U.S. patent application number 09/761105 was filed with the patent office on 2001-08-09 for exhaust-gas cleaning devices for engine.
Invention is credited to Kuji, Youichi, Kuroki, Masayuki, Taga, Junichi, Yokota, Kazuya.
Application Number | 20010011453 09/761105 |
Document ID | / |
Family ID | 18541429 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010011453 |
Kind Code |
A1 |
Taga, Junichi ; et
al. |
August 9, 2001 |
Exhaust-gas cleaning devices for engine
Abstract
An exhaust-gas cleaning device comprises an absorption quantity
detector (22) for determining the first and second amounts of NOx
and oxygen absorbed by a NOx-absorbing material and an
oxygen-absorbing material during first and second reference times
based on sensing signals fed from an oxygen concentration detector
(11), a NOx quantity estimator (23) for obtaining first and second
estimated values of the amount of NOx absorbed by the NOx-absorbing
material during the first and second reference times in accordance
with engine operating condition, and a deterioration detector (24)
for making a judgment on deterioration of the NOx-absorbing
material based on the first and second amounts of absorption of NOx
and oxygen determined by the absorption quantity detector (22) and
the first and second estimated values of the amount of NOx obtained
by the NOx quantity estimator (23).
Inventors: |
Taga, Junichi; (Fuchu-cho,
JP) ; Yokota, Kazuya; (Fuchu-cho, JP) ; Kuji,
Youichi; (Fuchu-cho, JP) ; Kuroki, Masayuki;
(Fuchu-cho, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Family ID: |
18541429 |
Appl. No.: |
09/761105 |
Filed: |
January 17, 2001 |
Current U.S.
Class: |
60/277 ;
60/311 |
Current CPC
Class: |
F01N 3/0807 20130101;
F01N 2570/16 20130101; F01N 13/009 20140601; F01N 2570/14 20130101;
F01N 3/0842 20130101; Y02T 10/40 20130101; F01N 3/0814 20130101;
F01N 2550/03 20130101; F01N 9/005 20130101; F01N 11/007 20130101;
Y02A 50/20 20180101; F02B 2075/125 20130101 |
Class at
Publication: |
60/277 ;
60/311 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 18, 2000 |
JP |
2000-13897(PAT) |
Claims
What is claimed is:
1. An exhaust-gas cleaning device for an engine, said exhaust-gas
cleaning device comprising: a NOx-absorbing material provided in an
exhaust passage to absorb NOx under oxygen-rich conditions where
the oxygen concentration is high and release absorbed NOx as the
oxygen concentration decreases; an oxygen-absorbing material
provided in the exhaust passage to absorb oxygen under oxygen-rich
conditions where the oxygen concentration is high and release
absorbed oxygen as the oxygen concentration decreases; an oxygen
concentration controlling means for controlling the oxygen
concentration in the exhaust passage; an oxygen concentration
detecting means provided downstream of the NOx-absorbing material
and the oxygen-absorbing material; an absorption quantity detecting
means which determines a first amount of absorption of NOx and
oxygen absorbed by the NOx-absorbing material and the
oxygen-absorbing material based on a sensing signal fed from the
oxygen concentration detecting means at a time when the oxygen
concentration upstream of the NOx-absorbing material and the
oxygen-absorbing material is just transferred from a high
concentration state maintained for a preset first reference time to
a low concentration state by controlling the oxygen concentration
with the oxygen concentration controlling means, and determines a
second amount of absorption of NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material based on a
sensing signal fed from the oxygen concentration detecting means at
a time when the oxygen concentration upstream of the NOx-absorbing
material and the oxygen-absorbing material is just transferred from
the high concentration state maintained for a preset second
reference time which is longer than the first reference time to the
low concentration state by controlling the oxygen concentration
with the oxygen concentration controlling means; a NOx quantity
estimating means which obtains a first estimated value of the
amount of NOx absorbed by the NOx-absorbing material during the
first reference time and a second estimated value of the amount of
NOx absorbed by the NOx-absorbing material during the second
reference time in accordance with engine operating condition; and a
deterioration detecting means which makes a judgment on
deterioration of the NOx-absorbing material based on the first
amount of absorption and the second amount of absorption of NOx and
oxygen determined by the absorption quantity detecting means and
the first estimated value and the second estimated value of the
amount of absorbed NOx obtained by the NOx quantity estimating
means.
2. An exhaust-gas cleaning device for an engine according to claim
1, wherein the deterioration detecting means makes a judgment on
deterioration of the NOx-absorbing material based on the difference
between the second amount of absorption and the first amount of
absorption of NOx and oxygen determined by the absorption quantity
detecting means and the difference between the second estimated
value and the first estimated value of the amount of NOx obtained
by the NOx quantity estimating means.
3. An exhaust-gas cleaning device for an engine according to claim
1, wherein the NOx quantity estimating means obtains the first
estimated value and the second estimated value of the amount of NOx
absorbed by the NOx-absorbing material in accordance with engine
speed and engine load.
4. An exhaust-gas cleaning device for an engine according to claim
1, wherein the absorption quantity detecting means is so
constructed as to determine the amount of NOx and oxygen absorbed
by the NOx-absorbing material and the oxygen-absorbing material
based on the time period required for the oxygen concentration
detected by the oxygen concentration detecting means to decrease to
a specific low concentration level from a point in time when the
oxygen concentration controlling means executes a control operation
for transferring the oxygen concentration upstream of the
NOx-absorbing material and the oxygen-absorbing material from the
high concentration state to the low concentration state.
5. An exhaust-gas cleaning device for an engine according to claim
1, wherein the oxygen concentration controlling means is so
constructed as to control the oxygen concentration in the exhaust
passage by controlling the air-fuel ratio in a combustion
chamber.
6. An exhaust-gas cleaning device for an engine according to claim
1, wherein the oxygen-absorbing material is provided upstream of
the NOx-absorbing material.
7. An exhaust-gas cleaning device for an engine, said exhaust-gas
cleaning device comprising: a NOx-absorbing material provided in an
exhaust passage to absorb NOx under oxygen-rich conditions where
the oxygen concentration is high and release absorbed NOx as the
oxygen concentration decreases; an oxygen-absorbing material
provided in the exhaust passage to absorb oxygen under oxygen-rich
conditions where the oxygen concentration is high and release
absorbed oxygen as the oxygen concentration decreases; an oxygen
concentration controlling means for controlling the oxygen
concentration in exhaust gas; an oxygen concentration detecting
means provided downstream of the NOx-absorbing material and the
oxygen-absorbing material; an absorption quantity detecting means
which determines a first amount of absorption of NOx and oxygen
absorbed by the NOx-absorbing material and the oxygen-absorbing
material based on a sensing signal fed from the oxygen
concentration detecting means at a time when the oxygen
concentration upstream of the NOx-absorbing material and the
oxygen-absorbing material is just transferred from a high
concentration state maintained for a preset first reference time to
a low concentration state by controlling the oxygen concentration
with the oxygen concentration controlling means, and determines a
second amount of absorption of NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material based on a
sensing signal fed from the oxygen concentration detecting means at
a time when the oxygen concentration upstream of the NOx-absorbing
material and the oxygen-absorbing material is just transferred from
the high concentration state maintained for a preset second
reference time which is longer than the first reference time to the
low concentration state by controlling the oxygen concentration
with the oxygen concentration controlling means; a first
deterioration detecting means which makes a judgment on
deterioration of the NOx-absorbing material based on the first
amount of absorption and the second amount of absorption of NOx and
oxygen determined by the absorption quantity detecting means; and a
second deterioration detecting means which makes a judgment on
deterioration of the oxygen-absorbing material; wherein the
judgment on deterioration of the NOx-absorbing material by the
first deterioration detecting means is inhibited when the
oxygen-absorbing material has been judged to have deteriorated by
the second deterioration detecting means.
8. An exhaust-gas cleaning device for an engine according to claim
7, wherein the absorption quantity detecting means is so
constructed as to determine the amount of NOx and oxygen absorbed
by the NOx-absorbing material and the oxygen-absorbing material
based on the time period required for the oxygen concentration
detected by the oxygen concentration detecting means to decrease to
a specific low concentration level from a point in time when the
oxygen concentration controlling means executes a control operation
for transferring the oxygen concentration upstream of the
NOx-absorbing material and the oxygen-absorbing material from the
high concentration state to the low concentration state.
9. An exhaust-gas cleaning device for an engine according to claim
7, wherein the oxygen concentration controlling means is so
constructed as to control the oxygen concentration in the exhaust
passage by controlling the air-fuel ratio in a combustion
chamber.
10. An exhaust-gas cleaning device for an engine according to claim
7, wherein the oxygen-absorbing material is provided upstream of
the NOx-absorbing material.
11. An exhaust-gas cleaning device for an engine, said exhaust-gas
cleaning device comprising: a NOx-absorbing material provided in an
exhaust passage to absorb NOx under oxygen-rich conditions where
the oxygen concentration is high and release absorbed NOx as the
oxygen concentration decreases; an oxygen-absorbing material
provided in the exhaust passage to absorb oxygen under oxygen-rich
conditions where the oxygen concentration is high and release
absorbed oxygen as the oxygen concentration decreases; an oxygen
concentration controller for controlling the oxygen concentration
in the exhaust passage; an oxygen concentration detector provided
downstream of the NOx-absorbing material and the oxygen-absorbing
material; an absorption quantity detector which determines a first
amount of absorption of NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material based on a
sensing signal fed from the oxygen concentration detector at a time
when the oxygen concentration upstream of the NOx-absorbing
material and the oxygen-absorbing material is just transferred from
a high concentration state maintained for a preset first reference
time to a low concentration state by controlling the oxygen
concentration with the oxygen concentration controller, and
determines a second amount of absorption of NOx and oxygen absorbed
by the NOx-absorbing material and the oxygen-absorbing material
based on a sensing signal fed from the oxygen concentration
detector at a time when the oxygen concentration upstream of the
NOx-absorbing material and the oxygen-absorbing material is just
transferred from the high concentration state maintained for a
preset second reference time which is longer than the first
reference time to the low concentration state by controlling the
oxygen concentration with the oxygen concentration controller; a
NOx quantity estimator which obtains a first estimated value of the
amount of NOx absorbed by the NOx-absorbing material during the
first reference time and a second estimated value of the amount of
NOx absorbed by the NOx-absorbing material during the second
reference time in accordance with engine operating condition; and a
deterioration detector which makes a judgment on deterioration of
the NOx-absorbing material based on the first amount of absorption
and the second amount of absorption of NOx and oxygen determined by
the absorption quantity detector and the first estimated value and
the second estimated value of the amount of absorbed NOx obtained
by the NOx quantity estimator.
12. An exhaust-gas cleaning device for an engine, said exhaust-gas
cleaning device comprising: a NOx-absorbing material provided in an
exhaust passage to absorb NOx under oxygen-rich conditions where
the oxygen concentration is high and release absorbed NOx as the
oxygen concentration decreases; an oxygen-absorbing material
provided in the exhaust passage to absorb oxygen under oxygen-rich
conditions where the oxygen concentration is high and release
absorbed oxygen as the oxygen concentration decreases; an oxygen
concentration controller for controlling the oxygen concentration
in exhaust gas; an oxygen concentration detector provided
downstream of the NOx-absorbing material and the oxygen-absorbing
material; an absorption quantity detector which determines a first
amount of absorption of NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material based on a
sensing signal fed from the oxygen concentration detector at a time
when the oxygen concentration upstream of the NOx-absorbing
material and the oxygen-absorbing material is just transferred from
a high concentration state maintained for a preset first reference
time to a low concentration state by controlling the oxygen
concentration with the oxygen concentration controller, and
determines a second amount of absorption of NOx and oxygen absorbed
by the NOx-absorbing material and the oxygen-absorbing material
based on a sensing signal fed from the oxygen concentration
detector at a time when the oxygen concentration upstream of the
NOx-absorbing material and the oxygen-absorbing material is just
transferred from the high concentration state maintained for a
preset second reference time which is longer than the first
reference time to the low concentration state by controlling the
oxygen concentration with the oxygen concentration controller; a
first deterioration detector which makes a judgment on
deterioration of the NOx-absorbing material based on the first
amount of absorption and the second amount of absorption of NOx and
oxygen determined by the absorption quantity detector; and a second
deterioration detector which makes a judgment on deterioration of
the oxygen-absorbing material; wherein the judgment on
deterioration of the NOx-absorbing material by the first
deterioration detector is inhibited when the oxygen-absorbing
material has been judged to have deteriorated by the second
deterioration detector.
Description
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0001] This invention relates to exhaust-gas cleaning devices for
an engine installed on a motor vehicle, for example.
[0002] Conventionally, the amount of nitrogen oxides (NOx) absorbed
by a NOx-absorbing material is detected for making a judgment on
deterioration of the NOx-absorbing material which is provided in an
exhaust passage. Basically, the amount of NOx absorbed by the
NOx-absorbing material can be determined by examining the
concentration of oxygen in exhaust gases that varies as NOx
released from the NOx-absorbing material is reduced. However, since
an exhaust-gas converting catalyst, in which the NOx-absorbing
material is provided, occludes oxygen, it is impossible to say
whether a change in the concentration of oxygen is caused by a
release of the absorbed NOx or by a release of occluded oxygen by
just detecting the change in oxygen concentration. It is therefore
impossible to accurately determine the amount of the absorbed
NOx.
[0003] Under such circumstances, Japanese Unexamined Patent
Publication No. 8-260949 proposes a deterioration detecting device
for detecting deterioration of a NOx-absorbing material provided in
an exhaust passage, in which the NOx-absorbing material absorbs NOx
and stores oxygen when the air-fuel ratio of exhaust gas is high
(lean exhaust gas), and releases the absorbed NOx and stored oxygen
when the air-fuel ratio of the exhaust gas becomes low (rich
exhaust gas). This deterioration detecting device comprises an
air-fuel ratio sensor provided in the exhaust passage, downstream
of the NOx-absorbing material, for generating an output signal
corresponding to the air-fuel ratio of the exhaust gas, an air-fuel
ratio switcher which switches the air-fuel ratio of the exhaust gas
from a higher value to a lower value when the amount of NOx
absorbed in the NOx-absorbing material is regarded as being almost
zero, or more specifically, when the air-fuel ratio has been made
high for a short time, and a stored oxygen quantity detector for
detecting the amount of oxygen stored in the NOx-absorbing material
based on the output signal generated by the air-fuel ratio sensor
when the air-fuel ratio of the exhaust gas flowing into the
NOx-absorbing material has been switched from a higher value to a
lower value, wherein, after the air-fuel ratio has been made high
for a long period by the air-fuel ratio switcher, the sum of the
amount of oxygen stored in the NOx-absorbing material and the
amount of NOx absorbed in the NOx-absorbing material is determined
based on the output signal generated by the air-fuel ratio sensor
when the air-fuel ratio of the exhaust gas flowing into the
NOx-absorbing material has been switched from the higher value to
the lower value, and the amount of NOx absorbed in the
NOx-absorbing material is calculated by subtracting the amount of
the stored oxygen from the aforementioned sum.
[0004] In the deterioration detecting device for detecting
deterioration of the NOx-absorbing material disclosed in the
aforementioned Publication, it is assumed that the period of time
required for the NOx-absorbing material to store the greatest
possible quantity (saturation level) of oxygen is sufficiently
short and NOx is scarcely absorbed by the NOx-absorbing material
during this time period. Based on this assumption, the
deterioration detecting device of regards a value detected from a
change in the air-fuel ratio of the exhaust gas that occurs when
the air-fuel ratio is switched to a lower value after once being
made high for a short time as being representative of the amount of
the stored oxygen, and calculates the amount of NOx absorbed in the
NOx-absorbing material from this detected value and the sum of the
amount of the stored oxygen and the amount of NOx. Even if the
aforementioned time period is short, however, the amount of NOx
absorbed in the NOx-absorbing material is not zero and oxygen is
produced when NOx released from the NOx-absorbing material is
reduced. Thus, the deterioration detecting device of the
Publication has a problem that its accuracy of judgment on
deterioration of the NOx-absorbing material inevitably lowers due
to errors occurring in detecting the amount of the stored
oxygen.
[0005] An exhaust-gas cleaning device for an engine in which
specific quantities of NOx-absorbing material and oxygen-absorbing
material are provided in an exhaust passage also has a problem that
it becomes impossible to accurately calculate the amount of
absorbed NOx in a manner disclosed in the aforementioned Patent
Publication when the functioning of the oxygen-absorbing material
deteriorates.
SUMMARY OF THE INVENTION
[0006] In view of the foregoing, it is an object of the invention
to provide an exhaust-gas cleaning device for an engine capable to
making a judgment on deterioration of a NOx-absorbing material with
a high degree of accuracy even when a specific quantity of
NOx-absorbing material and a large quantity of oxygen-absorbing
material are provided in an exhaust passage of the engine.
[0007] In a principal form of the invention, an exhaust-gas
cleaning device for an engine comprises a NOx-absorbing material
provided in an exhaust passage to absorb NOx under oxygen-rich
conditions where the oxygen concentration is high and release
absorbed NOx as the oxygen concentration decreases, an
oxygen-absorbing material provided in the exhaust passage to absorb
oxygen under oxygen-rich conditions where the oxygen concentration
is high and release absorbed oxygen as the oxygen concentration
decreases, an oxygen concentration controller for controlling the
oxygen concentration in the exhaust passage, an oxygen
concentration detector provided downstream of the NOx-absorbing
material and the oxygen-absorbing material, an absorption quantity
detector which determines a first amount of absorption of NOx and
oxygen absorbed by the NOx-absorbing material and the
oxygen-absorbing material based on a sensing signal fed from the
oxygen concentration detector at a time when the oxygen
concentration upstream of the NOx-absorbing material and the
oxygen-absorbing material is just transferred from a high
concentration state maintained for a preset first reference time to
a low concentration state by controlling the oxygen concentration
with the oxygen concentration controller, and determines a second
amount of absorption of NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material based on a
sensing signal fed from the oxygen concentration detector at a time
when the oxygen concentration upstream of the NOx-absorbing
material and the oxygen-absorbing material is just transferred from
the high concentration state maintained for a preset second
reference time which is longer than the first reference time to the
low concentration state by controlling the oxygen concentration
with the oxygen concentration controller, a NOx quantity estimator
which obtains a first estimated value of the amount of NOx absorbed
by the NOx-absorbing material during the first reference time and a
second estimated value of the amount of NOx absorbed by the
NOx-absorbing material during the second reference time in
accordance with engine operating condition, and a deterioration
detector which makes a judgment on deterioration of the
NOx-absorbing material based on the first amount of absorption and
the second amount of absorption of NOx and oxygen determined by the
absorption quantity detector and the first estimated value and the
second estimated value of the amount of absorbed NOx obtained by
the NOx quantity estimator.
[0008] In the exhaust-gas cleaning device thus constructed, the
judgment on deterioration of the NOx-absorbing material is made
based on the first and the second amounts of absorption of NOx and
oxygen determined by the absorption quantity detector and the first
and the second estimated values of the amount of absorbed NOx
obtained by the NOx quantity estimator. It is therefore possible to
perform the deterioration judgment operation in a proper manner
taking into account the amount of NOx absorbed in the NOx-absorbing
material during the first reference time. When a three-way
catalytic converter is provided upstream of the NOx-absorbing
material, for instance, the three-way catalytic converter at a low
temperature can be activated by reacting unburned fuel in exhaust
gas with oxygen released from the oxygen-absorbing material. Modern
exhaust-gas cleaning devices comprising a large quantity of
oxygen-absorbing material in the three-way catalytic converter have
a problem that the NOx-absorbing material tends to be wrongly
judged undeteriorated despite the fact that it is already
deteriorated, because a reference calculation time set for
calculating the amount of oxygen absorbed in the oxygen-absorbing
material required for saturating it can not be sufficiently
shortened and a specific amount of NOx is absorbed by the
NOx-absorbing material during the reference calculation time,
making it impossible to accurately determine the amount of oxygen
absorbed in the oxygen-absorbing material. It is possible to
overcome this problem of the prior art by employing the
aforementioned construction of the invention.
[0009] According to a unique feature of the invention, the
exhaust-gas cleaning device is constructed such that the
deterioration detector makes a judgment on deterioration of the
NOx-absorbing material based on the difference between the second
amount of absorption and the first amount of absorption of NOx and
oxygen determined by the absorption quantity detector and the
difference between the second estimated value and the first
estimated value of the amount of NOx obtained by the NOx quantity
estimator.
[0010] In this construction, the deterioration detector makes a
judgment on whether or not a specified quantity of NOx is absorbed
in the NOx-absorbing material in a proper manner based on, for
example, the ratio of the difference between the second amount of
absorption and the first amount of absorption of NOx and oxygen
determined by the absorption quantity detector to the difference
between the second estimated value and the first estimated value of
the amount of NOx obtained by the NOx quantity estimator.
[0011] According to another feature of the invention, the NOx
quantity estimator obtains the first estimated value and the second
estimated value of the amount of NOx absorbed by the NOx-absorbing
material in accordance with engine speed and engine load.
[0012] In this construction, the first estimated value and the
second estimated value of the amount of NOx corresponding to the
operating condition of the engine can be read from a map defined
using the engine speed and engine load as parameters, for
instance.
[0013] In another principal form of the invention, an exhaust-gas
cleaning device for an engine comprises a NOx-absorbing material
provided in an exhaust passage to absorb NOx under oxygen-rich
conditions where the oxygen concentration is high and release
absorbed NOx as the oxygen concentration decreases, an
oxygen-absorbing material provided in the exhaust passage to absorb
oxygen under oxygen-rich conditions where the oxygen concentration
is high and release absorbed oxygen as the oxygen concentration
decreases, an oxygen concentration controller for controlling the
oxygen concentration in exhaust gas, an oxygen concentration
detector provided downstream of the NOx-absorbing material and the
oxygen-absorbing material, an absorption quantity detector which
determines a first amount of absorption of NOx and oxygen absorbed
by the NOx-absorbing material and the oxygen-absorbing material
based on a sensing signal fed from the oxygen concentration
detector at a time when the oxygen concentration upstream of the
NOx-absorbing material and the oxygen-absorbing material is just
transferred from a high concentration state maintained for a preset
first reference time to a low concentration state by controlling
the oxygen concentration with the oxygen concentration controller,
and determines a second amount of absorption of NOx and oxygen
absorbed by the NOx-absorbing material and the oxygen-absorbing
material based on a sensing signal fed from the oxygen
concentration detector at a time when the oxygen concentration
upstream of the NOx-absorbing material and the oxygen-absorbing
material is just transferred from the high concentration state
maintained for a preset second reference time which is longer than
the first reference time to the low concentration state by
controlling the oxygen concentration with the oxygen concentration
controller, a first deterioration detector which makes a judgment
on deterioration of the NOx-absorbing material based on the first
amount of absorption and the second amount of absorption of NOx and
oxygen determined by the absorption quantity detector, and a second
deterioration detector which makes a judgment on deterioration of
the oxygen-absorbing material, wherein the judgment on
deterioration of the NOx-absorbing material by the first
deterioration detector is inhibited when the oxygen-absorbing
material has been judged to have deteriorated by the second
deterioration detector.
[0014] In the exhaust-gas cleaning device thus constructed, the
second deterioration detector judges whether or not the
oxygen-absorbing material has deteriorated, and if it is judged
undeteriorated, the first deterioration detector properly judges
whether or not the NOx-absorbing material has deteriorated based on
the second amount of absorption of NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material determined
by the absorption quantity detector. On the other hand, when the
oxygen-absorbing material is judged to have deteriorated, the
judgment on deterioration of the NOx-absorbing material by the
first deterioration detector is inhibited to thereby prevent
misjudgment by the first deterioration detector potentially caused
by the deterioration of the oxygen-absorbing material. Therefore,
this construction serves to improve the accuracy of judgment on
deterioration of the NOx-absorbing material by the first
deterioration detector.
[0015] According to a feature of the invention, the absorption
quantity detector is so constructed as to determine the amount of
NOx and oxygen absorbed by the NOx-absorbing material and the
oxygen-absorbing material based on the time period required for the
oxygen concentration detected by the oxygen concentration detector
to decrease to a specific low concentration level from a point in
time when the oxygen concentration controller executes a control
operation for transferring the oxygen concentration upstream of the
NOx-absorbing material and the oxygen-absorbing material from the
high concentration state to the low concentration state.
[0016] In this construction, the first and second amounts of
absorption of NOx and oxygen absorbed by the NOx-absorbing material
and the oxygen-absorbing material are obtained as large values when
the time period required for the oxygen concentration detected by
the oxygen concentration detector to decrease to the specific low
concentration level from the point in time when the oxygen
concentration controller executes the control operation for
transferring the oxygen concentration upstream of the NOx-absorbing
material and the oxygen-absorbing material from the high
concentration state to the low concentration state is long.
Contrary to this, when the aforementioned time period is short, the
first and second amounts of absorption of NOx and oxygen absorbed
by the NOx-absorbing material and the oxygen-absorbing material are
obtained as small values.
[0017] According to another feature of the invention, the oxygen
concentration controller is so constructed as to control the oxygen
concentration in the exhaust passage by controlling the air-fuel
ratio in a combustion chamber.
[0018] In this construction, the control operation for transferring
the oxygen concentration of the exhaust gas discharged from the
engine into the exhaust passage from the high concentration state
maintained for the preset first reference time to the low
concentration state and from the high concentration state
maintained for the preset second reference time to the low
concentration state is performed by controlling the air-fuel ratio
in the combustion chamber with the oxygen concentration controller
and, then, the absorption quantity detector determines the first
and second amounts of absorption of NOx and oxygen.
[0019] According to still another feature of the invention, the
oxygen-absorbing material is provided upstream of the NOx-absorbing
material.
[0020] This construction makes it possible to improve exhaust-gas
cleaning performance at low temperatures by using a large quantity
of the oxygen-absorbing material. When the exhaust gas in the
exhaust passage transfers from an oxygen-rich state to a state in
which the oxygen concentration has decreased, oxygen released from
the oxygen-absorbing material is supplied to the NOx-absorbing
material downstream of the oxygen-absorbing material. Since oxygen
produced when NOx released from the NOx-absorbing material is
reduced and oxygen released from the oxygen-absorbing material
together affect oxygen concentration values detected by the oxygen
concentration detector, significant effects are obtained by making
a judgment on deterioration of the NOx-absorbing material taking
into account the amount of NOx absorbed by the NOx-absorbing
material during the first reference time as stated above.
[0021] It will be recognized from the foregoing that the present
invention confers such advantageous effects that an exhaust-gas
cleaning device for an engine in which a specific quantity of
NOx-absorbing material and a large quantity of oxygen-absorbing
material are provided in an exhaust passage can make a judgment on
deterioration of the NOx-absorbing material with a high degree of
accuracy.
[0022] These and other objects, features and advantages of the
invention will become more apparent upon a reading of the following
detailed description in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is an explanatory diagram showing an exhaust-gas
cleaning device for an engine according to a preferred embodiment
of the invention;
[0024] FIGS. 2A-2B are time charts showing how the air-fuel ratio
and oxygen concentration detection value vary during a
deterioration judgment control operation;
[0025] FIG. 3 is a flowchart showing an air-fuel ratio control
operation;
[0026] FIG. 4 shows a first half of a flowchart of a deterioration
judgment control operation;
[0027] FIG. 5 shows a second half of the flowchart of the
deterioration judgment control operation;
[0028] FIG. 6 is a graph showing how detected values of oxygen
concentration vary with time; and
[0029] FIG. 7 is a block diagram showing an exhaust-gas cleaning
device according to another embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0030] FIG. 1 is an explanatory diagram showing an exhaust-gas
cleaning device for a direct injection gasoline engine according to
a preferred embodiment of the invention. A control system of the
engine to be installed on a motor vehicle comprises an intake
passage 2 and an exhaust passage 3 connected to an engine body 1, a
spark plug 4 fitted at the top of a combustion chamber of the
engine body 1, and a fuel injector 5 for injecting fuel directly
into the combustion chamber.
[0031] A surge tank 6 is provided in the intake passage 2 with a
throttle valve 7 provided upstream of the surge tank 6. Driven by
an electric actuator which operates in accordance with a control
signal output from an engine control unit (ECU) 8, the throttle
valve 7 regulates the amount of intake air introduced into the
combustion chamber. An intake shutter valve 10 for creating a swirl
and an intake valve (not shown) for opening and closing an intake
port are provided downstream of the surge tank 6.
[0032] An exhaust valve (not shown) for opening and closing an
exhaust port is provided in the exhaust passage 3. Further, a first
oxygen concentration detector 11 formed mainly of a .lambda.O.sub.2
sensor for detecting the concentration of oxygen contained in
exhaust gas expelled through the exhaust port, a three-way
catalytic converter 12 which reduces NOx, carbon monoxide (CO) and
hydrocarbons (HC) under conditions where the air-fuel ratio of the
exhaust gas is close to the stoichiometric air-fuel ratio, a lean
NOx catalytic converter 13 which reduces NOx even under oxygen-rich
conditions, and a second oxygen concentration detector 14 formed
mainly of a linear O.sub.2 sensor for detecting the air-fuel ratio
of the exhaust gas output from the lean NOx catalytic converter 13
are provided in the exhaust passage 3 in this order from the
upstream side.
[0033] The three-way catalytic converter 12 includes an
oxygen-absorbing material formed of ceria (CeO.sub.2) which absorbs
oxygen during lean mixture running in which the air-fuel ratio of
an air-fuel mixture in the combustion chamber is larger than the
stoichiometric air-fuel ratio, or under oxygen-rich conditions
where the oxygen concentration in the exhaust gas is high
(approximately 0.5% or above), and releases oxygen under
oxygen-poor conditions where the oxygen concentration in the
exhaust gas is low (less than approximately 0.5%). The three-way
catalytic converter 12 thus constructed simultaneously oxidizes HC
and CO and reduces NOx to remove or decrease these pollutants.
[0034] The lean NOx catalytic converter 13 includes a NOx-absorbing
material formed of alkali metal (especially potassium),
alkaline-earth metal (especially barium) or rare-earth metal which
absorbs NOx under oxygen-rich conditions where the oxygen
concentration in the exhaust gas is high (approximately 4% or
above), and releases NOx under oxygen-poor conditions where the
oxygen concentration in the exhaust gas is low (less than
approximately 0.5% to 1%). The lean NOx catalytic converter 13 thus
constructed causes the NOx-absorbing material to absorb NOx under
oxygen-rich conditions and release NOx as the exhaust gas becomes
poor in oxygen, and purify the exhaust gas by reducing NOx as a
result of an action of catalytic metal, such as noble metal, which
is located around the NOx-absorbing material.
[0035] As an alternative, the lean NOx catalytic converter 13 may
also include an oxygen-absorbing material formed of ceria
(CeO.sub.2). In this alternative construction, oxygen absorbed by
the oxygen-absorbing material under oxygen-rich conditions where
the oxygen concentration in the exhaust gas is high is released as
the oxygen concentration drops, and oxygen thus released is caused
to react with CO contained in the exhaust gas, whereby the lean NOx
catalytic converter 13 can be heated and maintained at appropriate
temperatures.
[0036] Referring to FIG. 1, the engine is provided with various
sensors, such as an airflow sensor 15 for detecting the amount of
intake air passing through the intake passage 2, a crank angle
sensor 16 for detecting engine speed, an accelerator pedal stroke
sensor 17 and an engine cooling water temperature sensor 18.
Sensing signals from these sensors are entered to the ECU 8.
[0037] The ECU 8 includes an operating condition identifier 19 for
judging the operating condition of the engine, a fuel injection
controller 20 for controlling the amount of injected fuel and
injection timing, an oxygen concentration controller 21 for
controlling the oxygen concentration in the exhaust passage 3 by
controlling the air-fuel ratio in the combustion chamber, an
absorption quantity detector 22 for detecting the amount of NOx and
oxygen absorbed by the NOx-absorbing material of the lean NOx
catalytic converter 13 and the oxygen-absorbing material of the
three-way catalytic converter 12 based on a sensing signal fed from
the second oxygen concentration detector 14, a NOx quantity
estimator 23 for calculating estimated quantities of NOx absorbed
by the NOx-absorbing material, and a deterioration detector 24 for
making a judgment on deterioration of the NOx-absorbing
material.
[0038] The operating condition identifier 19 judges whether the
engine is operating in a uniform charge combustion range under
high-load, high-to-low speed conditions or in a stratified charge
combustion range under low-load, low-speed conditions, for example,
based on an engine speed value detected by the crank angle sensor
16 and an engine load value detected by the accelerator pedal
stroke sensor 17, and transmits resultant judgment data to the fuel
injection controller 20.
[0039] The fuel injection controller 20 reads out a target engine
torque from a predefined map based on accelerator pedal stroke
detected by the accelerator pedal stroke sensor 17 and the engine
speed detected by the crank angle sensor 16, reads out a target
fuel injection quantity from a predefined map according to the
target torque and the actual amount of intake air detected by the
airflow sensor 15, and transmits a control signal corresponding to
the target fuel injection quantity to the fuel injector 5.
[0040] The fuel injection controller 20 also controls fuel
injection timing in accordance with the engine operating condition
determined by the operating condition identifier 19. When the
engine is already warm and operating in the uniform charge
combustion range under high-load, high-to-low speed conditions, for
example, the fuel injection controller 20 performs a control
operation for injecting the fuel during an intake stroke to produce
uniform charge combustion. When the engine is operating in the
stratified charge combustion range under low-load, low-speed
conditions, on the other hand, the fuel injection controller 20
performs a control operation for injecting the fuel in a latter
part of a compression stroke to produce stratified charge
combustion.
[0041] The oxygen concentration controller 21 controls the amount
of intake air by transmitting a control signal corresponding to the
engine operating condition to the actuator of the throttle valve 7
in such a way that the air-fuel ratio in a cylinder becomes far
higher than the stoichiometric air-fuel ratio in the stratified
charge combustion range and the air-fuel ratio in the cylinder
becomes richer in the uniform charge combustion range than in
stratified charge combustion range.
[0042] The oxygen concentration controller 21 also controls the
oxygen concentration in the exhaust gas by regulating the amount of
injected fuel in a manner described below referring to FIG. 2A to
enable the deterioration detector 24 to make a judgment on
deterioration of the NOx-absorbing material during steady-state
engine operation. Specifically, the oxygen concentration controller
21 first makes a setting to produce a rich mixture at a starting
point O of a deterioration judgment control operation, and at a
point T1 in time when a specific time (approximately 2 to 5
seconds) has elapsed from the starting point O, the oxygen
concentration controller 21 makes a setting to produce a lean
mixture such that the oxygen concentration in the exhaust gas
upstream of the three-way catalytic converter 12 and the lean NOx
catalytic converter 13 becomes high. Then, at a point T2 in time
when this high oxygen concentration state has been maintained for a
first reference time t1, which is set to approximately 10 to 20
seconds, the oxygen concentration controller 21 makes a setting to
produce a rich mixture to thereby transfer the exhaust gas to a low
oxygen concentration state.
[0043] At a point T3 in time when a specific time (approximately 2
to 5 seconds) has elapsed from the point T2, the oxygen
concentration controller 21 makes a setting to produce a lean
mixture for creating a high oxygen concentration state in the
exhaust gas. At a point T4 in time when this high oxygen
concentration state has been maintained for a second reference time
t2, which is set to approximately 150 seconds, the oxygen
concentration controller 21 makes a setting to produce a rich
mixture again to thereby transfer the exhaust gas to a low oxygen
concentration state. Then, at a point T5 in time when a specific
time (approximately 2 to 5 seconds) has elapsed from the point T4,
the deterioration judgment control operation is finished and an
ordinary control state is restored.
[0044] When making a judgment on deterioration of the NOx-absorbing
material by the deterioration judgment control operation mentioned
above, the oxygen concentration controller 21 performs a control
operation to make the amount of fuel injected during the first
reference time t1 smaller than that injected during the second
reference time t2 such that the air-fuel mixture becomes
significantly leaner during the first reference time t1 than during
the second reference time t2.
[0045] The absorption quantity detector 22 determines the amounts
of NOx and oxygen absorbed by the NOx-absorbing material of the
lean NOx catalytic converter 13 and the oxygen-absorbing material
of the three-way catalytic converter 12 based on time periods TAo
and TBo shown in FIG. 2B required for the oxygen concentration
detected by the second oxygen concentration detector 14 to decrease
to a specific low concentration level from the aforementioned
points T2 and T4, respectively, at which the oxygen concentration
controller 21 completes the setting for transferring the exhaust
gas upstream of the NOx-absorbing material and the oxygen-absorbing
material from the high oxygen concentration state to the low oxygen
concentration state.
[0046] Specifically, when the setting for transferring the exhaust
gas to the low oxygen concentration state has been made at the
points T2 and T4 at which the oxygen concentration controller 21
completes the operation for creating the high oxygen concentration
state in the exhaust gas performed during the first and second
reference times t1, t2, NOx and oxygen absorbed by the
NOx-absorbing material and the oxygen-absorbing material are
released. Therefore, the oxygen concentration value detected by the
second oxygen concentration detector 14 does not immediately change
to a value representative of a low oxygen concentration state, but
the detected value reaches a value equal to a specific value O2o
representative of the low oxygen concentration state at a time when
the time period TAo (TBo) has just elapsed.
[0047] Since the time periods TAo and TBo required for the oxygen
concentration detected by the second oxygen concentration detector
14 to transfer to the low oxygen concentration level from the
points T2 and T4, respectively, vary depending on the amount of NOx
and oxygen absorbed by the NOx-absorbing material and the
oxygen-absorbing material, it is possible to determine the amount
of NOx and oxygen absorbed by the NOx-absorbing material of the
lean NOx catalytic converter 13 and the oxygen-absorbing material
of the three-way catalytic converter 12 in terms of values
corresponding to the time periods TAo and TBo by means of the
absorption quantity detector 22.
[0048] The amount of NOx absorbed by the NOx-absorbing material of
the lean NOx catalytic converter 13 and the oxygen-absorbing
material of the three-way catalytic converter 12 varies with the
amount of NOx discharged from the engine into the exhaust passage 3
which varies in accordance with the operating condition of the
engine. Thus, the NOx quantity estimator 23 obtains a first
estimated value of the amount of NOx absorbed by the NOx-absorbing
material during the first reference time t1 and a second estimated
value of the amount of NOx absorbed by the NOx-absorbing material
during the second reference time t2 by reading out the amount of
NOx corresponding to the operating condition of the engine from a
map defined using the engine speed and engine load as
parameters.
[0049] The deterioration detector 24 calculates the difference
between a second amount of absorption and a first amount of
absorption of NOx and oxygen detected by the absorption quantity
detector 22 as well as the difference between the second estimated
value and the first estimated value of the amount of NOx obtained
by the NOx quantity estimator 23. The deterioration detector 24
then makes a judgment on deterioration of the NOx-absorbing
material by judging whether the ratio between the above two
differences. If the NOx-absorbing material is judged to have
deteriorated, the deterioration detector 24 transmits a control
signal to a display 25, causing it to indicate an anomaly of the
exhaust-gas cleaning device.
[0050] An air-fuel ratio control operation performed by the engine
exhaust-gas cleaning device of the present embodiment is now
described referring to a flowchart shown in FIG. 3. Following the
start of the control operation of the flowchart, data detected by
the individual sensors are entered to the ECU 8 (step S1). The ECU
8 then reads out and sets a basic fuel injection quantity Qb and a
basic injection point Ob corresponding to the engine operating
condition from a map (step S2) and judges whether the engine is in
a "lean" operating range (step S3).
[0051] If it is determined that the engine is in the "lean"
operating range (YES in step S3), a further judgment is made to
check whether conditions for determining that the NOx-absorbing
material has deteriorated have been satisfied (step S4). More
specifically, whether or not the deterioration judgment conditions
have been met is judged by verifying that the engine is in a warm
operating state, steady-state engine operation has continued for a
specific period of time, and the NOx-absorbing material has not
been judged to have deteriorated yet.
[0052] If the judgment result in step S4 is in the affirmative, the
count value of deterioration judgment time T which is counted by a
deterioration judgment timer is incremented by 1 (step S5), and a
further judgment is made to determine whether the count value of
the deterioration judgment time T exists in one of "rich" periods
shown in FIG. 2, that is, the period between the points O and T1,
the period between the points T2 and T3, or the period between the
points T4 and T5, (step S6).
[0053] If it is determined that the count value of the
deterioration judgment time T exists in a "rich" period (YES in
step S6), the basic fuel injection quantity Qb of the fuel is set
to a value Qbr1 which makes the excess-air factor .lambda. equal to
or less than 1 (step S7) and the basic injection point .theta.b is
set such that the fuel is separately injected in the intake and
compression strokes (step S8). Subsequently, a judgment is made to
determine whether a fuel injection point has been reached (step
S9). When the judgment result in step S9 becomes YES, an injection
control operation is executed to inject the fuel (step S10),
whereby an oxygen concentration control operation in the
deterioration judgment operation is performed.
[0054] On the other hand, if it is determined that the count value
of the deterioration judgment time T exists in one of "lean"
periods shown in FIG. 2, that is, the period between the points T1
and T2, or the period between the points T3 and T4, (NO in step
S6), the basic fuel injection quantity Qb of the fuel is set to a
value Qb1 which makes the excess-air factor .lambda. larger than 1
(step S11) and the operation flow proceeds to step S9, whereby an
oxygen concentration control operation in the deterioration
judgment operation is performed. If the count value of the
deterioration judgment time T exists in the period corresponding to
the earlier-mentioned first reference time t1 (between the points
T1 and T2), the basic fuel injection quantity Qb1 is set such that
the air-fuel ratio becomes higher (to produce a leaner mixture)
than when the count value of the deterioration judgment time T
exists in the period corresponding to the earlier-mentioned second
reference time t2 (between the points T3 and T4).
[0055] Also, if it is determined that the engine is not in the
"lean" operating range (NO in step S3), or if it is determined that
the deterioration judgment conditions for the NOx-absorbing
material have not been met (NO in step S4), the deterioration
judgment timer is initialized to reset the count value of the
deterioration judgment time T to 0 (step S12) and the operation
flow proceeds to step S9.
[0056] Next, the aforementioned deterioration judgment control
operation for making a judgment on deterioration of the
NOx-absorbing material performed by the exhaust-gas cleaning device
of the embodiment is described referring to a flowchart shown in
FIGS. 4 and 5. Following the start of this control operation, data
detected by the individual sensors are entered to the ECU 8 (step
S21), and a judgment is made to determine whether the value of the
deterioration judgment time T counted by the deterioration judgment
timer exists in the period between the starting point O and the
ending point T5 of the deterioration judgment control operation
(step S22).
[0057] If the judgment result in step S22 is in the affirmative, a
further judgment is made to determine whether the count value of
the deterioration judgment time T exists in the period
corresponding to the first reference time t1 (T1 to T2) (step S23).
If the judgment result in step S23 is in the affirmative (T1 T T2),
the NOx quantity estimator 23 reads out the first estimated value
NOx1 of the amount of NOx absorbed by the NOx-absorbing material
during the first reference time t1 from a map (not shown) in
accordance with the engine operating condition (step S24).
[0058] On the other hand, if it is determined that the count value
of the deterioration judgment time T does not exist in the period
corresponding to the first reference time t1 (T1 to T2) (NO in step
S23), a further judgment is made to determine whether the count
value of the deterioration judgment time T exists in the "rich"
period (T2 to T3) immediately following the first reference time t1
(step S25). If the judgment result in step S25 is in the
affirmative (T2<T T3), a further judgment is made to determine
whether the oxygen concentration value O2x detected by the second
oxygen concentration detector 14 has reached the specific value O2o
representative of a low oxygen concentration state (step S26).
[0059] If the judgment result in step S26 is in the affirmative,
the absorption quantity detector 22 sets the time period (T-T2)
elapsed from the end point T2 of the first reference time t1 to the
current deterioration judgment time T as a value corresponding to
the first amount of absorption TAo of NOx and oxygen absorbed by
the NOx-absorbing material of the lean NOx catalytic converter 13
and the oxygen-absorbing material of the three-way catalytic
converter 12 during the first reference time t1, and determines
this first amount of absorption TAo (step S27).
[0060] Next, it is judged whether the count value of the
deterioration judgment time T exists in the period corresponding to
the second reference time t2 (T3 to T4) (step S28). If the judgment
result in step S28 is in the affirmative (T3<T T4), the NOx
quantity estimator 23 reads out the second estimated value NOx2 of
the amount of NOx absorbed by the NOx-absorbing material during the
second reference time t2 from a map (not shown) in accordance with
the engine operating condition (step S29).
[0061] Then, it is judged whether the count value of the
deterioration judgment time T exists in the "rich" period (T4 to
T5) immediately following the second reference time t2 (step S30).
If the judgment result in step S30 is in the affirmative (T4<T
T5), a further judgment is made to determine whether the oxygen
concentration value O2x detected by the second oxygen concentration
detector 14 has reached the specific value O2o representative of a
low oxygen concentration state (step S31).
[0062] If the judgment result in step S31 is in the affirmative,
the absorption quantity detector 22 sets the time period (T-T4)
elapsed from the end point T4 of the second reference time t2 to
the current deterioration judgment time T as a value corresponding
to the second amount of absorption TBo of NOx and oxygen absorbed
by the NOx-absorbing material of the lean NOx catalytic converter
13 and the oxygen-absorbing material of the three-way catalytic
converter 12 during the second reference time t2, and determines
this second amount of absorption TBo (step S32).
[0063] Subsequently, the difference NOx between the second
estimated value NOx2 and the first estimated value NOx1 is obtained
by subtracting the first estimated value NOx1 of the amount of NOx
obtained in step S24 from the second reference time t2 of the
amount of NOx obtained in step S29 (step S33), and the difference
To between the second amount of absorption TBo obtained in step S32
and the first amount of absorption TAo obtained in step S27 is
calculated (step S34).
[0064] Here, a judgment is made to determine whether the ratio
To/NOx of the two differences is smaller than a reference value D
(step S35) to judge whether or not deterioration of the
NOx-absorbing material has occurred. If the judgment result in step
S35 is in the affirmative, the NOx-absorbing material is judged to
have deteriorated and the deterioration detector 24 transmits a
control signal to the display 25, causing it to indicate the
anomaly of the exhaust-gas cleaning device (step S36). If, on the
other hand, the judgment result in step S35 is in the negative, the
NOx-absorbing material is judged to have not deteriorated yet, the
above-described deterioration judgment control operation is
finished.
[0065] Since oxygen absorbed by the NOx-absorbing material of the
lean NOx catalytic converter 13 and the oxygen-absorbing material
of the three-way catalytic converter 12 reaches a saturation level
in a relatively short time, the greatest possible quantities
(saturation level) of oxygen are supposed to be absorbed in the
NOx-absorbing material and the oxygen-absorbing material. Thus, it
is possible to obtain the difference To between the amount of NOx
absorbed by the NOx-absorbing material during the second reference
time t2 and the amount of NOx absorbed by the NOx-absorbing
material during the first reference time t1 by subtracting the
first amount of absorption TAo of NOx and oxygen obtained in step
S27 from the second amount of absorption TBo of NOx and oxygen
obtained in step S32.
[0066] The difference NOx obtained by subtracting the first
estimated value NOx1 of the amount of NOx obtained in step S24 from
the second reference time t2 of the amount of NOx obtained in step
S29 gives an estimated value of the difference between the amount
of NOx absorbed by the NOx-absorbing material during the second
reference time t2 and the amount of NOx absorbed by the
NOx-absorbing material during the first reference time t1.
Therefore, if it is assumed that the result of estimation given by
the NOx quantity estimator 23 is correct and a rated amount of NOx
is absorbed in the NOx-absorbing material, the ratio To/ NOx of the
aforementioned two differences takes an approximately fixed
value.
[0067] On the contrary, if it is determined that the ratio To/ NOx
of the two differences is smaller than the predefined reference
value D, the amount of NOx absorbed by the NOx-absorbing material
is much smaller than the rated amount, so that the NOx-absorbing
material is judged to have deteriorated.
[0068] As shown in the foregoing discussion, the exhaust-gas
cleaning device of the embodiment is so constructed as to determine
the first amount of absorption TAo of NOx and oxygen absorbed by
the NOx-absorbing material and the oxygen-absorbing material based
on the sensing signal fed from the second oxygen concentration
detector 14 at the point in time when the exhaust gas upstream of
the NOx-absorbing material and the oxygen-absorbing material is
just transferred from the high concentration state maintained for
the preset first reference time t1 to the low concentration state
by controlling the oxygen concentration in the exhaust gas with the
oxygen concentration controller 21, determine the second amount of
absorption TBo of NOx and oxygen absorbed by the NOx-absorbing
material and the oxygen-absorbing material based on the sensing
signal fed from the second oxygen concentration detector 14 at the
point in time when the exhaust gas is just transferred from the
high concentration state maintained for the preset second reference
time t2, which is longer than the first reference time t1, to the
low concentration state by controlling the oxygen concentration in
the exhaust gas with the oxygen concentration controller 21, obtain
the first estimated value NOx1 of the amount of NOx absorbed by the
NOx-absorbing material during the first reference time t1 and the
second estimated value NOx2 of the amount of NOx absorbed by the
NOx-absorbing material during the second reference time t2 in
accordance with the engine operating condition, and make a judgment
on deterioration of the NOx-absorbing material based on the first
amount of absorption TAo and the second amount of absorption TBo of
NOx and oxygen and the first estimated value NOx1 and the second
estimated value NOx2 of the amount of absorbed NOx. It is therefore
possible to perform the deterioration judgment operation taking
into account the amount of NOx absorbed in the NOx-absorbing
material during the first reference time t1.
[0069] Therefore, it is possible to prevent such an undesirable
situation that the NOx-absorbing material is wrongly judged to be
normally working despite the fact that it is already deteriorated
due to the effects of the amount of NOx absorbed in the
NOx-absorbing material during the first reference time t1, and to
make a judgment on deterioration of the NOx-absorbing material with
a high degree of accuracy based on the sensing signal fed from the
second oxygen concentration detector 14 and the estimated values
given by the NOx quantity estimator 23, even when a specific
quantity of the NOx-absorbing material and a large quantity of the
oxygen-absorbing material are provided in the exhaust passage
3.
[0070] One of noticeable advantages of the exhaust-gas cleaning
device of the present embodiment is that it can make a judgment on
whether or not a specified quantity of NOx is absorbed in the
NOx-absorbing material in performing the deterioration judgment
operation in a proper and easy way by comparing the aforementioned
ratio To/NOx with the predefined reference value D. This is because
the exhaust-gas cleaning device is constructed to perform the
operation for judging on the deterioration of the NOx-absorbing
material by the deterioration detector 24 based on the difference
To between the second amount of absorption TBo and the first amount
of absorption TAo of NOx and oxygen determined by the absorption
quantity detector 22 and the difference NOx between the second
estimated value NOx2 and the first estimated value NOx1 of the
amount of NOx obtained by the NOx quantity estimator 23.
[0071] As an alternative to the foregoing embodiment, in which the
deterioration judgment to determine whether the specified quantity
of NOx has been absorbed in the NOx-absorbing material is made by
comparing the ratio To/NOx with the predefined reference value D,
the exhaust-gas cleaning device may be so constructed as to perform
the deterioration judgment operation by calculating the difference
between the aforementioned two differences NOx, To and comparing it
with a predefined reference value. In this alternative
construction, it is necessary to make the deterioration judgment
after converting the time periods TAo, TBo required for the oxygen
concentration detected by the second oxygen concentration detector
14 to decrease to the specific low concentration level into the
amounts of NOx and oxygen absorbed by the NOx-absorbing material
and the oxygen-absorbing material during the two periods TAo, TBo
and calculating the difference between the converted amounts.
[0072] Another advantage the foregoing embodiment is that it is
possible to obtain the first and second estimated values NOx1, NOx2
of the amount of NOx absorbed by the NOx-absorbing material taking
into account changes in the amount of NOx in the exhaust gas due to
variations in the engine operating condition on the occurrence of
such changes. This is because the exhaust-gas cleaning device is
constructed such that the NOx quantity estimator 23 obtains the
first and second estimated values NOx1, NOx2 of the amount of NOx
absorbed by the NOx-absorbing material in accordance with the
engine speed and load.
[0073] As another alternative to the foregoing embodiment, the
exhaust-gas cleaning device may be constructed such that the NOx
quantity estimator 23 obtains the first and second estimated values
NOx1, NOx2 of the amount of NOx absorbed by the NOx-absorbing
material in accordance with the temperature of engine cooling water
and the amount of exhaust gas recirculated into the intake air by
an exhaust recirculating system instead of or in addition to the
engine speed and load.
[0074] Still another advantage the foregoing embodiment is that it
is possible to determine the amounts of NOx and oxygen absorbed by
the NOx-absorbing material and the oxygen-absorbing material during
the first and second reference times t1, t2 in a proper and easy
way. This is because the exhaust-gas cleaning device is constructed
such that the absorption quantity detector 22 determines the
amounts of NOx and oxygen absorbed by the NOx-absorbing material
and the oxygen-absorbing material based on the time periods TAo and
TBo required for the oxygen concentration detected by the second
oxygen concentration detector 14 to decrease to the specific low
concentration level from the aforementioned points T2 and T4,
respectively, at which the oxygen concentration controller 21
completes the setting for transferring the exhaust gas upstream of
the NOx-absorbing material and the oxygen-absorbing material from
the high oxygen concentration state to the low oxygen concentration
state.
[0075] When the amount of NOx and oxygen absorbed in the
NOx-absorbing material of the lean NOx catalytic converter 13 and
the oxygen-absorbing material of the three-way catalytic converter
12 is large, the amount of NOx and oxygen released from these
materials after the first reference time t1 (second reference time
t2) has elapsed increases, and the time period TAo (TBo) required
for the oxygen concentration detected by the second oxygen
concentration detector 14 to decrease to the specific low
concentration level from the aforementioned point T2 (T4), at which
the setting for transferring the exhaust gas upstream of the
NOx-absorbing material and the oxygen-absorbing material from the
high oxygen concentration state to the low oxygen concentration
state is completed, tends to increase due to the influence of
oxygen released from the materials and produced by reduction of
NOx. It is therefore possible to obtain the first and second
amounts of absorption of NOx and oxygen from the time periods TAo
and TBo, respectively, in a proper and easy way.
[0076] As still another alternative to the foregoing embodiment,
the exhaust-gas cleaning device may be so constructed as to
determine the first and second amounts of absorption of NOx and
oxygen absorbed by the NOx-absorbing material and the
oxygen-absorbing material based on oxygen concentration values
detected by the second oxygen concentration detector 14 at the
points T2 and T4, at which the oxygen concentration controller 21
completes the setting for transferring the exhaust gas upstream of
the NOx-absorbing material and the oxygen-absorbing material from
the high oxygen concentration state to the low oxygen concentration
state.
[0077] Yet another advantage the foregoing embodiment is that it is
possible to determine the amount of absorption of NOx and oxygen by
the absorption quantity detector 22 with high accuracy by
performing the oxygen concentration control operation properly and
quickly. This is because the exhaust-gas cleaning device is
constructed such that it performs the control operation for
transferring the exhaust gas discharged from the engine into the
exhaust passage 3 to the low oxygen concentration state after
creating the high concentration state for the first reference time
t1 by controlling the air-fuel ratio in the combustion chamber with
the oxygen concentration controller 21.
[0078] When the three-way catalytic converter 12 including the
oxygen-absorbing material is provided upstream of NOx-absorbing
material provided in the lean NOx catalytic converter 13 as shown
in the foregoing embodiment, oxygen released from the
oxygen-absorbing material of the three-way catalytic converter 12
located on the upstream side, oxygen released from the
NOx-absorbing material of the lean NOx catalytic converter 13
located on the downstream side, and oxygen produced by reduction of
NOx released from the NOx-absorbing material individually affect
the oxygen concentration value detected by the second oxygen
concentration detector 14 during execution of the control operation
for transferring the exhaust gas in the exhaust passage 3 from an
oxygen-rich state (lean exhaust gas) to an oxygen-poor state (rich
exhaust gas). It is therefore difficult to make a judgment on
deterioration of the NOx-absorbing material with high accuracy
based on the oxygen concentration value detected by the second
oxygen concentration detector 14.
[0079] Specifically, when the control operation for transferring
the exhaust gas in the exhaust passage 3 from the oxygen-rich state
(lean exhaust gas) to the oxygen-poor state (rich exhaust gas) is
being executed, a comparison of values a detected by the first
oxygen concentration detector 11 disposed upstream of the lean NOx
catalytic converter 13 and the three-way catalytic converter 12,
values y detected by the second oxygen concentration detector 14
disposed downstream of the lean NOx catalytic converter 13 and the
three-way catalytic converter 12, and values P detected by a third
oxygen concentration detector disposed between the lean NOx
catalytic converter 13 and the three-way catalytic converter 12
gives data depicted in FIG. 6.
[0080] It is recognized from FIG. 6 that although oxygen
concentrations .beta.1, .beta.2 detected by the third oxygen
concentration detector disposed between the catalytic converters 12
and 13 deviate from the oxygen concentration .alpha. detected by
the first oxygen concentration detector 11 by an amount
corresponding to the quantity of oxygen released from the
oxygen-absorbing material of the three-way catalytic converter 12,
there is not a much difference between the value .beta.1 detected
when the duration of the oxygen-rich state is set to 30 seconds and
the value .beta.2 detected when the duration of the oxygen-rich
state is set to 180 seconds. This is because the oxygen
concentrations .beta.1, .beta.2 are not affected by NOx and oxygen
released from the NOx-absorbing material and the oxygen-absorbing
material of the lean NOx catalytic converter 13.
[0081] Compared to this, it is noticed that there is a substantial
difference between the value .gamma.1 detected when the duration of
the oxygen-rich state is set to 30 seconds and the value .gamma.2
detected when the duration of the oxygen-rich state is set to 180
seconds and, therefore, the influence of NOx released from the
NOx-absorbing material of the lean NOx catalytic converter 13 can
not be disregarded. This is because the second oxygen concentration
detector 14 is affected by oxygen produced when NOx released from
the NOx-absorbing material is reduced. For this reason, it is
particularly desirable that the exhaust-gas cleaning device can
make a correct judgment on deterioration of the NOx-absorbing
material based on the oxygen concentration value detected by the
second oxygen concentration detector 14 by constructing the
exhaust-gas cleaning device such that it performs the deterioration
judgment operation taking into account the amount of NOx absorbed
in the NOx-absorbing material during the first reference time t1 as
described earlier.
[0082] As previously mentioned, the exhaust-gas cleaning device of
the foregoing embodiment is constructed such that the air-fuel
mixture becomes significantly leaner during the first reference
time t1 than during the second reference time t2 by causing the
oxygen concentration controller 21 to make the amount of fuel
injected during the first reference time t1 smaller than that
injected during the second reference time t2 when making a judgment
on deterioration of the NOx-absorbing material. Therefore, the NOx
quantity estimator 23 can correctly estimate the amount of NOx
absorbed by the NOx-absorbing material by making the amounts of
NOx2 reduced per unit time in the two reference times t1, t2 equal
to a fixed value.
[0083] Specifically, it is possible to prevent degradation of the
activity of the three-way catalytic converter 12 and the lean NOx
catalytic converter 13 due to a drop in exhaust gas temperature
during the second reference time t2 by executing a control
operation for making the amount of fuel injected during the first
reference time t2 which is longer than the second reference time t1
smaller than that injected during the second reference time t1.
Therefore, the exhaust-gas cleaning device of the embodiment is
advantageous in that estimation of the amount of NOx by the NOx
quantity estimator 23 can be properly made by maintaining the
amount of oxygen produced per unit time as a result of reduction of
NOx at a fixed value when executing the deterioration judgment
control operation.
[0084] While the foregoing discussion has dealt with the preferred
embodiment in which the deterioration detector 24 makes a judgment
on deterioration of the NOx-absorbing material only, there may be
provided a first deterioration detector 24 for making a judgment on
deterioration of the NOx-absorbing material provided in the lean
NOx catalytic converter 13 based on the first and second amounts of
absorption detected by the absorption quantity detector 22 and a
second deterioration detector 26 for making a judgment on
deterioration of the oxygen-absorbing material provided in the
three-way catalytic converter 12 as shown in FIG. 7.
[0085] In this second embodiment, the second deterioration detector
26 is so constructed as to perform a feedback operation to switch
the air-fuel mixture in the combustion chamber between rich and
lean states based on oxygen concentration values detected by the
first and third oxygen concentration detectors, or to simply switch
the mixture in the combustion chamber between the rich and lean
states at fixed time intervals without using the oxygen
concentration values detected by the first and third oxygen
concentration detectors, for a specified time period which is set
to approximately 2 minutes such that the air-fuel ratio in the
combustion chamber eventually equals the stoichiometric air-fuel
ratio, and judge whether or not the oxygen-absorbing material has
deteriorated by comparing a sensing signal fed from the first
oxygen concentration detector 11 provided upstream of the three-way
catalytic converter 12 and the lean NOx catalytic converter 13
during the specified time period and a sensing signal fed from the
third oxygen concentration detector 27 formed mainly of a
.lambda.O.sub.2 sensor disposed between the three-way catalytic
converter 12 and the lean NOx catalytic converter 13 during the
specified time period.
[0086] More specifically, the second deterioration detector 26
compares the number of times the oxygen concentration value
detected by the first oxygen concentration detector 11 is switched
(from the lean state to the rich state) during the specified time
period and the number of times the oxygen concentration value
detected by the third oxygen concentration detector 27 is switched
during the specified time period, and judges that the
oxygen-absorbing material has deteriorated if the ratio of the
number of times the oxygen concentration value detected by the
third oxygen concentration detector 27 is switched to the number of
times the oxygen concentration value detected by the first oxygen
concentration detector 11 is switched is found to be larger than a
preset value. This judgment is based on the fact that the number of
times the oxygen concentration value detected by the third oxygen
concentration detector 27 is switched should be sufficiently
smaller than the number of times the oxygen concentration value
detected by the first oxygen concentration detector 11 is switched
if the oxygen-absorbing material is normal.
[0087] When the oxygen-absorbing material has been judged to have
deteriorated by the second deterioration detector 26, the judgment
on deterioration of the NOx-absorbing material by the first
deterioration detector 24 is inhibited or a control operation is
performed to change a reference value used in the deterioration
judgment operation, for example. This makes it possible to make a
judgment on deterioration of the NOx-absorbing material with high
accuracy, preventing misjudgment potentially caused by an increase
in the amounts of HC and CO supplied to the NOx-absorbing material
as a result of deterioration of the oxygen-absorbing material, as
well as by an increase in the rate of releasing NOx from the
NOx-absorbing material which occurs as HC and CO serve as reducing
agents.
[0088] While the oxygen concentration is controlled by adjusting
the amount of injected fuel in the foregoing embodiments, the
oxygen concentration may be controlled by adjusting the amount or
timing of secondary injection if the engine is of a type
constructed to make the secondary fuel injection during an
expansion stroke, or by adjusting the amount of secondary air if
the engine is of a type provided with an arrangement for supplying
secondary air to the exhaust passage 3, for example.
[0089] This application is based on Japanese Application Serial No.
2000-13897 filed in Japanese Patent Office on Jan. 18, 2000, the
contents of which are hereby incorporated by reference.
[0090] Although the present invention has been fully described by
way of example with reference to the accompanying drawings, it is
to be understood that various changes and modifications will be
apparent to those skilled in the art. Therefore, unless otherwise
such changes and modifications depart from the scope of the present
invention hereinafter defined, they should be construed as being
included therein.
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