U.S. patent application number 10/559826 was filed with the patent office on 2006-08-03 for exhaust purifying apparatus and exhaust purifying method for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Takayoshi Inaba, Shigehiro Matsuno, Hiroki Matsuoka, Yasuhiko Otsubo, Tatsuhisa Yokoi.
Application Number | 20060168939 10/559826 |
Document ID | / |
Family ID | 34962352 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060168939 |
Kind Code |
A1 |
Otsubo; Yasuhiko ; et
al. |
August 3, 2006 |
Exhaust purifying apparatus and exhaust purifying method for
internal combustion engine
Abstract
An exhaust purifying apparatus estimates an accumulation amount
of particulate matter trapped about a catalyst in an exhaust
system. When the estimated accumulation amount is equal to or more
than a permissible value, the apparatus executes PM elimination
control for supplying unburned fuel component to the catalyst. The
apparatus sets the estimated accumulation amount to zero at the
completion of the PM elimination control. When execution of the PM
elimination control becomes possible after suspension of the
control, the apparatus resumes the PM elimination control even if
the accumulation amount is less than the permissible value.
Therefore, The estimated accumulation amount is prevented from
being significantly deviated from the actual accumulation amount
due to suspension.
Inventors: |
Otsubo; Yasuhiko;
(Toyota-shi, JP) ; Yokoi; Tatsuhisa; (Toyota-shi,
JP) ; Matsuno; Shigehiro; (Toyota-shi, JP) ;
Matsuoka; Hiroki; (Susono-shi, JP) ; Inaba;
Takayoshi; (Aichi-ken, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
1500 K STREET N.W.
SUITE 700
WASHINGTON
DC
20005
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
DENSO CORPORATION
Kariya-shI
JP
|
Family ID: |
34962352 |
Appl. No.: |
10/559826 |
Filed: |
March 10, 2005 |
PCT Filed: |
March 10, 2005 |
PCT NO: |
PCT/JP05/04739 |
371 Date: |
December 6, 2005 |
Current U.S.
Class: |
60/274 ; 60/286;
60/295 |
Current CPC
Class: |
F01N 3/0253 20130101;
F01N 9/005 20130101; F01N 13/009 20140601; Y02T 10/40 20130101;
F01N 2560/06 20130101; F01N 2560/14 20130101; F02B 37/00 20130101;
F01N 3/035 20130101; Y02T 10/47 20130101; F01N 9/002 20130101; F01N
13/0097 20140603; F01N 2560/08 20130101 |
Class at
Publication: |
060/274 ;
060/286; 060/295 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 11, 2004 |
JP |
2004-068995 |
Claims
1. An exhaust purifying apparatus for an internal combustion
engine, wherein the apparatus estimates an accumulation amount of
particulate matter trapped about a catalyst in an exhaust system,
and wherein, when the estimated accumulation amount is equal to or
more than a permissible value, the apparatus executes PM
elimination control for supplying unburned fuel component to the
catalyst to increase the temperature of the catalyst and burning
the trapped particulate matter, and sets the estimated accumulation
amount to zero at the completion of the PM elimination control, and
when execution of the PM elimination control becomes possible after
suspension of the control, the apparatus resumes the PM elimination
control even if the accumulation amount of particulate matter about
the catalyst is less than the permissible value.
2. The exhaust purifying apparatus according to claim 1, wherein,
when resuming the PM elimination control, the smaller the
accumulation amount, the shorter the time for execution of the PM
elimination control is set by the apparatus.
3. The exhaust purifying apparatus according to claim 1, wherein,
at a final stage of the PM elimination control, the apparatus
executes burn-up control, in which performance and stopping of
concentrated intermittent fuel addition to a section of the exhaust
system that is upstream of the catalyst are repeated a
predetermined number of times.
4. The exhaust purifying apparatus according to claim 1, wherein,
when the estimated accumulation amount is less than a determination
value that is slightly more than zero, the apparatus executes
burn-up control, in which performance and stopping of concentrated
intermittent fuel addition to a section of the exhaust system that
is upstream of the catalyst are repeated a predetermined number of
times.
5. The exhaust purifying apparatus according to of claim 1, wherein
the apparatus discretely increases the temperature of the catalyst
after resuming the PM elimination control.
6. The exhaust purifying apparatus according to claim 5, wherein
the apparatus: burns, unburned fuel collected on the catalyst in an
early stage of the increase in the catalyst temperature; and
further increases the catalyst temperature thereafter, thereby
burning particulate matter collected on the catalyst.
7. An exhaust purifying method for an internal combustion engine,
the method comprising: estimating an accumulation amount of
particulate matter trapped about a catalyst in an exhaust system of
the internal combustion engine; executing PM elimination control
when the estimated accumulation amount is equal to or more than a
permissible value, in which control, unburned fuel component is
supplied to the catalyst to increase the temperature of the
catalyst and the trapped particulate matter is burned; setting the
estimated accumulation amount to zero at the completion of the PM
elimination control; and resuming the PM elimination control when
execution of the PM elimination control becomes possible after
suspension of the control, even if the accumulation amount of
particulate matter about the catalyst is less than the permissible
value.
8. The method according to claim 7, wherein, when the PM
elimination control is resumed, the smaller the accumulation
amount, the shorter the time for execution of the PM elimination
control is set.
9. The method according to claim 7, wherein, at a final stage of
the PM elimination control, burn-up control is executed, in which
performance and stopping of concentrated intermittent fuel addition
to a section of the exhaust system that is upstream of the catalyst
are repeated a predetermined number of times.
10. The method according to claim 7, wherein the temperature of the
catalyst is discretely increased after the PM elimination control
is resumed.
11. An exhaust purifying apparatus for an internal combustion
engine, comprising: an estimation unit that estimates an
accumulation amount of particulate matter trapped about a catalyst
in an exhaust system, and a control unit, wherein, when the
estimated accumulation amount is equal to or more than a
permissible value, the control unit executes PM elimination control
for supplying unburned fuel component to the catalyst to increase
the temperature of the catalyst and burning the trapped particulate
matter, and sets the estimated accumulation amount to zero at the
completion of the PM elimination control, and when execution of the
PM elimination control becomes possible after suspension of the
control, the control unit resumes the PM elimination control even
if the accumulation amount of particulate matter about the catalyst
is less than the permissible value.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purifying
apparatus and an exhaust purifying method for an internal
combustion engine.
BACKGROUND ART
[0002] As disclosed in Japanese Laid-Open Patent Publication No.
5-44434, a typical exhaust purifying apparatus applied to an
internal combustion engine such as a vehicle diesel engine includes
a PM filter that is located in the exhaust system. The PM filter
traps particulate matter, which is predominantly composed of soot
in exhaust gas.
[0003] In an internal combustion engine provided with such an
exhaust purifying apparatus, PM elimination control is performed to
prevent the PM filter from being clogged with particulate matter
(PM) when the accumulation amount of particulate matter in the PM
filter estimated from, for example, the operating condition of the
engine is more than or equal to a permissible value. In the PM
elimination control, fuel is added to exhaust gas in a section
upstream of the PM filter so that oxidation of unburned fuel
component on the catalyst of the PM filter generates heat to
increase the temperature of the catalyst. Accordingly, particulate
matter on the PM filter is burned. When it is determined that
particulate matter deposited on the PM filter is completely burned,
the estimated accumulation amount of particulate matter on the PM
filter is set to zero, and the PM elimination control is
completed.
[0004] The PM elimination control is sometimes suspended due to
stopping of the engine during the execution. When the engine is
started again and execution of the PM elimination control becomes
possible, the PM elimination control is not resumed if the
accumulation amount of particulate matter at the time of suspension
is less than the permissible value. However, if incomplete
execution of the PM elimination control due to suspension is
repeated several times, the following drawbacks are likely to occur
in relation to the estimated accumulation amount of particulate
matter.
[0005] The estimated accumulation amount of particulate matter may
contain an error in relation to the actual accumulation amount.
Such an error is eliminated by setting the estimated amount of
particulate matter to zero when the PM elimination control is
completed so that particulate matter deposited on the PM filter is
completely burned. However, after execution and suspension of the
PM elimination control are repeated a few times, accumulation of
particulate matter on the PM filter in normal operation of the
engine and burning of the particulate matter in the PM elimination
control up to its suspension are repeated without the estimated
particulate matter accumulation amount being set to zero. While the
accumulation amount of particulate matter is repeatedly increased
and reduced, the estimated accumulation amount can be greatly
deviated from the actual accumulation amount.
[0006] When the estimated accumulation amount of particulate matter
is significantly less than the actual accumulation amount,
execution of control for richening the exhaust air-fuel ratio based
on the estimated accumulation amount of particulate matter can
excessively increase the catalyst bed temperature of the PM filter.
Such an excessive catalyst bed temperature is caused in the
following manner. When unburned fuel component is supplied to the
PM filter based on the control for richening the exhaust air-fuel
ratio, oxidation of the unburned fuel component causes particulate
matter deposited on the PM filter to burn. At this time, a greater
amount of particulate matter than estimated has been accumulated,
and the heat generated when the particulate matter is burned is
increased accordingly.
SUMMARY OF THE INVENTION
[0007] Accordingly, it is an objective of the present invention to
provide an exhaust purifying apparatus of an internal combustion
engine that prevents an estimated accumulation amount of
particulate matter about a catalyst from being significantly
deviated from the actual accumulation amount due to suspension of
PM elimination control. The present invention further provides an
exhaust purifying method.
[0008] To achieve the foregoing and other objectives and in
accordance with the purpose of the present invention, the invention
provides an exhaust purifying apparatus for an internal combustion
engine. The apparatus estimates an accumulation amount of
particulate matter trapped about a catalyst in an exhaust system.
When the estimated accumulation amount is equal to or more than a
permissible value, the apparatus executes PM elimination control
for supplying unburned fuel component to the catalyst to increase
the temperature of the catalyst and burning the trapped particulate
matter. The apparatus sets the estimated accumulation amount to
zero at the completion of the PM elimination control. When
execution of the PM elimination control becomes possible after
suspension of the control, the apparatus resumes the PM elimination
control even if the accumulation amount of particulate matter about
the catalyst is less than the permissible value.
[0009] The present invention further provides an exhaust purifying
method for an internal combustion engine. The method includes
estimating an accumulation amount of particulate matter trapped
about a catalyst in an exhaust system of the internal combustion
engine. The method further includes executing PM elimination
control when the estimated accumulation amount is equal to or more
than a permissible value. In which control, unburned fuel component
is supplied to the catalyst to increase the temperature of the
catalyst and the trapped particulate matter is burned. The method
further includes setting the estimated accumulation amount to zero
at the completion of the PM elimination control. The method further
includes resuming the PM elimination control when execution of the
PM elimination control becomes possible after suspension of the
control, even if the accumulation amount of particulate matter
about the catalyst is less than the permissible value.
[0010] Other aspects and advantages of the invention will become
apparent from the following description, taken in conjunction with
the accompanying drawings, illustrating by way of example the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The invention, together with objects and advantages thereof,
may best be understood by reference to the following description of
the presently preferred embodiments together with the accompanying
drawings in which:
[0012] FIG. 1 is a diagrammatic view illustrating the overall
configuration of an internal combustion engine to which an exhaust
purifying apparatus according to the present invention is
applied;
[0013] FIG. 2 is a graph showing changes in combustion rate of
unburned fuel (HC) and PM about a catalyst relative to changes in
the catalyst bed temperature;
[0014] FIGS. 3(a) and 3(b) are time charts illustrating changes in
PM accumulation amount and catalyst bed temperature during PM
elimination control;
[0015] FIG. 4 is a graph showing the relationship between a PM
accumulation amount (determination value) Dc and holding period
t2;
[0016] FIG. 5 is a graph showing the relationship between a PM
accumulation amount (determination value) Db and holding period
t3;
[0017] FIGS. 6(a) and 6(b) are time charts illustrating the manner
of adding fuel and changes in the exhaust air-fuel ratio due to the
fuel addition during burn-up control;
[0018] FIGS. 7(a) and 7(b) are time charts illustrating changes in
the PM accumulation amount and the catalyst bed temperature when
the PM elimination control is executed and completed with the
estimated PM accumulation amount being deviated from the actual
accumulation amount;
[0019] FIGS. 8(a) and 8(b) are time charts illustrating changes in
the PM accumulation amount and the catalyst bed temperature when
the PM elimination control is repeatedly executed and
suspended;
[0020] FIGS. 9(a) and 9(b) are time charts illustrating changes in
the PM accumulation amount and the catalyst bed temperature when
the PM elimination control is first suspended and then resumed and
completed; and
[0021] FIG. 10 is a flowchart showing the procedure for resuming
the PM elimination control.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] An exhaust purifying apparatus for an internal combustion
engine according to a preferred embodiment of the present invention
will now be described with reference to FIGS. 1 to 10(d).
[0023] FIG. 1 illustrates the configuration of an internal
combustion engine 10 to which the exhaust purifying apparatus
according to this embodiment is applied. The internal combustion
engine 10 is a diesel engine that includes a common rail fuel
injection device, and a turbocharger 11. The engine 10 includes an
intake passage 12, combustion chambers 13, and an exhaust passage
14.
[0024] The intake passage 12 forms an intake system for the
internal combustion engine 10. In the most upstream section of the
intake passage 12, an air cleaner 15 is located. From the air
cleaner 15 toward the downstream side, the air flow meter 16, a
compressor 17 incorporated in the turbocharger 11, an intercooler
18, and an intake throttle valve 19 are provided in the intake
passage 12. The intake passage 12 is branched at an intake manifold
20 located downstream of the intake throttle valve 19, and
connected to each of the combustion chambers 13 of the internal
combustion engine 10 through intake ports 21.
[0025] In the exhaust passage 14, which forms part of the exhaust
system for the internal combustion engine 10, an exhaust port 22 is
connected to each combustion chamber 13. The exhaust ports 22 are
connected to an exhaust turbine 24 of the turbocharger 11 through
an exhaust manifold 23. In a section of the exhaust passage 14 that
is downstream of the exhaust turbine 24, a NOx catalytic converter
25, a PM filter 26, and an oxidation catalytic converter 27 are
provided in this order from the upstream side.
[0026] The NOx catalytic converter 25 supports an
occlusion-reduction NOx catalyst. The NOx catalyst occludes NOx in
exhaust gas when the concentration of oxygen in exhaust gas is
high, and emits the occluded NOx when the concentration of oxygen
in the exhaust gas is low. If a sufficient amount of unburned fuel
component, which functions as a reducing agent, exists in the
vicinity thereof, the NOx catalyst reduces emitted NOx to purify
the exhaust gas.
[0027] The PM filter 26 is made of a porous material and traps
particulate matter (PM), which is predominantly composed of soot,
in exhaust. Like the NOx catalytic converter 25, the PM filter 26
supports an occlusion-reduction NOx catalyst. The NOx catalyst of
the PM filter 26 reduces emitted NOx to purify the exhaust gas. The
reaction triggered by the NOx catalyst burns (oxidizes) and removes
the trapped PM.
[0028] The oxidation catalytic converter 27 supports an oxidation
catalyst. The oxidation catalyst oxidizes hydrocarbon (HC) and
carbon monoxide (CO) in exhaust gas to purify the exhaust gas.
[0029] In sections upstream of and downstream of the PM filter 26,
an input gas temperature sensor 28 and an output gas temperature
sensor 29 are provided, respectively. The input gas temperature
sensor 28 detects an input gas temperature, which is the
temperature of exhaust gas that flows into the PM filter 26. The
output gas temperature sensor 29 detects an output gas temperature,
which is the temperature of exhaust gas that has passed through the
PM filter 26. Also, a differential pressure sensor 30 is provided
in the exhaust passage 14. The differential pressure sensor 30
detects a pressure difference between a section upstream and a
section downstream of the PM filter 26. Oxygen sensors 31, 32 are
located in a section of the exhaust passage 14 that is upstream of
the NOx catalytic converter 25 and a section of the exhaust passage
14 between the PM filter 26 and the oxidation catalytic converter
27, respectively. The oxygen sensors 31, 32 detect the
concentration of oxygen in exhaust gas.
[0030] The internal combustion engine 10 further includes an
exhaust gas recirculation device (EGR device) for returning some of
the exhaust gas to the air in the intake passage 12. The EGR device
includes an EGR passage 33 that connects the exhaust passage 14
with the intake passage 12. The most upstream section of the EGR
passage 33 is connected to a section of the exhaust passage 14 that
is upstream of the exhaust turbine 24.
[0031] In the EGR passage 33, an EGR catalyst 34, an EGR cooler 35,
and an EGR valve 36 are provided in this order from the upstream
side. The EGR catalyst 34 reforms recirculated exhaust gas. The EGR
cooler 35 cools the reformed exhaust gas. The EGR valve 36 adjusts
the flow rate of the reformed and cooled exhaust gas. The most
downstream section of the EGR passage 33 is connected to a section
of the intake passage 12 that is downstream of the intake throttle
valve 19.
[0032] An injector 40 is provided in each combustion chamber 13 of
the internal combustion engine 10 to inject fuel to be combusted in
the combustion chamber 13. The injectors 40 are connected to a
common rail 42 with a high-pressure fuel pipe 41. High-pressure
fuel is supplied to the common rail 42 through a fuel pump 43. The
pressure of high-pressure fuel in the common rail 42 is detected by
a rail pressure sensor 44 attached to the common rail 42. The fuel
pump 43 is capable of supplying low-pressure fuel to a fuel adding
valve 46 through a low-pressure fuel pipe 45.
[0033] Various control procedures for the internal combustion
engine 10 are executed by an electronic control device 50. The
electronic control device 50 includes a CPU that executes various
computation processes related to control of the engine 10, a ROM
storing programs and data necessary for the control, a RAM for
temporarily storing the computation results of the CPU, and input
and output ports for inputting and outputting signals from and to
the outside.
[0034] In addition to the above described sensors, the input port
of the electronic control device 50 is connected to an NE sensor 51
for detecting the rotational speed of the engine 10, an
acceleration pedal sensor 52 for detecting the degree of depression
of an acceleration pedal, and a throttle valve sensor 53 for
detecting the opening degree of the intake throttle valve 19. The
output port of the electronic control device 50 is connected to a
drive circuit for driving the intake throttle valve 19, the EGR
valve 36, the injector 40, the fuel pump 43, and the fuel adding
valve 46.
[0035] Based on detected signals from the above described sensors,
the electronic control device 50 grasps the operating condition of
the engine 10. According to the grasped operating condition, the
electronic control device 50 outputs command signals to the drive
circuits of the devices connected to the output port. The
electronic control device 50 executes various control procedures
such as control of the opening degree of the intake throttle valve
19, EGR control based on the opening degree control of the EGR
valve 36, control of the amount, the timing and the pressure of
fuel injection from the injector 40, and control related to fuel
addition by the fuel adding valve 46.
[0036] In this embodiment, to prevent the NOx catalytic converter
25 and the PM filter 26 from being clogged with PM, PM elimination
control is performed, in which PM trapped by the NOx catalytic
converter 25 and the PM filter 26 are burned to purify exhaust gas.
In the PM elimination control, unburned fuel component is supplied
to the NOx catalytic converter 25 and the NOx catalyst of the PM
filter 26 so that the unburned fuel component is oxidized in
exhaust gas or on each catalyst to generate heat. Accordingly, the
catalyst is heated to a temperature of about 600 to 700.degree. C.,
and PM about the catalyst is burned.
[0037] During the PM elimination control, unburned fuel component
may be supplied to the catalysts by sub-injection (after injection)
in an exhaust stroke or an expansion stroke, which injection is
executed after fuel is injected from the injector 40 to be
combusted in the combustion chambers 13. Alternatively, unburned
fuel may be supplied by adding fuel to exhaust gas from the fuel
adding valve 46. To minimize extra fuel consumption due to the PM
elimination control, the amount of unburned fuel component added to
the catalysts in the PM elimination control is limited to the
minimum value that allows for a necessary increase in the
temperature of the catalysts.
[0038] In this embodiment, the PM elimination control is performed
when the following requirements are all satisfied.
[0039] The elimination of PM is requested. A request for the PM
elimination is made when clogging of the PM filter 26 is recognized
based on the fact that the PM accumulation amount of the PM filter
26 estimated from the engine operating condition reaches and
exceeds the highest value of a permissible range.
[0040] A detected value of the input gas temperature sensor 28
(input gas temperature thci) is more than or equal to a lower limit
temperature A (for example, 150.degree. C.) for performing the PM
elimination control. Also, the catalyst bed temperature of the NOx
catalyst, which is estimated from the history of the engine
operating condition is more than or equal to a lower limit
temperature B for performing the PM elimination control. The lower
limit temperatures A, B are the lowest values of the exhaust
temperature and the catalyst bed temperature that cause oxidation
sufficient to increase the catalyst bed temperature as unburned
fuel component is supplied.
[0041] The detected value of the input gas temperature sensor 28 is
less than an upper limit value C in a temperature range for
avoiding excessive temperature increase of the catalysts due to
heat generated by the PM elimination control.
[0042] The detected value of the output gas temperature sensor 29
is less than an upper limit value D in a temperature range for
avoiding excessive temperature increase of the catalysts due to the
PM elimination control.
[0043] Fuel addition to exhaust gas is permitted. That is, the
engine operating condition is in a range to permit the fuel
addition to exhaust gas. The addition of fuel to exhaust gas is
permitted in the internal combustion engine 10 as long as the
engine 10 is not stalling, the cylinders have been distinguished,
and the depression degree of the acceleration pedal is not
limited.
[0044] The PM elimination control will now be described with
reference to FIG. 2 to 6(b).
[0045] FIG. 2 is a graph showing changes in the combustion rates of
unburned fuel (HC) and PM collected on the surface of the catalysts
as the catalyst bed temperature increases at the NOx catalytic
converter 25 and the PM filter 26. As obvious from FIG. 2, unburned
fuel collected on the catalysts is burned at a relatively low
catalyst bed temperature (about 300.degree. C.), and PM collected
on the catalysts is burned when the catalyst bed temperature is
increased to a relatively high temperature, for example, in the
range between 600 and 700.degree. C., inclusive. Therefore, if the
catalyst bed temperature is suddenly increased to a value of about
700.degree. C., a great amount of unburned fuel and PM collected on
the catalysts are burned, and the generated heat can excessively
increase the catalyst bed temperature.
[0046] Such an excessive increase in the catalyst bed temperature
is prevented by discretely increasing the catalyst bed temperature
as shown in FIG. 3(b). That is, to burn unburned fuel (HC) and PM
collected on the surface of the catalysts in stages, the catalyst
bed temperature is increased to 300, 600, 630, and 650.degree. C.,
successively. Specifically, first the minimum amount of unburned
fuel component required for increasing the catalyst bed temperature
to 300.degree. C. is supplied to the catalysts. As the catalyst bed
temperature is increased toward 300.degree. C., unburned fuel
collected on the catalysts is burned. When the catalyst bed
temperature reaches 300.degree. C., the catalyst bed temperature is
increased to 600.degree. C. and is held at this temperature for
holding period t2. Then, the catalyst bed temperature is increased
to 630.degree. C. and is held at this temperature for holding
period t3. Finally, the catalyst bed temperature is increased to
650.degree. C.
[0047] As shown in FIG. 4, the holding period t2, during which the
catalyst bed temperature is held at 600.degree. C., is set shorter
as the PM accumulation amount (determination value) Dc at time Tc,
where the catalyst bed temperature is switched from 300.degree. C.
to 600.degree. C., is reduced. As shown in FIG. 5, the holding
period t3, during which the catalyst bed temperature is held at
630.degree. C., is set shorter as the PM accumulation amount
(determination value) Db at time Tb, where the catalyst bed
temperature is switched from 600.degree. C. to 630.degree. C., is
reduced. The holding periods t2 and t3 are varied according to the
PM accumulation amounts Dc, Db so that the time for the PM
elimination control to burn PM is minimized, and deterioration of
the fuel consumption due to the amount of fuel used in the control
is minimized.
[0048] As the catalyst bed temperature is increased in stages in
the above described PM elimination control, PM collected about the
catalysts is burned, and the PM accumulation amount is reduced as
shown in FIG. 3(a). However, at the upstream end of the NOx
catalytic converter 25 and the upstream end of the PM filter 26,
some PM remains even if the above described PM elimination control
is performed. The reason why PM remains is believed to be that PM
is likely to be deposited at the exhaust upstream end of the NOx
catalytic converter 25 and the exhaust upstream end of the PM
filter 26, and the supply of unburned fuel component in the PM
elimination control cannot supply a sufficient amount of unburned
fuel component per unit time to burn the PM completely.
Particularly, in the NOx catalytic converter 25, which is located
upstream of the PM filter 26, a greater amount of PM that is not
burned in the PM elimination control remains at the upstream
end.
[0049] Thus, at the final stage of the PM elimination control, that
is when the PM accumulation amount is reduced to a determination
value Da close to zero (for example, 0.3 g), burn-up control is
performed to burn PM that cannot be burned in the PM elimination
control. The overview of the burn-up control will be described with
reference to FIGS. 6(a) and 6(b). FIG. 6(a) shows the manner in
which the fuel adding valve 46 adds fuel, and FIG. 6(b) shows
changes in the exhaust air-fuel ratio.
[0050] As shown in FIG. 6(a), concentrated intermittent fuel
addition is repeatedly performed and stopped in the burn-up
control. The concentrated intermittent fuel addition increases the
amount of unburned fuel component and oxygen supplied to the
catalysts of the NOx catalytic converter 25 and the PM filter 26 to
a level sufficient for burning the PM that cannot be burned in the
PM elimination control. Therefore, the concentrated intermittent
fuel addition permits the PM to be burned.
[0051] The concentrated intermittent fuel addition unavoidably
causes the catalyst bed temperature to increase noticeably. Thus,
the fuel addition is periodically stopped, thereby suppressing
excessive increase in the catalyst bed temperature. As a result,
intermittent concentrated fuel addition is repeatedly performed and
stopped, and the exhaust air-fuel ratio is repeatedly reversed
between a rich state and a lean state as shown in FIG. 6(b). The
burn-up control is ended when the repetitions of performing and
stopping of the concentrated intermittent fuel addition has reached
a number (in this embodiment, three times) that is sufficient for
burning the PM remaining in the NOx catalytic converter 25 and the
PM filter 26.
[0052] The PM elimination control is completed based on the end of
the burn-up control. When the PM elimination control is completed,
the PM accumulation amount about the catalysts estimated from the
engine operating condition becomes zero. In other words, the PM
accumulation amount is set to zero when the PM elimination control
is completed.
[0053] The PM elimination control may be suspended during
execution. For example, when the engine 10 is stopped, the PM
elimination control is suspended even during the execution thereof.
Also, the PM elimination control is suspended when deactivation of
the catalyst occurs, in which the catalyst bed temperature is
lowered even if unburned fuel component is being supplied, due to a
drop of the exhaust temperature.
[0054] Such deactivation of a catalyst is caused by a vicious
circle in which a drop in the exhaust temperature during the PM
elimination control deactivates a catalyst and temporarily hampers
oxidation of unburned fuel, and the unburned fuel stays collected
on the catalyst and decreases the surface area of the catalyst that
is exposed to exhaust gas, and the degree of activation of the
catalyst is lowered further, and so on. If unburned fuel component
is supplied to each catalyst in the PM elimination control during
deactivation of the catalyst, the unburned fuel component is
emitted to the outside in an incomplete combustion state.
Therefore, the exhaust emission can be deteriorated. For example, a
great amount of smoke may be emitted. The PM elimination control is
thus suspended when the catalyst is deactivated.
[0055] However, if incomplete execution of the PM elimination
control due to suspension is repeated several times, the estimated
PM accumulation amount is greatly deviated from the actual
accumulation amount, which causes problems. The reason why
repetitive execution and suspension of the PM elimination control
causes the estimated PM accumulation amount to be deviated from the
actual accumulation amount will be described with reference to
FIGS. 7(a) to 8(b).
[0056] Since the PM accumulation amount used in the PM elimination
control is a value estimated from the engine operating condition,
the PM accumulation amount can be deviated from the actual
accumulation amount. For example, as shown in FIG. 7(a), the
estimated PM accumulation amount (solid line L1) can be deviated
from the actual accumulation amount (broken line L2). Normally,
such an error is eliminated by setting the estimated PM
accumulation to zero when PM collected about the catalyst is
completely burned and the PM elimination control is completed. That
is, as shown in FIG. 7(b), when the PM elimination control that has
been started at time T1, where the estimated PM accumulation amount
reaches and exceeds a permissible value, is completed at time T2,
the PM collected about each catalyst is completely burned, and the
estimated PM accumulation amount is set to zero. This allows the
estimated PM accumulation amount to correspond to the actual
accumulation amount, and an error between these values is
eliminated.
[0057] As described above, if the PM elimination control is
completed, an error between the estimated PM accumulation amount
and the actual accumulation amount is eliminated. However, if the
PM elimination control is suspended before it is completed, such an
error is not eliminated. For example, as shown in FIG. 8(a), when
the PM elimination control is suspended at time T3 due to stopping
of the engine 10 or deactivation of the catalysts, if the PM
accumulation amount has dropped to a value less than the
permissible value at the time of suspension, the PM elimination
control will not be resumed even if the engine 10 is started again
or the catalyst is activated so that the PM elimination control is
possible. In this case, since the PM elimination control is not
completed, setting the estimated PM accumulation amount to zero to
eliminate the error is not performed. Then, when the PM
accumulation amount reaches the permissible value again, the PM
elimination control is executed (time T4).
[0058] If execution and suspension of the PM elimination control
are repeated a few times (T4 to T7 in FIG. 8(b)), accumulation of
PM about each catalyst in a normal operation of the engine 10 and
burning of the PM in the PM elimination control are repeated
without the PM accumulation amount being set to zero. As a result,
the estimated PM accumulation amount is increased and decreased in
a manner as shown in FIG. 8(a), during which the estimated PM
accumulation amount can be significantly deviated from the actual
accumulation amount. When the estimated PM accumulation amount is
significantly less than the actual accumulation amount, for
example, when the actual accumulation amount is in a state
indicated by broken line L3 relative to the estimated PM
accumulation amount indicated by solid line L1 in FIG. 7(a), the
following conditions occur at the final stage of the PM elimination
control.
[0059] That is, the burn-up control, which should be started when
the estimated PM accumulation amount (L1) is reduced to the
determination value Da (0.3 g), is started in a state where the
amount of PM about each catalyst is significantly greater
(indicated by X in the FIG. 7(a)) than the determination value Da.
Then, when the unburned fuel component is supplied to each catalyst
during the control, oxidation of the unburned fuel component causes
PM to burn. However, since the actual amount of PM accumulation is
more than the assumed value (0.3 g), the heat of burning of the PM
is great, which can excessively increase the catalyst bed
temperature of the NOx catalytic converter 25 and the PM filter
26.
[0060] To avoid such a problem, when execution of the PM
elimination control is possible after suspension of the control,
the PM elimination control is resumed regardless of whether the PM
accumulation amount is less than the permissible value. The resumed
PM elimination control is executed until it is completed. FIGS.
9(a) and 9(b) show an example of changes in the PM accumulation
amount and the state of the PM elimination control in a case where
the resumption of the PM elimination control is executed.
[0061] In FIGS. 9(a) and 9(b), the execution of the PM elimination
control becomes possible at time T8 after it is suspended. Then,
even if the PM accumulation amount is less than the permissible
value, the PM elimination control is resumed and continued until it
is completed at time T9. By completing the PM elimination control,
PM accumulated about the catalysts is completely burned, and the
estimated PM accumulation value is set to zero, which eliminates an
error between the estimated PM accumulation amount and the actual
accumulation amount. Therefore, the problems described above, which
are caused by the error not being eliminated, are avoided.
[0062] The procedure for the resumption of the PM elimination
control will now be described with reference to FIG. 10, which
shows a control resumption routine. The control resumption routine
is executed as an interrupt by the electronic control device 50,
for example, at predetermined time intervals.
[0063] In the routine, whether the PM elimination control of the
previous cycle was suspended is determined based on a history of
the operating condition of the engine (S101). If the outcome is
positive, whether the PM elimination control is executable is
determined (S102). For example, whether the engine is running and
the deactivation of the catalyst is eliminated is determined
(S102). Whether the deactivation of the catalyst is eliminated is
determined based on whether the catalyst bed temperature has a
value that burns unburned fuel component collected on the catalysts
(for example 300.degree. C.) or based on whether the catalyst bed
temperature will soon have such a value because the engine load has
been high for a predetermined period.
[0064] When the outcome of step S102 is positive, the PM
elimination control is executed (S103) regardless of the current PM
accumulation amount, and continued until it is completed. Even if
the PM elimination control is suspended after it has been resumed
according to step S103, the PM elimination control will be
continued until it is completed since the control is repeatedly
resumed according to steps S101 to S103.
[0065] After being resumed, the PM elimination control discretely
increases the catalyst bed temperature. The holding periods t2 and
t3 for the catalyst bed temperatures of 600.degree. C. and
630.degree. C. are shortened as the PM accumulation amounts Dc and
Db are reduced. Also, at the final stage in the resumed PM
elimination control, the burn-up control is executed to completely
burn PM accumulated about the catalysts.
[0066] The above-described embodiment has the following
advantages.
[0067] (1) When the execution of the PM elimination control becomes
possible after it has been suspended, the PM elimination control is
resumed even if the current PM accumulation amount is less than the
permissible value. The resumed PM elimination control is continued
until it is completed so that PM accumulated about the catalysts is
completely burned. When the PM is completely burned and the PM
elimination control is completed, the estimated PM accumulation
value is set to zero, which eliminates an error between the
estimated PM accumulation amount and the actual accumulation
amount. Therefore, repetitive execution and suspension of the PM
elimination control without completion of the control are avoided.
Accordingly, an error between the estimated PM accumulation amount
and the actual accumulation amount, which would be increased during
the repetitive execution and suspension, is suppressed.
[0068] (2) When the PM elimination control is suspended, burning of
PM accumulated about each catalyst has progressed to a certain
degree. The less the remaining PM accumulation at the time of
suspension, the shorter the time for execution of the PM
elimination control after resumption can be set. That is, even a
shorter period for execution will be sufficient to completely burn
PM accumulated about each catalyst. Accordingly, in the resumed PM
elimination control, the less the PM accumulation amounts Dc and
Db, the shorter the holding periods are set for t2 and t3.
Accordingly, the execution time of the control is shortened in
accordance with a decrease in the PM accumulation amount.
Therefore, time required for completion of the control after
resumption is shortened. Also, degradation in the fuel consumption
due to a uselessly extended period for the control is avoided.
[0069] (3) In the PM elimination control resumed after suspension,
the burn-up control is executed at the final stage, so that PM
accumulated about each catalyst is completely burned and the actual
PM accumulation amount is reduced to zero. Therefore, when the PM
elimination control is completed after being resumed, a situation
is avoided in which PM remains about each catalyst even if the
estimated PM accumulation amount is set to zero and the estimated
PM accumulation amount does not correspond to the actual
accumulation amount.
[0070] The above-described embodiments may be modified as
follows.
[0071] The execution period of the PM elimination control does not
need to be varied according to the PM accumulation amount, but may
be fixed for a period that is sufficient for PM accumulated about
each catalyst to be completely burned by the PM elimination
control.
[0072] It may be configured so that the execution period is not
fixed in the PM elimination control that is resumed after
suspension and is fixed in the PM elimination control that is
completed without being suspended.
* * * * *