U.S. patent application number 15/475928 was filed with the patent office on 2017-10-05 for exhaust gas control device for internal combustion engine and control method thereof.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Eiji IKUTA, Keiichi MYOJO, Yuki NOSE, Yoshiyuki SHOGENJI.
Application Number | 20170284269 15/475928 |
Document ID | / |
Family ID | 59960735 |
Filed Date | 2017-10-05 |
United States Patent
Application |
20170284269 |
Kind Code |
A1 |
MYOJO; Keiichi ; et
al. |
October 5, 2017 |
EXHAUST GAS CONTROL DEVICE FOR INTERNAL COMBUSTION ENGINE AND
CONTROL METHOD THEREOF
Abstract
An exhaust gas control device for an internal combustion engine
includes an estimation unit configured to estimate a temperature of
a catalyst on the basis of an acquired operation state of the
internal combustion engine and a difference between a lean air-fuel
ratio and a rich air-fuel ratio which are set as target air-fuel
ratios during execution of catalyst regeneration control, a
determination unit configured to determine whether the estimated
temperature of the catalyst is higher than a threshold value during
execution of the catalyst regeneration control, and a prohibition
unit configured to prohibit the catalyst regeneration control when
it is determined that the estimated temperature of the catalyst is
higher than the threshold value during execution of the catalyst
regeneration control.
Inventors: |
MYOJO; Keiichi;
(Okazaki-shi, JP) ; SHOGENJI; Yoshiyuki;
(Toyota-shi, JP) ; NOSE; Yuki; (Kasugai-shi,
JP) ; IKUTA; Eiji; (Oobu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
59960735 |
Appl. No.: |
15/475928 |
Filed: |
March 31, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02T 10/22 20130101;
F01N 2900/0412 20130101; F01N 2900/08 20130101; F01N 2900/1404
20130101; F01N 2900/1402 20130101; F01N 2260/04 20130101; Y02T
10/12 20130101; F01N 3/101 20130101 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/08 20060101 F01N003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2016 |
JP |
2016-075168 |
Claims
1. An exhaust gas control device for an internal combustion engine,
the exhaust gas control device comprising: a catalyst configured to
purify exhaust gas discharged from a plurality of cylinders of the
internal combustion engine; and an electronic control unit
configured to i) acquire an operation state of the internal
combustion engine, ii) perform catalyst regeneration control, the
catalyst regeneration control being control of increasing a
temperature of the catalyst to regenerate the catalyst by setting a
target air-fuel ratio of at least one cylinder among the plurality
of cylinders to a rich air-fuel ratio which is lower than a
stoichiometric air-fuel ratio and setting target air-fuel ratios of
the other cylinders among the plurality of cylinders to a lean
air-fuel ratio which is higher than the stoichiometric air-fuel
ratio, iii) estimate the temperature of the catalyst based on the
acquired operation state of the internal combustion engine and a
difference between the lean air-fuel ratio and the rich air-fuel
ratio which are set as the target air-fuel ratios during execution
of the catalyst regeneration control, iv) determine whether or not
the estimated temperature of the catalyst is higher than a
threshold value during execution of the catalyst regeneration
control, and v) prohibit the catalyst regeneration control when the
electronic control unit determines that the estimated temperature
of the catalyst is higher than the threshold value during execution
of the catalyst regeneration control.
2. The exhaust gas control device according to claim 1, wherein the
electronic control unit is configured to prohibit the catalyst
regeneration control in a predetermined period when the electronic
control unit determines that the estimated temperature of the
catalyst is higher than the threshold value during execution of the
catalyst regeneration control.
3. A control method for an exhaust gas control device for an
internal combustion engine, the exhaust gas control device
including a catalyst configured to purify exhaust gas discharged
from a plurality of cylinders of the internal combustion engine and
an electronic control unit, the control method comprising: i)
acquiring an operation state of the internal combustion engine by
the electronic control unit; ii) performing catalyst regeneration
control by the electronic control unit, the catalyst regeneration
control being control of increasing a temperature of the catalyst
to regenerate the catalyst by setting a target air-fuel ratio of at
least one cylinder among the plurality of cylinders to a rich
air-fuel ratio which is lower than a stoichiometric air-fuel ratio
and setting target air-fuel ratios of the other cylinders among the
plurality of cylinders to a lean air-fuel ratio which is higher
than the stoichiometric air-fuel ratio; iii) estimating, by the
electronic control unit, the temperature of the catalyst based on
the acquired operation state of the internal combustion engine and
a difference between the lean air-fuel ratio and the rich air-fuel
ratio which are set as the target air-fuel ratios during execution
of the catalyst regeneration control; iv) determining, by the
electronic control unit, whether or not the estimated temperature
of the catalyst is higher than a threshold value during execution
of the catalyst regeneration control; and v) prohibiting, by the
electronic control unit, the catalyst regeneration control when the
electronic control unit determines that the estimated temperature
of the catalyst is higher than the threshold value during execution
of the catalyst regeneration control.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2016-075168 filed on Apr. 4, 2016 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND
1. Technical Field
[0002] The disclosure relates to an exhaust gas control device for
an internal combustion engine and a control method thereof.
2. Description of Related Art
[0003] A technique of increasing a temperature of a catalyst by
so-called dither control of setting a target air-fuel ratio of one
cylinder among a plurality of cylinders of an internal combustion
engine to a rich air-fuel ratio and setting target air-fuel ratios
of the other cylinders to a lean air-fuel ratio is known (for
example, see Japanese Patent Application Publication No. 9-088663
(JP 9-088663 A)).
SUMMARY
[0004] Regenerating a catalyst by increasing the temperature of the
catalyst up to a temperature range higher than a temperature range
in which the catalyst is activated by such a technique can be
considered.
[0005] Here, the rich air-fuel ratio and the lean air-fuel ratio
which are set as the target air-fuel ratios in the above-mentioned
technique are set to be variable depending on an operation state of
the internal combustion engine to decrease an influence on
drivability. Accordingly, while the control of regenerating a
catalyst is performed, there is a possibility that the temperature
of the catalyst will excessively increase over the temperature
range required for regenerating the catalyst depending on the
operation state of the internal combustion engine or the set rich
air-fuel ratio and lean air-fuel ratio.
[0006] Therefore, the disclosure provides an exhaust gas control
device for an internal combustion engine that suppresses an
excessive increase in temperature of a catalyst during execution of
catalyst regeneration control of setting a target air-fuel ratio of
one cylinder to a rich air-fuel ratio and setting target air-fuel
ratios of the other cylinders to a lean air-fuel ratio and a
control method thereof.
[0007] The above-mentioned problem can be solved by an exhaust gas
control device for an internal combustion engine, the exhaust gas
control device including: a catalyst configured to purify exhaust
gas discharged from a plurality of cylinders of the internal
combustion engine; an acquisition unit configured to acquire an
operation state of the internal combustion engine; a control unit
configured to perform catalyst regeneration control, the catalyst
regeneration control being control of increasing a temperature of
the catalyst to regenerate the catalyst by setting a target
air-fuel ratio of at least one cylinder among the plurality of
cylinders to a rich air-fuel ratio which is lower than a
stoichiometric air-fuel ratio and setting target air-fuel ratios of
the other cylinders among the plurality of cylinders to a lean
air-fuel ratio which is higher than the stoichiometric air-fuel
ratio; an estimation unit configured to estimate the temperature of
the catalyst on the basis of the acquired operation state of the
internal combustion engine and a difference between the lean
air-fuel ratio and the rich air-fuel ratio which are set as the
target air-fuel ratios during execution of the catalyst
regeneration control; a determination unit configured to determine
whether the estimated temperature of the catalyst is higher than a
threshold value during execution of the catalyst regeneration
control; and a prohibition unit configured to prohibit the catalyst
regeneration control when it is determined that the estimated
temperature of the catalyst is higher than the threshold value
during execution of the catalyst regeneration control. The aspect
of the disclosure may be defined as follows. An exhaust gas control
device for an internal combustion engine, the exhaust gas control
device including: a catalyst configured to purify exhaust gas
discharged from a plurality of cylinders of the internal combustion
engine; and an electronic control unit configured to i) acquire an
operation state of the internal combustion engine, ii) perform
catalyst regeneration control, the catalyst regeneration control
being control of increasing a temperature of the catalyst to
regenerate the catalyst by setting a target air-fuel ratio of at
least one cylinder among the plurality of cylinders to a rich
air-fuel ratio which is lower than a stoichiometric air-fuel ratio
and setting target air-fuel ratios of the other cylinders among the
plurality of cylinders to a lean air-fuel ratio which is higher
than the stoichiometric air-fuel ratio, iii) estimate the
temperature of the catalyst based on the acquired operation state
of the internal combustion engine and a difference between the lean
air-fuel ratio and the rich air-fuel ratio which are set as the
target air-fuel ratios during execution of the catalyst
regeneration control, iv) determine whether or not the estimated
temperature of the catalyst is higher than a threshold value during
execution of the catalyst regeneration control, and v) prohibit the
catalyst regeneration control when the electronic control unit
determines that the estimated temperature of the catalyst is higher
than the threshold value during execution of the catalyst
regeneration control. A control method for an exhaust gas control
device for an internal combustion engine, the exhaust gas control
device including a catalyst configured to purify exhaust gas
discharged from a plurality of cylinders of the internal combustion
engine and an electronic control unit, the control method
including: i) acquiring an operation state of the internal
combustion engine by the electronic control unit; ii) performing
catalyst regeneration control by the electronic control unit, the
catalyst regeneration control being control of increasing a
temperature of the catalyst to regenerate the catalyst by setting a
target air-fuel ratio of at least one cylinder among the plurality
of cylinders to a rich air-fuel ratio which is lower than a
stoichiometric air-fuel ratio and setting target air-fuel ratios of
the other cylinders among the plurality of cylinders to a lean
air-fuel ratio which is higher than the stoichiometric air-fuel
ratio; iii) estimating, by the electronic control unit, the
temperature of the catalyst based on the acquired operation state
of the internal combustion engine and a difference between the lean
air-fuel ratio and the rich air-fuel ratio which are set as the
target air-fuel ratios during execution of the catalyst
regeneration control; iv) determining, by the electronic control
unit, whether or not the estimated temperature of the catalyst is
higher than a threshold value during execution of the catalyst
regeneration control; and v) prohibiting, by the electronic control
unit, the catalyst regeneration control when the electronic control
unit determines that the estimated temperature of the catalyst is
higher than the threshold value during execution of the catalyst
regeneration control.
[0008] The temperature of the catalyst is accurately estimated on
the basis of the operation state of the internal combustion engine
and the magnitude of the difference between the lean air-fuel ratio
and the rich air-fuel ratio which are set as the target air-fuel
ratios during execution of the catalyst regeneration control. When
the estimated temperature of the catalyst is higher than the
threshold value, the catalyst regeneration control is prohibited
and thus an excessive increase in temperature of the catalyst is
suppressed.
[0009] The prohibition unit may be configured to prohibit the
catalyst regeneration control in a predetermined period when it is
determined that the estimated temperature of the catalyst is higher
than the threshold value during execution of the catalyst
regeneration control.
[0010] According to the disclosure, it is possible to provide an
exhaust gas control device for an internal combustion engine that
suppresses an excessive increase in temperature of a catalyst
during execution of catalyst regeneration control of setting a
target air-fuel ratio of one cylinder to a rich air-fuel ratio and
setting target air-fuel ratios of the other cylinders to a lean
air-fuel ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Features, advantages, and technical and industrial
significance of exemplary embodiments of the disclosure will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0012] FIG. 1 is a diagram schematically illustrating a
configuration of an exhaust gas control device;
[0013] FIG. 2 is a flowchart illustrating an example of
regeneration prohibition control which is performed by an ECU;
[0014] FIGS. 3A and 3B are diagrams illustrating an example of a
catalyst temperature map;
[0015] FIG. 4 is a timing chart illustrating an example of the
regeneration prohibition control;
[0016] FIG. 5 is a flowchart illustrating a modified example of the
regeneration prohibition control which is performed by the ECU;
and
[0017] FIG. 6 is a timing chart illustrating a modified example of
the regeneration prohibition control.
DETAILED DESCRIPTION OF EMBODIMENTS
[0018] FIG. 1 is a diagram schematically illustrating a
configuration of an exhaust gas control device 1 (an exhaust gas
control device for an internal combustion engine). As illustrated
in FIG. 1, the exhaust gas control device 1 includes a three-way
catalyst 31 that purifies exhaust gas of an internal combustion
engine 20. The internal combustion engine 20 combusts an air-fuel
mixture in a combustion chamber 23 in a cylinder block 21 to cause
a piston 24 to reciprocate. The internal combustion engine 20 is an
in-line four cylinder gasoline engine, but is not limited thereto
as long as it includes a plurality of cylinders and may be, for
example, a diesel engine.
[0019] An intake valve Vi that opens and closes an intake port and
an exhaust valve Ve that opens and closes an exhaust port are
provided for each cylinder in a cylinder head of the internal
combustion engine 20. An ignition plug 27 that ignites the air-fuel
mixture in the combustion chamber 23 is attached to a top of the
cylinder head for each cylinder.
[0020] The intake port of each cylinder is connected to a surge
tank 18 via a branch pipe for each cylinder. An intake pipe 10 is
connected to an upstream side of the surge tank 18 and an air
cleaner 19 is disposed at an upstream end of the intake pipe 10.
The intake pipe 10 is provided with an airflow meter 15 that
detects an amount of intake air and an electronically controlled
throttle valve 13 sequentially from the upstream side.
[0021] An injector 12 that injects fuel into the intake port is
installed in the intake port of each cylinder. The fuel injected
from the injector 12 is mixed with intake air to form an air-fuel
mixture, and the air-fuel mixture is suctioned into the combustion
chamber 23 when the intake valve Vi is opened, is compressed by the
piston 24, and is ignited and combusted by the ignition plug
27.
[0022] On the other hand, the exhaust port of each cylinder is
connected to an exhaust pipe 30 via a branch pipe for each
cylinder. The three-way catalyst 31 is provided in the exhaust pipe
30. The three-way catalyst 31 has an oxygen occlusion capacity and
purifies NOx, HC, and CO. In the three-way catalyst 31, one or more
catalyst layers including a catalyst carrier such as alumina
(Al.sub.2O.sub.3) and a catalyst metal such as platinum (Pt),
palladium (Pd), or rhodium (Rh) carried by the catalyst carrier are
formed on a substrate of cordierite or the like, particularly, a
honeycomb substrate. The three-way catalyst 31 is an example of a
catalyst that purifies exhaust gas discharged from a plurality of
cylinders of the internal combustion engine 20 and may be an
oxidation catalyst or a gasoline particulate filter coated with the
oxidation catalyst.
[0023] An air-fuel ratio sensor 33 that detects an air-fuel ratio
of exhaust gas is installed upstream from the three-way catalyst
31. The air-fuel ratio sensor 33 is a so-called wide-area air-fuel
ratio sensor, which can continuously detect an air-fuel ratio over
a relatively wide area and output a signal of a value proportional
to the air-fuel ratio.
[0024] The exhaust gas control device 1 includes an electronic
control unit (ECU) 50. The ECU 50 includes a central processing
unit (CPU), a random access memory (RAM), a read only memory (ROM),
and a storage unit. The ECU 50 performs various types of control by
executing a program stored in the ROM or the storage unit. The ECU
50 performs regeneration prohibition control to be described later.
The regeneration prohibition control is performed by an acquisition
unit, a control unit, an estimation unit, a determination unit, and
a prohibition unit of the ECU 50, which are functionally realized
by the CPU, the ROM, and the RAM. Details thereof will be described
later.
[0025] The ignition plug 27, the throttle valve 13, the injector
12, and the like are electrically connected to the ECU 50. In
addition to the airflow meter 15, the air-fuel ratio sensor 33, and
a crank angle sensor 25 that detects a crank angle of the internal
combustion engine 20, an accelerator opening level sensor 11 that
detects an accelerator opening level or various other sensors are
electrically connected to the ECU 50 via an A/D converter or the
like which is not illustrated. The ECU 50 controls the ignition
plug 27, the throttle valve 13, the injector 12, and the like to
acquire desired output power on the basis of detection values of
various sensors, and controls an ignition timing, an amount of fuel
injected, a fuel injection timing, a throttle opening level, and
the like.
[0026] Setting of a target air-fuel ratio by the ECU 50 will be
described below. In a normal state in which catalyst regeneration
control to be described later is not performed, the target air-fuel
ratio is set on the basis of a normal air-fuel ratio map based on
an engine speed and an engine load of the internal combustion
engine 20. The normal air-fuel ratio map is acquired by experiment
in advance and is stored in the ROM of the ECU 50.
[0027] For example, the target air-fuel ratio is set to a
stoichiometric air-fuel ratio in a low-speed and low-load area and
is set to be richer than the stoichiometric air-fuel ratio in a
high-speed and high-load area. When the target air-fuel ratio is
set, an amount of fuel injected into each cylinder is
feedback-controlled such that the air-fuel ratio detected by the
air-fuel ratio sensor 33 reaches the target air-fuel ratio. The
target air-fuel ratio based on the engine speed and the engine load
may be calculated using a calculation equation instead of the
normal air-fuel ratio map.
[0028] The ECU 50 performs catalyst regeneration control of
removing a sulfur compound (SO.sub.x) deposited on the three-way
catalyst 31 to regenerate purification capability of the three-way
catalyst 31 by increasing the temperature of the three-way catalyst
31 to within a predetermined temperature range. In the catalyst
regeneration control, so-called dither control of setting a target
air-fuel ratio of one cylinder among a plurality of cylinders to a
rich air-fuel ratio which is lower than the stoichiometric air-fuel
ratio and setting target air-fuel ratios in the other three
cylinders to a lean air-fuel ratio which is higher than the
stoichiometric air-fuel ratio is performed. An average of the
target air-fuel ratios of all the cylinders is set to be the
stoichiometric air-fuel ratio.
[0029] The target air-fuel ratios in the catalyst regeneration
control are similarly set on the basis of a regeneration air-fuel
ratio map based on the engine speed and the engine load. The
regeneration air-fuel ratio map is acquired by experiment in
advance and is stored in the ROM of the ECU 50. For example, the
rich air-fuel ratio is set to range from 9 to 12 and the lean
air-fuel ratio is set to range from 15 to 16. As the engine speed
and the engine load become greater, the rich air-fuel ratio is set
to be smaller and the lean air-fuel ratio is set to be larger.
[0030] The rich air-fuel ratio and the lean air-fuel ratio which
are set as the target air-fuel ratios on the basis of the
regeneration air-fuel ratio map are set to be variable depending on
the engine speed and the engine load of the internal combustion
engine 20 within a range in which an influence on drivability is
small. The target air-fuel ratios in the catalyst regeneration
control based on the engine speed and the engine load may be
calculated using a calculation equation instead of the regeneration
air-fuel ratio map. The catalyst regeneration control is not
performed in an idle operation state or when the accelerator
opening level is zero.
[0031] When the catalyst regeneration control is performed as
described above, surplus fuel discharged from a cylinder of which
the target air-fuel ratio is set to the rich air-fuel ratio is
attached to the three-way catalyst 31 and is combusted in a lean
atmosphere due to exhaust gas discharged at the lean air-fuel
ratio. Accordingly, the temperature of the three-way catalyst 31
increases to remove SO.sub.x.
[0032] However, during execution of the catalyst regeneration
control in which the three-way catalyst 31 is maintained at a high
temperature, there is a possibility of the temperature of the
three-way catalyst 31 excessively increasing over the temperature
range necessary for regeneration depending on the operation state
of the internal combustion engine 20 or the rich air-fuel ratio and
the lean air-fuel ratio which are set as the target air-fuel
ratios. Therefore, the ECU 50 performs regeneration prohibition
control of prohibiting the catalyst regeneration control during
execution of the catalyst regeneration control.
[0033] FIG. 2 is a flowchart illustrating an example of the
regeneration prohibition control which is performed by the ECU 50.
The control illustrated in FIG. 2 is repeatedly performed with a
predetermined cycle. The ECU 50 determines whether the catalyst
regeneration control is being performed (Step S1) and this control
ends when the determination result is negative.
[0034] When the determination result in Step S1 is affirmative, the
ECU 50 acquires the engine speed and the engine load of the
internal combustion engine 20 (Step S3). Specifically, the engine
speed is acquired on the basis of an output value from the crank
angle sensor 25, and the engine load is acquired on the basis of an
output value from the accelerator opening level sensor 11. The
process of Step S3 is an example of the process which is performed
by the acquisition unit that acquires the operation state of the
internal combustion engine 20.
[0035] Then, the ECU 50 acquires a magnitude of a difference
between the lean air-fuel ratio and the rich air-fuel ratio which
are set as the target air-fuel ratios in the catalyst regeneration
control as an air-fuel ratio difference (Step S5). Specifically, a
value obtained by subtracting the rich air-fuel ratio from the lean
air-fuel ratio is acquired as the air-fuel ratio difference. The
absolute value of the value obtained by subtracting the lean
air-fuel ratio from the rich air-fuel ratio may be acquired as the
air-fuel ratio difference.
[0036] Then, the ECU 50 estimates the temperature of the three-way
catalyst 31 (Step S7). Specifically, the temperature of the
three-way catalyst 31 is estimated on the basis of a catalyst
temperature map based on the acquired engine speed, the acquired
engine load, and the air-fuel ratio difference. The catalyst
temperature map is acquired by experiment in advance and is stored
in the ROM of the ECU 50. The process of Step S7 is an example of
the process which is performed by the estimation unit that
estimates the temperature of the three-way catalyst 31 on the basis
of the acquired operation state of the internal combustion engine
20 and the magnitude of the difference between the lean air-fuel
ratio and the rich air-fuel ratio which are set as the target
air-fuel ratios during execution of the catalyst regeneration
control.
[0037] FIGS. 3A and 3B illustrate an example of the catalyst
temperature map. The horizontal axis represents the engine speed,
the vertical axis represents the engine load, and a plurality of
isothermal lines are illustrated. FIG. 3A illustrates the catalyst
temperature map when the air-fuel ratio difference is relatively
large, and FIG. 3B illustrates the catalyst temperature map when
the air-fuel ratio difference is relatively small. Here,
temperatures T1 to T6 increase from the temperature T1 to the
temperature T6.
[0038] As illustrated in FIGS. 3A and 3B, the temperature of the
three-way catalyst 31 is estimated to be higher as the air-fuel
ratio difference becomes larger under the condition in which the
engine speed and the engine load are constant. The temperature of
the three-way catalyst 31 during execution of the catalyst
regeneration control can be accurately estimated on the basis of
the catalyst temperature map. The temperature of the three-way
catalyst 31 may be estimated on the basis of the engine speed, the
engine load, and the air-fuel ratio difference using a calculation
equation instead of the catalyst temperature map.
[0039] Then, the ECU 50 determines whether the estimated
temperature of the three-way catalyst 31 is higher than a threshold
value (Step S9) and this control ends when the determination result
is negative. The threshold value is a value for determining whether
the temperature of the three-way catalyst 31 increases excessively,
which is set to a value slightly smaller than a heat-resistance
upper-limit temperature of the three-way catalyst 31, and is, for
example, 900 degrees, but is not limited thereto. The process of
Step S9 is an example of the process which is performed by the
determination unit that determines whether the estimated
temperature of the three-way catalyst 31 is higher than the
threshold value during execution of the catalyst regeneration
control.
[0040] When the determination result in Step S9 is affirmative, the
ECU 50 prohibits the catalyst regeneration control (Step S11). In a
period in which the catalyst regeneration control is prohibited,
the target air-fuel ratios of all the cylinders are set to be the
same. Specifically, on the basis of the engine speed and the engine
load acquired in Step S3, the target air-fuel ratios are set on the
basis of the normal air-fuel ratio map which is used in the normal
operation. Accordingly, an excessive increase in temperature of the
three-way catalyst 31 is suppressed. In the period in which the
catalyst regeneration control is prohibited, the target air-fuel
ratios of all the cylinders may be set to the stoichiometric
air-fuel ratio. The process of Step S9 is an example of the process
which is performed by the prohibition unit that prohibits the
catalyst regeneration control when it is determined that the
estimated temperature of the three-way catalyst 31 is higher than
the threshold value under execution of the catalyst regeneration
control.
[0041] The regeneration prohibition control will be described below
with reference to a timing chart. FIG. 4 is a timing chart
illustrating an example of the regeneration prohibition control. In
FIG. 4, waveforms indicating a vehicle speed, an idle determination
flag, an engine speed, an engine load, an estimated temperature of
the three-way catalyst 31, a catalyst regeneration execution flag,
a target air-fuel ratio in a cylinder in which the target air-fuel
ratio is set to the lean air-fuel ratio, and a target air-fuel
ratio in a cylinder in which the target air-fuel ratio is set to
the rich air-fuel ratio are illustrated. In FIG. 4, for the purpose
of easy understanding, the estimated temperature of the three-way
catalyst 31 is also illustrated in the period in which the catalyst
regeneration control is not performed.
[0042] The catalyst regeneration execution flag is in an OFF state
at time t1 and the catalyst regeneration execution flag is turned
on when a catalyst regeneration request is issued at time t2.
Accordingly, the ECU 50 performs the catalyst regeneration control
of setting the target air-fuel ratio in one cylinder to the rich
air-fuel ratio and setting the target air-fuel ratios in the other
cylinders to the lean air-fuel ratio.
[0043] When the idle determination flag is turned on at time t3,
the engine 10 is determined to be in an idle operation state and
the catalyst regeneration execution flag is turned off to stop the
catalyst regeneration control even when the catalyst regeneration
request is issued. When the idle determination flag is turned off
at time t4, the engine 10 is determined to depart from the idle
operation state and the catalyst regeneration execution flag is
turned on to perform the catalyst regeneration control again. When
the idle determination flag is turned on again at time t5, the
catalyst regeneration execution flag is turned off to stop the
catalyst regeneration control.
[0044] When the idle determination flag is turned off at time t6,
the catalyst regeneration execution flag is turned on to perform
the catalyst regeneration control again. When the air-fuel ratio
difference between the lean air-fuel ratio and the rich air-fuel
ratio is increased after time t6 and the estimated temperature of
the three-way catalyst 31 becomes higher than the threshold value
at time t7, the catalyst regeneration execution flag is turned off
during a predetermined period to prohibit the catalyst regeneration
control. Thereafter, the idle determination flag is turned on at
time t8 and the estimated temperature of the three-way catalyst 31
further decreases.
[0045] Since the catalyst regeneration control is prohibited during
execution of the catalyst regeneration control as described above,
the temperature of the three-way catalyst 31 decreases and an
excessive increase in temperature is suppressed. Accordingly, it is
possible to reduce a possibility of the three-way catalyst 31
degrading due to heat. When the catalyst regeneration control is
prohibited, the state in which the catalyst regeneration control is
prohibited is maintained in this trip until an ignition key is
turned off. Accordingly, when a predetermined condition is
satisfied during the next trip, the catalyst regeneration control
can be performed to remove a sulfur compound deposited on the
three-way catalyst 31.
[0046] Instead of estimating the temperature of the three-way
catalyst 31 as in the above-mentioned technique, directly measuring
the temperature of the three-way catalyst 31 using a temperature
sensor or estimating the temperature of the three-way catalyst 31
using temperature sensors disposed upstream and downstream from the
three-way catalyst 31 on the basis of the difference between the
detected temperatures can be considered. However, in this case,
there is a possibility of the number of components increasing. In
this embodiment, since the temperature of the three-way catalyst 31
can be estimated without using such a temperature sensor, it is
possible to suppress an increase in the number of components.
[0047] A modified example of the regeneration prohibition control
will be described below. FIG. 5 is a flowchart illustrating a
modified example of the regeneration prohibition control which is
performed by the ECU 50. FIG. 6 is a timing chart illustrating a
modified example of the regeneration prohibition control. The
processes of Steps S1, S3, S5, S7, and S9 are the same as in the
regeneration prohibition control illustrated in FIG. 2 and thus
description thereof will not be repeated. The operation state such
as the engine speed in a period from time t1 to time t7 is the same
and thus description thereof will not be repeated.
[0048] As illustrated in FIG. 5, when the determination result in
Step S9 is affirmative, the ECU 50 prohibits the catalyst
regeneration control in a predetermined period (Step S11a). The
predetermined period is a period which is suitable for suppressing
an excessive increase in temperature of the three-way catalyst 31
and is acquired by experiment in advance and stored in the ROM of
the ECU 50. The predetermined period ranges, for example, from 500
ms to 1500 ms but is not limited thereto. When the period in which
the catalyst regeneration control is prohibited elapses, the
catalyst regeneration control is performed again in response to a
catalyst regeneration request.
[0049] As illustrated in FIG. 6, at time t7' at which a
predetermined period elapsed from time t7 at which the catalyst
regeneration control is prohibited, the prohibition of the catalyst
regeneration control is released, the catalyst regeneration
execution flag is turned on, and the catalyst regeneration control
is performed again. When the idle determination flag is turned on
at time t8 thereafter, the catalyst regeneration execution flag is
turned off and the catalyst regeneration control is stopped.
[0050] The excessive increase in temperature of the three-way
catalyst 31 can be suppressed by prohibiting the catalyst
regeneration control in the predetermined period in this way, the
temperature of the three-way catalyst 31 increases again when the
prohibition of the catalyst regeneration control is released after
the predetermined period elapses, and it is thus possible to remove
a sulfur compound deposited on the three-way catalyst 31 as early
as possible.
[0051] The predetermined period in which the catalyst regeneration
control is prohibited is not limited to the above-mentioned
example. For example, the predetermined period in which the
catalyst regeneration control is prohibited may be a period until
an idle operation flag is turned on after the catalyst regeneration
control is prohibited. In a state in which the idle operation flag
is turned on, the catalyst regeneration execution flag is turned
off as described above. Accordingly, when the operation state
becomes the idle operation state after the catalyst regeneration
control is prohibited, the catalyst regeneration control can
actually be performed only after the operation state is returned
from the idle operation. The predetermined period in which the
catalyst regeneration control is prohibited may be a period until
the accelerator opening level becomes zero after the catalyst
regeneration control is prohibited. In this case, since the
catalyst regeneration control is not performed in a state in which
the accelerator opening level is zero, the catalyst regeneration
control can actually be performed only after the accelerator
opening level becomes a value other than zero.
[0052] While an exemplary embodiment of the disclosure has been
described above in detail, the disclosure is not limited to this
specific exemplary embodiment, but can be modified in various forms
without departing from the gist of the disclosure described in the
appended claims.
[0053] In the above-mentioned embodiment, an in-line four cylinder
engine has been described as an example of the internal combustion
engine, but a V-type multi-cylinder engine including a catalyst for
each bank may be employed. In this case, in the catalyst
regeneration control, a target air-fuel ratio of one cylinder in
each bank is set to a rich air-fuel ratio, target air-fuel ratios
of the other cylinders are set to a lean air-fuel ratio, and the
catalyst regeneration control is performed for each bank.
* * * * *