U.S. patent application number 12/314053 was filed with the patent office on 2009-06-11 for catalyst degradation diagnosis device and diagnosis method for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Toru Kidokoro, Koichi Kimura, Koichi Kitaura.
Application Number | 20090145109 12/314053 |
Document ID | / |
Family ID | 40720221 |
Filed Date | 2009-06-11 |
United States Patent
Application |
20090145109 |
Kind Code |
A1 |
Kidokoro; Toru ; et
al. |
June 11, 2009 |
Catalyst degradation diagnosis device and diagnosis method for
internal combustion engine
Abstract
The oxygen storage capacity preceding a previous oxygen storage
capacity and the previous oxygen storage capacity of each of an
upstream catalyst and a downstream catalyst are measured, and a
present oxygen storage capacity of the downstream catalyst is
measured. Then, sulfur poisoning of the upstream catalyst and the
downstream catalyst is detected on the basis of an oxygen storage
capacity change amount of each of the upstream catalyst and the
downstream catalyst from the oxygen storage capacity preceding the
previous one to the previous oxygen storage capacity, and the
oxygen storage capacity change amount of the downstream catalyst
from the previous oxygen storage capacity to the present oxygen
storage capacity. The presence/absence of the sulfur poisoning can
be accurately detected by utilizing the difference between the
manners of sulfur poisoning of the two catalysts.
Inventors: |
Kidokoro; Toru; (Hadano-shi,
JP) ; Kimura; Koichi; (Numazu-shi, JP) ;
Kitaura; Koichi; (Odawara-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
TOYOTA-SHI
JP
|
Family ID: |
40720221 |
Appl. No.: |
12/314053 |
Filed: |
December 3, 2008 |
Current U.S.
Class: |
60/276 ;
60/277 |
Current CPC
Class: |
Y02T 10/40 20130101;
F01N 2560/14 20130101; F01N 3/0864 20130101; Y02T 10/12 20130101;
F01N 2900/1624 20130101; Y02T 10/22 20130101; F01N 2550/02
20130101; F01N 2560/025 20130101; F01N 13/009 20140601; F01N 3/101
20130101; F01N 11/007 20130101; F01N 2900/102 20130101; Y02T 10/47
20130101 |
Class at
Publication: |
60/276 ;
60/277 |
International
Class: |
F01N 11/00 20060101
F01N011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 5, 2007 |
JP |
2007-314921 |
Claims
1. A catalyst degradation diagnosis device for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst and the downstream catalyst, comprising: a
measurement device that measures an oxygen storage capacity of each
of the upstream catalyst and the downstream catalyst; and a sulfur
poisoning detection device that detects sulfur poisoning of the
upstream catalyst and the downstream catalyst based on an oxygen
storage capacity change amount of the upstream catalyst from an
oxygen storage capacity of the upstream catalyst preceding a
previous oxygen storage capacity of the upstream catalyst to the
previous oxygen storage capacity, an oxygen storage capacity change
amount of the downstream catalyst from an oxygen storage capacity
of the downstream catalyst preceding a previous oxygen storage
capacity of the downstream catalyst to the previous oxygen storage
capacity, and an oxygen storage capacity change amount of the
downstream catalyst from the previous oxygen storage capacity to a
present oxygen storage capacity of the downstream catalyst, when
the oxygen storage capacity of each of the upstream catalyst and
the downstream catalyst preceding the previous oxygen storage
capacity and the previous oxygen storage capacity of each of the
upstream catalyst and the downstream catalyst have been measured by
the measurement device, and the present oxygen storage capacity of
the downstream catalyst has been measured by the measurement
device.
2. The catalyst degradation diagnosis device according to claim 1,
further comprising: a determination device that determines the
presence/absence of degradation of the upstream catalyst and the
downstream catalyst based on the oxygen storage capacity measured
by the measurement device; and a determination prohibition device
that prohibits determination performed by the determination device
when the sulfur poisoning of the upstream catalyst and the
downstream catalyst is detected by the sulfur poisoning detection
device.
3. The catalyst degradation diagnosis device according to claim 2,
wherein the oxygen storage capacity preceding the previous oxygen
storage capacity and the previous oxygen storage capacity of each
of the upstream catalyst and the downstream catalyst are a value
measured during a trip preceding a previous trip and a value
measured during the previous trip, respectively, which are before
and after refueling.
4. The catalyst degradation diagnosis device according to claim 1,
wherein the oxygen storage capacity preceding the previous oxygen
storage capacity and the previous oxygen storage capacity of each
of the upstream catalyst and the downstream catalyst are a value
measured during a trip preceding a previous trip and a value
measured during the previous trip, respectively, which are before
and after refueling.
5. A catalyst degradation diagnosis device for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst and the downstream catalyst, comprising: a
measurement device that measures an oxygen storage capacity of each
of the upstream catalyst and the downstream catalyst; and a sulfur
poisoning detection device that detects sulfur poisoning of the
upstream catalyst and the downstream catalyst based on an oxygen
storage capacity change amount of the upstream catalyst from a
previous oxygen storage capacity of the upstream catalyst to a
present oxygen storage capacity of the upstream catalyst, and an
oxygen storage capacity change amount of the downstream catalyst
from a previous oxygen storage capacity of the downstream catalyst
to a present oxygen storage capacity of the downstream catalyst,
when the previous oxygen storage capacity and the present oxygen
storage capacity of each of the upstream catalyst and the
downstream catalyst have been measured by the measurement
device.
6. The catalyst degradation diagnosis device according to claim 5,
further comprising: a determination device that determines the
presence/absence of degradation of the upstream catalyst and the
downstream catalyst based on the oxygen storage capacity measured
by the measurement device; and a determination prohibition device
that prohibits determination performed by the determination device
when the sulfur poisoning of the upstream catalyst and the
downstream catalyst is detected by the sulfur poisoning detection
device.
7. A catalyst degradation diagnosis device for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst, comprising: a measurement device that
measures an oxygen storage capacity of the upstream catalyst; and a
sulfur poisoning detection device that detects sulfur poisoning of
the upstream catalyst based on an oxygen storage capacity change
amount of the upstream catalyst from a previous oxygen storage
capacity to a present oxygen storage capacity, when the previous
oxygen storage capacity and the present oxygen storage capacity
have been measured by the measurement device.
8. The catalyst degradation diagnosis device according to claim 7,
further comprising: a determination device that determines the
presence/absence of degradation of the upstream catalyst based on
the oxygen storage capacity measured by the measurement device; and
a determination prohibition device that prohibits determination
performed by the determination device when the sulfur poisoning of
the upstream catalyst is detected by the sulfur poisoning detection
device.
9. A catalyst degradation diagnosis method for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst and the downstream catalyst, comprising:
measuring an oxygen storage capacity of each of the upstream
catalyst and the downstream catalyst; and detecting sulfur
poisoning of the upstream catalyst and the downstream catalyst
based on an oxygen storage capacity change amount of the upstream
catalyst from an oxygen storage capacity of the upstream catalyst
preceding a previous oxygen storage capacity of the upstream
catalyst to the previous oxygen storage capacity, an oxygen storage
capacity change amount of the downstream catalyst from an oxygen
storage capacity of the downstream catalyst preceding a previous
oxygen storage capacity of the downstream catalyst to the previous
oxygen storage capacity, and an oxygen storage capacity change
amount of the downstream catalyst from the previous oxygen storage
capacity to a present oxygen storage capacity of the downstream
catalyst, when the oxygen storage capacity of each of the upstream
catalyst and the downstream catalyst preceding the previous oxygen
storage capacity and the previous oxygen storage capacity of each
of the upstream catalyst and the downstream catalyst have been
measured, and the present oxygen storage capacity of the downstream
catalyst has been measured.
10. The catalyst degradation diagnosis method according to claim 9,
further comprising: determining the presence/absence of degradation
of the upstream catalyst and the downstream catalyst based on the
oxygen storage capacity measured; and prohibiting determination
regarding the degradation when the sulfur poisoning of the upstream
catalyst and the downstream catalyst is detected.
11. The catalyst degradation diagnosis method according to claim
10, wherein the oxygen storage capacity preceding the previous
oxygen storage capacity and the previous oxygen storage capacity of
each of the upstream catalyst and the downstream catalyst are a
value measured during a trip preceding a previous trip and a value
measured during the previous trip, respectively, which are before
and after refueling.
12. The catalyst degradation diagnosis method according to claim 9,
wherein the oxygen storage capacity preceding the previous oxygen
storage capacity and the previous oxygen storage capacity of each
of the upstream catalyst and the downstream catalyst are a value
measured during a trip preceding a previous trip and a value
measured during the previous trip, respectively, which are before
and after refueling.
13. A catalyst degradation diagnosis method for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst and the downstream catalyst, comprising:
measuring an oxygen storage capacity of each of the upstream
catalyst and the downstream catalyst; and detecting sulfur
poisoning of the upstream catalyst and the downstream catalyst
based on an oxygen storage capacity change amount of the upstream
catalyst from a previous oxygen storage capacity of the upstream
catalyst to a present oxygen storage capacity of the upstream
catalyst, and an oxygen storage capacity change amount of the
downstream catalyst from a previous oxygen storage capacity of the
downstream catalyst to a present oxygen storage capacity of the
downstream catalyst, when the previous oxygen storage capacity and
the present oxygen storage capacity of each of the upstream
catalyst and the downstream catalyst have been measured.
14. The catalyst degradation diagnosis method according to claim
13, further comprising, determining the presence/absence of
degradation of tile upstream catalyst and the downstream catalyst
based on the oxygen storage capacity measured; and prohibiting
determination regarding the degradation when the sulfur poisoning
of the upstream catalyst and the downstream catalyst is
detected.
15. A catalyst degradation diagnosis method for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst, comprising: measuring an oxygen storage
capacity of the upstream catalyst; and detecting sulfur poisoning
of the upstream catalyst based on an oxygen storage capacity change
amount of the upstream catalyst from a previous oxygen storage
capacity to a present oxygen storage capacity, when the previous
oxygen storage capacity and the present oxygen storage capacity
have been measured.
16. The catalyst degradation diagnosis method according to claim
15, further comprising: determining the presence/absence of
degradation of the upstream catalyst based on the oxygen storage
capacity; and prohibiting determination regarding the degradation
when the sulfur poisoning of the upstream catalyst is detected.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2007-314921 filed on Dec. 5, 2007 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a device that diagnoses the
presence/absence of degradation of a catalyst disposed in an
exhaust passageway of an internal combustion engine, and to a
method for such diagnosis.
[0004] 2. Description of the Related Art
[0005] For example, in internal combustion engines for vehicles,
the exhaust system is provided with a catalyst for purifying
exhaust gas. Among such catalysts, there is a catalyst that has an
oxygen storage capability (O.sub.2 storage capability). This type
of catalyst adsorbs and retains excess oxygen present in exhaust
gas when the air-fuel ratio of exhaust gas flowing into the
catalyst becomes larger than a stoichiometric air-fuel ratio
(stoichiometric value), that is, becomes lean in fuel, and then
releases the adsorbed and retained oxygen when the air-fuel ratio
of the exhaust gas flowing into the catalyst becomes smaller than
the stoichiometric value, that is, becomes rich in fuel. For
example, in a gasoline engine, the air-fuel ratio control is
performed so that the exhaust gas flowing into the catalyst has an
air-fuel ratio in the vicinity of the stoichiometric value. If a
three-way catalyst having an oxygen storage capability is used,
deviations of the actual air-fuel ratio from the stoichiometric
value, which can occur depending on the operation condition, can be
absorbed to some extent due to the oxygen storage/release action of
the three-way catalyst.
[0006] If the catalyst degrades, the exhaust gas purification
efficiency of the catalyst declines. The degree of degradation of
the catalyst and the degree of decline of the oxygen storage
capability of the catalyst have a correlation since both the
degradation of the catalyst and the decline of the oxygen storage
capability of the catalyst are results of reactions via noble
metals. Therefore, degradation of the catalyst can be detected by
detecting a decline of the oxygen storage capability. In a method
generally employed to this end, an active air-fuel ratio control of
forcing a switch between a rich state and a lean state of the
air-fuel ratio of exhaust gas flowing into the catalyst is
performed. In conjunction with the execution of the active air-fuel
ratio control, the oxygen storage capacity of the catalyst is
measured in order to diagnose the presence/absence of degradation
of the catalyst.
[0007] Depending on the area of use of the vehicle, the fuel can
contain a relative high concentration of sulfur (S). In the case
where such a fuel is fed into the vehicle, there occurs a poisoning
(sulfur poisoning) in which the performance of the catalyst
declines due to the effect of sulfur components in exhaust gas. If
the sulfur poisoning occurs, the oxygen storage/release action of
the catalyst is impeded, so that the oxygen storage capacity of the
catalyst declines. However, if a fuel low in the sulfur
concentration is fed into the vehicle again, the poisoned state
will soon be dissolved. The performance decline of the catalyst due
to the sulfur poisoning is temporary and recoverable. In the
diagnosis regarding degradation of the catalyst, it is necessary to
avoid falsely diagnosing a temporary degradation caused by the
sulfur poisoning as an unrecoverable permanent degradation (thermal
degradation), which the diagnosis is intended to find. In
particular, it is necessary to avoid false diagnosis as degradation
with regard to the catalyst that is still normal while being in the
proximity of the border (criteria) between normality and
degradation.
[0008] As a countermeasure against this, it is conceivable to
perform a process in which when the catalyst is sulfur-poisoned,
the air-fuel ratio is transitionally corrected so as to desorb
sulfur, and after that, the diagnosis regarding degradation is
performed. This forced desorption of sulfur in this manner has a
problem of further degrading the catalyst.
[0009] According to vigorous studies of the present inventors, it
has been found that in the case where an upstream catalyst and a
downstream catalyst are disposed in the exhaust passageway of the
internal combustion engine, the manner of the sulfur poisoning is
different between the upstream catalyst and the downstream
catalyst. Therefore, if the sulfur poisoning of at least one of the
upstream and downstream catalysts can be detected by utilizing the
foregoing, finding, the misdiagnosis as mentioned above can be
prevented, and advantage can be achieved in the improvement of the
diagnosis accuracy and reliability.
SUMMARY OF THE INVENTION
[0010] In view of the foregoing circumstances, the invention has
been accomplished, and provides a catalyst degradation diagnosis
device and a catalyst degradation diagnosis method for an internal
combustion engine equipped with an upstream catalyst and a downs
catalyst disposed in an exhaust passageway of the internal
combustion engine which can detect the sulfur poisoning of at least
one of the two catalysts and improve the diagnosis accuracy and
reliability.
[0011] According to one aspect of the invention, there is provided
a catalyst degradation diagnosis device for an internal combustion
engine equipped with an upstream catalyst and a downstream catalyst
in an exhaust passageway of the internal combustion engine which
diagnoses presence/absence of degradation of the upstream catalyst
and the downstream catalyst, the device being characterized by
including: a measurement device that measures an oxygen storage
capacity of each of the upstream catalyst and the downstream
catalyst; and a sulfur poisoning detection device that detects
sulfur poisoning of the upstream catalyst and the downstream
catalyst based on an oxygen storage capacity change amount of the
upstream catalyst from an oxygen storage capacity of the upstream
catalyst preceding a previous oxygen storage capacity of the
upstream catalyst to the previous oxygen storage capacity, an
oxygen storage capacity change amount of the downstream catalyst
from an oxygen storage capacity of the downstream catalyst
preceding a previous oxygen storage capacity of the downstream
catalyst to the previous oxygen storage capacity, and an oxygen
storage capacity change amount of the downstream catalyst from the
previous oxygen storage capacity to a present oxygen storage
capacity of the downstream catalyst, when the oxygen storage
capacity of each of the upstream catalyst and the downstream
catalyst preceding the previous oxygen storage capacity and the
previous oxygen storage capacity of each of the upstream catalyst
and the downstream catalyst have been measured by the measurement
device, and the present oxygen storage capacity of the downstream
catalyst has been measured by the measurement device.
[0012] Besides, according to another aspect of the invention, there
is provided a catalyst degradation diagnosis method for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst and the downstream catalyst. This
diagnosis method includes the steps of: measuring an oxygen storage
capacity of each of the upstream catalyst and the downstream
catalyst; and detecting sulfur poisoning of the upstream catalyst
and the downstream catalyst based on an oxygen storage capacity
change amount of the upstream catalyst from an oxygen storage
capacity of the upstream catalyst preceding a previous oxygen
storage capacity of the upstream catalyst to the previous oxygen
storage capacity, an oxygen storage capacity change amount of the
downstream catalyst from an oxygen storage capacity of the
downstream catalyst preceding a previous oxygen storage capacity of
the downstream catalyst to the previous oxygen storage capacity,
and an oxygen storage capacity change amount of the downstream
catalyst from the previous oxygen storage capacity to a present
oxygen storage capacity of the downstream catalyst, when the oxygen
storage capacity of each of the upstream catalyst and the
downstream catalyst preceding the previous oxygen storage capacity
and the previous oxygen storage capacity of each of the upstream
catalyst and the downstream catalyst have been measured, and the
present oxygen storage capacity of the downstream catalyst has been
measured.
[0013] As a result of vigorous study by the present inventors, it
turned out that the decline of the oxygen storage capacity of a
catalyst caused by the sulfur component in fuel is attributed to
the following two phenomena. In a first phenomenon, SOx in exhaust
gas reacts with the catalyst component supported in a catalyst,
that is, a noble metal, and therefore lowers the reaction rate of
the noble metal, whereby the oxygen storage capacity of the
catalyst is lowered. In a second phenomenon, SOx in exhaust gas
adsorbs to a storage component of the catalyst, whereby the oxygen
storage capacity of the catalyst is lowered. In the case of the
upstream catalyst, the catalyst temperature thereof is higher than
that of the downstream catalyst Therefore, the second phenomenon,
that is, the sulfur adsorption onto the storage component, is
difficult to occur, and the decline of the oxygen storage capacity
of the upstream catalyst is predominantly caused by the first
phenomenon, that is, the decline of the reaction rate of the noble
metal. The decline of the oxygen storage capacity occurs
immediately after the fuel is changed from a low-sulfur fuel to a
high-sulfur fuel (i.e., immediately after the refueling with a
high-sulfur fuel). On the other hand, in the case of the downstream
catalyst, the first phenomenon and the second phenomenon occur so
that the oxygen storage capacity declines, and the decline of the
oxygen storage capacity gradually progresses after the change of
the fuel. Thus, the manners of sulfur poisoning of the two
catalysts are different from each other.
[0014] Considering the difference, the invention can suitably
detect the sulfur poisoning of the upstream catalyst and the
downstream catalyst. For example, the oxygen storage capacity
change amount of the upstream catalyst from the measurement of
oxygen storage capacity preceding the previous one to the previous
measurement, and the oxygen storage capacity change amount of the
downstream catalyst from the measurement preceding the previous one
to the previous measurement are larger than predetermined values,
it is tentatively estimated that there is a possibility of sulfur
poisoning. Then, if the oxygen storage capacity change amount of
the downstream catalyst from the previous measurement to the
present measurement is larger than a predetermined value, it is
finally detected that the two catalyst have sulfur poisoning. As
for the upstream catalyst, the second phenomenon is difficult to
occur, and therefore, if the oxygen storage capacity declined in
the period during the measurement preceding the previous one to the
previous measurement, the oxygen storage capacity does not exhibit
a large change after that, that is, in the present measurement. On
the other hand, as for the downstream catalyst, the second
phenomenon occurs as well as the first phenomenon, and therefore,
the oxygen storage capacity, after declining, does not stop
declining but further declines. Utilizing this difference, the
sulfur poisoning of a catalyst can be accurately detected, and the
diagnosis accuracy and reliability can be improved.
[0015] Besides, it is also preferable to determine the
presence/absence of degradation of the upstream catalyst and the
downstream catalyst based on the oxygen storage capacity measured,
and prohibit determination regarding the degradation when the su
poisoning of the upstream catalyst and the downstream catalyst is
detected. This makes it possible to prevent false determination of
the presence of degradation of a catalyst based on a measured
oxygen storage capacity value that has declined due to sulfur
poisoning.
[0016] Besides, it is also preferable that the oxygen storage
capacity preceding the previous oxygen storage capacity and the
previous oxygen storage capacity of each of the upstream catalyst
and the downstream catalyst be a value measured during a trip
preceding a previous trip and a value measured during the previous
trip, respectively, which are before and after refueling. Since the
decline of the oxygen storage capacity due to sulfur poisoning
occurs immediately after the fuel is changed, that is, after
refueling, it is preferable to check a change in the oxygen storage
capacity between before and after refueling.
[0017] Besides, according to still another aspect of the invention,
there is provided a catalyst degradation diagnosis device for an
internal combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst and the downstream catalyst, the device
being characterized by including: a measurement device that
measures an oxygen storage capacity of each of the upstream
catalyst and the downstream catalyst; and a sulfur poisoning
detection device that detects sulfur poisoning of the upstream
catalyst and the downstream catalyst based on an oxygen storage
capacity change amount of the upstream catalyst from a previous
oxygen storage capacity of the upstream catalyst to a present
oxygen storage capacity of the upstream catalyst, and an oxygen
storage capacity change amount of the downstream catalyst from a
previous oxygen storage capacity of the downstream catalyst to a
present oxygen storage capacity of the downstream catalyst, when
the previous oxygen storage capacity and the present oxygen storage
capacity of each of the upstream catalyst and the downstream
catalyst have been measured by the measurement device.
[0018] Besides, according to yet still another aspect of the
invention, there is provided a catalyst degradation diagnosis
method for an internal combustion engine equipped with an upstream
catalyst and a downstream catalyst in an exhaust passageway of the
internal combustion engine which diagnoses presence/absence of
degradation of the upstream catalyst and the downstream catalyst.
This diagnosis method includes the steps of: measuring an oxygen
storage capacity of each of the upstream catalyst and the
downstream catalyst; and detecting sulfur poisoning of the upstream
catalyst and the downstream catalyst based on an oxygen storage
capacity change amount of the upstream catalyst from a previous
oxygen storage capacity of the upstream catalyst to a present
oxygen storage capacity of the upstream catalyst, and an oxygen
storage capacity change amount of the downstream catalyst from a
previous oxygen storage capacity of the downstream catalyst to a
present oxygen storage capacity of the downstream catalyst, when
the previous oxygen storage capacity and the present oxygen storage
capacity of each of the upstream catalyst and the downstream
catalyst have been measured.
[0019] If sulfur poisoning occurs on the upstream catalyst and the
downstream catalyst, the oxygen storage capacities of the two
catalysts measured first following change of the fuel decline
greatly. Hence, the sulfur poisoning of the upstream catalyst and
the downstream catalyst can also be detected on the basis of the
oxygen storage capacity change amount of the upstream catalyst from
the previous measurement to the present measurement and the oxygen
storage capacity change amount of the downstream catalyst from the
previous measurement to the present measurement.
[0020] According to a further aspect of the invention, there is
provided a catalyst degradation diagnosis device for an internal
combustion engine equipped with an upstream catalyst and a
downstream catalyst in an exhaust passageway of the internal
combustion engine which diagnoses presence/absence of degradation
of the upstream catalyst, the device being characterized by
including: a measurement device that measures an oxygen storage
capacity of the upstream catalyst; and a sulfur poisoning detection
device that detects sulfur poisoning of the upstream catalyst based
on an oxygen storage capacity change amount of the upstream
catalyst from a previous oxygen storage capacity to a present
oxygen storage capacity, when the previous oxygen storage capacity
and the present oxygen storage capacity have been measured by the
measurement device.
[0021] Besides, according to a still further aspect of the
invention, there is provided a catalyst degradation diagnosis
method for an internal combustion engine equipped with an upstream
catalyst and a downstream catalyst in an exhaust passageway of the
internal combustion engine which diagnoses presence/absence of
degradation of the upstream catalyst. This diagnosis method
includes the steps of measuring an oxygen storage capacity of the
upstream catalyst; and detecting sulfur poisoning of the upstream
catalyst based on an oxygen storage capacity change amount of the
upstream catalyst from a previous oxygen storage capacity to a
present oxygen storage capacity, when the previous oxygen storage
capacity and the present oxygen storage capacity have been
measured.
[0022] In the case where an upstream catalyst and a downstream
catalyst are disposed in the exhaust passageway of an internal
combustion engine, the refueling with a high-sulfur fuel lowers the
oxygen storage capacity, particularly, of the upstream catalyst,
immediately after the change of the fuel. Hence, utilizing this,
the sulfur poisoning of the upstream catalyst can be detected on
the basis of the oxygen storage capacity change amount of the
upstream catalyst from the previous measurement to the present
measurement.
[0023] Thus, the invention achieves an excellent advantage as
follows. That is, in the case where the exhaust passageway of an
internal combustion engine is equipped with an upstream catalyst
and a downstream catalyst, the sulfur poisoning of at least one of
the catalysts can be detected, and therefore the diagnosis accuracy
and reliability can be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The features, advantages, and technical and industrial
significance of this invention will be described in the following
detailed description of example embodiments of the invention with
reference to the accompanying drawings, in which like numerals
denote like elements, and wherein:
[0025] FIG. 1 is a schematic diagram showing a construction of an
embodiment of the invention;
[0026] FIG. 2 is a schematic sectional view showing a construction
of a catalyst shown in FIG. 1;
[0027] FIG. 3 is a time chart regarding a first mode of a catalyst
degradation diagnosis;
[0028] FIG. 4 is a time chart similar to FIG. 3 for describing a
measurement method for an oxygen storage capacity;
[0029] FIG. 5 is a time chart regarding a second mode of We
catalyst degradation diagnosis;
[0030] FIG. 6 is a graph showing a change in the oxygen storage
capacity of an upstream catalyst between before and after the
change from a fuel to another in accordance with a first
embodiment;
[0031] FIG. 7 a flowchart showing a catalyst degradation diagnosis
process in accordance with the first embodiment;
[0032] FIG. 8 is a flowchart showing another catalyst degradation
diagnosis process in accordance with the first embodiment;
[0033] FIG. 9 is a graph showing a change in the oxygen storage
capacity of the upstream catalyst between before and after the
change of a fuel to another in accordance with a second
embodiment;
[0034] FIG. 10 is a graph showing changes in the oxygen storage
capacity of a downstream catalyst in accordance with the second
embodiment;
[0035] FIG. 11 is a flowchart showing a catalyst degradation
diagnosis process in accordance with the second embodiment;
[0036] FIG. 12 is a flowchart showing a second catalyst degradation
diagnosis process in accordance with the second embodiment; and
[0037] FIG. 13 is a flowchart continuing from FIG. 12, showing the
second catalyst degradation diagnosis process in accordance with
the second embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS
[0038] Best modes for carrying out the invention will be described
hereinafter with reference to the accompanying drawings. FIG. 1 is
a schematic diagram showing a construction of an embodiment of the
invention. As shown in FIG. 1, an internal combustion engine 1
generates power by burning a mixture of fuel and air within
combustion chambers 3 formed in a cylinder block 2 and thereby
reciprocating pistons 4 within the combustion chambers 3. The
internal combustion engine 1 is a multi-cylinder engine (only one
cylinder is shown) for a vehicle, and is a spark ignition type
internal combustion engine and, more specifically, a gasoline
engine.
[0039] A cylinder head of the internal combustion engine 1 is
provided with intake valves Vi that open and close intake ports,
and exhaust valves Ve that open and close exhaust ports. The intake
valves Vi and the exhaust valves Ve are opened and closed by
camshafts (not shown). Besides, in a top portion of the cylinder
head, ignition plugs 7 for igniting mixture within combustion
chambers 3 are attached separately for each cylinder.
[0040] The intake ports of the individual cylinders are connected
to a surge tank 8 that is an intake assembly chamber, via branch
pipes that correspond to the individual cylinders. An intake pipe
13 that forms an intake assembly passageway is connected to an
upstream side of the surge tank 8. An air cleaner 9 is provided on
an upstream end of the intake pipe 13. An air flow meter 5 for
detecting the amount of intake air, and an electronically
controlled throttle valve 10 are incorporated in the intake pipe 13
in that order from the upstream side. Incidentally, the intake
ports, the surge tank 8 and the intake pipe 13 form an intake
passageway.
[0041] Injectors (fuel injection valves) 12 that inject fuel into
the intake passage, particularly, into the intake ports, are
provided separately for each cylinder. The fuel injected from each
injector 12 is mixed with intake air to form a mixture, which is in
turn taken into a corresponding one of the combustion chambers 3
when the corresponding intake valve Vi opens, and is then
compressed by the piston 4, and is ignited to burn by the ignition
plug 7.
[0042] On the other hand, the exhaust ports of the individual
cylinders are connected to an exhaust pipe 6 that forms an exhaust
assembly passageway via branch pipes of the cylinders. An upstream
catalyst 11 and a downstream catalyst 19 each of which is formed by
a three-way catalyst that has an oxygen storage capability are
attached in series to the exhaust pipe 6. Incidentally, the exhaust
ports, the branch pipes and the exhaust pipe 6 form an exhaust
passageway. The upstream catalyst 11 is attached at a position that
is relatively near to the combustion chambers 3, so as to be
activated in an early period by utilizing exhaust heat. On the
other hand, the downstream catalyst 19 is attached at a position
relatively far from the combustion chambers 3, for example, under a
floor of the vehicle, or the like.
[0043] A pre-catalyst sensor 17 is disposed at an upstream side of
the upstream catalyst 11, and an inter-catalyst sensor 18 is
disposed between the upstream catalyst 11 and the downstream
catalyst 19, and a post-catalyst sensor 21 is disposed at a
downstream side of the downstream catalyst 19. Each of the
pre-catalyst sensor 17, the inter-catalyst sensor 18 and the
post-catalyst sensor 21 is an air-fuel ratio sensor for detecting
the air-fuel ratio of exhaust gas. In particular, the pre-catalyst
sensor 17 is made up of a so-called wide-range air-fuel ratio
sensor, and is therefore able to continuously detect the air-fuel
ratio over a relatively wide range, and outputs a signal having a
value proportional to the detected air-fuel ratio. On other hands,
the inter-catalyst sensor 18 and the post-catalyst sensor 21 are
each made up of a so-called O.sub.2 sensor, and has a
characteristic in which the output value sharply changes at the
stoichiometric air-fuel ratio.
[0044] The ignition plugs 7, the throttle valve 10, the injectors
12, etc. are electrically connected to an electronic control unit
(hereinafter, abbreviated as "ECU") 20 as a control device. The ECU
20 includes a CPU, a ROM, a RAM, input/output ports, memory
devices, etc. (none of which is shown). The ECU 20 is also
electrically connected to the air flow meter 5, the pre-catalyst
sensor 17, the inter-catalyst sensor 18, and the post-catalyst
sensor 21, and furthermore to a crank angle sensor 14 that detects
the crank angle of the internal combustion engine 1, an accelerator
operation amount sensor 15 that detects the accelerator operation
amount, and other various sensors, via A/D converters or the like
(not shown). The ECU 20 controls the ignition plugs 7, the throttle
valve 10, the injectors 12, etc., and therefore controls the
ignition timing, the amount of fuel injection, the fuel injection
timing, the throttle opening degree, etc. so that a desired output
of the engine is obtained, on the basis of the detection values
from various sensors, and the like.
[0045] Each of the upstream catalyst 11 and the downstream catalyst
19 removes NOx, HCs and CO simultaneously when the air-fuel ratio
A/F of the exhaust gas that flows into the catalyst is equal to the
stoichiometric air-fuel ratio (stoichiometric value, for example,
A/Fs=14.6). In response to this, the ECU 20 controls the air-fuel
ratio of mixture so that during usual operation of the internal
combustion engine, the air-fuel ratio of the mixture discharged
from the combustion chambers 3 which flows into the upstream
catalyst 11, that is, the pre-catalyst air-fuel ratio A/Ffr,
becomes equal to the stoichiometric air-fuel ratio. Concretely, the
ECU 20 sets a target air-fuel ratio A/Ft equal to the
stoichiometric air-fuel ratio, and feedback-controls the amount of
fuel injection injected from the injectors 12 so that the
pre-catalyst air-fuel ratio A/Ffr detected by -the pre-catalyst
sensor 17 becomes equal to the target air-fuel ratio A/Ft.
Therefore, the air-fuel ratio of the exhaust gas that flows into
the catalyst 11 is kept in the vicinity of the stoichiometric
air-fuel ratio, so that the catalyst 11 delivers its maximum
purification performance. In this air-fuel ratio feedback control,
a main feedback control of making the pre-catalyst air-fuel ratio
A/Ffr equal to the stoichiometric air-fuel ratio, and a subsidiary
feedback control of making the inter-catalyst air-fuel ratio A/Fmd
detected by the inter-catalyst sensor 18 equal to the
stoichiometric air-fuel ratio are performed. The subsidiary
feedback control is performed for the purpose of eliminating the
deviation of the center air-fuel ratio caused by degradation of the
pre-catalyst sensor 17 or the like.
[0046] Herein, the upstream catalyst 11 and the downstream catalyst
19, which are the objects of the degradation diagnosis, will be
described in more detail. Since the upstream catalyst 11 and the
downstream catalyst 19 have the same construction, the upstream
catalyst 11 will be taken as an example in the description below.
As shown in FIG. 2, in the catalyst 11, a surface of a support base
(not shown) is coated with a coating material 31, and a catalyst
component 32 in a fine powder state is retained in a large quantity
on the coating material 31 in a dispersed arrangement, and is
exposed within the catalyst 11. The catalyst component 32 is mainly
made up of a noble metal, such as Pt, Pd, etc., and serves as
active sites for reactions of exhaust gas components, such as NOx,
HCs and CO. On the other hand, the coating material 31 contains an
oxygen storage component that plays a role of a promoter that
accelerates the reactions on the interface between exhaust gas and
the catalyst component 32, and that is capable of absorbing and
releasing oxygen according to the air-fuel ratio of the atmosphere
gas. The oxygen storage component is made up of, for example,
cerium dioxide CeO.sub.2, or zirconia. For example, if the
atmosphere gas around the catalyst component 32 and the coating
material 31 is richer than the stoichiometric air-fuel ratio,
oxygen stored in the oxygen storage component present around the
catalyst component 32 is released therefrom. As a result, the
released oxygen oxidizes unburnt components, such as HCs and CO,
thus removing the components. Conversely, if the atmosphere gas
around the catalyst component 32 and the coating material 31 is
leaner than the stoichiometric air-fuel ratio, the oxygen storage
component present around the catalyst component 32 absorbs oxygen
from the atmosphere gas, resulting in the reductive removal of
NOx.
[0047] Due to this oxygen storage/release action, the three exhaust
gas components, that is, NOx, HCs and CO, can be simultaneously
removed although the exhaust air-fuel ratio may fluctuate to some
extent about the stoichiometric air-fuel ratio during an ordinary
air-fuel ratio control. Therefore, it is also possible to perform
exhaust gas purification by intentionally oscillating the
pre-catalyst air-fuel ratio A/Ffr to small extents about the
stoichiometric air-fuel ratio and therefore repeatedly performing
the storage and the release of oxygen. It is to be understood that
"storage" used herein means retention of a substance (solid,
liquid, gas, molecules) in the form of at least one of adsorption,
adhesion, absorption, trapping, occlusion, and others.
[0048] By the way, in the catalyst 11 in a brand-new state, the
catalyst component 32 in a fine particle state as described above
is uniformly arranged in a large quantity in a dispersed fashion,
so that a state of high probability of contact between exhaust gas
and the catalyst component 32 is maintained. However, as the
catalyst 11 degrades over time due to thermal stress, disappearance
of a portion of the catalyst component 32 is observed, and some
particles of the catalyst component 32 are thermally solidified by
exhaust heat, thus entering a sintered state (see broken lines in
FIG. 2). This brings about a decline in the probability of contact
between exhaust gas and the catalyst component 32, and becomes a
cause of lowering the purification rate. Besides this, the amount
of the coating material 31 present around the catalyst component
32, that is, the amount of the oxygen storage component, decreases,
and the oxygen storage capability declines.
[0049] The degree of degradation of the catalyst 11 and the degree
of decline of the oxygen storage capability of the catalyst 11 are
in a correlation. Therefore, in this embodiment, the degree of
degradation of the catalyst 11 is detected by detecting the oxygen
storage capability of the catalyst 11. Incidentally, the oxygen
storage capability of the catalyst 11 is represented by the
magnitude of the oxygen storage capacity (OSC, that is, the O.sub.2
storage capacity in the unit of g) that is a maximum amount of
oxygen that can be stored by the catalyst 11 as it is at the
present time.
[0050] Hereinafter, the catalyst degradation diagnosis in this
embodiment will be described. Incidentally, for the sake of
convenience, a mode of the degradation diagnosis (first mode) in
which the diagnosis is limited to the upstream catalyst 11, which
has a great influence on the emission quality, will firstly be
described, and a mode of the degradation diagnosis (second mode) in
which the downstream catalyst 19 is also included as an object of
the diagnosis will be described later.
[0051] The catalyst degradation diagnosis of this embodiment is
basically a diagnosis based on the foregoing Cmax method. At the
time of the degradation diagnosis of the catalyst 11, the ECU 20
executes an active air-fuel ratio control In the active air-fuel
ratio control, the air-fuel ratio of mixture and, therefore, the
pre-catalyst air-fuel ratio A/Ffr is alternately switched between
the rich side and the lean side of a predetermined center air-fuel
ratio A/Fc in a forced fashion (actively). Incidentally, the
air-fuel ratio given by a change to the fuel-rich side will be
termed the rich air-fuel ratio A/Fr, and the air-fuel ratio given
by a change to the fuel-lean side will be termed the lean air-fuel
ratio A/Fl. When the pre-catalyst air-fuel ratio A/Ffr is changed
to the rich side or the lean side by the active air-fuel ratio
control, the oxygen storage capacity OSC of the catalyst 11 is
measured.
[0052] The degradation diagnosis of the catalyst 11 is executed
when the internal combustion engine 1 is in a steady operation and
the catalyst 11 is within an activation temperature range. As for
the measurement of the temperature of the catalyst 11 (catalyst bed
temperature), a temperature sensor may be used for direct detection
In this embodiment, however, the temperature of the catalyst 11 is
estimated from the state of operation of the internal combustion
engine. For example, the ECU 20 estimates the temperature Tc of the
catalyst 11, utilizing a pre-set map, on the basis of the intake
air amount Ga detected by the air flow meter 5. Incidentally, the
parameters used to estimate the catalyst temperature may also
include parameters other than the intake air amount Ga, for
example, the engine rotation speed Ne (rpm), etc.
[0053] FIGS. 3A and 3B show the output from the pre-catalyst sensor
17 and the output of the inter-catalyst sensor. 18 at the time of
execution of the active air-fuel ratio control, respectively, by
solid lines. Besides, in FIG. 3A, the target air-fuel ratio A/Ft
generated within the ECU 20 is shown by a broken line. The output
values of the pre-catalyst sensor 17 and the inter-catalyst sensor
18 are correspond to values of the pre-catalyst air-fuel ratio
A/Ffr and the inter-catalyst air-fuel ratio A/Fmd,
respectively.
[0054] As shown in FIG. 3A, the target air-fuel ratio A/Ft is
forced to alternately switch between an air-fuel ratio (rich
air-fuel ratio A/Fr) that is apart from the stoichiometric air-fuel
ratio (stoichiometric value) A/Fs serving as a center air-file
ratio to the rich side by a predetermined amplitude (rich-side
amplitude Ar, where Ar>0) and an air-fuel ratio (lean air-fuel
ratio A/Fl) that is apart from the stoichiometric air-fuel ratio to
the lean side by a predetermined amplitude (lean-side amplitude Al,
where Al>0). Then, following the switching of the target
air-fuel ratio A/Ft, the pre-catalyst air-fuel ratio A/Ffr as the
actual value switches with a small time delay relative to the
target air-fuel ratio A/Ft. From this, it can be understood that
the target air-fuel ratio A/Ft and the pre-catalyst air-fuel ratio
A/Ffr are equivalent to each other, except that there is a time
delay therebetween.
[0055] In the example shown in FIGS. 3A and 3B, the rich-side
amplitude Ar and the lean-side amplitude Al are substantially
equal. For example, the stoichiometric air-fuel ratio A/Fs=14.6,
the rich air-fuel ratio A/Fr=14.1, the lean air-fuel ratio
A/Fl=15.1, and the rich-side amplitude Ar=the lean-side amplitude
Al=0.5. Compared with the case of usual air-fuel ratio control, the
amplitude of the air-fuel ratio in the case of the active air-fuel
ratio control is large, that is, the values of the lean-side
amplitude Al in the control are large.
[0056] Incidentally, the timing at which the target air-fuel ratio
A/Ft is switched is a timing at which the output of the
inter-catalyst sensor 18 switched from the rich side to the lean
side or from the lean side to the rich side. As shown in the
diagrams, the output voltage of the inter-catalyst sensor 18
sharply changes at the stoichiometric air-fuel ratio A/Fs. That is,
when the inter-catalyst air-fuel ratio A/Fmd is smaller than the
stoichiometric air-fuel ratio A/Fs, that is, on the rich side
thereof, the output voltage of the inter-catalyst sensor 18 becomes
equal to or greater than a rich-side criterion value VR. When the
inter-catalyst air-fuel ratio A/Fmd is larger than the
stoichiometric air-fuel ratio A/Fs, that is, on the lean side
thereof, the output voltage of the inter-catalyst sensor 18 becomes
less than or equal to a lean-side criterion value VL. Herein,
VR>VL, for example, VR=0.59 (V), and VL=0.21 (V).
[0057] As shown in FIGS. 3A and 3B, when the output voltage of the
inter-catalyst sensor 18 changes from a value on the rich side to
the lean side to become equal to the lean-side criterion value VL
(time t1), the target air-fuel ratio A/Ft is switched from the lean
air-fuel ratio A/Fl to the rich air-fuel ratio A/Fr. After that,
when the output voltage of the inter-catalyst sensor 18 changes
from a value on the lean side to the rich side to become equal to
the rich-side criterion value VR (time t2), the target air-fuel
ratio A/Ft is switched from the rich air-fuel ratio A/Fr to the
lean air-fuel ratio A/Fl.
[0058] While the active air-fuel ratio control of performing the
changing of the air-fuel ratio as described above is executed, the
oxygen storage capacity OSC of the catalyst 11 is measured in the
following manner to determine whether or not the catalyst 11 has
degraded.
[0059] Referring to FIGS. 3A and 3B, before time t1, the target
air-fuel ratio A/Ft is set at the lean air-fuel ratio A/Fl, and a
fuel-lean gas flows into the catalyst 11. At this time, the
catalyst 11 continues absorbing oxygen. However, when oxygen is
absorbed to the fill capacity, no more oxygen can be absorbed, so
that the lean gas passes through the catalyst 11, and flows out at
the downstream side of the catalyst 11. If this happens, the
inter-catalyst air-fuel ratio A/Fmd changes to the lean side. At
the time point (t1) at which the output voltage of the
inter-catalyst sensor 18 reaches the lean-side criterion value VL,
the target air-fuel ratio A/Ft is switched to the rich air-fuel
ratio A/Fr, or is inverted. In this manner, the target air-fuel
ratio A/Ft is inverted, with the output of the inter-catalyst
sensor 18 serving as a trigger.
[0060] Then, a rich gas flows into the catalyst 11. At this time,
the catalyst 11 continues releasing the oxygen that has been
stored. Therefore, exhaust gas of substantially the stoichiometric
air-fuel ratio A/Fs flows out at the downstream side of the
catalyst 11, so that the inter-catalyst air-fuel ratio A/Fmd does
not become fuel-rich and therefore the output of the inter-catalyst
sensor 18 does not become inverted. As oxygen continues to be
released from the catalyst 11, the oxygen stored in the catalyst 11
is entirely released. From that time point on, no more oxygen can
be released, and therefore fuel-rich gas passes through the
catalyst 11, and flows out at the downstream side of the catalyst
11. Hence, the inter-catalyst air-fuel ratio A/Fmd changes to the
rich side. At the time point (t2) at which the output voltage of
the inter-catalyst sensor 18 reaches the rich-side criterion value
VR, the target air-fuel ratio A/Ft is switched to the lean air-fuel
ratio A/Fl.
[0061] The greater the oxygen storage capacity OSC, the longer the
time during which oxygen can continue to be absorbed or released.
That is, in the case where the catalyst has degraded, the inversion
cycle of the target air-fuel ratio A/Ft (e.g., the time from t1 to
t2) becomes longer. As the degradation of the catalyst advances,
the inversion cycle of the target air-fuel ratio A/Ft becomes
shorter.
[0062] Therefore, utilizing this, the oxygen storage capacity OSC
is measured as follows. As shown in FIGS. 4A and 4B, immediately
after the target air-fuel ratio A/Ft is switched to the rich
air-fuel ratio A/Fr as the time t1, the pre-catalyst air-fuel ratio
A/Ffr as the actual value switches to the rich air-fuel ratio A/Fr
with a slight delay. From the time point t11 at which the
pre-catalyst air-fuel ratio A/Ffr reaches the stoichiometric
air-fuel ratio A/Fs till the time point t2 at which the target
air-fuel ratio A/Ft becomes inverted, the oxygen storage capacity
dOSC (the instantaneous value of the oxygen storage capacity) at
every predetermined small amount of time is calculated, and the
oxygen storage capacity dOSC at every predetermined small amount of
time is integrated from the time t11 to the time t2. In this
manner, the oxygen storage capacity, that is, the released amount
of oxygen (OSC(1) in FIG; 4A), in the present oxygen release cycle
is measured.
dOSC=.DELTA.A/F.times.Q.times.K=|A/Ffr-A/Fs|.times.Q.times.K
(1)
[0063] In the equation, Q is the amount of fuel injection. The
short fall or excess of air with respect to the stoichiometric
value can be calculated by multiplying the air-fuel ratio
difference .DELTA.A/F by the fuel injection amount Q. In addition,
K is a constant that represents the proportion of the oxygen (about
0.23) contained in air.
[0064] Basically, the oxygen storage capacity OSC obtained by one
measurement process is used, that is, compared with a predetermined
degradation criterion value OSCs in order to determine whether or
not the catalyst has degraded. That is, if the measured oxygen
storage capacity OSC is greater than the degradation criterion
value OSCs, it is determined that the catalyst is normal. If oxygen
storage capacity OSC is less than or equal to the degradation
criterion value OSCs, it is determined that the catalyst has
degraded. However, in order to improve accuracy in this embodiment,
the oxygen storage capacity (amount of oxygen stored in this case)
is also measured in the oxygen storage cycle during which the
target air-fuel ratio A/Ft is on the lean side, and an average
value of these oxygen storage capacities is measured as one unit of
oxygen storage capacity in accordance with one storage-release
cycle. Furthermore, the storage-release cycle is repeated a
plurality of times to obtain values of a plurality of units of
oxygen storage capacity, and the average value of the obtained
values is used as a final measured oxygen storage capacity
value.
[0065] As for the measurement of the oxygen storage capacity
(stored oxygen amount) in the oxygen storage cycle, after the
target air-fuel ratio A/Ft is switched to the lean air-fuel ratio
A/Fl at a time t2 as shown in FIG. 4, an oxygen storage capacity
dOSC at every predetermined small amount of time is calculated
using the foregoing equation (1), and the oxygen storage capacity
dOSC at every predetermined small amount of time is integrated,
during a period from a time point t21 at which the pre-catalyst
air-fuel ratio A/Ffr reaches the stoichiometric air-fuel ratio A/Fs
to a time point t3 at which the target air-fuel ratio A/Ft is next
inverted to the rich side. In this manner, the oxygen storage
capacity OSC in the oxygen storage cycle, that is, the stored
oxygen amount (OSC(2) in FIG. 4), is measured. The oxygen storage
capacity OSC1 measured during the previous cycle and the oxygen
storage capacity OSC2 measured during the present cycle should
become substantially equal values.
[0066] Next, using the measured value of the oxygen storage
capacity, determination regarding the degradation of the catalyst
is performed. That is, the measured value of the oxygen storage
capacity OSC is compared with a predetermined degradation criterion
value OSCs. If the value of the oxygen storage capacity OSC is
greater than the degradation criterion value OSCs, it is determined
that the catalyst is normal. If the value of oxygen storage
capacity OSC is less than or equal to the degradation criterion
value OSCs, it is determined that the catalyst has degraded. In
addition, in the case where it is determined that the catalyst has
degraded, it is preferable to activate a warning device, such as a
check lamp, in order to inform a user of that fact What has been
described above is a basic content of a mode (first mode) of the
degradation diagnosis in the case where the determination is
limited to the upstream catalyst 11.
[0067] Incidentally, the method of the first mode is also
applicable singly to the downstream catalyst 19. In this case, the
output of the pre-catalyst sensor 17 is substituted by the output
of the inter-catalyst sensor 18, and the output of the
inter-catalyst sensor 18 is substituted by the output of the
post-catalyst sensor 21.
[0068] Next, a mode (second mode) of the degradation diagnosis that
includes the upstream catalyst 11 and the downstream catalyst 19
will be described. In the second mode, the switch timing of the
target air-fuel ratio A/Ft in the active air-fuel ratio control is
made to coincide with the inversion timing of the post-catalyst
sensor 21 on the downstream side of the downstream catalyst 19, and
the storage or release of oxygen is performed simultaneously and
serially with respect to the upstream catalyst 11 and the
downstream catalyst 19, the oxygen storage capacity OSC of the
upstream catalyst 11 and the oxygen storage capacity OSC of the
downstream catalyst 19 are simultaneously and serially
measured.
[0069] FIGS. 5A to 5E show how various values change during
execution of the active air-fuel ratio control. FIG. 5A shows the
target air-fuel ratio A/Ft, FIG. 5B shows the output of the
inter-catalyst sensor 18, FIG. 5C shows the actual inter-catalyst
air-fuel ratio A/Fmd, FIG. 5D shows the output of the post-catalyst
sensor 21, and FIG. 5E shows the post-catalyst air-fuel ratio
A/Frr.
[0070] In the example shown in FIGS. 5A to 5E, prior to a time T1,
the target air-fuel ratio A/Ft (and the pre-catalyst air-fuel ratio
A/Ffr)) is kept at a rich air-fuel ratio, and the oxygen stored in
the upstream catalyst 11 is released to purity exhaust gas, so that
an exhaust gas of a substantially stoichiometric air-fuel ratio
flows out at the downstream end of the upstream catalyst 11.
Therefore, during that time, the inter-catalyst air-fuel ratio
A/Fmd is kept substantially at the stoichiometric air-fuel ratio.
Then, after the oxygen stored in the upstream catalyst 11 is
entirely released, a rich gas containing unburned components flows
out at the downstream end of the upstream catalyst 11.
Corresponding to this, the output of the inter-catalyst sensor 18
becomes inverted from the lean side to the rich side. In the
example shown in FIGS. 5A to 5E, the output of the inter-catalyst
sensor 18 reaches the rich-side criterion value VR at the time
T1.
[0071] However, unlike the first mode, the target air-fuel ratio
A/Ft is not switched at this time point. Therefore, after the time
T1, an exhaust gas of a rich air-fuel ratio flows into the
downstream catalyst 19. As such a rich gas flows in, the downstream
catalyst 19 purifies the rich gas while releasing the oxygen stored
therein. Thus, an exhaust gas of a substantially stoichiometric
air-fuel ratio flows out at the downstream end of the downstream
catalyst 19. Therefore, during that time, the post-catalyst
air-fuel ratio A/Frr is substantially maintained at the
stoichiometric air-fuel ratio.
[0072] In the course of time, when the oxygen stored in the
downstream catalyst 19 is entirely released, rich gas flows out at
the downstream end of the downstream catalyst 19. Corresponding to
this, the output of the post-catalyst sensor 21 becomes inverted
from the lean side to the rich side. In the example shown, the
output of the post-catalyst sensor 21 reaches the rich-side
criterion value VR at the time 12. At the time point at which the
output of the post-catalyst sensor 21 becomes inverted to the rich
side, it can be determined that both the upstream catalyst 11 and
the downstream catalyst 19 have completely released the oxygen
stored therein. Therefore, simultaneously, the target air-fuel
ratio A/Ft is switched to a lean air-fuel ratio so that the
pre-catalyst air-fuel ratio A/Ffr becomes inverted to the lean
side. Thus, at the timing at which the output of the post-catalyst
sensor 21 becomes inverted, the target air-fuel ratio A/Ft is
switched.
[0073] From the time T2 on, exhaust gas of lean air-fuel ratio
flows into the upstream catalyst 11, so that the upstream catalyst
11 purifies exhaust gas while storing excess oxygen in the exhaust
gas. Therefore, after a slight delay time elapses from the time T2,
the inter-catalyst air-fuel ratio A/Fmd changes to the vicinity of
the stoichiometric air-fuel ratio. Then, after oxygen is stored in
the upstream catalyst 11 to its full capacity, lean gas flows out
at the downstream end of the upstream catalyst 11. Corresponding to
this, the output of the inter-catalyst sensor 18 becomes inverted
from the rich side to the lean side. In the example shown in FIGS.
5A to 5E, the output of the inter-catalyst sensor 18 reaches the
lean-side criterion value VL at a time T3.
[0074] As described above, at this time point, the target air-fuel
ratio A/Ft cannot be switched yet. Then, from the time T3 on,
exhaust gas of lean air-fuel ratio flows into the downstream
catalyst 19. As fuel-lean gas flows in, the downstream catalyst 19
purifies the lean gas while storing excess oxygen. Therefore,
exhaust gas of a substantially stoichiometric air-fuel ratio flows
out at the downstream end of the catalyst, and the post-catalyst
air-fuel ratio A/Frr is kept substantially at the stoichiometric
air-fuel ratio.
[0075] In the course of time, when oxygen is stored in the
downstream catalyst 19 to its full capacity, lean gas flows out at
the downstream end of the downstream catalyst 19. Corresponding to
this, the output of the post-catalyst sensor 21 becomes inverted
from the rich side to the lean side. In the example shown in FIGS.
5A to 5E, the output of the post-catalyst sensor 21 reaches the
lean-side criterion value VL at a time T4. At the time point at
which the output of the post-catalyst sensor 21 becomes inverted to
the lean side, it can be determined that both the upstream catalyst
11 and the downstream catalyst 19 have stored oxygen to their fuel
capacities. Therefore, simultaneously with this, the target
air-fuel ratio A/Ft is switched to the rich air-fuel ratio so that
the pre-catalyst air-fuel ratio A/Ffr becomes inverted to the rich
side.
[0076] Then, likewise, a rich gas flows into the upstream catalyst
11, so that the upstream catalyst 11 released the oxygen stored.
Then, after the upstream catalyst 11 has completely released
oxygen, rich gas flows out at the downstream end of the upstream
catalyst 11, so that the output of the inter-catalyst sensor 18
becomes inverted to the rich side (time T5). At this time point,
the target air-fuel ratio A/Ft is not switched.
[0077] As a result, rich gas flows into the downstream catalyst 19,
so that the downstream catalyst 19 releases oxygen. Then, after the
downstream catalyst 19 has completely released oxygen, rich gas
flows out at the downstream end of the downstream catalyst 19, so
that the output of the post-catalyst sensor 21 becomes inverted to
the rich side (time T6). Simultaneously with this inversion, the
target air-fuel ratio A/Ft is switched to the lean side.
[0078] Along with the active air-fuel ratio control as described
above, the oxygen storage capacities OSC1, OSC2 of the upstream
catalyst 11 and the downstream catalyst 19 are measured on the
basis of the foregoing equation (1), as in the first mode. That is,
when oxygen is stored into the upstream catalyst 11 during the time
T2 to T3, the oxygen storage capacity dOSC at every predetermined
small amount of time is calculated using the equation (1), and the
oxygen storage capacity dOSC at every predetermined small amount of
time is integrated from the time point at which the output of the
pre-catalyst sensor 17 changes to the lean side and reaches a value
substantially equivalent to the stoichiometric air-fuel ratio until
the time point T3 at which the output of the inter-catalyst sensor
18 becomes inverted to the lean side, as in the example shown in
FIGS. 4A and 4B. At this time, the value of the air-fuel ratio
difference .DELTA.A/F used is a difference between the
stoichiometric air-fuel ratio and the air-fuel ratio converted from
the output of the pre-catalyst sensor 17.
[0079] Subsequently, when oxygen is stored into the downstream
catalyst 19 during the time T3 to T4, the oxygen storage capacity
dOSC at every predetermined small amount of time is calculated
using the equation (1), and the oxygen storage capacity dOSC at
every predetermined small amount of time is integrated from the
time point T3 at which the output of the inter-catalyst sensor 18
becomes inverted to the lean side until the time point T4 at which
the output of the post-catalyst sensor 21 becomes inverted to the
lean side. At this time, since lean gas passes through the upstream
catalyst 11 and flows into the downstream catalyst 19, the value of
the air-fuel ratio difference .DELTA.A/F used is a difference
between the stoichiometric air-fuel ratio and the air-fuel ratio
converted from the output of the pre-catalyst sensor 17.
[0080] After that, when the target air-fuel ratio A/Ft is switched
to the rich air-fuel ratio, the oxygen storage capacity of the
upstream catalyst 11 is measured during the time T4 to T5, and the
oxygen storage capacity of the downstream catalyst 19 is measured
during the time T5 to T6 by substantially the same method as
described above.
[0081] The measurement data about the oxygen storage capacity
measured as described above are stored in the ECU 20, and are
subjected to the above-described averaging process to calculate
final measured oxygen storage capacity values OSC1, OSC2 of the
upstream catalyst 11 and the downstream catalyst 19. Then, these
measured values OSC1, OSC2 are compared with degradation criterion
values OSC1s, OSC2s, respectively, which are individually set
beforehand. According to results of the magnitude comparison, the
normality/degradation of the catalyst is determined separately for
each of the upstream catalyst 11 and the downstream catalyst 19.
Incidentally, in the case where at least one of the catalysts is
determined as being degraded, it is preferable to activate a
warning device, for example, a check lamp or the like, in order to
inform a user of the thus determined fact.
[0082] Next, detection of the sulfur poisoning of the catalysts in
the embodiment will be described. As described above, in the case
where a high-sulfur concentration fuel is used, there is a problem
of the sulfur poisoning of a catalyst. If the sulfur poisoning of a
catalyst occurs, the oxygen storage capacity of the catalyst
temporarily declines, leading to a misdiagnosis or the like. As a
result of vigorous study by the present inventors, it turned out
that the upstream catalyst 11 and the downstream catalyst 19
experience the sulfur poisoning in different manners.
[0083] That is, the following two phenomena of the oxygen storage
capacity of a catalyst declining due to the sulfur component in
fuel exist. In a first phenomenon, SOx in exhaust gas reacts with
the catalyst component 32 supported in a catalyst, that is, a noble
metal, and therefore lowers the reaction rate of the noble metal,
whereby the oxygen storage capacity of the catalyst is lowered. The
decline of the oxygen storage capacity occurs immediately after the
fuel is changed from a low-sulfur fuel to a high-sulfur fuel (i.e.,
immediately after the refueling with a high-sulfur fuel). The
amount of decline is dependent on the sulfur concentration of the
fuel and the magnitude of the oxygen storage capacity that the
catalyst has. In a second phenomenon, SOx in exhaust gas adsorbs to
the storage component (coating material) 31 of the catalyst,
whereby the oxygen storage capacity of the catalyst is lowered. The
decline of the oxygen storage capacity in this phenomenon gradually
progresses after the fuel is changed from a low-sulfur fuel to a
high-sulfur fuel.
[0084] With attention focused on the upstream catalyst 11,
high-temperature exhaust gas from the combustion chamber 3
initially and constantly flows into the upstream catalyst 11, and
therefore the temperature of the catalyst is high. Therefore, the
second phenomenon, that is, the sulfur adsorption onto the storage
component 31, is difficult to occur, and the decline of the oxygen
storage capacity of the upstream catalyst 11 is predominantly
caused by the first phenomenon, that is, the decline of the
reaction rate of the noble metal.
[0085] On the other hand, with regard to the downstream catalyst
19, since the catalyst temperature thereof is lower than that of
the upstream catalyst 11, the second phenomenon occurs as well as
the first phenomenon, lowering the oxygen storage capacity. That
is, while the oxygen storage capacity of the upstream catalyst 11
sharply declines immediately following the change of fuel, and
subsequently remains substantially unchanged, the oxygen storage
capacity of the downstream catalyst 19 continues to gradually
decline after sharply declining following the change of fuel.
[0086] Therefore, in this embodiment, in view of the difference in
the manner of sulfur poisoning, the sulfur poisoning of each
catalyst is detected, and the degradation determination regarding a
catalyst is prohibited as follows.
[0087] Firstly, a first embodiment in which the object of the
degradation diagnosis is limited to the upstream catalyst 11 will
be described. FIG. 6 shows a change in the oxygen storage capacity
OSC1 of the upstream catalyst 11 between before and after the fuel
is changed to a high-sulfur fuel. In the description herein, the
"present" is synonymous with the present time, and means or
indicates a timing or a period that serves as a reference. The
"previous" indicates a timing or a period that immediately precedes
the "present" timing or period. The "preceding the previous"
indicates a timing or a period that immediately precedes the
"previous" timing or period. The "trip" is a period of time during
which the engine (or the engine system) is on. In the example shown
in FIG. 6, the fuel is changed during an engine-off period between
the previous trip and the present trip. What is shown by a solid
line in FIG. 6 is an actual change in the oxygen storage
capacity.
[0088] As show in FIG. 6, during the previous trip, the oxygen
storage capacity OSC1n-1 (previous OSC) of the upstream catalyst 11
was measured, and during the present trip, the oxygen storage
capacity OSC1n (present OSC) of the upstream catalyst 11 is
measured From the previous oxygen storage capacity OSC1n-1, the
present oxygen storage capacity OSC1n immediately following the
change of the fuel greatly changes, more concretely, greatly
declines. Hence, if the amount of change or the amount of decline
from the previous oxygen storage capacity OSC1n-1 to the present
oxygen storage capacity OSC1n is calculated, the calculated value
can be used as a basis to detect that a high-sulfur fuel has been
fed into the fuel tank or the like and also detect that the
upstream catalyst 11 has been poisoned with sulfur. The oxygen
storage capacity change amount (or decline amount) .DELTA.OSC1 is
defined herein simply as a difference between the previous oxygen
storage capacity OSC1n-1 and the present oxygen storage capacity
OSC1n, that is, .DELTA.OSC1=OSC1n-1-OSC1n. However, other defines
are also possible. For example, oxygen storage capacity change
amount .DELTA.OSC1 may also be defined as
.DELTA.OSC1=(OSC1n-1-OSC1n)/OSC1n-1.
[0089] A catalyst degradation diagnosis process according to the
first embodiment will be described with reference to FIG. 7. This
process is executed by the ECU 20.
[0090] Firstly in step S101, it is determined whether or not a
precondition for starting the diagnosis has been satisfied. The
precondition is satisfied, for example, if the engine is in a
steady operation state and the catalyst 11 and the air-fuel ratio
sensors 17, 18, 21 have reached their predetermined activation
temperatures. The steady operation state of the engine may be
considered to be present, for example, if the fluctuation widths of
the intake air amount Ga and the engine rotation speed Ne are
within their respective predetermined ranges. Incidentally, the
precondition is not limited to the foregoing examples. If the
precondition has not been satisfied, the process ends. On the other
hand, if the precondition has been satisfied, the process proceeds
to step S102.
[0091] In step S102, the oxygen storage capacity OSC1 of the
upstream catalyst 11 is measured. Since the diagnosis object is
only the upstream catalyst 11 herein, it is preferable to adopt an
oxygen storage capacity measurement method as described above in
conjunction with the first mode shown in FIGS. 3A, 3B, 4A and 4B.
However, it is also permissible to adopt an oxygen storage capacity
measurement method as described above in conjunction with the
second mode shown in FIGS. 5A to 5E and use only the measured
oxygen storage capacity values of the upstream catalyst 11.
[0092] Subsequently in step S103, it is determined whether or not
at least one of misfire and air-fuel ratio abnormality has been
detected, concretely, whether or not such a detection history
exists. That is, this embodiment is provided with an abnormality
detection device that detects at least one of misfire and air-fuel
ratio abnormality. For example, when fluctuations of the engine
rotation speed Ne are larger than a predetermined value, or when a
greatly rich air-fuel ratio is detected by the pre-catalyst sensor
17, or when the hydrogen concentration in exhaust gas is higher
than a predetermined value in the case where a hydrogen sensor is
provided in the exhaust passageway upstream of the upstream
catalyst, it can be determined that misfire has occurred. Besides,
for example, when the correction value of the feedback air-fuel
ratio control is maintained at a value that is such as to correct
the air-fuel ratio to the rich side and that is a predetermined
limit value, it can be determined that there has occurred an
air-fuel ratio abnormality because of insufficient supply of the
fuel caused by a failure in the fuel system.
[0093] In the case where a poor-quality fuel or an unsuitable kind
of fuel (e.g., diesel) or the like is fed, there sometimes occurs
abnormality combustion or misfire causing a melt loss of a
catalyst. Therefore, in step S103, it is determined whether or not
there has occurred a factor of catalyst melt loss. If an
affirmative determination is made in step S103, a usual-degradation
determination is performed in step S107. That is, the oxygen
storage capacity OSC1 of the upstream catalyst 11 measured in step
S102 is compared with the degradation criterion value OSC1s. If
OSC1>OSC1s, it is determined that the upstream catalyst 11 is
normal. If OSC1.ltoreq.OSC1s, it is determined that the upstream
catalyst 11 has degraded. In the case of a melt loss of the
catalyst, which is a permanent degradation that is unrecoverable,
the degradation determination is performed as usual, and the
degradation thereof, it so determined, is indicated to a user by
turning on a check lamp so as to prompt the user to replace the
catalyst.
[0094] On the other hand, if a negative determination is made in
step S103, the process proceeds to step S104, in which the oxygen
storage capacity change amount .DELTA.OSC1 of the upstream catalyst
11 is calculated. That is, using the oxygen storage capacity OSC1n
measured in step S102 in the present process and the oxygen storage
capacity OSC1n-1 measured in step S102 in the previous process, the
oxygen storage capacity change amount .DELTA.OSC1 is calculated as
in .DELTA.OSC1=OSC1n-1-OSC1n.
[0095] Next, the oxygen storage capacity change amount .DELTA.OSC1
is compared with a predetermined threshold value .alpha. (see FIG.
6). If .DELTA.OSC1.ltoreq..alpha., the process proceeds to step
S107, in which the degradation determination is performed on the
basis of the measured oxygen storage capacity value OSC1 of the
upstream catalyst 11 measured in step S102, as usual.
[0096] On the other hand, when .DELTA.OSC1>.alpha., which means
that the oxygen storage capacity OSC1 of the upstream catalyst 11
has greatly changed (declined), the process proceeds to step S106.
In step S106, it is determined that the sulfur poisoning of the
upstream catalyst 11 is present, and simultaneously the degradation
determination based on the measured oxygen storage capacity value
OSC1 of the upstream catalyst 11 is prohibited. Thus, the sulfur
poisoning of the upstream catalyst 11 is suitably detected, and the
degradation determination based on the measured oxygen storage
capacity value OSC1 that has declined due to the sulfur poisoning
is prohibited, whereby the misdiagnosis can be prevented and the
diagnosis accuracy and reliability can be improved.
[0097] Next, another catalyst degradation diagnosis process
according to the first embodiment of will be described with
reference to FIG. 8. This process is executed by the ECU 20. The
same steps as those in the process in FIG. 7 are merely represented
by the reference numerals obtained by changing the third digit from
the right in each step number from "1" to "2", and detailed
description of those steps are omitted. The following description
is given mainly on the differences from the process shown in FIG.
7.
[0098] This another process is different from the foregoing process
in that step S202A is added subsequently to step S202, and step
S208 is added subsequently to step S206. After step S202, step
S202A is executed. In step S202A, it is determined whether or not
the measurement of the oxygen storage capacity in step S202 is the
first measurement of the oxygen storage capacity performed after
the fuel has been changed. Since the decline of the oxygen storage
capacity caused by the sulfur poisoning of the upstream catalyst 11
suddenly occurs immediately after the change of the fuel, it is
advantageous for the improvement of the diagnosis accuracy to add
information regarding the presence/absence of change of the fuel.
In this case, a detection device that detects the presence/absence
of the execution of the change of the fuel is provided and, for
example, the detection device has a remaining amount sensor for the
fuel tank (fuel gage) or a sensor that detects the opening of the
refill lid. In the case where it is detected that the remaining
amount of fuel in the fuel tank has increased or that the refill
lid has been opened, it is considered that the change of the fuel
has been performed.
[0099] If a negative determination is made in step S202A, the
process proceeds to step S207, in which the usual degradation
determination is performed. On the other hand, if an affirmative
determination is made in step S202A, the process of steps S203 to
S208 is performed, in which the presence/absence of the feeding of
a high-sulfur fuel is detected, and therefore the presence/absence
of the sulfur poisoning of the upstream catalyst 11 is detected.
That is, in this another process, the detection of the sulfur
poisoning of the upstream catalyst 11 is executed only at the time
of the first measurement of the oxygen storage capacity performed
after the change of the fuel.
[0100] Besides, after in step S206, the sulfur poisoning of the
upstream catalyst 11 is detected, and the degradation determination
regarding the upstream catalyst 11 is prohibited, the process
proceeds to step S203, in which the check lamp is turned on, and
simultaneously a diagnosis code indicating the sulfur poisoning and
the like is recorded in the ECU 20. This promotes the user to have
the feeding of a low-sulfur fuel again. Alternatively, at a later
stage in the maintenance of the vehicle, it can be made known to a
service person that the upstream catalyst 11 has temporarily been
poisoned with sulfur and the catalyst replacement is
unnecessary.
[0101] Next, a second embodiment in which both the upstream
catalyst 11 and the downstream catalyst 19 are included as
degradation diagnosis objects will be described. FIGS. 9 and 10
show changes of the oxygen storage capacities of the upstream
catalyst 11 and the downstream catalyst 19 between before and after
the fuel is changed. In the examples shown in FIGS. 9 and 10, the
change of the fuel is performed between the trip preceding the
previous trip and the previous trip.
[0102] As shown in FIGS. 9 and 10, during each of the trip
preceding the previous one, the previous trip, and the present
trip, the oxygen storage capacities of the upstream catalyst 11 and
the downstream catalyst 19 are measured. In the case of the
upstream catalyst 11 shown in FIG. 9, the previous oxygen storage
capacity OSC1n-1, immediately following the change of the fuel,
greatly declined from the oxygen storage capacity OSC1n-2 preceding
the previous one, similarly to the case shown in FIG. 6. However,
in the case of the upstream catalyst 11, the second phenomenon,
that is, the sulfur adsorption to the oxygen storage component 31,
is hard to occur, and therefore substantially no further decline of
the oxygen storage capacity is observed. That is, the present
oxygen storage capacity OSC1n remains substantially unchanged from
the previous oxygen storage capacity OSC1n-1.
[0103] On the other hand, in the case of the downstream catalyst 19
shown in FIG. 10, the previous oxygen storage capacity OSC2n-1,
immediately following the change of the fuel, greatly declines from
the oxygen storage capacity OSC2n-2 preceding the previous one, and
farther decline is observed subsequently to the great decline. That
is, present oxygen storage capacity OSC2n further declines from the
previous oxygen storage capacity OSC2n-1. The amount of decline
from the previous capacity to the present capacity is less than the
amount of decline from the capacity preceding the previous one to
the previous capacity, that is, there is a tendency of the oxygen
storage capacity OSC2 of the downstream catalyst 19 greatly
declining immediately following the change of the fuel, and
subsequently continuing to gradually decline. A reason for this is
that as described above, in the case where a high-sulfur fuel is
fed, the downstream catalyst 19 undergoes the first phenomenon,
that is, the decline of the reaction rate of the noble metal, and
also the second phenomenon, that is, the sulfur adsorption to the
oxygen storage component 31. Since the sulfur adsorption gradually
occurs, the oxygen storage capacity also gradually declines.
Considering the foregoing the oxygen storage capacity change
characteristic, the catalyst degradation diagnosis is executed as
follows.
[0104] Hereinafter, a catalyst degradation diagnosis process
according to the second embodiment will be described with reference
to FIG. 11. This process is executed by the ECU 20.
[0105] Firstly in step S301, similar to step S101, it is determined
whether or not a precondition for starting the diagnosis has been
satisfied. If the precondition has not been satisfied, the process
ends. On the other hand, if the precondition has been satisfied,
the process proceeds to step S302.
[0106] In step S302, the oxygen storage capacity OSC1 of the
upstream catalyst 11 is measured. Subsequently in step S303, the
oxygen storage capacity OSC2 of the downstream catalyst 19 is
measured. Since the upstream catalyst 11 and the downstream
catalyst 19 are diagnosis objects, an oxygen storage capacity
measurement method as described above in conjunction with the
second mode shown in FIG. 5 is adopted.
[0107] Subsequently in step S304, the oxygen storage capacity
change amount .DELTA.OSC1 of the upstream catalyst 11 is
calculated. Subsequently in step S305, the oxygen storage capacity
change amount .DELTA.OSC2 of the downstream catalyst 19 is
calculated. That is, by the method as in step S104, the oxygen
storage capacity change amount of the upstream catalyst 11
.DELTA.OSC1=OSC1n-1-OSC1n during the period from the previous
process to the present process is calculated. Subsequently, the
oxygen storage capacity change amount of the downstream catalyst 19
.DELTA.OSC2=OSC2n-1-OSC2n during the period from the previous
process to the present process is calculated.
[0108] After that, in step S306, the oxygen storage capacity change
amount .DELTA.OSC1 of the upstream catalyst 11 is compared with a
predetermined threshold value .alpha. (see FIG. 9). If
.DELTA.OSC1>.alpha., the process proceeds to step S307, in which
the oxygen storage capacity change amount .DELTA.OSC2 of the
downstream catalyst 19 is compared with a predetermined threshold
value .beta. (see FIG. 10). If .DELTA.OSC2>.beta., the process
proceeds to step S308, in which it is determined that the sulfur
poisoning is present regarding both the upstream catalyst 11 and
the downstream catalyst 19, and simultaneously the degradation
determination based on the measured oxygen storage capacity values
OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst
19 is prohibited. In this manner, the sulfur poisoning of each of
the upstream catalyst 11 and the downstream catalyst 19 is suitably
detected, and the degradation determination regarding each catalyst
based on the measured oxygen storage capacity value has been
greatly lowered by the sulfur poisoning is prohibited, so that the
misdiagnosis can be prevented and the diagnosis accuracy and
reliability can be improved.
[0109] On the other hand, if in step S306 it is determined
.DELTA.OSC1.ltoreq..alpha., or if in step S307 it is determined
that .DELTA.OSC2.ltoreq..beta., the process proceeds to step S309,
in which the usual degradation determination regarding the upstream
catalyst 11 and the downstream catalyst 19 is performed. That is,
the measured oxygen storage capacity values OSC1, OSC2 of the
upstream catalyst 11 and the downstream catalyst 19 measured in
steps S102 and S103 are compared with their corresponding
degradation criterion values OSC1s, OSC2s, respectively. If the
measured value of a catalyst is greater than the degradation
criterion value, it is determined that the catalyst is normal. If
the measured value of a catalyst is less than or equal to the
degradation criterion value, it is determined that the catalyst has
degraded. For example, in the case where .DELTA.OSC1>.alpha.
regarding the upstream catalyst 11 and .DELTA.OSC2.ltoreq..beta.
regarding the downstream catalyst 19, the usual degradation
determination is performed in step S309. In this case, the
possibility of melt loss is high with regard to the upstream
catalyst 11, and therefore it is determined that the upstream
catalyst 11 has degraded in the degradation determination of step
S309. Hence, it is possible to detect a permanent degradation
caused by a melt loss of one of the catalysts.
[0110] As can be understood from the foregoing description, in the
diagnosis process, the presence/absence of the sulfur poisoning is
detected on the basis of the measured oxygen storage capacity
values obtained at two serial measurement timings. However, in
another catalyst degradation diagnosis process described below, the
presence/absence of sulfur poisoning is detected with regard to the
two catalysts, on the basis of the measured oxygen storage capacity
values obtained at three serial measurement timings.
[0111] FIGS. 12 and 13 show flowcharts of other catalyst
degradation diagnosis processes according to the second embodiment.
The processes are executed by the ECU 20. The process shown in FIG.
12 is executed in a cycle preceding the process shown in FIG. 13.
Correspondence of the processes shown in FIGS. 12 and 13 to the
examples shown in FIGS. 9 and 10 can be made as follows. For
example, the process shown in FIG. 12 corresponds to a process
executed during the previous trip which is shown in FIGS. 9 and 10,
and the process shown in FIG. 13 corresponds to the process
executed during the present trip which is shown in FIGS. 9 and
10.
[0112] Steps S401 to S407 and 9409 in the process shown in FIG. 12
are the same as steps S301 to S307 and S309 in the process shown in
FIG. 11. Besides, step S408 in the process in FIG. 12 is changed in
content from step S308 in the process in FIG. 11. In step S408, a
sulfur poisoning detection provisional flag is set, instead of
execution of the determination regarding sulfur poisoning and the
prohibition of the degradation determination. That is, with regard
to the upstream catalyst 11 and the downstream catalyst 19, when
the oxygen storage capacity change amounts .DELTA.OSC1, .DELTA.OSC2
indicated in FIGS. 9 and 10 which occurred during a period from the
measurement preceding the previous one to the previous measurement
are greater than predetermined values .alpha., .beta.,
respectively, the possibility of sulfur poisoning is tentatively
estimated. Besides, the degradation determination is suspended,
that is, is substantially prohibited.
[0113] Next, the process shown in FIG. 13 is executed. In FIG. 13,
steps S501 to S503 are the same as steps S301 to S303 in the
process shown in FIG. 11. After step S503, step S504 is executed,
in which it is determined whether or not the sulfur poisoning
detection provisional flag has been set. If the flag has not been
set, the process proceeds to step S509, in which the usual
degradation determination with respect to the upstream catalyst 11
and the downstream catalyst 19 is performed as in step S309.
[0114] If the flag has been set, the process proceeds to step S505,
in which the oxygen storage capacity change amount .DELTA.OSC2 of
the downstream catalyst 19 from the previous process time to the
present process time (the oxygen storage capacity change amount
from the previous measurement to the present measurement in FIGS. 9
and 10) is calculated. Next, the process proceeds to step S506, in
which the calculated oxygen storage capacity change amount
.DELTA.OSC2 is compared with a predetermined threshold value
.gamma. (see FIG. 10). If .DELTA.OSC2.ltoreq..gamma., the process
proceeds to step S509, in which the usual degradation determination
with respect to the upstream catalyst 11 and the downstream
catalyst 19 is performed.
[0115] On the other hand, if .DELTA.OSC>.gamma., the process
proceeds to step S507, in which it is finally determined that the
upstream catalyst 11 and the downstream catalyst 19 have sulfur
poisoning, and simultaneously the degradation determination with
respect to the upstream catalyst 11 and the downs catalyst 19 is
prohibited. Then, in step S508, the sulfur poisoning detection
provisional flag is cleared. After that, the process ends.
[0116] If in the process in FIG. 12, a great decline in the oxygen
storage capacity of each catalyst is detected, it is checked
whether or not the oxygen storage capacity of the downstream
catalyst has further declined in the process shown in FIG. 13,
before the final sulfur poisoning detection is performed.
Therefore, the detection accuracy can be heightened, and the
diagnosis accuracy and reliability can be improved, and
misdiagnosis can be more reliably prevented.
[0117] While embodiments of the invention have been described in
detail above, various other embodiments of the invention can also
be conceived. For example, the use and the type of the internal
combustion engine are arbitrary; for example, the engine may be
used for purposes other than the vehicle, and may also be of a
die-injection type, or the like. The post-catalyst sensor may also
be a wide-range air-fuel ratio sensor similar to the pre-catalyst
sensor, and the pre-catalyst sensor used may be an O.sub.2 sensor,
similarly to the post-catalyst sensor. Sensors that detect the
exhaust gas air-fuel ratio over a wide range, including a
wide-range air-fuel ratio sensor, and an O.sub.2 sensor, are
referred to as "air-fuel ratio sensors". The invention is
applicable to not only three-way catalysts, but all kinds of
catalysts that have oxygen storage capability.
[0118] Although FIGS. 6, 9 and 10 show examples in which the oxygen
storage capacity is measured only once per one trip, this is not
restrictive. The invention is also applicable to the cases where
the oxygen storage capacity is measured a plurality of times per
one trip, or once in a plurality of trips. Besides, the invention
is applicable to the foregoing degrade diagnosis process.
[0119] While the invention has been described with reference to
exemplary embodiments thereof, it is to be understood that the
invention is not limited to the exemplary embodiments or
constructions. To the contrary, the invention is intended to cover
various modifications and equivalent arrangements. In addition,
while the various elements of the exemplary embodiments are shown
in various combinations and configurations, which are exemplary,
other combinations and configurations, including more, less or only
a single element, are also within the spirit and scope of the
invention.
* * * * *