U.S. patent application number 12/224997 was filed with the patent office on 2009-02-26 for exhaust gas purification device for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Nobumoto Ohashi.
Application Number | 20090049825 12/224997 |
Document ID | / |
Family ID | 38514070 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090049825 |
Kind Code |
A1 |
Ohashi; Nobumoto |
February 26, 2009 |
Exhaust Gas Purification Device For Internal Combustion Engine
Abstract
A NOx absorbent is arranged in an exhaust passage of an internal
combustion engine, and a fuel supply valve (28) is arranged in the
exhaust passage upstream of the NOx absorbent. If the temperature
of the NOx absorbent is lower than a predetermined temperature when
the NOx must be released from the NOx absorbent, the air-fuel ratio
of exhaust gas flowing through the NOx absorbent is first switched
from a basic lean air-fuel ratio to and maintained at a lean
air-fuel ratio with a lower leanness for a predetermined lean time,
and is then switched to a rich air-fuel ratio. If the temperature
of the NOx absorbent is higher than the predetermined temperature
when the NOx must be released from the NOx absorbent, the air-fuel
ratio of the exhaust gas flowing through the NOx absorbent is
switched to the rich air-fuel ratio without being switched to the
lean air-fuel ratio with a lower leanness.
Inventors: |
Ohashi; Nobumoto;
(Susono-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi
JP
|
Family ID: |
38514070 |
Appl. No.: |
12/224997 |
Filed: |
April 26, 2007 |
PCT Filed: |
April 26, 2007 |
PCT NO: |
PCT/JP2007/059435 |
371 Date: |
September 11, 2008 |
Current U.S.
Class: |
60/285 |
Current CPC
Class: |
F01N 3/0842 20130101;
F01N 2250/14 20130101; F02B 37/00 20130101; Y02T 10/26 20130101;
F02D 41/024 20130101; Y02T 10/12 20130101; F01N 3/0871 20130101;
F02D 2200/0802 20130101; F01N 2610/03 20130101; F02D 41/0275
20130101 |
Class at
Publication: |
60/285 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2006 |
JP |
2006-123785 |
Claims
1. An exhaust gas purification device for an internal combustion
engine having an exhaust passage, combustion being carried out
under a basic lean air-fuel ratio, comprising: a NOx absorbent
arranged in the exhaust passage, the NOx absorbent absorbing NOx
contained in exhaust gas therein when the air-fuel ratio of exhaust
gas is lean and releasing absorbed NOx therefrom when the air-fuel
ratio of exhaust gas is switched to rich; and control means for
controlling the air-fuel ratio of exhaust gas flowing through the
NOx absorbent, wherein, when NOx must be released from the NOx
absorbent, the air-fuel ratio of exhaust gas flowing through the
NOx absorbent is first switched from the basic lean air-fuel ratio
to and maintained at a lean air-fuel ratio with a lower leanness
for a predetermined lean time, and is then switched to a rich
air-fuel ratio.
2. An exhaust gas purification device for an internal combustion
engine according to claim 1, wherein the air-fuel ratio of the
exhaust gas flowing through the NOx absorbent is returned to and
maintained at the basic lean air-fuel ratio until the NOx must be
released from the NOx absorbent once more.
3. An exhaust gas purification device for an internal combustion
engine according to claim 1, further comprising means for obtaining
a temperature of the NOx absorbent, wherein, if the temperature of
the NOx absorbent is lower than a predetermined temperature when
the NOx must be released from the NOx absorbent, the air-fuel ratio
of exhaust gas flowing through the NOx absorbent is first switched
from the basic lean air-fuel ratio to and maintained at the lean
air-fuel ratio with a lower leanness for the predetermined lean
time, and is then switched to the rich air-fuel ratio, if the
temperature of the NOx absorbent is higher than the predetermined
temperature when the NOx must be released from the NOx absorbent,
the air-fuel ratio of exhaust gas flowing through the NOx absorbent
is switched to the rich air-fuel ratio without being switched to
the lean air-fuel ratio with a lower leanness.
4. An exhaust gas purification device for an internal combustion
engine according to claim 3, further comprising means for obtaining
a degree of deterioration of the NOx absorbent, wherein the
predetermined temperature when the degree of deterioration of the
NOx absorbent is high is set higher than that when the degree of
deterioration is low.
5. An exhaust gas purification device for an internal combustion
engine according to claim 1, further comprising means for obtaining
a temperature of the NOx absorbent, wherein the lean time is set in
accordance with the obtained temperature of the NOx absorbent.
6. An exhaust gas purification device for an internal combustion
engine according to claim 1, further comprising means for obtaining
an amount of the exhaust gas flowing through the NOx absorbent,
wherein the lean time is set in accordance with the amount obtained
in the exhaust gas.
7. An exhaust gas purification device for an internal combustion
engine according to claim 1, further comprising means for obtaining
a degree of deterioration of the NOx absorbent, wherein the lean
time is set in accordance with the obtained degree of
deterioration.
8. An exhaust gas purification device for an internal combustion
engine according to claim 1, wherein the lean time is set to make
the temperature of the NOx absorbent or the increment thereof,
obtained by maintaining the air-fuel ratio of the exhaust gas
inflowing the NOx absorbent at the lean air-fuel ratio with a lower
leanness for the lean time, equal to a target value.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an exhaust gas purification
device for an internal combustion engine.
BACKGROUND ART
[0002] There is known an internal combustion engine wherein a NOx
absorbent is arranged in the exhaust passage of the engine in which
an NOx absorbent absorbs NOx contained in the exhaust gas therein
when the air-fuel ratio of the exhaust gas is lean and releases
absorbed NOx therefrom when the air-fuel ratio of the exhaust gas
is switched to rich, wherein a fuel supply valve is arranged in the
exhaust passage upstream of the NOx absorbent, and wherein fuel is
supplied from the fuel supply valve to the NOx absorbent to make
the air-fuel ratio of exhaust gas flowing through the NOx absorbent
temporarily rich, when the NOx must be released from the NOx
absorbent (see Japanese Unexamined Patent Publication No. 11-62666,
for example). In the engine, NOx generated when combustion is
carried out under a lean air-fuel ratio is absorbed in the NOx
absorbent. On the other hand, when the NOx absorption capacity has
reached a saturated state, the air-fuel ratio is temporarily made
rich to release NOx from the NOx absorbent and reduce the NOx.
[0003] However, for example if an engine is idled for a long time,
the temperature of the NOx absorbent is lowered since the
temperature of exhaust gas inflowing through the NOx absorbent at
this time is low. When the temperature of the NOx absorbent is low
as mentioned above, the release rate of NOx from the NOx absorbent
is low. Therefore, if the air-fuel ratio of exhaust gas is simply
switched to rich, it may be impossible to obtain an adequate
release of NOx from the NOx absorbent.
DISCLOSURE OF THE INVENTION
[0004] It is, therefore, an object of the present invention to
provide an exhaust gas purification device for an internal
combustion engine, which is capable of obtaining an adequate
release of NOx from an NOx absorbent even when the temperature of
the NOx absorbent is low.
[0005] According to the present invention, there is provided an
exhaust gas purification device for an internal combustion engine
having an exhaust passage, combustion being carried out under a
basic lean air-fuel ratio, comprising: a NOx absorbent arranged in
the exhaust passage, the NOx absorbent absorbing NOx contained in
exhaust gas therein when the air-fuel ratio of exhaust gas is lean
and releasing absorbed NOx therefrom when the air-fuel ratio of
exhaust gas is switched to rich; and control means for controlling
the air-fuel ratio of exhaust gas flowing through the NOx
absorbent, wherein, when NOx must be released from the NOx
absorbent, the air-fuel ratio of exhaust gas flowing through the
NOx absorbent is first switched from the basic lean air-fuel ratio
to and maintained at a lean air-fuel ratio with a lower leanness
for a predetermined lean time, and is then switched to a rich
air-fuel ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is an overall view of an internal combustion engine
of a compression ignition type;
[0007] FIG. 2 is a sectional side view of a NOx storing
catalyst;
[0008] FIGS. 3A and 3B are sectional views of a surface part of a
catalyst carrier;
[0009] FIGS. 4A and 4B are views of the structure of a particulate
filter;
[0010] FIG. 5 is a time chart explaining a NOx release control;
[0011] FIG. 6 is a map illustrating the amount of NOx adsorbed per
unit time dNOx;
[0012] FIGS. 7A and 7B are time charts illustrating variations of
the air-fuel ratio of flowing exhaust gas AFEG;
[0013] FIG. 8 is a map illustrating a predetermined temperature
TcS;
[0014] FIGS. 9A to 9D are maps illustrating lean time tL,
respectively; and
[0015] FIG. 10 is a flowchart for executing the NOx release
control.
BEST MODE FOR CARRYING OUT THE INVENTION
[0016] FIG. 1 illustrates a case where the present invention is
applied to an internal combustion engine with a compression type
ignition. Alternatively, the present invention may also be applied
to an internal combustion engine with a spark type ignition.
[0017] Referring to FIG. 1, numeral 1 indicates an engine body, 2 a
combustion chamber of each cylinder, 3 an electrically-controlled
fuel injector for injecting fuel into each combustion chamber 2, 4
an intake manifold, and 5 an exhaust manifold. The intake manifold
4 is connected through an intake duct 6 to an outlet of a
compressor 7a of a turbocharger 7. The inlet of the compressor 7a
is connected via an air flow meter 8 to an air cleaner 9. An
electrically-controlled throttle valve 10 is arranged in the intake
duct 6. Further, a cooling device 11 is arranged around the intake
duct 6 for cooling intake air flowing through the intake duct 6. In
the embodiment shown in FIG. 1, engine cooling water is guided into
the cooling device 11 and cools intake air. On the other hand, the
exhaust manifold 5 is connected to an inlet of an exhaust turbine
7b of the exhaust turbocharger 7, while the outlet of the exhaust
turbine 7b is connected to an exhaust aftertreatment system 20.
[0018] The exhaust manifold 5 and the intake manifold 4 are
interconnected through an exhaust gas recirculation (hereinafter
referred to as an "EGR") passage 12. The EGR passage 12 is provided
with an electrically-controlled EGR control valve 13. Further, a
cooling device 14 is arranged around the EGR passage 12 for cooling
EGR gas flowing through the EGR passage 12. In the embodiment shown
in FIG. 1, engine cooling water is guided into the cooling device
14 and cools the EGR gas. Each fuel injector 3 is connected through
a fuel feed tube 15 to a common rail 16. This common rail 16 is
supplied with fuel from an electrically-controlled type variable
discharge fuel pump 20. Fuel supplied into the common rail 16 is
supplied through each fuel feed tube 15 to the fuel injector 3.
[0019] The exhaust aftertreatment system 20 comprises an exhaust
pipe 21 connected to an outlet of the exhaust turbine 7b, a
catalytic converter 22 connected to the exhaust pipe 21, and an
exhaust pipe 23 connected to the catalytic converter 22. A NOx
storing catalyst 24 and a particulate filter 25 are arranged in the
catalytic converter 22 in order, starting from the upstream side.
In addition, a temperature sensor 26 for detecting the temperature
of exhaust gas discharged from the catalytic converter 22 and an
air-fuel ratio sensor 27 for detecting the air-fuel ratio of
exhaust gas discharged from the catalytic converter 22 are arranged
in the exhaust pipe 23. The temperature of exhaust gas discharged
from the catalytic converter 22 represents the temperature of the
NOx storing catalyst 24 and the particulate filter 25.
[0020] On the other hand, the exhaust manifold 5 is provided with a
fuel supply valve 28. The fuel supply valve 28 is supplied with
fuel from the common rail 16, the fuel is fed from the fuel supply
valve 28 to the exhaust manifold 5. In the embodiment according to
the present invention, fuel is comprised of light oil. The fuel
supply valve 28 may be arranged in the exhaust pipe 21,
alternatively.
[0021] An electronic control unit 30 is comprised of a digital
computer provided with read only memory (ROM) 32, random access
memory (RAM) 33, a microprocessor (CPU) 34, an input port 35, and
an output port 36, all connected to each other by a bidirectional
bus 31. The output signals of the air flow meter 8, the temperature
sensor 26 and the air-fuel ratio sensor 27 are input through
corresponding AD converters 37 to the input port 35. Further,
connected to the accelerator pedal 39 is a load sensor 40
generating output voltage proportional to the amount of the
depression L of an accelerator pedal 39. Outputted voltage of the
load sensor 40 is input through a corresponding AD converter 37 to
the input port 35. Furthermore, connected to the input port 35 is a
crank angle sensor 41 generating an output pulse each time the
crankshaft turns, for example, by 15 degrees. The CPU 34 calculates
engine speed N based on the output pulse from the crank angle
sensor 41. On the other hand, the output port 36 is connected
through corresponding drive circuits 38 to the fuel injectors 3,
driver for the throttle valve 10, EGR control valve 13, fuel pump
20, and fuel supply valve 28.
[0022] FIG. 2 shows the structure of the NOx storing catalyst 24.
In the embodiment shown in FIG. 2, the NOx storing catalyst 24 is
formed of a honeycomb structure and is provided with a plurality of
exhaust gas passages 61 separated from each other by partitions 60.
The opposite surfaces of the partitions 60 carry a catalyst carrier
comprised of, for example, alumina. FIGS. 3A and 3B schematically
show the cross-section of the surface part of this catalyst carrier
65. As shown in FIGS. 3A and 3B, the catalyst carrier 65 carries a
precious metal catalyst 66 diffused on its surface. Further, the
catalyst carrier 65 is formed with a layer of a NOx absorbent 67 on
its surface.
[0023] In the embodiment according to the present invention,
platinum Pt is used as the precious metal catalyst 66. As the
ingredient for forming the NOx absorbent 67, for example, at least
one element selected from potassium K, sodium Na, cesium Cs, or
another alkali metal, barium Ba, calcium Ca, or another alkali
earth, lanthanum La, yttrium Y, or another rare earth may be
used.
[0024] The ratio of air and fuel (hydrocarbons) supplied to the
engine intake passage, combustion chambers 2, and exhaust passage
upstream of the NOx storing catalyst 24 is referred to as an
air-fuel ratio of the exhaust gas. The NOx absorbent 67 performs
NOx absorption and release action of absorbing the NOx when the
air-fuel ratio of the exhaust gas is lean and releasing the
absorbed NOx when the oxygen concentration in the exhaust gas
falls.
[0025] That is, if in the case of using barium Ba as the ingredient
forming the NOx absorbent 67, when the air-fuel ratio of exhaust
gas is lean, that is, when the oxygen concentration in exhaust gas
is high, the NO contained in the exhaust gas is oxidized on the
platinum Pt 66 such as shown in FIG. 3A to become NO.sub.2, and is
then absorbed in the NOx absorbent 67 and diffused in the NOx
absorbent 67 in the form of nitric acid ions NO.sub.3.sup.- while
bonding with the barium carbonate BaCO.sub.3. In this way, NOx is
absorbed in the NOx absorbent 67. If the oxygen concentration in
the exhaust gas is high, NO.sub.2 is produced on the surface of the
platinum Pt 66. If the NOx absorbing capability of the NOx
absorbent 67 is not saturated, the NO.sub.2 is absorbed in the NOx
absorbent 67 and nitric acid ions NO.sub.3.sup.- are produced.
[0026] In contrast, when the air-fuel ratio of the exhaust gas is
made rich or a stoichiometric air-fuel ratio, since the oxygen
concentration in the exhaust gas falls, the reaction proceeds in
the reverse direction (NO.sub.3.sup.-->NO.sub.2), and therefore
nitric acid ions NO.sub.3.sup.- in the NOx absorbent 67 are
released from the NOx absorbent 67 in the form of NO.sub.2. The
released NOx is then reduced to unburned hydrocarbons or CO that is
included in exhaust gas.
[0027] In the engine shown in FIG. 1, combustion under a lean
air-fuel ratio is continued, and the air-fuel ratio of the exhaust
gas flowing through the NOx absorbent 67 is thus maintained lean so
long as the fuel supply from the fuel supply valve 28 is stopped.
The NOx included in exhaust gas is absorbed into the NOx absorbent
67 at this stage. However, if combustion under a lean air-fuel
ratio is continued, the NOx absorbing capability of the NOx
absorbent 67 will end up becoming saturated, and therefore NOx will
no longer be able to be absorbed by the NOx absorbent 67.
Therefore, in the embodiment according to the present invention,
before the absorbing capability of the NOx absorbent 67 becomes
saturated, fuel is supplied from the fuel supply valve 28 so as to
temporarily make the air-fuel ratio of the exhaust gas rich, and
thereby release NOx from the NOx absorbent 67.
[0028] FIGS. 4A and 4B show the structure of the particulate filter
25. Note that FIG. 4A is a front view of the particulate filter 25,
while FIG. 4B is a side sectional view of the particulate filter
25. As shown in FIGS. 4A and 4B, the particulate filter 25 forms a
honeycomb structure and is provided with a plurality of exhaust
passages 70 and 71 extending parallel with each other. These
exhaust passages are comprised of exhaust gas inflow passages 70
with downstream ends sealed by plugs 72 and exhaust gas outflow
passages 71 with upstream ends sealed by plugs 73. Note that the
hatched portions in FIG. 4A show plugs 73. Therefore, the exhaust
gas inflow passages 70 and exhaust gas outflow passages 71 are
arranged alternately through thin wall partitions 74. In other
words, the exhaust gas inflow passages 70 and exhaust gas outflow
passages 71 are arranged so that each exhaust gas inflow passage 70
is surrounded by four exhaust gas outflow passages 71, and each
exhaust gas outflow passage 71 is surrounded by four exhaust gas
inflow passages 70.
[0029] The particulate filter 25 is formed from a porous material
such as cordierite. Therefore, exhaust gas flowing into the exhaust
gas inflow passages 70 flows out into the adjoining exhaust gas
outflow passages 71 through the surrounding partitions 74 as shown
by the arrows in FIG. 4B.
[0030] In the embodiment according to the present invention, the
peripheral walls of the exhaust gas inflow passages 70 and exhaust
gas outflow passages 71, that is, the opposite surfaces of the
partitions 74 and the inside walls of the micropores of the
partitions 74 also carry a catalyst carrier comprised of, for
example, alumina. As shown in FIGS. 3A and 3B, the catalyst carrier
65 carries a precious metal catalyst 66 diffused on its surface.
Further, the catalyst carrier 65 is formed with a layer of the NOx
absorbent 67 on its surface.
[0031] Therefore, combustion under a lean air-fuel ratio is carried
out, NOx contained in the exhaust gas is also absorbed in the NOx
absorbent 67 carried on the particulate filter 25. The thus
absorbed NOx is released and reduced by supplying fuel from the
fuel supply valve 28.
[0032] On the other hand, the particulate matter contained in the
exhaust gas is trapped on the particulate filter 25 and
successively oxidized. However, if the amount of the particulate
matter trapped becomes greater than the amount of the particulate
matter oxidized, the particulate matter will gradually be deposited
on the particulate filter 25. In this case, if the amount of
particulate matter deposited increases, engine output may be
decreased. Therefore, it is necessary to remove the deposited
particulate matter when the amount of particulate matter deposited
increases. In this case, if raising the temperature of the
particulate filter 25 under an excess of air to about 600.degree.
C., the deposited particulate matter is oxidized and removed.
[0033] In the embodiment according to the present invention, when
the amount of the particulate matter deposited on the particulate
filter 25 exceeds an allowable amount, fuel is supplied from the
fuel supply valve 28 while the air-fuel ratio of the exhaust gas
flowing in the particulate filter 25 is maintained lean, and then
raising the temperature of the particulate filter 25 by the
oxidation heat of the thus supplied fuel, and thereby oxidizing and
removing the deposited particulate matter.
[0034] Note that the NOx storing catalyst 24 may be omitted in FIG.
1. In addition, in FIG. 1, a particulate filter that does not carry
NOx absorbent 67 may be used as a particulate filter 25.
[0035] In the embodiment according to the present invention,
whenever a cumulative amount .SIGMA.NOx of NOx absorbed in the NOx
absorbent 67 exceeds an allowable amount MAX as indicated by X in
FIG. 5, fuel is supplied from the fuel supply valve 28 in the form
of successive pulses, and thereby the air-fuel ratio of the exhaust
gas flowing through the NOx absorbent 67, which is carried on the
NOx storing catalyst 24 and the particulate filter 25, is switched
to rich temporarily. As a result, NOx is released from the NOx
absorbent 67 and is reduced. Alternatively, fuel may be supplied to
the NOx absorbent 67 by injecting additional fuel from the fuel
injectors 3 during the power or exhaust stroke.
[0036] In this case, in the embodiment according to the present
invention, the amount of NOx dNOx absorbed in the NOx absorbent 67
per unit of time is stored in ROM 32 in advance in the form of a
map as shown in FIG. 6 as a function of the required torque TQ and
engine speed N. The cumulative NOx amount .SIGMA.NOx is calculated
by a cumulation of the NOx amount of dNOx.
[0037] However, as mentioned at the beginning of this
specification, when the temperature of the NOx absorbent 67 is low,
it may be impossible to obtain an adequate release of NOx from the
NOx absorbent if the air-fuel ratio of the exhaust gas is simply
switched to rich.
[0038] Therefore, in the embodiment according to the present
invention, the temperature Tc of the NOx absorbent 67 is first
detected, and the air-fuel ratio of the exhaust gas flowing to the
NOx absorbent 67 is switched to a rich air-fuel ratio or is changed
depending on the absorbent temperature Tc. This will be explained
with reference to FIGS. 7A and 7B.
[0039] FIG. 7A shows a case where the temperature Tc of the NOx
absorbent 67 is lower than a predetermined temperature TcS. As
shown in FIG. 7A, fuel supply from the fuel supply valve 28 is not
carried out until the timing indicated by X, that is, until the
cumulative NOx amount .SIGMA.NOx exceeds the allowable amount MAX
and NOx must be released from the NOx absorbent 67 (see FIG. 5). At
this time, the air-fuel ratio AFEG of exhaust gas flowing through
the NOx absorbent 67 is maintained at a lean air-fuel ratio. If the
lean air-fuel ratio at this time is a basic lean air-fuel ratio
AFLB, the basic air-fuel ratio AFLB then conforms to the air-fuel
ratio in the combustion chambers 2, in the engine shown in FIG.
1.
[0040] When NOx must be released from the NOx absorbent 67 as
indicated by X in FIG. 7A, fuel from the fuel supply valve 28 is
switched to start the air-fuel ratio of the inflowing exhaust gas
AFEG from the basic lean air-fuel ratio AFLB to a lean air-fuel
ratio with a lower leanness AFLL. When the air-fuel ratio of the
inflowing exhaust gas AFEG is maintained at a lean air-fuel ratio
with a lower leanness AFLL for a lean time tL, it is followed by
the air-fuel ratio of the inflowing exhaust gas AFEG being switched
to a rich air-fuel ratio AFR. When the air-fuel ratio of the
inflowing exhaust gas AFEG is maintained at a rich air-fuel ratio
AFR for a rich time tR, the fuel supply is then stopped and the
air-fuel ratio of the inflowing exhaust gas AFEG is returned to a
basic lean air-fuel ratio AFLB.
[0041] When the air-fuel ratio of the inflowing exhaust gas AFEG is
switched to and maintained at the lean air-fuel ratio with a lower
leanness AFLL, the amount of unburned HC and CO contained in the
exhaust gas is increased, compared to when the air-fuel ratio of
the inflowing exhaust gas AFEG is a basic lean air-fuel ratio AFLB.
The increased amount of unburned HC and CO will be oxidized in the
NOx absorbent 67 under the presence of excess oxygen, and thus the
temperature Tc of the NOx absorbent 67 increases rapidly.
Therefore, the air-fuel ratio of inflowing exhaust gas AFEG is
switched to the rich air-fuel ratio AFR after the temperature Tc of
the NOx absorbent 67 is high, and an adequate NOx release from the
NOx absorbent 67 is accordingly obtained.
[0042] In addition, in the embodiment according to the present
invention, the air-fuel ratio of the inflowing exhaust gas AFEG is
returned from the rich air-fuel ratio AFR back to the basic lean
air-fuel ratio AFLB, and is maintained at the basic lean air-fuel
ratio AFLB until the NOx must be released from the NOx absorbent 67
again as shown in FIG. 5. In other words, fuel from the fuel supply
valve 28 is stopped when the air-fuel ratio of the inflowing
exhaust gas AFEG is returned back to the basic lean air-fuel ratio
AFLB until the cumulative NOx amount .SIGMA.NOx exceeds the
allowable amount MAX again. This ensures that an increment in the
temperature of the NOx absorbent 67 is carried out only when it is
necessary, and that supplied fuel is used effectively for NOx
release and reduction. Note that NOx is well absorbed in the NOx
absorbent 67 even when the temperature Tc of the NOx absorbent 67
is lower than the predetermined temperature TcS.
[0043] In contrast, if the temperature Tc of the NOx absorbent 67
is higher than the predetermined temperature TcS when the NOx must
be released from the NOx absorbent 67, as indicated by X in FIG.
7B, the air-fuel ratio of the inflowing exhaust gas AFEG is
immediately switched to the rich air-fuel ratio AFR, without being
switched to the lean air-fuel ratio with a lower leanness AFLL.
When the air-fuel ratio of the inflowing exhaust gas AFEG is
maintained at the rich air-fuel ratio AFR for the rich time tR, the
fuel is stopped and the air-fuel ratio of the inflowing exhaust gas
AFEG is returned to the basic lean air-fuel ratio AFLB. That is, in
this case, it is not necessary for the temperature Tc of the NOx
absorbent 67 to be increased.
[0044] As can be understood from the above explanation, the
predetermined temperature TcS is a temperature required for a good
release of NOx from the NOx absorbent 67. The temperature necessary
for a good release of NOx from the NOx absorbent 67 will vary
depending on the degree of deterioration of the NOx absorbent 67.
Therefore, in the embodiment according to the present invention,
the degree of the deterioration DET of the NOx absorbent 67 is
first detected, and the predetermined temperature TcS is then
determined depending on the degree of deterioration DET of the NOx
absorbent 67. Specifically, the predetermined temperature TcS is
set higher as the degree of deterioration DET becomes higher, as
shown in FIG. 8. The predetermined temperature TcS is stored in ROM
32 in advance, in the form of a map as shown in FIG. 8. Note that
there are many procedures for obtaining the degree of deterioration
DET of the NOx absorbent 67. For example, the degree of
deterioration DET of the NOx absorbent 67 may be judged to be
higher as the increment of the temperature Tc of the NOx absorbent
67 obtained when fuel is supplied from the fuel supply valve 28 to
the NOx absorbent 67 is smaller.
[0045] On the other hand, TcY indicated in FIG. 7A is the
temperature Tc of the NOx absorbent 67 when the lean time tL has
elapsed from when the air-fuel ratio of inflowing exhaust gas AFEG
is switched to the lean air-fuel ratio with a lower leanness AFLL.
If the temperature TcY conforms approximately to the predetermined
temperature TcS mentioned above, an adequate NOx release will be
obtained while the amount of fuel from the fuel supply valve 28 is
kept low. Therefore, the lean time tL is the amount of time
required to increase the temperature Tc of the NOx absorbent 67 to
approximately the predetermined temperature TcS when the air-fuel
ratio of the inflowing exhaust gas AFEG is maintained at the lean
air-fuel ratio with a lower leanness AFLL.
[0046] In this case, the lean time tL becomes longer as the
temperature Tc of the NOx absorbent 67 becomes lower as shown in
FIG. 9A, as the amount of intake air Ga becomes larger as shown in
FIG. 9B, and as the degree of deterioration DET of the NOx
absorbent 67 becomes higher as shown in FIG. 9C. In the embodiment
according to the present invention, the lean time tL is stored in
ROM 32 in advance, in the form of a map shown in FIG. 9D, as a
function of the temperature Tc and the degree of deterioration DET
of the NOx absorbent 67 and the amount of intake air Ga. Here, the
amount of intake air Ga represents the amount of exhaust gas
flowing through the NOx absorbent 67.
[0047] Note that, when fuel supply from the fuel supply valve 28 is
carried out, the air-fuel ratio of the inflowing exhaust gas AFEG
is made leaner by reducing the number of fuel supply pulses per
unit time, and is made richer by increasing the number fuel pulses
per unit time.
[0048] FIG. 10 shows a routine of the NOx release control.
[0049] Referring to FIG. 10, the routine proceeds to step 100 where
the amount of NOx .SIGMA.NOx absorbed in the NOx absorbent 67 is
calculated. Specifically, in the embodiment according to the
present invention, the amount of NOx dNOx adsorbed in the NOx
absorbent 67 per unit time is calculated using the map shown in
FIG. 6, and is then added to the absorbed NOx amount .SIGMA.NOx. In
the following step 101, it is determined whether the absorbed NOx
amount .SIGMA.NOx exceeds the allowable amount MAX. When the amount
is .SIGMA.NOx.ltoreq.MAX, the processing cycle is ended. In
contrast, when the amount is .SIGMA.NOx>MAX, the routine
proceeds to step 102 where the predetermined temperature TcS is
calculated using the map shown in FIG. 8. In the following step
103, it is determined whether the temperature Tc of the NOx
absorbent 67 is lower than the predetermined temperature TcS. When
the amount is Tc<TcS, the routine proceeds to step 104, where
the lean time tL is calculated using the map shown in FIG. 9D. In
the following step 105, the fuel supply valve 28 supplies fuel to
maintain the air-fuel ratio of inflowing exhaust gas AFEG at the
lean air-fuel ratio with a lower leanness AFLL for the lean time
tL. Then, the routine proceeds to step 106. In contrast, when the
amount is Tc.gtoreq.TcS, the routine jumps from step 103 to step
106. In step 106, the fuel supply valve 28 supplies fuel to
maintain the air-fuel ratio of the inflowing exhaust gas AFEG at
the rich air-fuel ratio AFR for the rich time tR. In the following
step 107, the absorbed NOx amount .SIGMA.NOx is returned to
zero.
* * * * *