U.S. patent application number 09/759316 was filed with the patent office on 2001-08-09 for exhaust purifying apparatus and method for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Harima, Kenji, Ichinose, Hiroki, Itou, Keiji, Itou, Takaaki, Katoh, Kenji, Tanaka, Hiroshi.
Application Number | 20010011455 09/759316 |
Document ID | / |
Family ID | 18552035 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010011455 |
Kind Code |
A1 |
Harima, Kenji ; et
al. |
August 9, 2001 |
Exhaust purifying apparatus and method for internal combustion
engine
Abstract
An exhaust purifying apparatus for an internal combustion engine
includes a catalyst that purifies exhaust gas from the engine, a
first passage that allows exhaust gas to flow from the engine to
the catalyst, and a second passage that allows exhaust gas to flow
from the engine to the catalyst. The first passage includes an
accelerated cooling portion whose cross section is designed so that
a relatively large quantity of heat is released from the exhaust
gas in the first passage, and the second passage has a cross
section designed so that a relatively small quantity of heat is
released from the exhaust gas in the second passage. The apparatus
further includes a flow amount controller that controls amounts of
exhaust gas flowing through the first and second passages, such
that the amount of exhaust gas flow through the first passage is
made larger than that through the second passage when the
temperature of exhaust gas emitted from the engine is to be lowered
by a relatively large degree before reaching the catalyst, and such
that the amount of exhaust gas flow through the second passage is
made larger than that through the first passage when the
temperature of exhaust gas emitted from the engine is to be lowered
by a relatively small degree before reaching the catalyst.
Inventors: |
Harima, Kenji; (Susono-shi,
JP) ; Itou, Takaaki; (Mishima-shi, JP) ;
Ichinose, Hiroki; (Fujinomiya-shi, JP) ; Katoh,
Kenji; (Sunto-gun, JP) ; Tanaka, Hiroshi;
(Susono-shi, JP) ; Itou, Keiji; (Susono-shi,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
277 S. WASHINGTON STREET, SUITE 500
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
|
Family ID: |
18552035 |
Appl. No.: |
09/759316 |
Filed: |
January 16, 2001 |
Current U.S.
Class: |
60/288 ; 60/274;
60/285 |
Current CPC
Class: |
F01N 3/0835 20130101;
F01N 2570/12 20130101; F01N 3/2046 20130101; F01N 2570/04 20130101;
F01N 3/0878 20130101; F01N 2570/14 20130101; F01N 3/22 20130101;
F01N 3/085 20130101; F01N 3/30 20130101; F01N 3/0871 20130101; Y02T
10/12 20130101; F01N 13/009 20140601; F01N 13/0097 20140603; F01N
3/0885 20130101; Y02A 50/20 20180101; F01N 3/0814 20130101; F01N
3/0807 20130101; F01N 3/2006 20130101; F01N 3/0842 20130101; F01N
13/011 20140603; F01N 2430/06 20130101; F01N 3/05 20130101; F01N
2240/02 20130101 |
Class at
Publication: |
60/288 ; 60/274;
60/285 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 3, 2000 |
JP |
2000-026281 |
Claims
What is claimed is:
1. An exhaust purifying apparatus for an internal combustion
engine, the exhaust purifying apparatus comprising: a catalyst that
purifies exhaust gas emitted from the internal combustion engine; a
first passage disposed between the engine and the catalyst to allow
exhaust gas to flow therethrough from the engine to the catalyst,
said first passage including an accelerated cooling portion having
a cross section that permits a first quantity of heat to be
released from the exhaust gas in the first passage; a second
passage disposed between the engine and the catalyst to allow
exhaust gas to flow therethrough from the engine to the catalyst,
said second passage having a cross section that permits a second
quantity of heat that is smaller than said first quantity of heat
to be released from the exhaust gas in the second passage; and a
flow amount controller that controls amounts of exhaust gas flowing
through the first passage and through the second passage, such
that: (i) the amount of exhaust gas flowing through the first
passage is made larger than the amount of exhaust gas flowing
through the second passage when a temperature of exhaust gas
emitted from the engine is to be lowered by a relatively large
degree before the exhaust gas flows into the catalyst, and (ii) the
amount of exhaust gas flowing through the second passage is made
larger than the amount of exhaust gas flowing through the first
passage when the temperature of exhaust gas emitted from the engine
is to be lowered by a relatively small degree before the exhaust
gas flows into the catalyst.
2. An exhaust purifying apparatus according to claim 1, wherein the
cross section of the accelerated cooling portion of the first
passage is generally arcuate, and the cross section of the second
passage is generally circular.
3. An exhaust purifying apparatus according to claim 2, wherein the
accelerated cooling portion of the first passage substantially
surrounds the second passage.
4. An exhaust purifying apparatus according to claim 1, wherein the
first passage and the second passage have substantially equal
lengths.
5. An exhaust purifying apparatus according to claim 1, wherein the
first passage and the second passage have cross-sectional areas
that are substantially equal.
6. An exhaust purifying apparatus according to claim 1, wherein the
catalyst comprises a lean NOx catalyst capable of purifying exhaust
gas with a lean air-fuel ratio of NOx.
7. An exhaust purifying apparatus according to claim 1, wherein the
flow amount controller controls the amounts of exhaust gas flowing
through the first passage and through the second passage, based on
a catalyst midbed temperature of the catalyst or a parameter
effecting the catalyst midbed temperature.
8. An exhaust purifying apparatus according to claim 7, wherein the
flow amount controller controls the amounts of exhaust gas flowing
through the first passage and through the second passage so as to
control a catalyst midbed temperature of the catalyst to be within
a predetermined temperature range in which the catalyst provides a
purification efficiency of not lower than a specified value.
9. An exhaust purifying apparatus according to claim 1, wherein the
flow amount controller controls the amount of exhaust gas flowing
through the second passage to be larger than the amount of exhaust
gas flowing through the first passage during an S-poisoning
recovery process for releasing SOx that has been absorbed by the
catalyst.
10. An exhaust purifying apparatus according to claim 9, further
comprising an exhaust air-fuel controller that controls an air-fuel
ratio of exhaust gas that flows into the catalyst during the
S-poisoning recovery process to be substantially equal to or richer
than a stoichiometric air-fuel ratio.
11. An exhaust purifying apparatus according to claim 9, wherein
the flow amount controller controls the amounts of flow so as to
increase a proportion of the exhaust gas flowing through the first
passage immediately after completion of the S2 poisoning recovery
process.
12. An exhaust purifying apparatus according to claim 1, further
comprising an HC adsorbent provided in the first passage, wherein
the flow amount controller causes all the exhaust gas to flow
through the first passage during a start of the internal combustion
engine.
13. An exhaust purifying apparatus according to claim 12, wherein
the HC adsorbent is disposed downstream of the accelerated cooling
portion of the first passage.
14. An exhaust purifying apparatus according to claim 12, wherein
the catalyst comprises a selective reduction type NOx catalyst that
reduces or decomposes NOx in the presence of hydrocarbon in an
oxygen-excess atmosphere, said second passage being provided with
an occlusion-reduction type NOx catalyst that absorbs NOx when
incoming exhaust gas has a lean air-fuel ratio, and releases and
reduces the absorbed NOx into N.sub.2 when the concentration of
oxygen contained in the incoming exhaust gas is reduced, said flow
amount controller causes exhaust gas to flow mainly through the
first passage when the engine is operating with a high load.
15. An exhaust purifying apparatus according to claim 14, further
comprising a common passage located upstream of the first passage
and the second passage, said common passage allowing exhaust gas to
flow therethrough before entering the first and second passages,
said common passage being provided with an additional selective
reduction type NOx catalyst that reduces or decomposes NOx in the
presence of hydrocarbon in an oxygen-excess atmosphere.
16. An exhaust purifying apparatus according to claim 1, further
comprising: a temperature sensor disposed at one of: (i) a joint
passage into which a downstream portion of the first passage and a
downstream portion of the second passage merge, (ii) an exhaust
passage disposed downstream of the joint passage, and (iii) the
catalyst; and a diagnostic unit that determines whether the flow
amount controller operates normally or not, based on a change in
the temperature detected by the temperature sensor when the flow
amount controller is operated to change a ratio of the amounts of
exhaust gas flowing through the first passage and through the
second passage.
17. An exhaust purifying apparatus according to claim 1, further
comprising: a pressure sensor disposed in the first passage to
detect a pressure of exhaust gas; and a diagnostic unit that
determines whether the flow amount controller operates normally or
not, based on a change in the pressure detected by the pressure
sensor when the flow amount controller is operated to change a
ratio of the amounts of exhaust gas flowing through the first
passage and through the second passage.
18. A method of purifying exhaust emitted from an internal
combustion engine by utilizing a catalyst that purifies exhaust gas
emitted from the internal combustion engine, the method comprising:
disposing a first passage between the engine and the catalyst to
allow exhaust gas to flow through the first passage from the engine
to the catalyst, the first passage including an accelerated cooling
portion having a cross section that permits a first quantity of
heat to be released from the exhaust gas in the first passage;
disposing a second passage between the engine and the catalyst to
allow exhaust gas to flow through the second passage from the
engine to the catalyst, the second passage having a cross section
that permits a second quantity of heat that is smaller than the
first quantity of heat to be released from the exhaust gas in the
second passage; and controlling amounts of exhaust gas flowing
through the first passage and through the second passage,
including: (i) making the amount of exhaust gas flowing through the
first passage larger than the amount of exhaust gas flowing through
the second passage when a temperature of exhaust gas emitted from
the engine is to be lowered by a relatively large degree before the
exhaust gas flows into the catalyst, and (ii) making the amount of
exhaust gas flowing through the second passage larger than the
amount of exhaust gas flowing through the first passage when the
temperature of exhaust gas emitted from the engine is to be lowered
by a relatively small degree before the exhaust gas flows into the
catalyst.
19. A method according to claim 18, further comprising making the
cross section of the accelerated cooling portion of the first
passage generally arcuate, and making the cross section of the
second passage generally circular.
20. A method according to claim 19, wherein the accelerated cooling
portion of the first passage is disposed to substantially surround
the second passage.
21. A method according to claim 18, further comprising making a
length of the first passage substantially equal to a length of the
second passage.
22. A method according to claim 18, further comprising making
cross-sectional areas of the first passage and of the second
passage substantially equal.
23. A method according to claim 18, wherein the amounts of exhaust
gas flowing through the first passage and through the second
passage are controlled based on a catalyst midbed temperature of
the catalyst or a parameter effecting the catalyst midbed
temperature.
24. A method according to claim 23, wherein the amounts of exhaust
gas flowing through the first passage and through the second
passage are controlled so as to control a catalyst midbed
temperature of the catalyst to be within a predetermined
temperature range in which the catalyst provides a purification
efficiency of not lower than a specified value.
25. A method according to claim 18, wherein the amount of exhaust
gas flowing through the second passage is controlled to be larger
than the amount of exhaust gas flowing through the first passage
during an S-poisoning recovery process for releasing SOx that has
been absorbed by the catalyst.
26. A method according to claim 25, further comprising controlling
an air-fuel ratio of exhaust gas that flows into the catalyst
during the S-poisoning recovery process to be substantially equal
to or richer than a stoichiometric air-fuel ratio.
27. A method according to claim 25, wherein the amounts of flow are
controlled so as to increase a proportion of the exhaust gas
flowing through the first passage immediately after completion of
the S-poisoning recovery process.
28. A method according to claim 18, wherein an HC adsorbent is
provided in the first passage, and wherein all the exhaust gas is
controlled to flow through the first passage during a start of the
internal combustion engine.
29. A method according to claim 28, wherein: the catalyst comprises
a selective reduction type NOx catalyst that reduces or decomposes
NOx in the presence of hydrocarbon in an oxygen-excess atmosphere;
the second passage is provided with an occlusion-reduction type NOx
catalyst that absorbs NOx when incoming exhaust gas has a lean
air-fuel ratio, and releases and reduces the absorbed NOx into
N.sub.2 when the concentration of oxygen contained in the incoming
exhaust gas is reduced; and the controlling step causes exhaust gas
to flow mainly through the first passage when the engine is
operating with a high load.
30. A method according to claim 18, further comprising: sensing a
temperature at one of: (i) a joint passage into which a downstream
portion of the first passage and a downstream portion of the second
passage merge, (ii) an exhaust passage disposed downstream of the
joint passage, and (iii) the catalyst; and determining whether the
flow amount controlling step is being performed normally or not,
based on a change in the sensed temperature when the controlling
step changes a ratio of the amounts of exhaust gas flowing through
the first passage and through the second passage.
31. A method according to claim 18, further comprising: detecting a
pressure of exhaust gas in the fist passage; and determining
whether the flow amount controlling step is being performed
normally or not, based on a change in the detected pressure when
the controlling step changes a ratio of the amounts of exhaust
flowing through the first passage and the second passage.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2000-026281 filed on Feb. 3, 2000 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 an exhaust purifying apparatus and
method for purifying exhaust gas discharged from an internal
combustion engine through the use of a catalytic converter.
[0004] 2. Description of Related Art
[0005] As a measure for reducing the amount of harmful components
of exhaust gas discharged from an internal combustion engine into
the atmosphere, an exhaust system is known which purifies the
exhaust gas of the harmful components by using the oxidizing or
reducing effect of a catalyst.
[0006] In general, this type of catalyst has an activation
temperature range, and is not able to substantially remove harmful
components in exhaust gas when the catalyst midbed temperature is
outside the activation temperature range. This type of catalyst
also has a temperature characteristic that its exhaust purifying
capability considerably varies in accordance with the catalyst
midbed temperature even within the activation temperature
range.
[0007] The temperature of a catalyst disposed in an exhaust passage
of the engine significantly depends on the temperature of exhaust
gas flowing through the catalyst. Since the exhaust gas temperature
changes considerably in accordance with the operating state of the
internal combustion engine, the catalyst midbed temperature also
changes depending on the operating state of the engine.
[0008] Therefore, if measures are not taken to control the catalyst
midbed temperature, the exhaust purifying capability of the
catalyst changes from moment to moment and does not stabilize,
which may undesirably result in fluctuations in the concentrations
of harmful components in exhaust gas escaping into the
atmosphere.
[0009] In exhaust purifying systems using catalysts, it is
particularly important to determine how to keep the catalyst midbed
temperature within the activation temperature range and,
furthermore, how to stabilize the catalyst midbed temperature
within a temperature range in which the exhaust purifying
capability is high, in order to improve exhaust purifying
performance.
[0010] Furthermore, the catalyst is likely to undergo heat
deterioration when exposed to high temperatures. It is therefore
desirable to prevent inadvertent flow of high-temperature exhaust
gas to the catalyst.
[0011] Typical fuels for internal combustion engines contain
sulfur. When such a fuel is burned, sulfur contained in the fuel is
caused to burn and produce oxides of sulfur (SOx), such as SO.sub.2
and SO.sub.3, which become exhaust gas components. When the exhaust
gas containing SOx reaches the catalyst, SOx is apt to be absorbed
into the catalyst and form sulfates, and the like. Since the
sulfates are stable, the sulfates are unlikely to be decomposed and
released, and tend to be accumulated in the catalyst. If the amount
of SOx accumulated in the catalyst increases, the ability of the
catalyst to reduce other harmful components (HC, CO, NOx) in
exhaust gas may deteriorate. This is generally called
"S-poisoning".
[0012] In order to maintain a high exhaust purifying capability of
the catalyst for a long time, therefore, it is necessary to recover
the catalyst from S-poisoning by decomposing SOx accumulated in the
catalyst and releasing decomposed SOx from the catalyst. This
S-poisoning recovery process requires the catalyst to be placed in
a high-temperature atmosphere having a certain temperature or
higher. In this case, too, appropriate control of the catalyst
midbed temperature is very important in order to efficiently
recover the catalyst from S-poisoning.
[0013] Japanese Patent Laid-Open Publication No. 8-105318 discloses
a technology relating to catalyst temperature control performed at
the time of the aforementioned S-poisoning recovery process. In an
exhaust purifying apparatus disclosed in this publication, an
exhaust manifold of an engine capable of operation in a lean-burn
mode and a catalyst capable of substantially removing NOx in
exhaust gas discharged from the engine (a generally termed
lean-burn NOx catalyst) are connected by a first exhaust passage
and by a second exhaust passage that are disposed in parallel with
each other.
[0014] In this apparatus, the channel length of the first exhaust
passage is set to be greater than the channel length of the second
exhaust passage. With this arrangement, the temperature of exhaust
gas flowing through the first exhaust passage decreases by a larger
extent than that of exhaust gas flowing through the second exhaust
passage. In other words, a greater exhaust gas cooling effect can
be achieved by heat dissipation from the first exhaust passage than
from the second exhaust passage.
[0015] Furthermore, a control valve is mounted in the second
exhaust passage in such a manner that exhaust gas is caused to flow
through the first exhaust passage while the control valve is
closed, and that exhaust gas is caused to flow through the second
exhaust passage while the control valve is opened.
[0016] When the S-poisoning recovery process is performed on the
catalyst, the catalyst midbed temperature needs to be kept high.
Therefore, the control valve is placed in an open position so that
exhaust gas flows through the second exhaust passage, which has a
shorter channel length, and is less likely to cool exhaust gas.
When the S-poisoning recovery process is not performed, the control
valve is placed in a closed position so that exhaust gas flows
through the first exhaust passage, which has a greater channel
length, and is more likely to cool exhaust gas.
[0017] In the above-described known catalyst temperature control
system, however, the first exhaust passage and the second exhaust
passage must be formed with largely different channel lengths in
order to provide significantly different exhaust-gas cooling
effects. As a result, the channel length of the first exhaust
passage becomes very long, and a catalyst temperature control unit
becomes large in size, thus causing a problem in installing the
control system on the vehicle.
SUMMARY OF THE INVENTION
[0018] It is therefore an object of the invention to provide an
exhaust purifying apparatus that is simple and compact in
construction, wherein the catalyst midbed temperature can be
suitably controlled.
[0019] To accomplish the above and/or other objects, one aspect of
the invention provides an exhaust purifying apparatus for an
internal combustion engine having a catalyst, a first passage, a
second passage and a flow amount controller. The catalyst purifies
exhaust gas emitted from the internal combustion engine. The first
passage is disposed between the engine and the catalyst to allow
exhaust gas to flow therethrough from the engine to the catalyst.
The first passage includes an accelerated cooling portion whose
cross section is designed so that a first quantity of heat is
released from the exhaust gas in the first passage. The second
passage is disposed between the engine and the catalyst to allow
exhaust gas to flow therethrough from the engine to the catalyst.
The second passage has a cross section that is designed so that a
second quantity of heat that is smaller than the first quantity of
heat is released from the exhaust gas in the second passage. The
flow amount controller controls amounts of exhaust gas flowing
through the first passage and the second passage, such that: (i)
the amount of exhaust gas flowing through the first passage is made
larger than that flowing through the second passage when the
temperature of exhaust gas emitted from the engine is to be lowered
by a relatively large degree before the exhaust gas flows into the
catalyst; and (ii) the amount of exhaust gas flowing through the
second passage is made larger than that flowing through the first
passage when the temperature of exhaust gas emitted from the engine
is to be lowered by a relatively small degree before the exhaust
gas flows into the catalyst. By controlling the amounts of exhaust
gas flowing through the first passage and through the second
passage in this manner, the midbed temperature of the catalyst can
be controlled to be within a suitable range.
[0020] In the exhaust purifying apparatus as described above, the
flow amount controller may consist of a flow amount control valve.
Such a flow amount control valve may be of the electromagnetically
driven type, or of the hydraulically driven type.
[0021] In the apparatus of the invention, "(i) the amount of
exhaust gas flowing through the first passage is made larger than
that flowing through the second passage" includes the case where
the entire amount of exhaust gas is caused to flow through the
first passage while no exhaust gas is caused to flow through the
second passage. Likewise, "(ii) the amount of exhaust gas flowing
through the second passage is made larger than that flowing through
the first passage" includes the case where the entire amount of
exhaust gas is caused to flow through the second passage while no
exhaust gas is caused to flow through the first passage.
[0022] In one preferred embodiment of the invention, the
accelerated cooling portion of the first passage has a generally
arcuate shape in cross section, and the second passage has a
generally circular shape in cross section. It is, however, to be
understood that the cross-sectional shape of the first passage is
not limited to the generally arcuate shape, but may be selected
from other shapes including a generally U-like shape, a rectangular
shape with one side eliminated, and a polygonal shape having a
plurality of segments connected in a non-linear manner. The first
passage may be provided with radiating fins formed on the outer
wall surface thereof, so as to permit a relatively large quantity
of heat to be released from exhaust gas passing through the first
passage.
[0023] In another preferred embodiment of the invention, the
accelerated cooling portion of the first passage substantially
surrounds the second passage. With this arrangement, the exhaust
purifying apparatus can be made compact or small in size, and can
be more easily installed on the vehicle.
[0024] Preferably, the first passage and the second passage have
substantially equal lengths. The resulting exhaust purifying
apparatus can be made compact, which is advantageous in
installation of the apparatus in the vehicle.
[0025] Preferably, the first passage and the second passage have
cross-sectional areas that are substantially equal. The exhaust
purifying apparatus having this feature does not suffer from a
difference in the exhaust resistance between the first passage and
the second passage.
[0026] In the exhaust purifying apparatus as described above, the
catalyst may be a lean NOx catalyst capable of purifying exhaust
gas with a lean air-fuel ratio of NOx. The lean NOx catalyst may be
an occlusion-reduction type NOx catalyst that absorbs NOx when
incoming exhaust gas has a lean air-fuel ratio, and releases and
reduces the absorbed NOx into N.sub.2 when the concentration of
oxygen contained in the incoming exhaust gas is reduced, or may be
a selective reduction type NOx catalyst that reduces or decomposes
NOx in the presence of hydrocarbon in an oxygen-excess atmosphere.
It is, however, to be understood that the catalyst is not limited
to the lean NOx catalyst, but may be a three-way catalyst.
[0027] In the exhaust purifying apparatus as described above, the
flow amount controller controls the amounts of exhaust gas flowing
through the first passage and through the second passage, based on
a catalyst midbed temperature of the catalyst or a parameter
effecting the catalyst midbed temperature. In this manner, the
catalyst midbed temperature can be controlled with improved
accuracy. Here, the parameter effecting the catalyst midbed
temperature may be, for example, an exhaust gas temperature, or an
operating state of the engine. The exhaust gas temperature can be
used as the parameter since the catalyst midbed temperature greatly
depends upon the exhaust gas temperature. The operating state of
the engine can be used as the parameter since the temperature of
exhaust gas discharged from the engine can be estimated according
to the operating state of the engine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The foregoing and further objects, features and advantages
of the invention will become apparent from the following
description of preferred embodiments with reference to the
accompanying drawings, wherein like numerals are used to represent
like elements and wherein:
[0029] FIG. 1 is a schematic diagram illustrating the construction
of a first embodiment of the invention in the form of an exhaust
purifying apparatus for an internal combustion engine;
[0030] FIG. 2 is a perspective view of a principal portion of the
exhaust purifying apparatus of the first embodiment;
[0031] FIG. 3 is a vertical cross-sectional view of an accelerated
cooling portion of the exhaust purifying apparatus of the first
embodiment;
[0032] FIG. 4 is a diagram illustrating the configuration of an ECU
in the first embodiment;
[0033] FIG. 5 is a flowchart illustrating a basic control routine
to be executed in the first embodiment;
[0034] FIG. 6 is a flowchart illustrating an HC adsorption/purge
control routine to be executed in the first embodiment;
[0035] FIG. 7 is a flowchart illustrating a NOx catalyst
temperature control routine to be executed in the first
embodiment;
[0036] FIG. 8 is a flowchart illustrating an S-poisoning recovery
control routine to be executed in the first embodiment;
[0037] FIG. 9 is a flowchart illustrating a secondary air control
routine to be executed in the first embodiment;
[0038] FIG. 10 is a graph indicating an example of the relationship
between the midbed temperature of an occlusion-reduction type NOx
catalyst and its NOx removal efficiency;
[0039] FIG. 11 is a graph indicating a relationship between the
vehicle speed and the catalyst midbed temperature measured when
exhaust gas is caused to flow through only one of a first passage
and a second passage;
[0040] FIG. 12 is a diagram illustrating the construction of a
principal portion of a second embodiment of the invention in the
form of an exhaust purifying apparatus for an internal combustion
engine;
[0041] FIG. 13 is a perspective view of the principal portion of
the exhaust purifying apparatus of the second embodiment;
[0042] FIG. 14 is a schematic perspective view of an exhaust
switching valve used in the second embodiment;
[0043] FIG. 15 is a cross-sectional view of a first passage and a
second passage in the second embodiment;
[0044] FIG. 16 is a diagram illustrating the construction of a
principal portion of another embodiment of the engine exhaust
purifying apparatus of the invention; and
[0045] FIG. 17 is a diagram illustrating a principal portion of
still another embodiment of the engine exhaust purifying apparatus
of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0046] Referring to FIGS. 1 to 17, preferred embodiments of an
exhaust purifying apparatus for an internal combustion engine of
the invention will be described in detail. In each of the following
embodiments, the engine exhaust purifying apparatus of the
invention is employed in a vehicular direct injection type
lean-burn gasoline engine capable of lean combustion.
[0047] Referring first to FIGS. 1 to 11, an exhaust purifying
apparatus for an internal combustion engine according to a first
embodiment of the invention will be described. Initially, the
construction of the engine exhaust purifying apparatus according to
the first embodiment will be described with reference to FIGS. 1 to
4.
[0048] An engine body 1 of the first embodiment shown in FIG. 1 is
a main body of an in-line four-cylinder engine. Intake ports of the
cylinders of the engine body 1 are connected to a surge tank 3 via
corresponding intake branch pipes 2. The surge tank 3 is connected
to an air cleaner 5 via an intake pipe 4. A throttle valve 6 is
provided within the intake pipe 4. An air flow meter 8 that is
disposed upstream of the throttle valve 6 is adapted to output an
electric signal corresponding to the amount of air flowing in the
intake pipe 4. Furthermore, the cylinders are provided with fuel
injection valves 7 that inject fuel directly into the cylinders.
The fuel injection valves 7 and an ECU 90 as described later
constitute an exhaust air-fuel ratio control unit.
[0049] Exhaust ports of the respective cylinders of the engine body
1 are connected to an exhaust pipe 10 via an exhaust manifold 9, so
that the exhaust gases flowing from these cylinders meet or combine
in the exhaust pipe 10. The exhaust pipe 10 is provided with an
upstream-side O.sub.2 sensor 11 that outputs an electric signal
corresponding to the oxygen concentration of exhaust gas flowing in
the exhaust pipe 10.
[0050] The exhaust pipe 10 is connected to a casing 13 that houses
a catalyst 12 for cold start. The cold-start catalyst 12 is a
catalyst for substantially purifying exhaust gas upon a start of
the engine at which a NOx catalyst 61 as described later has not
been activated. For example, the cold-start catalyst 12 consists of
a two-way catalyst formed by loading an alumina support with a
precious metal such as platinum Pt.
[0051] The casing 13 is connected to a first passage 20 and to a
second passage 40 via an exhaust pipe 14. The first passage 20 and
the second passage 40 have substantially equal lengths, and are
disposed in parallel with each other.
[0052] The first passage 20 includes a tubular portion 21 having a
cylindrical shape (with a circular cross section) which is
connected to the exhaust pipe 14. The tubular portion 21 is
connected to an accelerated cooling portion 23 that has a generally
arcuate cross-sectional shape as shown in FIG. 3, with a
sectional-shape changing portion 22 interposed therebetween.
Furthermore, the accelerated cooling portion 23 is connected to a
tubular portion 25 having a cylindrical shape, with a
sectional-shape changing portion 24 interposed therebetween. The
tubular portion 25 is connected to a casing 27 that houses an HC
adsorbent 26. The casing 27 is connected to a tubular portion 28
having a cylindrical shape.
[0053] The accelerated cooling portion 23 is formed with a
generally arcuate shape in cross section to provide increased heat
dissipation, so as to cause the temperature of exhaust gas flowing
in the first passage 20 to fall or decrease at a relatively high
rate. A cross-sectional shape and dimensions of the accelerated
cooling portion 23 and a length of this cooling portion 23 in an
axial direction thereof (i.e., the length as measured in the
direction of flow of exhaust gas) are set so as to achieve a
desired heat dissipation effect.
[0054] As shown in FIG. 3, an interfering plate 29 is provided in
one of opposite arc end portions of the accelerated cooling portion
23, for averaging different flow rates of exhaust gas flowing in
the accelerated cooling portion 23, which flow rates are
distributed along the arc of the cooling portion 23. Thus, the
accelerated cooling portion 23 achieves efficient heat dissipation
by averaging the flow rates of the exhaust gas.
[0055] The sectional-shape changing portion 22 that extends from
the tubular portion 21 with a circular cross section has a
gradually varying cross-sectional shape, so as to smoothly connect
the tubular portion 21 to the accelerated cooling portion 23 having
an arcuate cross-sectional shape. Similarly, the sectional-shape
changing portion 24 that extends from the accelerated cooling
portion 23 with an arcuate cross-sectional shape has a gradually
varying cross-sectional shape, so as to smoothly connect the
accelerated cooling portion 23 to the tubular portion 25 with a
circular cross section.
[0056] The HC adsorbent 26 is an adsorbent for adsorbing unburned
HC that cannot be removed by the cold-start catalyst 12 at the time
of, for example, a cold start of the engine.
[0057] A first exhaust switching valve 52 adapted to be opened and
closed by a first actuator 51 is mounted in the tubular portion 21
of the first passage 20. The second passage 40, on the other hand,
has a cylindrical shape over the entire length thereof, and a
second exhaust switching valve 54 adapted to be opened and closed
by a second actuator 53 is mounted in the second passage 40.
[0058] In this embodiment, the first exhaust switching valve 52 and
the second exhaust switching valve 54 constitute a flow amount
control unit (or system) that controls the amount of exhaust gas
flow in the first passage 20 and the amount of exhaust gas flow in
the second passage 40.
[0059] With the first passage 20 and the second passage 40
constructed as described above, if the exhaust gas of the same
temperature is caused to flow through the first passage 20 and the
second passage 40 at the same flow rate, the temperature of the
exhaust gas decreases by a greater degree when it passes through
the first passage 20 than when it passes through the second passage
40.
[0060] The tubular portion 28 of the first passage 20 and the
second passage 40 are both connected to a junction pipe (junction
passage) 60. The junction pipe 60 is connected to a casing 62 that
houses an occlusion-reduction type NOx catalyst (hereinafter,
simply referred to as "NOx catalyst") 61, as one type of lean-bum
NOx catalyst. The casing 62 is connected to a casing 65 that houses
a three-way catalyst 64, via an exhaust pipe 63. Furthermore, the
casing 65 is connected to a muffler (not shown) via an exhaust pipe
66.
[0061] The NOx catalyst 61 is formed by, for example, loading an
alumina support with a precious metal, such as platinum Pt,
palladium Pd, rhodium Rh, or iridium Ir, and at least one substance
selected from alkali metals, such as potassium K, sodium Na,
lithium Li, and cesium Cs, alkaline earths, such as barium Ba and
calcium Ca, and rare earths, such as lanthanum La and yttrium
Y.
[0062] The NOx catalyst 61 performs NOx absorbing and releasing
operations; the NOx catalyst 61 absorbs NOx when the exhaust
air-fuel ratio is on a fuel-lean side, and releases the thus
absorbed NOx when the oxygen concentration in the incoming exhaust
gas is reduced. NOx released from the NOx catalyst 61 is reduced
into N.sub.2 by a reducing agent. Here, the exhaust air-fuel ratio
refers to the ratio of the total amount of the air to the total
amount of fuel and the total amount of the reducing agent supplied
into the engine intake passage and a portion of the exhaust passage
upstream of the NOx catalyst 61. When neither fuel (hydrocarbon)
nor air is supplied into the exhaust passage upstream of the NOx
catalyst 61, the exhaust air-fuel ratio equals the ratio of the
total amount of air to the total amount of fuel supplied into the
cylinders.
[0063] In the exhaust pipe 63 are mounted an exhaust temperature
sensor (temperature detector) 67 that outputs an electric signal
proportional to the temperature of exhaust gas flowing in the
exhaust pipe 63, and a downstream-side O.sub.2 sensor 68 that
outputs an electric signal corresponding to the oxygen
concentration of the exhaust gas flowing in the exhaust pipe
63.
[0064] In this embodiment, the exhaust gas temperature detected by
the exhaust temperature sensor 67 is also used to represent the
catalyst midbed temperature of the NOx catalyst 61 or the catalyst
midbed temperature of the three-way catalyst 64.
[0065] A secondary air supply pipe 69 is connected to the junction
pipe 60. A secondary air supplying device 70 connected to the
secondary air supply pipe 69 is operable to supply secondary air
into the junction pipe 60.
[0066] As shown in FIG. 4, the engine-controlling electronic
control unit (ECU) 90, which is in the form of a digital computer,
includes a ROM (read-only memory) 92, a RAM (random access memory)
93, a CPU (microprocessor) 94, a B-RAM (backup RAM) 95 constantly
supplied with power, an input port 96, and an output port 97. These
components of the ECU 90 are interconnected by a bidirectional bus
91.
[0067] Output signals of the air flow meter 8, the upstream-side
O.sub.2 sensor 11, the exhaust temperature sensor 67, the
downstream-side O.sub.2 sensor 68, etc., are received by the input
port 96 of the ECU 90 via corresponding A/D converters 98.
Furthermore, output pulses representing the engine speed N are
transmitted from an engine speed sensor (not shown) to the input
port 96 of the ECU 90.
[0068] The output port 97 of the ECU 90 is electrically connected
to the fuel injection valves 7 of the cylinders, ignition plugs
(not shown) of the cylinders, the first actuator 51, the second
actuator 53, the secondary air supplying device 70, etc., via
corresponding drive circuits 99.
[0069] In the meantime, air-fuel ratio control is performed on the
engine of this embodiment by varying the air-fuel ratio depending
upon the operating state of the engine. That is, if lean-burn
conditions are established in the engine, the air-fuel ratio of a
mixture to be burned in each cylinder is controlled to a fuel-lean
side of the stoichiometric air-fuel ratio (which will be referred
to as "lean control"). If the lean-burn conditions are not
established, the air-fuel ratio of a mixture to be burned in each
cylinder is controlled to the stoichiometric air-fuel ratio (which
will be referred to as "stoichiometric control"). For example, when
the engine load is higher than a set load, or when an engine
warm-up operation is being performed, or when the NOx catalyst 61
is not in an activated state, it is determined that the lean-burn
conditions are not established, and the stoichiometric control is
performed. In other cases, it is determined that the lean-burn
conditions are established, and the lean control is performed.
[0070] Since the air-fuel ratio of exhaust gas discharged from the
engine during the lean control is on the fuel-lean side, NOx
contained in the exhaust gas is absorbed into the NOx catalyst 61
during the lean control. However, since the NOx absorbing
capability of the NOx catalyst 61 is limited, it is necessary to
release NOx from the NOx catalyst 61 before the NOx absorbing
capability of the NOx catalyst 61 becomes saturated. In this
embodiment, therefore, when the amount of NOx absorbed in the NOx
catalyst 61 exceeds a predetermined amount, the air-fuel ratio of a
mixture to be burned in each cylinder is temporarily set to the
fuel-rich side of the stoichiometric air-fuel ratio so as to
release NOx from the NOx catalyst 61 and reduce NOx. This air-fuel
ratio control is termed lean/rich spike control.
[0071] The NOx removal efficiency of the NOx catalyst 61 varies
depending on the catalyst midbed temperature as indicated in FIG.
10. The NOx removal efficiency of the NOx catalyst 61 is low when
the catalyst midbed temperature is excessively low or high. The NOx
catalyst 61 exhibits considerably high NOx removal efficiencies
within a predetermined temperature range.
[0072] In this embodiment, therefore, the first passage 20, which
provides a relatively large exhaust gas temperature decrease, and
the second passage 40, which provides a relatively small exhaust
gas temperature decrease, are selectively used depending upon the
situation, so as to keep the catalyst midbed temperature of the NOx
catalyst 61 within a temperature range in which the NOx removal
efficiency is high (hereinafter, referred to as "high-NOx-removal
temperature range"). By controlling the catalyst midbed temperature
of the NOx catalyst 61 in this manner, the NOx catalyst 61 is made
less likely to be exposed to high temperatures, whereby the
progress of heat deterioration of the NOx catalyst 61 can be
retarded. The switching between the exhaust gas channels is carried
out by using the first exhaust switching valve 52 and the second
exhaust switching valve 54. The temperature control of the NOx
catalyst 61 will be described in detail below.
[0073] Fuel normally contains sulfur (S). When sulfur contained in
fuel is burned, oxides of sulfur (SOx), such as SO.sub.2 and
SO.sub.3, are produced. The NOx catalyst 61 also absorbs the SOx in
exhaust gas. While SOx form sulfates in the NOx catalyst 61, the
sulfates thus formed are stable and less likely to be decomposed.
That is, sulfates cannot be decomposed only by passing rich
air-fuel ratio exhaust gas through the NOx catalyst 61, but remain
in the NOx catalyst 61. As the amount of sulfates produced
increases, the NOx absorbing capability of the NOx catalyst 61 is
reduced. This is generally termed "S-poisoning".
[0074] However, sulfates produced in the NOx catalyst 61 can be
decomposed and released from the NOx catalyst 61 in the form of
SO.sub.3 by controlling the air-fuel ratio of incoming exhaust gas
to an air-fuel ratio that is slightly to the fuel-rich side of the
stoichiometric air-fuel ratio (hereinafter, referred to as
"slightly rich air-fuel ratio") when the temperature of the NOx
catalyst 61 is higher than a predetermined temperature
(hereinafter, referred to as "SOx releasing temperature"). In this
embodiment, therefore, when the amount of SOx absorbed in the NOx
catalyst 61 exceeds a predetermined prescribed amount, the air-fuel
ratio of the incoming exhaust gas is controlled to a slightly rich
air-fuel ratio (e.g., about 13.5 to about 14.3) and the NOx
catalyst 61 is heated so that SOx is released from the NOx catalyst
61. This process will be referred to as "S-poisoning recovery
process" of the NOx catalyst 61. SO.sub.3 released from the NOx
catalyst 61 by the S-poisoning recovery process is immediately
reduced into SO.sub.2 by HC and CO present in the incoming exhaust
gas.
[0075] During the S-poisoning recovery process in this embodiment,
secondary air is supplied to the junction pipe 60 in order to
increase the temperature of the NOx catalyst 61 so that SOx is
released from the NOx catalyst 61. During the S-poisoning recovery
process, the engine is operated at a slightly rich air-fuel ratio,
so that exhaust gas contains a large amount of unburned HC.
Therefore, when secondary air is supplied into the junction pipe
60, unburned HC contained in the exhaust gas is oxidized on the NOx
catalyst 61 by oxygen present in a large amount in the secondary
air. Reaction heat produced by the oxidation increases the
temperature of the NOx catalyst 61. Furthermore, during the
S-poisoning recovery process, the exhaust gas is caused to flow
through the second passage 40, so as to suppress an exhaust gas
temperature decrease and thereby accelerate heating of the NOx
catalyst 61.
[0076] Immediately after the S-poisoning recovery process, the
catalyst midbed temperature of the NOx catalyst 61 is very high,
and the NOx removal efficiency is low. Thus, after the S-poisoning
recovery process is completed in this embodiment, the exhaust gas
channel is switched from the second passage 40 to the first passage
20 that provides a relatively large exhaust gas temperature
decrease, and secondary air is supplied to the junction pipe 60, in
order to quickly cool the NOx catalyst 61 to a temperature at which
the NOx removal efficiency is high. This process will be referred
to as "NOx catalyst cooling process".
[0077] The S-poisoning recovery process and the NOx catalyst
cooling process will be described in detail below.
[0078] When the air-fuel ratio is controlled to the stoichiometric
air-fuel ratio, and when the air-fuel ratio is controlled to a
slightly rich air-fuel ratio, the ECU 90 performs main feedback
control of the amount of fuel injected, based on an output value of
the upstream-side O.sub.2 sensor 11, and also performs subsidiary
feedback control of the amount of fuel injected, based on an output
value of the downstream-side O.sub.2 sensor 68 for further improved
control performance.
[0079] The operation of the exhaust purifying apparatus for the
engine according to the present embodiment will be now described
with reference to the flowcharts of FIGS. 5 to 9.
[0080] The flowchart of FIG. 5 illustrates a basic control routine
of the engine exhaust purifying apparatus of this embodiment. The
basic control routine is pre-stored in the ROM 92 of the ECU 90,
and is repeatedly executed at certain time intervals set in advance
by the CPU 94.
[0081] First, the ECU 90 executes HC adsorption/purge control in
step 100. The HC adsorption/purge control is performed so as to
pass exhaust gas through the first passage 20 and cause the HC
adsorbent 26 to adsorb unburned HC in exhaust gas during a cold
start of the engine, so that unburned HC produced in a large amount
during the cold start is prevented from being released to the
atmosphere. In addition, the adsorbed HC is desorbed from the HC
adsorbent 26 when the engine is brought into a suitable operating
state, and the HC thus desorbed is oxidized and removed by the NOx
catalyst 61 or by the three-way catalyst 64 disposed downstream of
the HC adsorbent 26. The HC adsorption/purge control will be
described in detail below.
[0082] After executing the HC adsorption/purge control, the ECU 90
proceeds to step 200 to perform temperature control of the NOx
catalyst 61. Under the NOx catalyst temperature control, the
catalyst midbed temperature of the NOx catalyst 61 always remains
within an optimal temperature range in which the NOx removal
efficiency is high. More specifically, when the catalyst midbed
temperature is about to be on the high-temperature side outside the
optimal temperature range, the ECU 90 causes exhaust gas to flow
through the first passage 20 so as to increase heat dissipation, so
that the temperature of exhaust gas flowing into the NOx catalyst
61 is lowered and the catalyst midbed temperature of the NOx
catalyst 61 is reduced. Conversely, when the catalyst midbed
temperature is about to be on the low temperature side outside the
optimal temperature range, the ECU 90 causes exhaust gas to flow
through the second passage 40 so as to reduce heat dissipation, so
that the temperature of exhaust gas flowing into the NOx catalyst
61 is raised and the catalyst midbed temperature of the NOx
catalyst 61 is increased. The NOx catalyst midbed temperature
control will be described in detail later.
[0083] After executing the NOx catalyst temperature control, the
ECU 90 proceeds to step 300 in which the ECU 90 updates an
S-poisoning counter related to the NOx catalyst 61. The S-poisoning
counter functions to make up or adjust a physical quantity, such as
an integrated amount of fuel consumed or an integrated amount of
exhaust gas, which can be a substitute for the S consumption
amount, so as to estimate the amount of S-poisoning of the NOx
catalyst 61. The S-poisoning counter is reset when the S-poisoning
recovery process is completed.
[0084] Subsequently, the ECU 90 proceeds to step 400 in which the
ECU 90 determines whether at least one of the following conditions
is met: a condition that the count value of the S-poisoning counter
is equal to or larger than a preset value, and a condition that an
exhaust cooling flag F1 is "1". If the count value of the
S-poisoning counter is equal to or larger than the preset value, it
means that the S-poisoning of the NOx catalyst 61 has progressed
and it is time to execute the S-poisoning recovery process. If the
exhaust cooling flag F1 is "1", it is time to supply the secondary
air to the exhaust passage in order to promptly cool the NOx
catalyst 61 to a temperature at which the exhaust can be purified
of NOx after the S-poisoning recovery process.
[0085] If the count value of the S-poisoning counter is equal to or
larger than the preset value, or if the exhaust cooling flag F1 is
"1", an affirmative decision (YES) is obtained in step 400, and the
ECU 90 goes to step 500 to perform an S-poisoning recovery control.
In the S-poisoning recovery control, SOx absorbed in the NOx
catalyst 61 is desorbed from the NOx catalyst 61 and discharged in
the form of SO.sub.2. In addition, the S-poisoning recovery control
is performed so as to lower the catalyst midbed temperature of the
NOx catalyst 61 down to a temperature at which the catalyst 61
shows a high NOx removing capability, since the NOx catalyst 61 has
a high temperature and a low NOx removing capability immediately
after the desorption of SOx from the catalyst 61. The S-poisoning
control will be described in detail later.
[0086] If a negative decision (NO) is obtained in step 400, or
after the ECU 90 executes the S-poisoning recovery control in step
500, the ECU 90 proceeds to step 600 to perform secondary air
control. The secondary air control is executed to supply secondary
air into the exhaust passage when necessary. After executing step
600, the ECU 90 temporarily ends execution of the routine.
[0087] Next, the HC adsorption/purge control of step 100 will be
described with reference to an HC adsorption/purge control routine
illustrated in FIG. 6.
[0088] First in step 101, the ECU 90 accesses a storage area of an
adsorption completion flag F2 and a storage area of an adsorption
prohibition flag F3 that are set in advance in predetermined areas
of the RAM 93, and determines whether at least one of the following
conditions is met: a condition that the adsorption completion flag
F2 is "0", and a condition that the adsorption prohibition flag F3
is "0".
[0089] In the storage area of the adsorption completion flag F2,
"1" is stored when a predetermined amount of HC has been adsorbed
on the HC adsorbent 26 during a cold start of the engine, and the
initial value "0" is stored when the engine is stopped. Thus, the
initial value "0" is always stored in the storage area of the
adsorption completion flag F2 every time the engine is started.
[0090] In the storage area of the adsorption prohibition flag F3,
"1" is stored when the exhaust gas temperature becomes equal to or
higher than a predetermined value, and the initial value "0" is
stored when the engine is stopped. Thus, the initial value "0" is
always stored in the storage area of the adsorption prohibition
flag F3 every time the engine is started.
[0091] If the adsorption completion flag F2 is "0" or if the
adsorption prohibition flag F3 is "0", the ECU 90 makes an
affirmative decision in step 101, and proceeds to step 102 to
determine whether an HC adsorption condition is established. The HC
adsorption condition is established when the cold-start catalyst 12
has not reached the activation temperature. Whether the condition
is established or not is determined based on the engine cooling
water temperature, the integrated amount of exhaust gas that has
been emitted since the start of the engine, or the like. Therefore,
the HC adsorption condition is established when the engine is
started in a cold state in which a great amount of unburned HC is
produced.
[0092] If an affirmative decision (YES) is obtained in step 102,
the ECU 90 proceeds to step 103 to operate the first actuator 51 so
as to fully open the first exhaust switching valve 52, and operate
the second actuator 53 so as to completely close the second exhaust
switching valve 54, thereby causing exhaust gas to flow through the
first passage 20. When the HC adsorption condition is met, the HC
adsorbent 26 is at an adsorption temperature and the exhaust gas
temperature is low. If exhaust gas is passed through the first
passage 20, therefore, unburned HC contained in the exhaust gas is
adsorbed to the HC adsorbent 26. Furthermore, since exhaust gas is
cooled while passing through the accelerated cooling portion 23 of
the first passage 20, the temperature of the exhaust gas flowing
into the HC adsorbent 26 is lowered, with results of an increase in
the HC adsorption rate of the HC adsorbent 26 and an increase in
the HC adsorption time.
[0093] After executing step 103, the ECU 90 proceeds to step 104 in
which the ECU 90 determines whether the integrated amount of
exhaust gas after the HC adsorption condition is established is
equal to or larger than a predetermined value. The predetermined
value of the integrated amount of exhaust gas herein is set at an
amount that is needed before the temperature of the HC adsorbent 26
becomes equal to or higher than an HC release temperature, and is
empirically determined and stored in advance in the ROM 92.
[0094] If an affirmative decision (YES) is obtained in step 104,
the ECU 90 proceeds to step 105 to rewrite the value in the storage
area of the adsorption completion flag F2 from "0" to "1". The ECU
90 then proceeds to step 109. If a negative decision (NO) is
obtained in step 104, on the other hand, the ECU 90 proceeds from
step 104 to step 109. Therefore, as long as the HC adsorption
condition is established, exhaust gas flows through the first
passage 20 until the integrated amount of exhaust gas following the
establishment of the HC adsorption condition reaches or exceeds the
predetermined value, and an affirmative decision (YES) is obtained
in step 104.
[0095] Conversely, if a negative decision (NO) is obtained in step
102, the ECU 90 proceeds to step 106 to operate the first actuator
51 so as to completely close the first exhaust switching valve 52,
and operate the second actuator 53 so as to fully open the second
exhaust switching valve 54, thereby causing exhaust gas to flow
through the second passage 40.
[0096] The reason for switching the channel of exhaust gas from the
first passage 20 to the second passage 40 is as follows. When the
adsorption condition is not established, the cold-start catalyst 12
has reached the activation temperature so that the cold-start
catalyst 12 is able to substantially remove HC from exhaust gas.
Furthermore, when the adsorption condition is not established, it
can be assumed that the temperature of the HC adsorbent 26 has
become equal to or higher than the HC release temperature.
Therefore, if the exhaust gas is caused to flow through the first
passage 20 in this situation, there is a danger that HC may be
released from the HC adsorbent 26 to degrade exhaust emission.
[0097] After executing step 106, the ECU 90 proceeds to step 107 to
determine whether the exhaust gas temperature is equal to or higher
than a predetermined temperature, based on the output value of the
exhaust temperature sensor 67. In step 107, the exhaust gas
temperature is used to represent the catalyst midbed temperature of
the three-way catalyst 64. The exhaust gas temperature being equal
to or higher than the predetermined level means that the
temperature of the three-way catalyst 64 is equal to or higher than
its activation temperature. The predetermined level of the exhaust
gas temperature is empirically determined and stored in advance in
the ROM 92.
[0098] If an affirmative decision (YES) is obtained in step 107, it
is considered that the three-way catalyst 64 is activated, and
therefore the ECU 90 proceeds to step 108. In step 108, the ECU 90
rewrites the value in the storage area of the adsorption
prohibition flag F3 from "0" to "1". After that, the ECU 90
proceeds to step 109. Conversely, if a negative decision (NO) is
obtained in step 107, the ECU 90 proceeds from step 107 to step
109.
[0099] In step 109, the ECU 90 determines whether the adsorption
prohibition flag F3 is "1". If a negative decision (NO) is obtained
in step 109, the ECU 90 temporarily ends execution of the routine.
That is, the ECU 90 does not proceed to steps 110-115 and 116, and
does not perform HC purge from the HC adsorbent 26 until the
adsorption prohibition flag F3 becomes "1".
[0100] When the ECU 90 executes this routine after rewriting the
adsorption completion flag F2 to "1" in step 105 and rewriting the
adsorption prohibition flag F3 to "1" in step 108, a negative
decision (NO) is obtained in step 101, and the ECU 90 proceeds from
step 101 to step 109.
[0101] If the ECU 90 makes an affirmative decision (YES) in step
109, the ECU 90 proceeds to step 110 to access a storage area of an
HC purge completion flag F4 that is set in advance in a
predetermined area of the RAM 93, and determine whether "0" is
stored in the storage area.
[0102] In the storage area of the HC purge completion flag F4, "1"
is stored when the integrated value of the amount of exhaust gas
during the HC purge from the HC adsorbent 26 becomes equal to or
greater than a predetermined value, and the initial value "0" is
stored when the engine is stopped. Thus, the initial value "0" is
stored in the storage area of the HC purge completion flag F4 every
time the engine is started.
[0103] If an affirmative decision (YES) is obtained in step 110,
the ECU 90 proceeds to step 111 to determine whether an HC purge
execution condition is established. The HC purge execution
condition is that the catalyst midbed temperature of the three-way
catalyst 64 is equal to or higher than the activation temperature
AND the engine is being operated in a lean control mode or is in
fuel-cut operation. This is because desorption of HC from the HC
adsorbent 26 when the air-fuel ratio of exhaust gas is on the lean
side results in reduced burdens on the NOx catalyst 61 and the
three-way catalyst 64, thus preventing deterioration of emission.
Furthermore, if the catalyst midbed temperature of the three-way
catalyst 64 is lower than the activation temperature, HC purged
from the HC adsorbent 26 cannot be substantially removed by the
three-way catalyst 64, resulting in degraded emission. In this
embodiment, the catalyst midbed temperature of the three-way
catalyst 64 is represented by the exhaust gas temperature detected
by the exhaust temperature sensor 67.
[0104] If an affirmative decision (YES) is obtained in step 111,
namely, if the HC purge execution condition is established, the ECU
90 proceeds to step 112. In step 112, the ECU 90 operates the first
actuator 51 so as to fully open the first exhaust switching valve
52, and operates the second actuator 53 so as to completely close
the second exhaust switching valve 54, thereby causing exhaust gas
to flow through the first passage 20.
[0105] When exhaust gas is caused to flow through the first passage
20, HC that has been adsorbed on the HC adsorbent 26 are purged
since the temperature of the HC adsorbent 26 is equal to or higher
than the HC release temperature. Purged HC flow together with
exhaust gas through the NOx catalyst 61 and the three-way catalyst
64, and is substantially removed by oxidation at the NOx catalyst
61 or the three-way catalyst 64.
[0106] After executing step 112, the ECU 90 proceeds to step 113 to
update a purge integrated gas amount counter that integrates the
amount of exhaust gas during HC purge from the HC adsorbent 26 from
the start of the purge, and thus integrate the amount of exhaust
gas during the HC purge. The purge integrated gas amount counter is
reset to initial value "0" when the engine is stopped.
[0107] After integrating the exhaust gas during HC purge in step
113, the ECU 90 proceeds to step 114 to determine whether the count
value of the purge integrated gas amount counter is equal to or
larger than a predetermined value, that is, whether the integrated
amount of exhaust gas during HC purge is equal to or larger than a
predetermined value. The predetermined value of the integrated
amount of exhaust gas during HC purge is an amount of exhaust gas
that is needed in order to purge the entire amount of HC adsorbed
on the HC adsorbent 26, and is empirically determined and stored in
advance in the ROM 92.
[0108] If a negative decision (NO) is obtained in step 114, the ECU
90 temporarily ends this routine. Therefore, exhaust gas is caused
to flow through the first passage 20 and HC purge from the HC
adsorbent 26 continues until the integrated amount of exhaust gas
during the HC purge becomes equal to or greater than the
predetermined value.
[0109] If an affirmative decision (YES) is obtained in step 114,
the ECU 90 proceeds to step 115 to rewrite the value in the storage
area of the HC purge completion flag F4 from "0" to "1".
Subsequently, the ECU 90 temporarily ends execution of the
routine.
[0110] When the ECU 90 executes the next and subsequent cycles of
this routine, the ECU 90 makes a negative decision (NO) in step
110, and then temporarily ends the routine. In this case,
therefore, the ECU 90 does not execute steps 111-115 and 116.
[0111] If the purge execution condition is not established or
satisfied any longer during the next cycle of the routine while HC
is being purged from the HC adsorbent 26, the ECU 90 makes a
negative decision (NO) in step 111, and proceeds to step 116. In
step 116, the ECU 90 operates the first actuator 51 so as to
completely close the first exhaust switching valve 52, and operates
the second actuator 53 so as to fully open the second exhaust
switching valve 54, thereby causing exhaust gas to flow through the
second passage 40. As a result, HC purge from the HC adsorbent 26
is temporarily stopped. The HC purge is restarted when the purge
execution condition is satisfied again.
[0112] As described above, by executing the HC adsorption/purge
control, unburned HC discharged upon a start of the engine, in
particular, unburned HC discharged in a large amount during a cold
start of the engine, is prevented from being discharged into the
atmosphere.
[0113] Next, the NOx catalyst temperature control of step 200 in
the basic control routine in FIG. 5 will be described with
reference to a NOx catalyst temperature control routine illustrated
in FIG. 7.
[0114] First in step 201, the ECU 90 determines whether both the
adsorption completion flag F2 and the HC purge completion flag F4
are "1". If a negative decision (NO) is obtained in step 201, the
ECU 90 proceeds to step 202. If an affirmative decision (YES) is
obtained in step 201, the ECU 90 proceeds to step 203.
[0115] In step 202, the ECU 90 determines whether the adsorption
prohibition flag F3 is "1". If an affirmative decision (YES) is
obtained in step 202, the ECU 90 proceeds to step 203. If a
negative decision (NO) is obtained in step 202, the ECU 90
temporarily ends execution of this routine. Thus, the ECU 90 does
not proceed to step 203 unless purge of HC adsorbed to the HC
adsorbent 26 is completed or the three-way catalyst 64 has been
activated.
[0116] In step 203, the ECU 90 accesses a storage area of an
S-poisoning recovery control flag F5, and determines whether "0" is
stored therein. In the storage area of the S-poisoning recovery
control flag F5, "1" is stored when the S-poisoning recovery
process of desorbing SOx absorbed in the NOx catalyst 61 is being
executed, and "0" is stored when the NOx catalyst cooling process
is being executed after the S-poisoning recovery process has been
completed.
[0117] If a negative decision is obtained in step 203, that is, if
the S-poisoning recovery process is being performed on the NOx
catalyst 61, the ECU 90 temporarily ends execution of the
routine.
[0118] If an affirmative decision (YES) is obtained in step 203,
the ECU 90 proceeds to step 204 to determine whether the catalyst
midbed temperature of the NOx catalyst 61 is lower than a specified
value a. In this embodiment, the exhaust gas temperature detected
by the exhaust temperature sensor 67 represents the catalyst midbed
temperature of the NOx catalyst 61.
[0119] If the ECU 90 makes an affirmative decision (YES) in step
204, the ECU 90 proceeds to step 205. In step 205, the ECU 90
operates the first actuator 51 so as to completely close the first
exhaust switching valve 52, and operates the second actuator 53 so
as to fully open the second exhaust switching valve 54, thereby
causing exhaust gas to flow through the second passage 40. As
described above, the temperature of exhaust gas flowing through the
second passage 40 decreases by a smaller degree than that of
exhaust gas flowing through the first passage 20. Therefore, the
temperature of exhaust gas flowing into the NOx catalyst 61 can be
increased. As a result, the catalyst midbed temperature of the NOx
catalyst 61 can be kept within a temperature range in which a high
NOx removal efficiency is achieved.
[0120] If a negative decision (NO) is obtained in step 204, the ECU
90 proceeds to step 206 in which the ECU 90 determines whether the
catalyst midbed temperature of the NOx catalyst 61 is equal to or
higher than a specified value .beta.. The specified value .beta. is
higher than the above-indicated specified value
.alpha.(.beta.>.alpha.).
[0121] If an affirmative decision (YES) is obtained in step 206,
the ECU 90 proceeds to step 207. In step 207, the ECU 90 operates
the first actuator 51 so as to fully open the first exhaust
switching valve 52, and operates the second actuator 53 so as to
completely close the second exhaust switching valve 54, thereby
causing exhaust gas to flow through the first passage 20. The
temperature of the exhaust gas flowing through the first passage 20
decreases by a greater degree than that of exhaust gas flowing
through the second passage 40. Thus, the temperature of exhaust gas
flowing into the NOx catalyst 61 is lowered, so that the catalyst
midbed temperature of the NOx catalyst 61 can be kept within a
temperature range in which a high NOx removal efficiency is
achieved.
[0122] If a negative decision (NO) is obtained in step 206, the ECU
90 temporarily ends execution of the routine. Therefore, when the
catalyst midbed temperature of the NOx catalyst 61 is equal to or
greater than the specified value .alpha. but is less than the
specified value .beta., the current exhaust gas channel continues
to be used without being switched to the other channel, thus
reducing the frequency of switching between the exhaust gas
channels.
[0123] The specified value .alpha., which serves as a threshold for
the exhaust gas channel switching, is a minimum temperature at
which the NOx removal efficiency can be kept high. The specified
value .beta. is a maximum temperature at which the NOx removal
efficiency can be kept high. The specified values .alpha., .beta.
may be simply fixed values, or may be changed as follows: since the
temperature characteristic of the NOx removal efficiency varies
depending on the degree of heat deterioration of the NOx catalyst,
the degree of heat deterioration of the NOx catalyst may be
detected, and the specified values .alpha., .beta. may be changed
in accordance with the detected degree of heat deterioration.
[0124] FIG. 11 indicates an empirically obtained example of the
relationship between the vehicle speed and the catalyst midbed
temperature of the NOx catalyst 61 when exhaust gas is caused to
flow through the first passage 20 or the catalyst midbed
temperature of the NOx catalyst 61 when exhaust gas is caused to
flow through the second passage 40. As indicated in FIG. 11, when
exhaust gas is passed through the first passage 20, the NOx
catalyst 61 cannot be kept within a high-NOx removal temperature
region if the vehicle speed is low. When exhaust gas is passed
through the second passage 40, the NOx catalyst 61 cannot be kept
within the high-NOx removal temperature region if the vehicle speed
is high. If the exhaust gas channel is switched between the first
passage 20 or the second passage 40 in accordance with the catalyst
midbed temperature of the NOx catalyst 61 as described above, it
becomes possible to keep the catalyst midbed temperature of the NOx
catalyst 61 within the high-NOx removal temperature region over a
broad vehicle-speed range from a low vehicle speed to a high
vehicle speed.
[0125] Next, the S-poisoning recovery control of step 500 in the
basic control routine of FIG. 5 will be described with reference to
an S-poisoning recovery control routine as illustrated in FIG.
8.
[0126] First in step 501, the ECU 90 determines whether the exhaust
cooling flag F1 is "0".
[0127] If an affirmative decision (YES) is obtained in step 501, it
means that the count value of the above-indicated S-poisoning
counter is equal to or greater than a predetermined value. In this
case, the ECU 90 proceeds to step 502 in which the ECU 90 sets the
S-poisoning recovery control flag F5 to "1". The S-poisoning
recovery control flag F5=1 means that the S-poisoning recovery
process is being performed.
[0128] Subsequently to step 502, the ECU 90 proceeds to step 503 in
which the ECU 90 (1) causes exhaust gas to flow through the second
passage 40, (2) supplies secondary air into the exhaust gas, and
(3) operates the engine by controlling the air-fuel ratio to a
slightly rich air-fuel ratio.
[0129] More specifically, the ECU 90 first operates the first
actuator 51 so as to completely close the first exhaust switching
valve 52, and operates the second actuator 53 so as to fully open
the second exhaust switching valve 54, thereby causing exhaust gas
to flow through the second passage 40. As a result, the temperature
fall of exhaust gas occurring until the exhaust gas reaches the NOx
catalyst 61 is reduced. Next, the ECU 90 operates the secondary air
supplying device 70 to supply secondary air from the secondary air
supply pipe 69 into the junction pipe 60 disposed upstream of the
NOx catalyst 61. Then, the ECU 90 controls the air-fuel ratio to a
slightly rich air-fuel ratio by using a fuel injection control
device.
[0130] If the engine is operated at a slightly rich air-fuel ratio,
exhaust gas having a slightly rich air-fuel ratio and containing a
large amount of unburned HC passes through the second passage 40,
and flows into the NOx catalyst 61.
[0131] The slightly rich exhaust gas containing a large amount of
unburned HC and the secondary air supplied from the secondary air
supply pipe 69 meet in the junction pipe 60 to form an
oxygen-excess exhaust gas containing a large amount of unburned HC,
which then flows into the NOx catalyst 61. As a result, oxygen and
unburned HC in the exhaust gas undergo oxidizing reaction on the
NOx catalyst 61, and heat produced by the reaction increases the
catalyst midbed temperature of the NOx catalyst 61. When the
catalyst midbed temperature of the NOx catalyst 61 becomes equal to
or higher than the SOx release temperature (e.g., 650.degree. C.),
SOx absorbed in the NOx catalyst 61 is desorbed from the NOx
catalyst 61, and is released in the form of SO.sub.2.
[0132] Unburned HC that was not oxidized on the NOx catalyst 61 is
oxidized on the three-way catalyst 64 disposed downstream of the
NOx catalyst 61. Therefore, the degree of the slight richness
achieved by control for establishing a slightly rich air-fuel ratio
in step 503 is preferably controlled such that unburned HC passing
through the NOx catalyst 61 can be substantially removed by the
three-way catalyst 64.
[0133] Exhaust gas passes through the cold-start catalyst 12 before
flowing into the second passage 40. When passing through the
cold-start catalyst 12, however, exhaust gas is not supplied with
secondary air yet and therefore has a very low oxygen
concentration. Thus, the amount of unburned HC oxidized on the
cold-start catalyst 12 is very small, and most unburned HC
contained in exhaust gas flows into the NOx catalyst 61.
[0134] Furthermore, by (1) changing the exhaust gas channel, (2)
supplying secondary air, and (3) performing slightly rich control
in this order as mentioned above with respect to the processing of
step 503, it is possible to prevent emission deterioration at the
time of a start of the S-poisoning recovery process.
[0135] After executing step 503, the ECU 90 proceeds to step 504 to
determine whether the catalyst midbed temperature of the NOx
catalyst 61 is equal to or higher than the SOx release temperature,
and whether the output value of the downstream-side O.sub.2 sensor
indicates a fuel-rich atmosphere or not. In this embodiment, the
exhaust gas temperature detected by the exhaust temperature sensor
67 is used to represent the catalyst midbed temperature of the NOx
catalyst 61.
[0136] In order to efficiently desorb SOx from the NOx catalyst 61
and release it in the form of SO.sub.2, it is necessary that the
catalyst midbed temperature of the NOx catalyst 61 be equal to or
higher than the SOx release temperature and that the air-fuel ratio
of exhaust gas at the NOx catalyst 61 be slightly rich. If the
output value of the downstream-side O.sub.2 sensor disposed
downstream of the NOx catalyst is on the rich side, it can be
determined that a slightly rich atmosphere exists in the casing 62
of the NOx catalyst 61 as well.
[0137] Furthermore, it is preferable to control the amount of
secondary air to be supplied by controlling the secondary air
supplying device 70 so that the air-fuel ratio of exhaust gas
flowing into the NOx catalyst 61 becomes equal to such an air-fuel
ratio that allows efficient reduction of SOx. It is also possible
to control the air-fuel ratio of exhaust gas flowing into the NOx
catalyst 61 by maintaining a constant amount of secondary air
supplied and controlling the air-fuel ratio of a mixture to be
supplied to the engine.
[0138] If an affirmative decision (YES) is obtained in step 504, it
follows that SOx is being released from the NOx catalyst 61, and
the ECU 90 proceeds to step 505. In step 505, the ECU 90 updates
the integrated gas amount counter that integrates the amount of
exhaust gas during the release of SOx.
[0139] Conversely, if a negative decision (NO) is obtained in step
504, the ECU 90 proceeds to step 511 to determine whether the
catalyst midbed temperature of the NOx catalyst 61 is lower than a
predetermined value T2 and whether the exhaust cooling flag F1 is
"1". Since the exhaust cooling flag F1 is "0" in the current cycle,
the ECU 90 makes a negative decision (NO) in step 511, and then
temporarily ends execution of the routine.
[0140] After executing step 505, the ECU 90 proceeds to step 506 to
determine whether the count value of the integrated gas amount
counter is equal to or greater than a specified value. The
specified value is a count value corresponding to an amount of gas
that is needed for substantially completely releasing SOx absorbed
in the NOx catalyst 61. The specified value is empirically
determined and stored in advance in the ROM 92.
[0141] If a negative decision (NO) is obtained in step 506, the ECU
90 proceeds to step 511 to determine whether the catalyst midbed
temperature of the NOx catalyst 61 is lower than the predetermined
value T2 and the exhaust cooling flag F1 is "1". Since the exhaust
cooling flag F1 is still "0" at this point of time, the ECU 90
makes a negative decision in step 511, and then temporarily ends
execution of the routine.
[0142] Conversely, if an affirmative decision (YES) is obtained in
step 506, the S-poisoning recovery process of the NOx catalyst 61
is considered as having been completed, and the ECU 90 proceeds to
step 507. In step 507, the ECU 90 starts a cooling process for
cooling the NOx catalyst 61 by (1) operating the first actuator 51
so as to fully open the first exhaust switching valve 52 and
operating the second actuator 53 so as to completely close the
second exhaust switching valve 54, thereby causing exhaust gas to
flow through the first passage 20, (2) switching air-fuel ratio
control performed by the fuel injection control device from the
slightly rich control to a feedback stoichiometric control based
solely on the output value of the upstream-side O.sub.2 sensor 11,
(3) setting the exhaust cooling flag F1 to "1", (4) resetting the
S-poisoning counter, and (5) resetting the integrated gas amount
counter for use in the S-poisoning recovery process.
[0143] Immediately after the S-poisoning recovery process of the
NOx catalyst 61 is completed, the catalyst midbed temperature of
the NOx catalyst 61 is considerably high and within a temperature
range in which the NOx removal efficiency is low. If exhaust gas
having a lean air-fuel ratio flows through the NOx catalyst 61 in
this condition, emission will deteriorate. It is, therefore,
desirable to quickly reduce the catalyst midbed temperature of the
NOx catalyst 61 after the S-poisoning recovery process is
completed. If exhaust gas is caused to flow through the first
passage 20, the temperature decrease of the exhaust gas occurring
before the exhaust gas reaches the junction pipe 60 increases due
to the cooling accelerating effect of the accelerated cooling
portion 23, whereby the catalyst midbed temperature of the NOx
catalyst 61 can be quickly reduced.
[0144] Since the secondary air continues to be supplied into the
junction pipe 60, exhaust gas that has been mixed with the
secondary air and further cooled flows into the NOx catalyst 61,
thus further accelerating the cooling of the NOx catalyst 61.
[0145] While the catalyst midbed temperature of the NOx catalyst 61
remains within a high temperature range in which the NOx removal
efficiency is low, the exhaust air-fuel ratio is kept at the
stoichiometric air-fuel ratio, so that exhaust gas can be purified
by the NOx catalyst 61 and the three-way catalyst 64. In this
manner, emission deterioration immediately after completion of the
S-poisoning recovery process can be prevented.
[0146] The reason why the stoichiometric control of the air-fuel
ratio in step 507 is the feedback control based solely on the
output value of the upstream-side O.sub.2 sensor 11 is as follows.
For a while after completion of the S-poisoning recovery process,
the output value of the downstream-side O.sub.2 sensor 68 is on the
rich side, and the air-fuel ratio may be falsely corrected toward
the lean side if subsidiary feedback control based on the output
value of the downstream-side O.sub.2 sensor 68 is also
performed.
[0147] When the ECU 90 executes the next cycle of this routine
after setting the exhaust cooling flag F1 to "1" in step 507, the
ECU 90 makes a negative decision in step 501, and proceeds to step
508 to set the S-poisoning recovery control flag F5 to "0".
[0148] Subsequently, the ECU 90 proceeds to step 509 to determine
whether the catalyst midbed temperature of the NOx catalyst 61 is
lower than a predetermined temperature T1 and whether the feedback
stoichiometric control based on the output value of the
upstream-side O.sub.2 sensor 11 is being executed. The
predetermined temperature T1 is a threshold for determining which
of stoichiometric exhaust gas and lean exhaust gas is to be caused
to flow.
[0149] If an affirmative decision (YES) is obtained in step 509,
the ECU 90 proceeds to step 510 to switch the air-fuel ratio
control performed by the fuel injection control device from the
feedback stoichiometric control based only on the output value of
the upstream-side O.sub.2 sensor 11 to normal control (i.e.,
air-fuel ratio control in accordance with the operating state of
the engine). After that, the ECU 90 proceeds to step 511.
[0150] If a negative decision (NO) is obtained in step 509, the ECU
90 proceeds to step 511. Thus, after the air-fuel ratio control is
switched to the feedback stoichiometric control based only on the
output value of the upstream-side O.sub.2 sensor 11, a negative
decision (NO) is obtained in step 509 and the feedback
stoichiometric control based only on the output value of the
upstream-side O.sub.2 sensor 11 is continued until the catalyst
midbed temperature of the NOx catalyst 61 becomes lower than the
predetermined temperature T1.
[0151] In step 511, the ECU 90 determines whether the catalyst
midbed temperature of the NOx catalyst 61 is lower than the
predetermined temperature T2 and the exhaust cooling flag F1 is
"1". The predetermined temperature T2 is set to a value that is
smaller than the predetermined temperature T1 (e.g., T1=550.degree.
C., and T2=500.degree. C.).
[0152] If the catalyst midbed temperature of the NOx catalyst 61 is
higher than or equal to the predetermined temperature T2, a
negative decision (NO) is obtained in step 511, and the ECU 90
temporarily ends execution of the routine. In this case, therefore,
the secondary air continues to be supplied into the junction pipe
60 so as to continue cooling of the NOx catalyst 61.
[0153] If the catalyst midbed temperature of the NOx catalyst 61
becomes lower than the predetermined temperature T2, an affirmative
decision (YES) is obtained in step 511. Then, the ECU 90 proceeds
to step 512 to rewrite the exhaust cooling flag F1 to "0". When the
exhaust cooling flag F1 is set to "0", the supply of secondary air
into the junction pipe 60 is stopped as described below in
conjunction with the secondary air control. In this manner, the NOx
catalyst cooling process following completion of the S-poisoning
recovery process is finished.
[0154] Furthermore, since "0" is stored in the exhaust cooling flag
F1 in step 512, a negative decision is obtained in step 400 in the
next cycle of the basic control routine of FIG. 5, and the
S-poisoning recovery control of step 500 is not executed.
[0155] In this embodiment, when an affirmative decision (YES) is
obtained in step 506 and the S-poisoning recovery process is
completed, the ECU 90 controls the engine air-fuel ratio to be
stoichiometric in step 507 while continuing the supply of secondary
air to a location upstream of the NOx catalyst 61, and thus causes
stoichiometric exhaust gas and secondary air to flow into the NOx
catalyst 61 via the first passage 20. However, this process may
result in a shift of the air-fuel ratio of exhaust gas flowing into
the NOx catalyst 61 toward the fuel-lean side, possibly resulting
in a reduction of the NOx removal efficiency when the catalyst
midbed temperature of the NOx catalyst 61 is high (e.g., equal to
or higher than the predetermined temperature T1 in the illustrated
embodiment). To deal with this problem, it is also possible to cool
the NOx catalyst 61 while keeping the NOx removal efficiency at a
high level, by temporarily stopping the supply of secondary air to
the NOx catalyst 61 upon completion of the S-poisoning recovery
process, then controlling the engine air-fuel ratio to be
substantially stoichiometric, and causing stoichiometric exhaust
gas to flow into the NOx catalyst 61 via the first passage 20 while
suspending supply of secondary air to the NOx catalyst 61 until the
catalyst midbed temperature of the NOx catalyst 61 becomes lower
than the predetermined temperature T1. Then, once the catalyst
midbed temperature of the NOx catalyst 61 becomes lower than the
predetermined temperature T1, secondary air may be supplied to the
NOx catalyst 61 to more efficiently cool the NOx catalyst 61. When
the catalyst midbed temperature of the NOx catalyst 61 then becomes
lower than the predetermined temperature T2, the supply of
secondary air may be stopped. In this case, the engine air-fuel
ratio control may be changed to lean control upon the re-start of
the supply of secondary air, or the stoichiometric control may be
continued.
[0156] In the foregoing embodiment, after an affirmative decision
(YES) is obtained in step 506 and the S-poisoning recovery process
is completed, the process of cooling the NOx catalyst 61 is
performed by causing stoichiometric exhaust gas to flow until an
affirmative decision (YES) is obtained in step 509. However, if
exhaust gas having a lean air-fuel ratio is passed through the
first passage 20 immediately after completion of the S-poisoning
recovery process, and the catalyst midbed temperature of the NOx
catalyst 61 can be immediately reduced below the predetermined
temperature T2, in other words, if the accelerated cooling portion
23 is designed so as to achieve such a great exhaust cooling
effect, the process of cooling the NOx catalyst 61 by passing
stoichiometric exhaust gas through the NOx catalyst 61 may be
omitted, thus avoiding a reduction in fuel economy that would be
otherwise caused by stoichiometric engine operation.
[0157] Next, the secondary air control of step 600 in the basic
control routine in FIG. 5 will be described with reference to a
secondary air control routine illustrated in FIG. 9.
[0158] First in step 601, the ECU 90 determines whether at least
one of the following conditions is satisfied: a condition that the
S-poisoning recovery control flag F5 is "1", and a condition that
the exhaust cooling flag F1 is "1".
[0159] If the S-poisoning recovery control flag F5 is "1", or if
the exhaust cooling flag F1 is "1", the ECU 90 makes an affirmative
decision (YES) in step 601, and proceeds to step 602. In step 602,
the ECU 90 operates the secondary air supplying device 70 to supply
secondary air from the secondary air supply pipe 69 to the junction
pipe 60.
[0160] If the S-poisoning recovery control flag F5 is "0" and the
exhaust cooling flag F1 is "0", the ECU 90 makes a negative
decision (NO) in step 601, and proceeds to step 603. In step 603,
the ECU 90 stops the operation of the secondary air supplying
device 70, thereby stopping the supply of secondary air from the
secondary air supply pipe 69 to the junction pipe 60.
[0161] Therefore, if the ECU 90 makes an affirmative decision in
step 400 and proceeds to the S-poisoning recovery control of step
500 in the basic control routine, the secondary air continues to be
supplied to the junction pipe 60 until the exhaust cooling flag F1
is rewritten into "0" in step 512 in the S-poisoning recovery
control routine.
[0162] Next, an exhaust purifying apparatus for an internal
combustion engine according to a second embodiment of the invention
will be described with reference to FIGS. 12 to 15. The exhaust
purifying apparatus of the second embodiment is made even more
compact or smaller in size than that of the first embodiment.
[0163] As shown in FIGS. 12 and 13, in the second embodiment, a
part of the interior of an exhaust pipe 14 disposed downstream of a
casing 13 that houses a cold-start catalyst 12 is divided by a
partition 31 into upper and lower passages 32, 41 each having a
generally semicircular cross-sectional shape.
[0164] A portion of the exhaust pipe 14 located above the partition
31 (i.e., the passage 32) is connected to an accelerated cooling
portion 35 having a generally arcuate cross-sectional shape via a
sectional shape changing portion 34. The accelerated cooling
portion 35 is connected to an upper half portion of a junction pipe
60 via a sectional shape changing portion 36. The junction pipe 60
has a cylindrical shape, and is connected to a casing 62 that
houses a NOx catalyst 61 as in the first embodiment. A secondary
air supply pipe 69 is connected to the junction pipe 60. The
cross-sectional shape of the sectional shape changing portion 34
gradually changes from the upper half portion of the exhaust pipe
14 for smooth connection with the accelerated cooling portion 35
having an arcuate cross-sectional shape. The cross-sectional shape
of the sectional shape changing portion 36 gradually changes from
the accelerated cooling portion 35 having an arcuate
cross-sectional shape for smooth connection with the upper half
portion of the junction pipe 60.
[0165] A portion of the exhaust pipe 14 located below the partition
31 (i.e., the passage 41) is connected to a tubular portion 43
having a cylindrical shape via a sectional shape changing portion
42. The tubular portion 43 is connected to a lower half portion of
the junction pipe 60 via a sectional shape changing portion 44. The
cross-sectional shape of the sectional shape changing portion 42
gradually changes from the lower half portion of the exhaust pipe
14 for smooth connection with the tubular portion 43 having a
cylindrical shape. The cross-sectional shape of the sectional shape
changing portion 44 gradually changes from the tubular portion 43
for smooth connection with the lower half portion of the junction
pipe 60.
[0166] The accelerated cooling portion 35 is positioned relative to
the tubular portion 43 so as to surround the tubular portion 43, as
shown in FIG. 15.
[0167] An exhaust switching valve 56 is disposed in the passages
32, 41 of the exhaust pipe 14 such that the switching valve 56 is
selectively opened and closed by an actuator 55. In the exhaust
switching valve 56, as shown in FIG. 14, a valve body 56a for
opening and closing the passage 32 and a valve body 56b for opening
and closing the passage 41 are connected to a single valve shaft
56c that is rotatably mounted to extend through the partition 31,
in such a fashion that the plane of the valve body 56a and the
plane of the valve body 56b form right angles. The valve shaft 56c
is rotated by the actuator 55. Thus, when the valve body 56a of the
exhaust switching valve 56 is positioned so as to completely close
the passage 32, the valve body 56b is positioned so as to fully
open the passage 41. When the valve body 56a is positioned so as to
fully open the passage 32, the valve body 56b is positioned so as
to completely close the passage 41.
[0168] Furthermore, in the second embodiment, an HC adsorbent 26 is
located in a downstream end portion of the accelerated cooling
portion 35.
[0169] In the second embodiment, the passage 32 of the exhaust pipe
14, the sectional shape changing portion 34, the accelerated
cooling portion 35 and the sectional shape changing portion 36
constitute a first passage 20. Also, the passage 41 of the exhaust
pipe 14, the sectional shape changing portion 42, the tubular
portion 43 and the sectional shape changing portion 44 constitute a
second passage 40. Furthermore, in the second embodiment, the
exhaust switching valve 56 forms a flow amount control system that
controls the amount of exhaust gas flow in the first passage 20 and
the amount of exhaust gas flow in the second passage 40. Other
structures are substantially the same as those of the first
embodiment, and will not be described herein.
[0170] In the second embodiment in which the first passage 20
includes the accelerated cooling portion 35, when exhaust gas at
the same temperature is caused to flow through the first passage 20
or the second passage 40 at the same flow rate, the temperature of
the exhaust gas decreases by a larger degree when the exhaust gas
flows through the first passage 20 than when it flows through the
second passage 40. Therefore, by selectively using the first
passage 20 and the second passage 40, the catalyst midbed
temperature of the NOx catalyst 61 can be kept within a high-NOx
removal temperature range and heat deterioration of the NOx
catalyst 61 can be retarded or suppressed, as in the first
embodiment.
[0171] Furthermore, in the second embodiment, the accelerated
cooling portion 35 is disposed in such a manner as to surround the
tubular portion 43, and the flow amount control unit is constituted
by the single exhaust switching valve 56, while the HC adsorbent 26
is housed in the accelerated cooling portion 35. Therefore, the
exhaust purifying apparatus of this embodiment can be made more
compact or smaller in size than that of the first embodiment.
[0172] In the first embodiment, the exhaust gas temperature is used
to represent the catalyst midbed temperature of the NOx catalyst
61, and the exhaust temperature sensor 67 for detecting the
temperature of exhaust gas is disposed downstream of the NOx
catalyst 61. However, it is also possible to dispose the exhaust
temperature sensor 67 upstream of the NOx catalyst 61. Furthermore,
it is possible to provide a catalyst temperature sensor in the
casing 62 and use this temperature sensor to directly detect the
catalyst midbed temperature of the NOx catalyst 61.
[0173] In the illustrated first embodiment, the channel through
which the exhaust gas flows is switched between the first passage
20 and the second passage 40, depending upon the exhaust gas
temperature detected by the exhaust temperature sensor 67. It is,
however, to be understood that the exhaust gas temperature sensor
or a catalyst temperature sensor for detecting the catalyst midbed
temperature of the NOx catalyst 61 is not an essential constituent
element of the invention.
[0174] For example, the channel selection may be accomplished as
follows. Assuming all possibilities of the engine operating state
(the engine speed, engine load, and the air-fuel ratio), the
catalyst midbed temperatures of the NOx catalyst 61 when the entire
amount of exhaust gas flows through the first passage 20 and when
the entire amount of exhaust gas flows through the second passage
40 are empirically determined in advance. Based on the results of
experiments, one of the channels that achieves higher NOx removal
efficiency is determined in accordance with the current engine
operating state. A map of correspondence between the engine
operating states and the exhaust passages (the first passage 20 and
the second passage 40) is prepared and stored in the ROM 92. With
reference to this map, the ECU 90 controls the exhaust switching
valves 52, 54.
[0175] In the first embodiment, the first and second exhaust
switching valves 52, 54 are controlled so that exhaust gas flows
through one of the first passage 20 and the second passage 40. With
the valves 52, 54 controlled in this manner, however, the
temperature of exhaust gas flowing into the NOx catalyst 61 may be
rapidly or sharply changed, which may greatly affect the resulting
emissions. In order to prevent such rapid changes in the catalyst
midbed temperature of the NOx catalyst 61 and perform accurate
catalyst temperature control, the first and second exhaust
switching valves 52, 54 may be constructed as being able to control
the flow amount therethrough, and are controlled in the following
manner. Within a temperature range for switching of the exhaust
channel, the first and second exhaust switching valves 52, 54 are
controlled so as to control the ratio of the amount of flow through
the first passage 20 to that of flow through the second passage 40,
so that the exhaust gas is caused to flow through both the first
passage 20 and the second passage 40 at a suitable flow amount
ratio.
[0176] The above-indicated temperature range for switching of the
exhaust channel corresponds to a temperature range that is higher
than a threshold a for switching from the first passage 20 to the
second passage 40 and is lower than a threshold .beta. for
switching from the second passage 40 to the first passage 20 in the
first embodiment. An example of a method for setting a flow amount
ratio between the first passage 20 and the second passage 40
includes empirically determining optimal flow amount ratios for
achieving higher NOx removal efficiencies with respect to different
temperatures within the exhaust channel switching temperature
range, and mapping and storing those ratios in advance in the ROM
92. The ECU 90 then selects a suitable flow amount ratio referring
to the map, depending upon the exhaust gas temperature detected by
the exhaust temperature sensor 67.
[0177] Furthermore, where a system is designed such that the
exhaust gas channel is switched based on the operating state of the
engine, the catalyst midbed temperatures of the NOx catalyst 61 may
be measured when the entire amount of exhaust gas flows through the
first passage 20 and when the entire amount of exhaust gas flows
through the second passage 40, assuming all possibilities of the
engine operating state (engine speed, engine load and air-fuel
ratio), and may be mapped and stored in the ROM 92. With this
system, the catalyst midbed temperature of the NOx catalyst 61
corresponding to each flow amount ratio between the first passage
20 and the second passage 40 may be estimated by performing
interpolation on the map stored in the ROM 92, thus permitting the
ECU 90 to select a flow amount ratio that provides an optimal
catalyst midbed temperature most suitable for NOx removal or
exhaust purification.
[0178] In the first embodiment, the catalyst whose midbed
temperature is to be controlled by switching channels, that is, the
catalyst housed in the casing 62, is the occlusion-reduction type
NOx catalyst 61. However, the catalyst housed in the casing 62 may
also be a selective reduction type NOx catalyst 57 (see FIG. 16)
that is a kind of lean-bum NOx catalyst.
[0179] The high-NOx removal temperature range of a selective
reduction type NOx catalyst is at a higher temperature side,
compared to the high-NOx removal temperature range of an
occlusion-reduction type NOx catalyst. Therefore, in the case where
a selective reduction type NOx catalyst is employed, it is
preferable to increase the NOx removal efficiency at low exhaust
gas temperatures by providing an occlusion-reduction type NOx
catalyst 58 in the second passage 40 through which relatively
low-temperature exhaust gas flows, as shown in FIG. 16. This
arrangement makes it possible to reduce the load or burden imposed
on the selective reduction type NOx catalyst 57, that is, the NOx
concentration in exhaust gas flowing into the selective reduction
type NOx catalyst 57, when exhaust gas flows through the second
passage 40.
[0180] Furthermore, in the exhaust purifying apparatus as shown in
FIG. 16, a basic channel switching control is substantially the
same as that of the first embodiment, and exhaust gas is caused to
flow through the first passage 20 during high-load engine operation
in which the exhaust gas temperature becomes high. During this
operation, HC adsorbed on the HC adsorbent 26 is desorbed from the
adsorbent 26, and flows along with exhaust gas into the selective
reduction type NOx catalyst 57, to serve as a reducing agent for
NOx removal. Accordingly, the NOx removal efficiency is improved
during high-load engine operation, for example, during high-speed
vehicle running.
[0181] Furthermore, as shown in FIG. 17, a casing 16 that houses a
selective reduction type NOx catalyst 15 may also be provided
between the exhaust pipe 14 and the casing 13 that houses the
cold-start catalyst 12. With this arrangement, NOx can be removed
at a high removal efficiency by the selective reduction type NOx
catalyst 15 when high-temperature exhaust gas is discharged from
the engine during high-load engine operation, for example, during
high-speed vehicle running. Furthermore, the load or burden on the
downstream-side selective reduction type NOx catalyst 57 can be
reduced. In this construction, the casing 16 forms a common passage
upstream of the first passage 20 and the second passage 40.
[0182] In the exhaust purifying apparatus of the first embodiment,
it is possible to determine whether the first and second exhaust
switching valves 52, 54 are normally operating, based on the
magnitude of a change in the exhaust gas temperature detected by
the exhaust temperature sensor 67 when the exhaust gas channel in
operation is changed by the opening/closing control of the first
and second exhaust switching valves 52, 54. This is because when
the first and second exhaust switching valves 52, 54 are normally
operating, the magnitude of a change in the temperature of the NOx
catalyst 61 that occurs upon switching of the exhaust gas channel
is substantially determined in accordance with the engine operating
state.
[0183] Here, a relationship between the engine operating state and
the magnitude of the temperature change upon switching of the
channel during normal operations of the first and second exhaust
switching valves 52, 54 is empirically obtained. A map of reference
temperature differences is prepared based on the thus obtained
relationship, taking account of allowable errors, and the map thus
prepared is stored in the ROM 92. It is thus possible to determine
whether the first and second exhaust switching valves 52, 54 are
normally operating, by comparing the magnitude of a temperature
change calculated based on exhaust gas temperatures detected by the
exhaust temperature sensor 67 before and after switching of the
channel, with a reference temperature difference read from the map
in accordance with the engine operating state.
[0184] The determination as to normality/abnormality of the
operation of the exhaust switching valve(s) may also be carried out
with the exhaust purifying apparatus of the second embodiment.
[0185] Furthermore, the operation normality/abnormality
determination regarding the exhaust switching valves may also be
accomplished based on a pressure change occurring upon switching of
the channel, as well as the magnitude of a temperature change upon
switching of the channel as mentioned above. This modification will
be described in conjunction with the exhaust purifying apparatus of
the second embodiment.
[0186] In the exhaust purifying apparatus of the second embodiment
shown in FIG. 12, the pressure in the pipe upstream of the HC
adsorbent 26 is higher when the passage 32 is opened by the valve
body 56a than when the passage 32 is closed by the valve body 56a.
The magnitude of a pressure change that occurs when the valve body
56a is switched from the open state to the closed state or,
conversely, when the valve body 56a is switched from the closed
state to the open state, may be substantially determined in advance
in accordance with the engine operating state.
[0187] For example, a pressure sensor (pressure detector) 37 for
detecting the in-pipe pressure between the valve body 56a and the
HC adsorbent 26 is provided in the exhaust pipe 14. A relationship
between engine operating states and pressure changes occurring upon
switching of the channel when the exhaust switching valve 56
normally operates are empirically determined beforehand. A map of
reference ranges of pressure change is prepared based on the
relationship, taking account of allowable errors, and is stored in
the ROM 92. It is thus possible to determine whether the exhaust
switching valve 56 is normally operating, by comparing the
magnitude of a pressure change calculated based on the levels of
pressure detected by the pressure sensor 37 before and after
switching of the channel, and a reference range of pressure change
read from the map in accordance with the current engine operating
state,.
[0188] Although the exhaust switching valve 56 is provided upstream
of the HC adsorbent 26 in the second embodiment as shown in FIG.
12, the exhaust switching valve 56 may also be disposed, together
with the partition 31, in the junction pipe 60 downstream of the HC
adsorbent 26. In this case, too, a pressure sensor may be provided
in a section between the HC adsorbent 26 and the valve body 56a of
the exhaust switching valve 56, so that whether the exhaust
switching valve 56 is normally operating or not can be determined
based on the change between the pressure levels detected by the
pressure sensor before and after the exhaust switching.
[0189] The normality/abnormality determination regarding the
exhaust switching valve based on the pressure change may also be
implemented in the exhaust purifying apparatus of the first
embodiment.
[0190] While the invention has been described with reference to
preferred embodiments thereof, it is to be understood that the
invention is not limited to the preferred 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 preferred 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.
* * * * *