U.S. patent application number 11/630323 was filed with the patent office on 2007-12-27 for control method and control device for exhaust gas control apparatus.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Kotaro Hayashi, Kohei Yoshida.
Application Number | 20070294998 11/630323 |
Document ID | / |
Family ID | 35005783 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070294998 |
Kind Code |
A1 |
Yoshida; Kohei ; et
al. |
December 27, 2007 |
Control Method and Control Device for Exhaust Gas Control
Apparatus
Abstract
It is an object of the invention to provide a technology for
appropriately recovering a decreased HC oxidizing ability of
rhodium (Rh), in an exhaust gas control apparatus including a
catalyst that contains rhodium (Rh) and a particulate filter (5).
In this exhaust gas control apparatus, rich-spike control is
prohibited and a NOx storage reduction catalyst is placed in a
reduction atmosphere during a period in which a temperature of the
NOx storage reduction catalyst is equal to or higher than a
predetermined temperature, in a course of decreasing the
temperature of the NOx storage reduction catalyst after a PM
trapping ability forcible recovery process of the particulate
filter (5) is completed. Thus, the decreased HC oxidizing ability
of rhodium (Rh) is recovered.
Inventors: |
Yoshida; Kohei; (Susono-shi,
JP) ; Hayashi; Kotaro; (Mishima-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
1, TOYOTA-CHO
TOYOTA-SHI
JP
471-8571
|
Family ID: |
35005783 |
Appl. No.: |
11/630323 |
Filed: |
June 30, 2005 |
PCT Filed: |
June 30, 2005 |
PCT NO: |
PCT/IB05/01861 |
371 Date: |
December 21, 2006 |
Current U.S.
Class: |
60/274 |
Current CPC
Class: |
F01N 3/0871 20130101;
F01N 3/0885 20130101; F01N 2610/03 20130101; F01N 3/0821 20130101;
F02D 41/029 20130101; B01D 53/96 20130101; B01D 53/9422 20130101;
Y02T 10/20 20130101; F02D 41/0275 20130101; F02M 26/15 20160201;
Y02T 10/12 20130101; B01D 2255/91 20130101; B01D 2255/1025
20130101 |
Class at
Publication: |
060/274 |
International
Class: |
F01N 3/18 20060101
F01N003/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2004 |
JP |
2004-198310 |
Claims
1. A control method for an exhaust gas control apparatus formed by
integrally or separately arranging a catalyst that contains rhodium
and a particulate filter for trapping particulate matter, in an
exhaust system of an internal combustion engine, comprising:
performing a particulate matter trapping ability forcible recovery
process for forcibly recovering a particulate matter trapping
ability of the particulate filter by increasing a temperature of
the particulate filter and a temperature of the catalyst, and
placing the catalyst in a reduction atmosphere in a course of
decreasing the temperature of the catalyst after the particulate
matter trapping ability forcible recovery process is completed.
2. The control method for an exhaust gas control apparatus
according to claim 1, wherein the catalyst is placed in a reduction
atmosphere in a period in which the temperature of the catalyst is
equal to or higher than a predetermined temperature.
3. The control method for an exhaust gas control apparatus
according to claim 2, wherein the predetermined temperature is
approximately 400.degree. C.
4. The control method for an exhaust gas control apparatus
according to claim 1, wherein: the catalyst is a NOx storage
reduction catalyst, and a process for recovering a NOx storage
reduction ability of the catalyst is prohibited from being
performed, when the catalyst is placed in the reduction
atmosphere.
5. The control method for an exhaust gas control apparatus
according to claim 4, wherein an air-fuel ratio of exhaust gas when
the catalyst is placed in the reduction atmosphere is made higher
than an air-fuel ratio of the exhaust gas during the process for
recovering the NOx storage reduction ability of the catalyst.
6. A control device for an exhaust gas control apparatus,
comprising: recovery portion that forcibly recovers a particulate
matter trapping ability of a particulate filter that is provided in
an exhaust system of an internal combustion engine by increasing a
temperature of the particulate filter and a temperature of a
catalyst that is provided integrally with or separately from the
particulate filter in the exhaust system and that contains rhodium;
and NOx reducing ability recovery portion that places the catalyst
in a reduction atmosphere in a course of decreasing the temperature
of the catalyst after the particulate matter trapping ability of
the particulate filter has been forcibly recovered.
7. The control method for an exhaust gas control apparatus
according to claim 2, wherein: the catalyst is a NOx storage
reduction catalyst, and a process for recovering a NOx storage
reduction ability of the catalyst is prohibited from being
performed, when the catalyst is placed in the reduction
atmosphere.
8. The control method for an exhaust gas control apparatus
according to claim 3, wherein: the catalyst is a NOx storage
reduction catalyst, and a process for recovering a NOx storage
reduction ability of the catalyst is prohibited from being
performed, when the catalyst is placed in the reduction
atmosphere.
9. The control method for an exhaust gas control apparatus
according to claim 7, wherein an air-fuel ratio of exhaust gas when
the catalyst is placed in the reduction atmosphere is made higher
than an air-fuel ratio of the exhaust gas during the process for
recovering the NOx storage reduction ability of the catalyst.
10. The control method for an exhaust gas control apparatus
according to claim 8, wherein an air-fuel ratio of exhaust gas when
the catalyst is placed in the reduction atmosphere is made higher
than an air-fuel ratio of the exhaust gas during the process for
recovering the NOx storage reduction ability of the catalyst.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a control method and control device
for an exhaust gas control apparatus including a catalyst that
contains rhodium (Rh), and a particulate filter.
[0003] 2. Description of the Related Art
[0004] In an exhaust gas control apparatus for an internal
combustion engine, which is mounted in, for example, a vehicle, a
catalyst that contains platinum (Pt) deteriorates with use. With
the aim of addressing such a problem, there is a known method in
which, when the catalyst deteriorates, the catalyst is placed in a
rich atmosphere for a predetermined period, whereby the
deteriorated catalyst is recovered. A technology related to such a
method is disclosed in, for example, Japanese Utility Model
Application Publication No. 63-128221.
[0005] As an exhaust gas control apparatus for a compression
ignition internal combustion engine (i.e., diesel engine), an
exhaust gas control apparatus is known which is formed by
integrally or separately arranging a catalyst that contains rhodium
(Rh) and a particulate filter for trapping particulate matter
(hereinafter, referred to as "PM") in an exhaust system.
[0006] In this type of exhaust gas control apparatus, while the PM
trapping ability of the particulate filter is recovered, the
catalyst is exposed to a high-temperature and lean atmosphere. If
the catalyst that contains rhodium (Rh) is exposed to the
high-temperature and lean atmosphere, rhodium (Rh) moves to the
inside of a catalyst carrier, resulting in a decrease in the NOx
reducing ability of the catalyst.
[0007] Such a decreased NOx reducing ability of the catalyst is
recovered, when the catalyst is exposed to a rich atmosphere at a
high temperature of 400.degree. C. or higher. However, there is a
problem that, since the temperature of the exhaust gas released
from the compression ignition internal combustion engine is low,
increasing the temperature of the catalyst to be 400.degree. C. or
higher decreases fuel efficiency.
SUMMARY OF THE INVENTION
[0008] The invention is made in light of the above-mentioned
circumstances. It is therefore an object of the invention to
provide a control method and control device which can appropriately
recover a decreased reducing ability of a catalyst in an exhaust
gas control apparatus for an internal combustion engine, the
exhaust gas control apparatus being formed by integrally or
separately arranging the catalyst that contains rhodium (Rh) and a
particulate filter in an exhaust system of the internal combustion
engine.
[0009] According to an aspect of the invention, there is provided a
control method for an exhaust gas control apparatus formed by
integrally or separately arranging a catalyst that contains rhodium
(Rh) and a particulate filter in an exhaust system of an internal
combustion engine, characterized in that the catalyst is placed in
a reduction atmosphere in a course of decreasing a catalyst
temperature after a PM trapping ability forcible recovery process
for the particulate filter is completed.
[0010] In order to recover the PM trapping ability of the
particulate filter, the temperature of the particulate filter is
increased by forcibly increasing the temperature of exhaust gas
and/or forcibly increasing the amount of reaction heat in the
catalyst. Thus, the so-called PM trapping ability forcible recovery
process is performed so as to oxidize and remove the PM trapped in
the particulate filter.
[0011] When the PM trapping ability forcible recovery process is
performed, the catalyst is exposed to a high-temperature and lean
atmosphere together with the particulate filter. Accordingly,
rhodium (Rh) moves to the inside of a catalyst carrier. If rhodium
(Rh) moves to the inside of the catalyst carrier, the reducing
ability of the catalyst is decreased.
[0012] When the catalyst is exposed to the reduction atmosphere at
a high temperature, the rhodium (Rh), which has moved to the inside
of the catalyst carrier, outcrops to a surface of the catalyst
carrier.
[0013] Note that, if the temperature of the catalyst is forcibly
increased only in order to recover the decreased reducing ability
of the catalyst, the fuel efficiency may be considerably
decreased.
[0014] In order to address such a problem, the catalyst is placed
in the reduction atmosphere in a period, in which the temperature
of the catalyst has been sufficiently decreased and re-heating of
the catalyst need not be performed, in the course of decreasing the
catalyst temperature after the PM trapping ability forcible
recovery process is completed. Thus, the decreased the reducing
ability of the catalyst can be recovered by using heat obtained
during the PM trapping ability forcible recovery process. As a
result, an extra temperature increasing process for recovering the
decreased reducing ability of the catalyst need not be performed,
and a decrease in the fuel efficiency is suppressed.
[0015] The decreased reducing ability of the catalyst is
appropriately recovered, when the catalyst is exposed to the
reduction atmosphere at a high temperature of approximately
400.degree. C. or higher. Accordingly, the catalyst may be placed
in the reduction atmosphere in a period, in which the catalyst
temperature is approximately 400.degree. C. or higher, in the
course of decreasing the catalyst temperature after the PM trapping
ability forcible recovery process is completed. In this case, an
amount of reducing agent required to generate the reduction
atmosphere can be minimized.
[0016] In the invention, a NOx storage reduction catalyst may be
used as the catalyst that contains rhodium (Rh). Since the NOx
storage ability of the NOx storage reduction catalyst is limited,
the NOx storage ability needs to be recovered when required, in the
exhaust gas control apparatus including the NOx storage reduction
catalyst.
[0017] As a method for recovering the NOx storage ability of the
NOx storage reduction catalyst, so-called rich-spike control is
effective. In the rich-spike control, an air-fuel ratio of the
exhaust gas flowing into the catalyst is made rich by supplying a
reducing agent into the exhaust gas flowing upstream of the
catalyst.
[0018] In the exhaust gas control apparatus including the
particulate filter and the NOx storage reduction catalyst, the
rich-spike control may be performed after the PM trapping ability
forcible recovery process for the particulate filter is
completed.
[0019] If the rich-spike control is performed when the NOx reducing
ability of the catalyst has been decreased, although the NOx stored
in the NOx storage reduction catalyst is released, the released NOx
cannot be reduced sufficiently. Accordingly, the NOx may be
released into the air without being reduced. In addition, with an
increase in the amount of NOx that has not been reduced, the amount
of reducing agent that has not reacted with NOx may increase.
[0020] Meanwhile, if the rich-spike control is performed after the
PM trapping ability forcible recovery process is completed, the
catalyst is exposed to the high-temperature and rich atmosphere.
Accordingly, the decreased NOx reducing ability of the catalyst may
be recovered. In the rich-spike control, however, the air-fuel
ratio of the exhaust gas is made rich intermittently, and the
length of each period in which the air-fuel ratio of the exhaust
gas is rich is relatively short. It is therefore difficult to
sufficiently recover the decreased the NOx reducing ability of the
catalyst. Further, the conventional type of rich-spike control is
performed without the characteristics of rhodium (Rh) taken into
consideration. Accordingly, the catalyst is not always placed in
the rich atmosphere when the temperature of the catalyst in an
appropriate temperature range.
[0021] According to the invention, if the catalyst that contains
rhodium (Rh) is the NOx storage reduction catalyst, the rich-spike
control may be prohibited after the PM trapping ability forcible
recovery process is completed, whereby the catalyst is placed in
the reduction atmosphere.
[0022] When the catalyst is placed in the reduction atmosphere,
preferably an air-fuel ratio of the exhaust gas is made higher than
that when the rich atmosphere is generated by the rich-spike
control for the following reason.
[0023] If the rich atmosphere similar to that generated by the
rich-spike control is formed when the NOx reducing ability of the
catalyst has been decreased, a relatively large amount of NOx is
released from the NOx storage reduction catalyst, and therefore an
amount of NOx released into the air without being reduced may
increase.
[0024] Examples of a method for placing the catalyst in the
reduction atmosphere include a method in which a small amount of
reducing agent is supplied to the exhaust gas at intervals shorter
than those in the rich-spike control, and a method in which an
air-fuel ratio in the internal combustion engine is made low.
[0025] In the exhaust gas control apparatus including the NOx
storage reduction catalyst and the particulate filter, a process
for recovering the NOx storage reduction catalyst from sulfur
poisoning (hereinafter, referred to as a "sulfur poisoning recovery
process for the NOx storage reduction catalyst") may be performed
subsequent to the PM trapping ability forcible recovery
process.
[0026] The control according to the invention is different from the
sulfur poisoning recovery process in the following point. In the
control according to the invention, the catalyst is placed in the
reduction atmosphere without forcibly increasing the temperature of
the catalyst and without forcibly maintaining the temperature of
the catalyst. In contrast to this, in the sulfur poisoning recovery
process, the catalyst is placed in the reduction atmosphere while
the temperature of the catalyst is forcibly increased and
maintained.
[0027] If the sulfur poisoning recovery process is performed, the
catalyst is exposed to the high-temperature and rich atmosphere.
Therefore the decreased NOx reducing ability of the catalyst can be
recovered.
[0028] Accordingly, when the sulfur poisoning recovery process is
performed subsequent to the PM trapping ability forcible recovery
process, the control according to the invention is prohibited. On
the other hand, when the sulfur poisoning recovery process is not
performed subsequent to the PM trapping ability forcible recovery
process, the control according to the invention is performed. In
this case, the catalyst is prevented from unnecessarily being
placed in the reduction atmosphere, and therefore the fuel
efficiency is prevented from being decreased.
[0029] According to another aspect of the invention, there is
provided a control device for an exhaust gas control apparatus
including a particulate filter provided in an exhaust system of an
internal combustion engine, and a catalyst that is provided
integrally with or separately from the particulate filter in the
exhaust system and that contains rhodium, the control device being
characterized by including recovery means for increasing a
temperature of the particulate filter and a temperature of the
catalyst, thereby forcibly recovering a PM trapping ability of the
particulate filter; and NOx reducing ability recovery means for
placing the catalyst in a reduction atmosphere in a course of
decreasing the temperature of the catalyst after the PM trapping
ability of the particulate filter is forcibly recovered.
[0030] It is to be understood that "storage" used herein means
retention of a substance (solid, liquid, gas molecules) in the form
of at least one of adsorption, adhesion, absorption, trapping,
occlusion, and others.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The above-mentioned embodiment and other embodiments,
objects, features, advantages, technical and industrial
significance of this invention will be better understood by reading
the following detailed description of the exemplary embodiments of
the invention, when considered in connection with the accompanying
drawings, in which:
[0032] FIG. 1 is a view schematically showing a structure of an
internal combustion engine to which the invention is applied;
[0033] FIG. 2 is a graph showing a temperature at which a NOx
reducing ability of a NOx storage reduction catalyst is
activated;
[0034] FIG. 3 is a flowchart showing a routine of catalyst's NOx
reducing ability recovery control; and
[0035] FIG. 4 is a graph showing a concrete method for performing
an exhaust gas enriching process.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0036] In the following description, the invention will be
described in more detail in terms of exemplary embodiments.
[0037] An internal combustion engine 1 shown in FIG. 1 is a
compression ignition internal combustion engine (i.e., diesel
engine). The internal combustion engine 1 is provided with an
intake passage 2 and an exhaust passage 3. An intake throttle valve
4 is provided in the intake passage 2. A particulate filter 5 is
provided in the exhaust passage 3. The particulate filter 5
supports a NOx storage reduction catalyst that contains rhodium
(Rh).
[0038] A reducing agent supply valve 6, which injects fuel from the
internal combustion engine 1 as a reducing agent, is provided in
the exhaust passage 3 at a position upstream of the particulate
filter 5. An exhaust gas temperature sensor 7 is provided in the
exhaust passage 3 at a position downstream of the particulate
filter 5.
[0039] An EGR passage 8 permits communication between the intake
passage 2 and the exhaust passage 3. An EGR valve 9 is provided in
the EGR passage 8.
[0040] Each of the intake throttle valve 4, the reducing agent
supply valve 6, the exhaust gas temperature sensor 7, and the EGR
valve 9 is electrically connected to an ECU 10.
[0041] The ECU 10 performs known controls such as fuel injection
control and EGR control based on operation states of the exhaust
gas temperature sensor 7 and the internal combustion engine 1. The
ECU 10 also performs catalyst's NOx reducing ability recovery
control that is a main feature of the invention. Hereafter, the
catalyst's NOx reducing ability recovery control will be described
in detail.
[0042] Since the PM trapping ability of the particulate filter 5 is
limited, the ECU 10 performs the PM trapping ability forcible
recovery process before the limit of the PM trapping ability is
reached. In the PM trapping ability forcible recovery process, the
ECU 10 increases the temperature of the exhaust gas and/or
increases the amount of reaction heat in the NOx storage reduction
catalyst by performing post-injection and/or supplying fuel from
the reducing agent supply valve 6 into the exhaust gas, thereby
forcibly increasing the temperature of the particulate filter
5.
[0043] When the PM trapping ability forcible recovery process for
the particulate filter 5 is performed, the NOx storage reduction
catalyst supported by the particulate filter 5 is also exposed to
the high-temperature and rich atmosphere. At this time, rhodium
(Rh) contained in the NOx storage reduction catalyst moves to the
inside of the catalyst carrier. As a result, the NOx reducing
ability of the NOx storage reduction catalyst (especially, a
hydrocarbon (HC) oxidizing ability) is decreased.
[0044] If the HC oxidizing ability of the NOx storage reduction
catalyst is decreased, the NOx reducing ability of the NOx storage
reduction catalyst is decreased. Namely, if the HC oxidizing
ability of the NOx storage reduction catalyst is decreased, when
the NOx storage ability of the NOx storage reduction catalyst is
recovered, that is, when the rich spike control, in which fuel
(hydrocarbon (HC)) is intermittently supplied from the reducing
agent supply valve 6 into the exhaust gas, is performed, it becomes
difficult for the hydrocarbon (HC) to transform into a reaction
activated substance in the NOx storage reduction catalyst.
Accordingly, the NOx released from the NOx storage reduction
catalyst may be released into the air without being reduced, and
the hydrocarbon supplied to the NOx storage reduction catalyst may
be released into the air without reacting with NOx.
[0045] FIG. 2 is a graph showing the temperature at which the NOx
reducing ability of the NOx storage reduction catalyst is
activated. Before the PM trapping ability forcible recovery process
for the particulate filter 5 is performed, the NOx reducing ability
of the NOx storage reduction catalyst is activated at a temperature
of approximately 300.degree. C. In contrast to this, after the PM
trapping ability forcible recovery process is performed, the NOx
reducing ability of the NOx storage reduction catalyst is not
activated until the temperature increases to be approximately
350.degree. C. or higher.
[0046] The temperature of the exhaust gas released from the
compression ignition internal combustion engine is approximately
300.degree. C. at times other than a period in which the internal
combustion engine is operated at high load. As described above, an
increase in the temperature, at which the NOx reducing ability is
activated, increases a possibility that the amounts of NOx and HC
released into the air increase when the rich-spike control is
performed.
[0047] Accordingly, when the PM trapping ability forcible recovery
process is performed, the decreased HC oxidizing ability of the NOx
storage reduction catalyst needs to be recovered. In order to
recover the decreased HC oxidizing ability, rhodium (Rh), which has
moved to the inside of the catalyst carrier, needs to outcrop to
the surface of the catalyst carrier again.
[0048] Rhodium (Rh), which has moved to the inside of the catalyst
carrier, outcrops to the surface of the catalyst carrier, when the
catalyst is exposed to the reduction atmosphere at a high
temperature of 400.degree. C. or higher. Therefore, if the exhaust
gas flowing in the NOx storage reduction catalyst is made rich
after the temperature of the NOx storage reduction catalyst is
increased to be 400.degree. C. or higher, the decreased HC
oxidizing ability can be recovered.
[0049] Examples of an effective method for increasing the
temperature of the NOx storage reduction catalyst to be 400.degree.
C. or higher, that is, a temperature in a high temperature range,
include a method in which the temperature of the exhaust gas is
increased by performing post injection and a method in which the
amount of reaction heat in the NOx storage reduction catalyst is
increased by supplying fuel into the exhaust gas. However, there is
a problem common to these methods, that is, a decrease in the fuel
efficiency.
[0050] Accordingly, in the catalyst's NOx reducing ability recovery
control according to the embodiment, the particulate filter 5 is
placed in the reduction atmosphere (rich atmosphere) in the period
in which the temperature of the NOx storage reduction catalyst is
400.degree. C. or higher, in the course of decreasing the
temperature of the catalyst after the PM trapping ability forcible
recovery process is completed.
[0051] Hereafter, the catalyst's NOx reducing ability recovery
control will be described with reference to FIG. 3. FIG. 3 is a
flowchart showing the routine of the catalyst's NOx reducing
ability recovery control. The catalyst's NOx reducing ability
recovery control routine is stored in ROM of the ECU 10 in advance.
The catalyst's NOx reducing ability recovery control routine is an
interrupt routine that is performed by the ECU 10 when the PM
trapping ability forcible recovery process is completed.
[0052] In the catalyst's NOx reducing ability recovery control
routine, the ECU 10 initially determines in step S101 whether a PM
trapping ability forcible recovery completion flag shows "1". The
PM trapping ability forcible recovery completion flag is stored in
RAM or the like in advance. When the PM trapping ability forcible
recovery process is completed, "1" is stored. When the catalyst's
NOx reducing ability recovery control is completed, "0" is
stored.
[0053] When it is determined in step S101 that the PM trapping
ability forcible recovery completion flag shows "0", the ECU 10
ends the routine. On the other hand, when it is determined in step
S101 that the PM trapping ability forcible recovery completion flag
shows "1", the ECU 10 then performs step S102.
[0054] In step S102, the ECU 10 receives a signal Tout which
indicates a temperature of the exhaust gas released from the
particulate filter 5 (hereinafter, referred to as "an outflow
exhaust gas temperature Tout"), and which is output from the
exhaust gas temperature sensor 7.
[0055] In step S103, the ECU 10 determines whether the outflow
exhaust gas temperature Tout received in step S102 is equal to or
higher than a predetermined temperature Ts (e.g., 400.degree.
C.).
[0056] When it is determined in step S103 that the outflow exhaust
gas temperature Tout is equal to nor higher than the predetermined
temperature (Tout<Ts), the ECU estimates that a bed temperature
of the NOx storage reduction catalyst is lower than the
predetermined temperature Ts, and then performs step S110. In step
S110, the ECU 10 changes the value of the PM trapping ability
forcible recovery completion flag to "0", and then ends the
routine.
[0057] On the other hand, when it is determined in step S103 that
the outflow exhaust gas temperature Tout is equal to or higher than
the predetermined temperature Ts (Tout.gtoreq.Ts), the ECU 10
estimates that the bed temperature of the NOx storage reduction
catalyst is equal to or higher than the predetermined temperature
T.sub.s, and then performs step S104.
[0058] In step S104, the ECU 10 prohibits the rich-spike
control.
[0059] In step S105, the ECU 10 performs an exhaust gas enriching
process for making the exhaust gas flowing into the particulate
filter 5 rich. In the exhaust gas enriching process, the ECU 10
controls the reducing agent supply valve 6 such that fuel is
intermittently supplied into the exhaust gas.
[0060] At this time, the ECU 10 controls the reducing agent supply
valve 6 such that the amount of fuel supplied from the reducing
agent supply valve 6 during each supply become smaller than that in
the rich-spike control, and the interval between the fuel supplies
become shorter than that in the rich-spike control, as shown in
FIG. 4.
[0061] The amount of fuel supplied from the reducing agent supply
valve 6 during each supply is made smaller than that in the
rich-spike control for the following reason. If the same amount of
hydrocarbon (HC) as that in the rich-spike control is supplied to
the NOx storage reduction catalyst when the HC oxidizing ability of
the NOx storage reduction catalyst has been decreased, the amount
of NOx released from the NOx storage reduction catalyst increases,
and the amount of NOx released into the air without being reduced
also increases.
[0062] The amount of fuel supplied from the reducing agent supply
valve 6 during each supply is made smaller than that in the
rich-spike control also for the following reason. If the same
amount of hydrocarbon (HC) as that in the rich-spike control is
supplied to the NOx storage reduction catalyst when the HC
oxidizing ability of the NOx storage reduction catalyst has been
decreased, the amount of hydrocarbon (HC) that is released into the
air without reacting with NOx may increase.
[0063] The interval between the fuel supplies is made shorter than
that in the rich-spike control for the following reason. The
temperatures of the particulate filter 5 and the NOx storage
reduction catalyst rapidly decrease after the PM trapping ability
forcible recovery process is completed. Accordingly, if the fuel is
supplied with the same intervals as those in the rich-spike
control, the temperature of the NOx storage reduction catalyst may
decrease to be the predetermined temperature Ts or lower, before
the HC oxidizing ability is recovered.
[0064] In step S106, the ECU 10 receives the signal (i.e., outflow
exhaust gas temperature) Tout output from the exhaust gas
temperature sensor 7 again.
[0065] In step S107, the ECU 10 determines whether the outflow
exhaust gas temperature Tout received in step S106 has decreased to
be lower than the predetermined temperature Ts.
[0066] When it is determined in step S107 that the outflow exhaust
gas temperature Tout has not decreased to be lower than the
predetermined temperature Ts (Tout.gtoreq.Ts), the ECU 10
determines that the bed temperature of the NOx storage reduction
catalyst is still equal to or higher than the predetermined
temperature Ts, and then performs step S105 and the following steps
again.
[0067] On the other hand, when it is determined in step S107 that
the outflow exhaust gas temperature Tout has decreased to be lower
than the predetermined temperature Ts (Tout<Ts), the ECU 10
determines that the bed temperature of the NOx storage reduction
catalyst has decreased to be lower than the predetermined
temperature Ts, and then performs step S108.
[0068] In step S108, the ECU 10 ends the exhaust gas enriching
process.
[0069] In step S109, the ECU 10 removes prohibition of the
rich-spike control.
[0070] In step S110, the ECU 10 changes the value of the PM
trapping ability forcible recovery process completion flag to
"0".
[0071] When the ECU 10 performs the catalyst's NOx reducing ability
recovery control routine in the above-mentioned manner, the HC
oxidizing ability of the NOx storage reduction catalyst can be
recovered by using the heat obtained during the PM trapping ability
forcible recovery process. As a result, a decrease in the fuel
efficiency due to an increase in the temperature of the NOx storage
reduction catalyst can be suppressed.
[0072] In the embodiment, the amount of fuel supplied into the
exhaust gas during each supply is made smaller than that in the
rich-spike control. Also, in the embodiment, the interval between
the fuel supplies is made shorter than that in the rich-spike
control. Accordingly, the HC oxidizing ability of the NOx storage
reduction catalyst can be recovered in the period in which the bed
temperature of the NOx storage reduction catalyst is equal to or
higher than the predetermined temperature Ts. In addition, the
amount of NOx released into the air without being reduced and the
amount of hydrocarbon (HC) that are released into the air without
reacting with NOx can be decreased.
[0073] If the exhaust gas enriching process is performed after the
PM trapping ability forcible recovery process is completed, the bed
temperature of the NOx storage reduction catalyst may be maintained
at a temperature equal to or higher than the predetermined
temperature Ts for a long time due to the heat generated by
reaction of rhodium (Rh) and hydrocarbon (HC). In such a case, any
one of the following methods may be employed; (1) the exhaust gas
enriching process is completed when the performance time of the
exhaust gas enriching process becomes equal to or longer than a
predetermined time, (2) the temperature of the NOx storage
reduction catalyst is gradually decreased by decreasing the fuel
supply amount with an increase in the number of times of fuel
supply, and (3) an interval is provided every time fuel supply has
been performed a predetermined number of times such that the
temperature of the NOx storage reduction catalyst is decreased in a
stepwise manner.
[0074] In the embodiment, supplying fuel into the exhaust gas from
the reducing agent supply valve 6 is employed as a concrete method
for performing the exhaust gas enriching process. However, the
air-fuel ratio of the exhaust gas released from the internal
combustion engine 1 may be decreased by increasing the amount of
the EGR gas.
[0075] In the embodiment, the particulate filter 5 and the NOx
storage reduction catalyst are integrally provided in the exhaust
passage 3. However, the particulate filter 5 and the NOx storage
reduction catalyst may be separately provided in the exhaust
passage 3.
[0076] For example, the particulate filter 5 and the NOx storage
reduction catalyst may be provided in the exhaust passage 3 in
series (preferably, the NOx storage reduction catalyst is provided
upstream of the particulate filter 5). Note that, in this case, the
reducing agent supply valve 6 needs to be provided upstream of the
NOx storage reduction catalyst.
[0077] Hereafter, the other embodiments will be described.
[0078] When the amount of sulfur contained in the fuel used in the
internal combustion engine 1 is large, sulfur poisoning (i.e., S
poisoning) occurs in the NOx storage reduction catalyst.
Accordingly, the sulfur poisoning recovery process may be performed
subsequent to the PM trapping ability forcible recovery
process.
[0079] In the sulfur poisoning recovery process, the catalyst is
placed in the rich atmosphere while the temperature of the NOx
storage reduction catalyst is maintained at a high temperature.
Accordingly, the decreased HC oxidizing ability of the NOx storage
reduction catalyst can be recovered.
[0080] Therefore, when performing the sulfur poisoning recovery
process subsequent to the PM trapping ability forcible recovery
process, the ECU 10 prohibits the catalyst's NOx reducing ability
recovery control. On the other hand, when not performing the sulfur
poisoning recovery process subsequent to the PM trapping ability
forcible recovery process, the ECU 10 performs the catalyst's NOx
reducing ability recovery control.
[0081] In this case, the catalyst's NOx reducing ability recovery
control is prevented from being unnecessarily performed.
Accordingly, fuel consumption due to the catalyst's NOx reducing
ability recovery control can be suppressed.
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