execution flag F1. In this case, in step
100 in the routine shown in FIG. 14, it is determined that F1=0, so
that the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0228] Incidentally, if in step 700 it is determined that F2=0, the
routine proceeds to step 703, in which "0" is input to the
inter-cylinder air/fuel ratio imbalance determination execution
flag F1. Then, the routine ends. That is, when in step 700 it is
determined that F2=0, it means that the condition as the
prerequisite for executing the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance has
not been satisfied. Then, the routine proceeds to step 703, in
which "0" is input to the inter-cylinder air/fuel ratio imbalance
determination execution flag F1. In this case, in step 100 in the
routine shown in FIG. 14, it is determined that F1=0, so that the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is not performed.
[0229] By the way, when the mixture burns in the combustion
chambers 21, hydrogen (H.sub.2) is produced. The amount of hydrogen
produced by the combustion of the mixture is larger when the
mixture whose air/fuel ratio is richer than the stoichiometric
air/fuel ratio is burned than when the mixture whose air/fuel ratio
is leaner than the stoichiometric air/fuel ratio is burned.
Therefore, the amount of hydrogen in the exhaust gas discharged
from the combustion chambers 21 is larger when the mixture whose
air/fuel ratio is richer than the stoichiometric air/fuel ratio is
burned than when the mixture whose air/fuel ratio is leaner than
the stoichiometric air/fuel ratio is burned. That is, the exhaust
gas whose air/fuel ratio is richer than the stoichiometric air/fuel
ratio contains a larger amount of hydrogen than the exhaust gas
whose air/fuel ratio is leaner than the stoichiometric air/fuel
ratio.
[0230] The molecules of hydrogen are smaller than the molecules of
oxygen (O.sub.2), carbon monoxide (CO), and hydrocarbons (HCs).
Therefore, the hydrogen in the exhaust gas that comes to the
upstream-side air/fuel ratio sensor 55 diffuses in the diffusion
resistance layer 559 at higher speed than the oxygen, carbon
monoxide and hydrocarbons contained in the exhaust gas. Then, the
hydrogen affects the amount of oxygen ions that flow in the solid
electrolyte layer 551 of the upstream-side air/fuel ratio sensor
55. Specifically, in the comparison between the case where an
exhaust gas whose air/fuel ratio is richer than the stoichiometric
air/fuel ratio comes to the upstream-side air/fuel ratio sensor 55
and the case where an exhaust gas whose air/fuel ratio is leaner
than the stoichiometric air/fuel ratio comes to the upstream-side
air/fuel ratio sensor 55, the amount of oxygen ions that flow in
the solid electrolyte layer 551 of the upstream-side air/fuel ratio
sensor 55 is larger when the air/fuel ratio of the exhaust gas that
comes to the upstream-side air/fuel ratio sensor 55 is richer than
the stoichiometric air/fuel ratio than when the air/fuel ratio of
the exhaust gas that comes to the upstream-side air/fuel ratio
sensor 55 is leaner than the stoichiometric air/fuel ratio provided
that the degree of richness of the rich air/fuel ratio of the
exhaust gas relative to the stoichiometric air/fuel ratio is equal
to the degree of leanness of the lean air/fuel ratio of the exhaust
gas relative to the stoichiometric air/fuel ratio. In other words,
the characteristic of output of the upstream-side air/fuel ratio
sensor 55 varies greatly according to the amount of hydrogen in the
exhaust gas that comes to the upstream-side air/fuel ratio sensor
55, that is, according to the air/fuel ratio of the exhaust gas
that comes to the upstream-side air/fuel ratio sensor 55.
[0231] When the air/fuel ratio of the mixture is being controlled
to the stoichiometric air/fuel ratio, exhaust gas whose air/fuel
ratio is richer than the stoichiometric air/fuel ratio and exhaust
gas whose air/fuel ratio is leaner than the stoichiometric air/fuel
ratio alternately comes to the upstream-side air/fuel ratio sensor
55. Therefore, in that case, if the characteristic of the output of
the upstream-side air/fuel ratio sensor 55 varies greatly according
to the air/fuel ratio of the exhaust gas that comes to the
upstream-side air/fuel ratio sensor 55, the air/fuel ratio of the
mixture cannot be accurately controlled to the stoichiometric
air/fuel ratio even if the air/fuel ratio of the mixture is
intended to be controlled to the stoichiometric air/fuel ratio on
the basis of the output value of the upstream-side air/fuel ratio
sensor 55.
[0232] Therefore, in order to prevent the output characteristic of
the upstream-side air/fuel ratio sensor 55 from varying according
to the air/fuel ratio of the exhaust gas that comes to the
upstream-side air/fuel ratio sensor 55, it is appropriate to employ
an air/fuel ratio detection element 55a as shown in FIG. 21 which
is similar to the air/fuel ratio detection element 55a shown in
FIG. 12 but has a catalyst 561 in the through holes 558. The
catalyst 561 is disposed so as to close the through holes 558.
Besides, the catalyst 561 is a porous body, and supports a
catalytic substance that accelerates oxidation-reduction reactions
and an oxygen storing material that achieves an oxygen
storage/release capability, as in the upstream-side catalyst
43.
[0233] In the air/fuel ratio detection element 55a shown in FIG.
21, the exhaust gas that comes to the upstream-side air/fuel ratio
sensor 55 flows into the diffusion resistance layer 554 through the
catalyst 561. When the exhaust gas passes through the catalyst 561,
the catalyst 561 accelerates the oxidation of hydrogen in the
exhaust gas, thereby lessening the amount of oxygen in the exhaust
gas. According to this arrangement, when exhaust gas whose air/fuel
ratio is richer than the stoichiometric air/fuel ratio comes to the
upstream-side air/fuel ratio sensor 55, the amount of hydrogen in
the exhaust gas is lessened before the exhaust gas flows into the
diffusion resistance layer 554. As a result, the variation of the
characteristic of the output of the upstream-side air/fuel ratio
sensor 55 commensurate with the air/fuel ratio of the exhaust gas
that comes to the upstream-side air/fuel ratio sensor 55 can be
eliminated.
[0234] Incidentally, the functions of the air/fuel ratio detection
element 55a illustrated in FIG. 21 are the same as the functions of
the air/fuel ratio detection element 55a shown in FIG. 12, except
that exhaust gas flows into the diffusion resistance layer 554
through the catalyst 561.
[0235] In the case where the upstream-side air/fuel ratio sensor 55
includes the air/fuel ratio detection element 55a that is equipped
with the catalyst 561 shown in FIG. 21, the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance performed while the air/fuel ratio of the mixture is
being controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio achieves more effects, compared with
the determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance performed while the
air/fuel ratio of the mixture is being controlled to the
stoichiometric air/fuel ratio.
[0236] That is, in the case where the air/fuel ratio of the mixture
is being controlled to the stoichiometric air/fuel ratio, exhaust
gas whose air/fuel ratio is richer than the stoichiometric air/fuel
ratio and exhaust gas whose air/fuel ratio is leaner than the
stoichiometric air/fuel ratio alternately comes to the
upstream-side air/fuel ratio sensor 55, as described above. In this
case, in order for the upstream-side air/fuel ratio sensor 55 to
output values that very accurately correspond to the air/fuel ratio
of exhaust gas that comes to the sensor 55, the flow direction of
oxygen ions flowing in the solid electrolyte layer 551 of the
air/fuel ratio detection element 55a of the upstream-side air/fuel
ratio sensor 55 needs to instantly reverse when the air/fuel ratio
of the exhaust gas that comes to the upstream-side air/fuel ratio
sensor 55 changes between the rich side of the stoichiometric
air/fuel ratio and the lean side of the stoichiometric air/fuel
ratio. However, the direction of flow of oxygen ions in the solid
electrolyte layer 551 does not instantly reverse, as described
above. In the case where the air/fuel ratio detection element 55a
of the upstream-side air/fuel ratio sensor 55 has the catalyst 561
in the through holes 558, the direction of flow of oxygen ion in
the solid electrolyte layer 551 reverses even less readily.
Therefore, when the air/fuel ratio of the mixture is being
controlled to the stoichiometric air/fuel ratio, the output value
of the upstream-side air/fuel ratio sensor 55 is even less likely
to very accurately show the air/fuel ratio of the exhaust gas that
comes to the upstream-side air/fuel ratio sensor 55 (i.e., the
air/fuel ratio of the mixture).
[0237] However, if the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is being controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio, the exhaust
gas that comes to the upstream-side air/fuel ratio sensor 55 at
that time is only the exhaust gas whose air/fuel ratio is richer
than the stoichiometric air/fuel ratio, and therefore it is not
required that the direction of flow of oxygen ions in the solid
electrolyte layer 554 of the upstream-side air/fuel ratio sensor 55
reverse during the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance. Therefore, in the
case where the upstream-side air/fuel ratio sensor 55 includes the
air/fuel ratio detection element 55a that is equipped with the
catalyst 561, the determination of the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance performed while
the air/fuel ratio of the mixture is being controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio
achieves more effects, compared with the determination of the
presence or absence of the state of inter-cylinder air/fuel ratio
imbalance performed while the air/fuel ratio of the mixture is
being controlled to the stoichiometric air/fuel ratio.
[0238] Incidentally, in the foregoing embodiment, the determination
of the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance utilizes the absolute value of an average slope of
a line that is followed by the upstream-side air/fuel ratio sensor
output value (i.e., the output value of the upstream-side air/fuel
ratio sensor 55), that is, the absolute value of the time-dependent
rate of change in the upstream-side air/fuel ratio sensor output
value.
[0239] However, instead of using the absolute value of the
time-dependent rate of change in the upstream-side air/fuel ratio
sensor output value, the determination of the presence or absence
of the state of inter-cylinder air/fuel ratio imbalance may use,
for example, the largest value of the absolute values of values of
the first order time differential of the upstream-side air/fuel
ratio sensor output value, the largest value of the absolute values
of values of the second order time differential of the
upstream-side air/fuel ratio sensor output value, or the length of
the signal trace between two different upstream-side air/fuel ratio
sensor output values with a predetermined time interval
therebetween.
[0240] In the case where the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance
uses the largest value of the absolute values of values of the
first order time differential of the upstream-side air/fuel ratio
sensor output value (hereinafter, referred to as "maximum value of
the first order time differential of the upstream-side air/fuel
ratio sensor output value"), the maximum value of the first order
time differential of the upstream-side air/fuel ratio sensor output
value is larger when the state of inter-cylinder air/fuel ratio
imbalance is present than when the state of inter-cylinder air/fuel
ratio imbalance is not present. Therefore, in the case where the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance uses the maximum value of
the first order time differential of the upstream-side air/fuel
ratio sensor output value, the largest value of the maximum values
of the first order time differential of the upstream-side air/fuel
ratio sensor output value that are obtained when it has to be
determined that the state of inter-cylinder air/fuel ratio
imbalance is not present is determined beforehand as a threshold
value. If the maximum value of the first order time differential of
the upstream-side air/fuel ratio sensor output value is greater
than this predetermined threshold value, it is determined that the
state of inter-cylinder air/fuel ratio imbalance is present. If the
maximum value of the first order time differential of the
upstream-side air/fuel ratio sensor output value is less than or
equal to the predetermined threshold value, it is determined that
the state of inter-cylinder air/fuel ratio imbalance is not
present.
[0241] In the case where the determination of the presence or
absence of the state of inter-cylinder air/fuel ratio imbalance
uses the largest value of the absolute values of values of the
second order time differential of the upstream-side air/fuel ratio
sensor output value (hereinafter, referred to as "maximum value of
the second order time differential of the upstream-side air/fuel
ratio sensor output value"), the maximum value of the second order
time differential of the upstream-side air/fuel ratio sensor output
value is larger when the state of inter-cylinder air/fuel ratio
imbalance is present than when the state of inter-cylinder air/fuel
ratio imbalance is not present. Therefore, in the case where the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance uses the maximum value of
the second order time differential of the upstream-side air/fuel
ratio sensor output value, the largest value of the maximum values
of the second order time differential of the upstream-side air/fuel
ratio sensor output value that are obtained when it has to be
determined that the state of inter-cylinder air/fuel ratio
imbalance is not present is determined beforehand as a threshold
value. If the maximum value of the second order time differential
of the upstream-side air/fuel ratio sensor output value is greater
than this predetermined threshold value, it is determined that the
state of inter-cylinder air/fuel ratio imbalance is present. If the
maximum value of the second order time differential of the
upstream-side air/fuel ratio sensor output value is less than or
equal to the predetermined threshold value, it is determined that
the state of inter-cylinder air/fuel ratio imbalance is not
present.
[0242] Besides, in the case where the determination of the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
uses the length of the signal trace between two different output
values of the upstream-side air/fuel ratio sensor (hereinafter,
referred to as "length of the signal trace of the output of the
upstream-side air/fuel ratio sensor"), the length of the signal
trace of the output of the upstream-side air/fuel ratio sensor is
longer when the state of inter-cylinder air/fuel ratio imbalance is
present than when the state of inter-cylinder air/fuel ratio
imbalance is not present. Therefore, in the case where the
determination of the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance uses the length of the
signal trace of the output of the upstream-side air/fuel ratio
sensor, the largest value of the lengths of the signal trace of the
output of the upstream-side air/fuel ratio sensor that are obtained
when it has to be determined that the state of inter-cylinder
air/fuel ratio imbalance is not present is determined beforehand as
a threshold value. If the length of the signal trace of the output
of the upstream-side air/fuel ratio sensor is longer than this
predetermined threshold value, it is determined that the state of
inter-cylinder air/fuel ratio imbalance is present. If the length
of the signal trace of the output of the upstream-side air/fuel
ratio sensor is less than or equal to the predetermined threshold
value, it is determined that the state of inter-cylinder air/fuel
ratio imbalance is not present.
[0243] Incidentally, the foregoing manners of the determination of
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance in the first to sixth embodiments may be combined
as appropriate. Therefore, the embodiment of the invention may be a
construction in which in an internal combustion engine in which the
air/fuel ratio of the mixture is controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for at least one of
the purposes of: determining the presence or absence of abnormality
in the output of the downstream-side air/fuel ratio sensor 56;
determining the presence or absence of degradation of the oxygen
storage/release capability of the upstream-side catalyst 43;
determining the presence or absence of abnormality in the response
of the upstream-side air/fuel ratio sensor 55; quickly raising the
temperature of the internal combustion engine 10 when the internal
combustion engine 10 is started; causing the upstream-side catalyst
43 to release stored oxygen therefrom after the fuel-cut control is
stopped; and lowering the temperature of the upstream-side catalyst
43 when the output demanded of the internal combustion engine 10 is
very large, the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance is determined while the air/fuel ratio of
the mixture is being controlled to the air/fuel ratio richer than
the stoichiometric air/fuel ratio.
[0244] Besides, in the internal combustion engine 10, if there is a
case where the air/fuel ratio of the mixture is controlled to an
air/fuel ratio richer than the stoichiometric air/fuel ratio for a
purpose that is other than the purposes of: determining the
presence or absence of abnormality in the output of the
downstream-side air/fuel ratio sensor 56; determining the presence
or absence of degradation of the oxygen/release capability of the
upstream-side catalyst 43; determining the presence or absence of
abnormality in the response of the upstream-side air/fuel ratio
sensor 55; quickly raising the temperature of the internal
combustion engine 10 when the internal combustion engine 10 is
started; causing the upstream-side catalyst 43 to release stored
oxygen therefrom after the fuel-cut control is stopped; and
lowering the temperature of the upstream-side catalyst 43 when the
output demanded of the internal combustion engine 10 is very large,
and that is also other than the purpose of determining the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance,
it is also permissible to determine the presence or absence of the
state of inter-cylinder air/fuel ratio imbalance while the air/fuel
ratio of the mixture is being controlled to the air/fuel ratio
richer than the stoichiometric air/fuel ratio. Therefore, the
embodiment of the invention may also be a construction in which in
an internal combustion engine in which the air/fuel ratio of the
mixture is controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio for a purpose other than the purpose
of determining the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance, and in which the presence
or absence of the state of inter-cylinder air/fuel ratio imbalance
is determined while the air/fuel ratio of the mixture is being
controlled to the air/fuel ratio richer than the stoichiometric
air/fuel ratio for a purpose other than the purpose of determining
the presence or absence of the state of inter-cylinder air/fuel
ratio imbalance.
[0245] In particular, in the internal combustion engine 10, in
order to determine the presence or absence of abnormality in the
output of the downstream-side air/fuel ratio sensor 56, the
air/fuel ratio of the mixture is controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio over a predetermined
period of time. On the other hand, in the internal combustion
engine 10, in order to determine the presence or absence of
degradation of the oxygen storage/release capability of the
upstream-side catalyst 43, firstly the air/fuel ratio of the
mixture is controlled to an air/fuel ratio richer than the
stoichiometric air/fuel ratio, and then the air/fuel ratio of the
mixture is controlled to an air/fuel ratio leaner than the
stoichiometric air/fuel ratio when it has been detected by the
downstream-side air/fuel ratio sensor 56 that exhaust gas whose
air/fuel ratio is richer than the stoichiometric air/fuel ratio has
begun to flow out from the upstream-side catalyst 43. Specifically,
both when the presence or absence of abnormality in the output of
the downstream-side air/fuel ratio sensor 56 is to be determined
and when the presence or absence of degradation of the oxygen
storage/release capability of the upstream-side catalyst 43 is to
be determined, the air/fuel ratio of the mixture is controlled to
the air/fuel ratio richer than the stoichiometric air/fuel ratio.
Therefore, if the presence or absence of abnormality in the output
of the downstream-side air/fuel ratio sensor 56 and the presence or
absence of degradation of the oxygen storage/release capability of
the upstream-side catalyst 43 are determined in a series of
processes, good efficiency is achieved.
[0246] Therefore, in the internal combustion engine 10, the
presence or absence of abnormality in the output of the
downstream-side air/fuel ratio sensor 56 and the presence or
absence of degradation of the oxygen storage/release capability of
the upstream-side catalyst 43 may be determined as follows. That
is, in the internal combustion engine 10, when the presence or
absence of abnormality in the output of the downstream-side
air/fuel ratio sensor 56 and the presence or absence of degradation
of the oxygen storage/release capability of the upstream-side
catalyst 43 are to be determined, firstly the air/fuel ratio of the
mixture is controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio over a predetermined period. The
predetermined period herein is set at a time that is sufficiently
long for the oxygen stored in the upstream-side catalyst 43 to run
out when exhaust gas whose air/fuel ratio is richer than the
stoichiometric air/fuel ratio continues to flow into the
upstream-side catalyst 43. Then, when the foregoing predetermined
period elapses after the air/fuel ratio of the mixture begins to be
controlled to the air/fuel ratio richer than the stoichiometric
air/fuel ratio, it is determined whether the output value of the
downstream-side air/fuel ratio sensor 56 shows an air/fuel ratio of
exhaust gas that corresponds to the air/fuel ratio of the mixture
that is being controlled to the air/fuel ratio richer than the
stoichiometric air/fuel ratio.
[0247] If it is determined that the then downstream-side air/fuel
ratio sensor output value (i.e., the then output value of the
downstream-side air/fuel ratio sensor 56) shows an air/fuel ratio
of exhaust gas that corresponds to the air/fuel ratio of the
mixture that is being controlled to the air/fuel ratio richer than
the stoichiometric air/fuel ratio, it is determined that
abnormality in the output of the downstream-side air/fuel ratio
sensor 56 is not present and that the downstream-side air/fuel
ratio sensor 56 is normal. On the other hand, if it is determined
that the then downstream-side air/fuel ratio sensor output value
does not show an air/fuel ratio of exhaust gas that corresponds to
the air/fuel ratio of the mixture that is being controlled to the
air/fuel ratio richer than the stoichiometric air/fuel ratio, it is
determined that abnormality in the output of the downstream-side
air/fuel ratio sensor 56 is present. When the predetermined period
elapses after the air/fuel ratio of the mixture begins to be
controlled to the air/fuel ratio richer than the stoichiometric
air/fuel ratio, the air/fuel ratio of the mixture is controlled to
an air/fuel ratio leaner than the stoichiometric air/fuel ratio.
Then, when it is determined by the downstream-side air/fuel ratio
sensor 56 that exhaust gas whose air/fuel ratio is leaner than the
stoichiometric air/fuel ratio has begun to flow out from the
upstream-side catalyst 43, the amount of oxygen stored in the
upstream-side catalyst 43, that is, the maximum amount of oxygen
that can be stored into the upstream-side catalyst 43, that is, the
maximum storable amount of oxygen, is calculated on the basis of
the amount of time that elapses from when exhaust gas whose
air/fuel ratio is leaner than the stoichiometric air/fuel ratio
begins to flow into the upstream-side catalyst 43 following the
beginning of the control of the air/fuel ratio of the mixture to
the air/fuel ratio leaner than the stoichiometric air/fuel ratio
till when it is detected by the downstream-side air/fuel ratio
sensor 56 that exhaust gas whose air/fuel ratio is leaner than the
stoichiometric air/fuel ratio has begun to flow out from the
upstream-side catalyst 43, and on the basis of the degree of
leanness of the air/fuel ratio of the mixture obtained when the
air/fuel ratio is being controlled to the air/fuel ratio leaner
than the stoichiometric air/fuel ratio. Then, it is determined
whether the calculated maximum storable amount of oxygen is greater
that a predetermined threshold value. The predetermined threshold
value herein is set to a maximum storable amount of oxygen of such
a degree that it can be said that the upstream-side catalyst 43 of
the oxygen storage/release capability has not degraded. Then, if it
is determined that the calculated maximum storable amount of oxygen
is greater than the predetermined threshold value, it is determined
that degradation of the oxygen storage/release capability of the
upstream-side catalyst 43 is not present. On the other hand, if it
is determined that the calculated maximum storable amount oxygen is
less than or equal to the predetermined threshold value, it is
determined that degradation of the oxygen storage/release
capability of the upstream-side catalyst 43 is present.
[0248] By the way, there is known a so-called hybrid system as
shown in FIG. 22 which includes an electric motor M in addition to
the foregoing internal combustion engine 10 in order to produce
drive force for driving a vehicle. The hybrid system shown in FIG.
22 has a drive force switching mechanism P for switching the
transmission path for the drive force for driving a vehicle 70
according to the state of travel of the vehicle 70, and a
transmission TM that transmits the drive force transmitted thereto
from the drive force switching mechanism P to a drive force
transmission system of front wheels 71 of the vehicle 70.
[0249] The electric motor M is an alternating-current electric
motor, and is driven by alternating-current electric power supplied
from an inverter I that converts direct-current electric power
supplied from a battery B into a predetermined alternating-current
electric power. Besides, drive force switching mechanism P is able
to switch the transmission path for drive force among a mode in
which the transmission path for drive force is established only
between the electric motor M and the transmission TM, a mode in
which the transmission path for drive force is established only
between the internal combustion engine 10 and the transmission TM,
and a mode in which the transmission path for drive force is
established between the electric motor M and the transmission TM
and between the internal combustion engine 10 and the transmission
TM.
[0250] In this hybrid system, in the case where in the internal
combustion engine 10, the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is controlled to an air/fuel ratio
richer than the stoichiometric air/fuel ratio for the purpose of
determining the presence or absence of abnormality in the output of
the downstream-side air/fuel ratio sensor 56, or the purpose of
determining the presence or absence of degradation of the oxygen
storage/release capability of the upstream-side catalyst 43, or the
purpose of determining the presence or absence of abnormality in
the response of the upstream-side air/fuel ratio sensor 55, or the
purpose of quickly raising the temperature of the internal
combustion engine 10 when the internal combustion engine 10 is
started, or the purpose of causing the upstream-side catalyst 43 to
release stored oxygen therefrom after the fuel-cut control is
stopped, or the purpose of lowering the temperature of the
upstream-side catalyst 43 when the output demanded of the internal
combustion engine 10 is very large, it is possible to achieve more
effects than in the case where the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance is determined when the
air/fuel ratio of the mixture is controlled to the air/fuel ratio
richer than the stoichiometric air/fuel ratio only for the purpose
of determining the presence or absence of the state of
inter-cylinder air/fuel ratio imbalance.
[0251] That is, as described above, in the hybrid system, the
transmission path for drive force is sometimes established only
between the electric motor M and the transmission TM. In such
cases, operation of the internal combustion engine 10 is stopped.
Therefore, the occasions on which the internal combustion engine 10
is operated are correspondingly less frequent in the hybrid system,
so that it can be said that in the hybrid system, there are less
frequent occasions on which the air/fuel ratio of the mixture in
the internal combustion engine 10 can be controlled to an air/fuel
ratio richer than the stoichiometric air/fuel ratio. If in the
internal combustion engine 10, the presence or absence of the state
of inter-cylinder air/fuel ratio imbalance is determined
simultaneously with the control of the air/fuel ratio of the
mixture to an air/fuel ratio richer than the stoichiometric
air/fuel ratio which is performed for the purpose of determining
the presence or absence of abnormality in the output of the
downstream-side air/fuel ratio sensor 56, or of determining the
presence or absence of degradation of the oxygen storage/release
capability of the upstream-side catalyst 43, or of determining the
presence or absence of abnormality in the response of the
upstream-side air/fuel ratio sensor 55, or of quickly raising the
temperature of the internal combustion engine 10 when the internal
combustion engine 10 is started, or of causing the upstream-side
catalyst 43 to release stored oxygen therefrom after the fuel-cut
control is stopped, or of lowering the temperature of the
upstream-side catalyst 43 when the output demanded of the internal
combustion engine 10 is very large, it is possible to achieve an
effect of correspondingly increasing the frequency of the occasions
to determine the presence or absence of the state of inter-cylinder
air/fuel ratio imbalance.
* * * * *