U.S. patent application number 09/904875 was filed with the patent office on 2002-01-24 for device for purifying the exhaust gas of an internal combustion engine.
This patent application is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Asanuma, Takamitsu, Hayashi, Kotaro, Hirota, Shinya, Itoh, Kazuhiro, Kimura, Koichi, Matushita, Souichi, Nakatani, Koichiro, Tanaka, Toshiaki, Tsukasaki, Yukihiro.
Application Number | 20020007629 09/904875 |
Document ID | / |
Family ID | 26596761 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020007629 |
Kind Code |
A1 |
Asanuma, Takamitsu ; et
al. |
January 24, 2002 |
Device for purifying the exhaust gas of an internal combustion
engine
Abstract
A device for purifying the exhaust gas of an internal combustion
engine is disclosed. The device comprises a particulate filter
arranged in the exhaust system, which carry a catalyst for
absorbing and reducing NO.sub.x. The catalyst absorbs NO.sub.x when
the air-fuel ratio in the surrounding atmosphere thereof is lean
and releases the absorbed NO.sub.x to purify NO.sub.x by reduction
when the air-fuel ratio is the stoichiometric or rich. The device
further comprises a catalytic apparatus for purifying NO.sub.x
arranged in the exhaust system upstream of the particulate filter,
which has an oxidation function.
Inventors: |
Asanuma, Takamitsu;
(Susono-shi, JP) ; Tanaka, Toshiaki; (Numazu-shi,
JP) ; Hirota, Shinya; (Susono-shi, JP) ; Itoh,
Kazuhiro; (Mishima-shi, JP) ; Nakatani, Koichiro;
(Susono-shi, JP) ; Kimura, Koichi; (Susono-shi,
JP) ; Hayashi, Kotaro; (Mishima-shi, JP) ;
Matushita, Souichi; (Numazu-shi, JP) ; Tsukasaki,
Yukihiro; (Susono-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
Toyota Jidosha Kabushiki
Kaisha
1, Toyota-cho
Toyota-shi
JP
471-8571
|
Family ID: |
26596761 |
Appl. No.: |
09/904875 |
Filed: |
July 16, 2001 |
Current U.S.
Class: |
60/297 ; 60/299;
60/311 |
Current CPC
Class: |
F02M 26/28 20160201;
F02D 41/403 20130101; Y02T 10/40 20130101; F01N 2570/16 20130101;
F02B 23/0672 20130101; F02D 41/028 20130101; F02D 2041/389
20130101; F02D 2011/102 20130101; F01N 13/009 20140601; F02B 23/06
20130101; Y02T 10/125 20130101; Y02T 10/47 20130101; F01N 3/0821
20130101; F01N 3/0842 20130101; F01N 3/085 20130101; F02B 23/0669
20130101; F01N 3/0233 20130101; F01N 3/023 20130101; F01N 2330/06
20130101; F01N 3/0814 20130101; F01N 3/0878 20130101; F02B 3/06
20130101; Y02T 10/12 20130101; F02D 21/08 20130101; F02D 41/0275
20130101; F01N 2570/04 20130101; F02D 41/0057 20130101; F01N 3/031
20130101; F01N 3/035 20130101; F02D 41/029 20130101; Y02T 10/44
20130101; F02D 41/402 20130101; F01N 3/0231 20130101 |
Class at
Publication: |
60/297 ; 60/299;
60/311 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 21, 2000 |
JP |
2000-226224 |
Oct 18, 2000 |
JP |
2000-318344 |
Claims
1. A device for purifying the exhaust gas of an internal combustion
engine comprising: a particulate filter arranged in the exhaust
system, which carry a catalyst for absorbing and reducing NO.sub.x,
said catalyst absorbing NO.sub.x when the air-fuel ratio in the
surrounding atmosphere thereof is lean and releasing the absorbed
NO.sub.x to purify NO.sub.x by reduction when said air-fuel ratio
is the stoichiometric or rich; and a catalytic apparatus for
purifying NO.sub.x arranged in the exhaust system upstream said
particulate filter, which has an oxidation function.
2. A device for purifying the exhaust gas of an internal combustion
engine according to claim 1, further comprising bypassing means to
make possible the exhaust gas bypass said particulate filter
downstream said catalytic apparatus.
3. A device for purifying the exhaust gas of an internal combustion
engine according to claim 2, wherein said catalytic apparatus
carries said catalyst for absorbing and reducing NO.sub.x, during
the recovery process of the SO.sub.x pollution of said catalytic
apparatus, said bypassing means makes the exhaust gas bypass said
particulate filter.
4. A device for purifying the exhaust gas of an internal combustion
engine according to claim 2, wherein said catalytic apparatus
carries said catalyst for absorbing and reducing NO.sub.x,
immediately after the finishing of the recovery process of the
SO.sub.x pollution of said catalytic apparatus, said bypassing
means does not make the exhaust gas bypass said particulate filter
and thus the exhaust gas passes through said particulate
filter.
5. A device for purifying the exhaust gas of an internal combustion
engine comprising: a particulate filter arranged in the exhaust
system, which carries an oxidation catalyst; and a catalytic
apparatus for purifying NO.sub.x arranged in the exhaust system
upstream said particulate filter.
6. A device for purifying the exhaust gas of an internal combustion
engine according to claim 5, wherein said particulate filter
carries an oxygen absorbing agent.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a device for purifying the
exhaust gas of an internal combustion engine.
[0003] 2. Description of the Related Art
[0004] The exhaust gas of an internal combustion engine and,
particularly, of a diesel engine, contains NO.sub.x. Therefore, it
has been suggested that a filter for absorbing and reducing
NO.sub.x should be arranged in the exhaust system. The particulate
filter absorbs NO.sub.x in the form of nitric acid ions when the
oxygen concentration of the surrounding atmosphere thereof is high
and releases the absorbed NO.sub.xwhen the oxygen concentration of
the surrounding atmosphere becomes low. Therefore, the particulate
filter absorbs NO.sub.x favorably from an exhaust gas of a diesel
engine in which the combustion takes place in an excess air
condition. If the air-fuel ratio of the surrounding atmosphere is
periodically made rich or stoichiometric and thus the oxygen
concentration is made low, NO.sub.x is released from the
particulate filter and thereafter the released NO.sub.x can be
reduced by the reducing material such as HC and thus NO.sub.x can
be purified before it is emitted into the outside of the engine
exhaust system.
[0005] By the way, the exhaust gas of a diesel engine also contains
particulates comprising carbon as a chief component. The
particulates are required to be treated before they are emitted
into the outside of the engine exhaust system. Thus, it has been
suggested that a particulate filter should be arranged in the
exhaust system to trap the particulate. If such a particulate
filter carries the above-mentioned catalyst for absorbing and
reducing NO.sub.x, the particulate filter can absorb NO.sub.x and
can also oxidize and remove the particulates. Thus, it is effective
for the purifying of the exhaust gas that a particulate filter
carrying the catalyst for absorbing and reducing NO.sub.x is
arranged in the exhaust system.
[0006] The structure of the particulate filter is usually a
wall-flow type in which the exhaust gas passes through the pores of
the trapping wall. Therefore, the area for carrying the catalyst on
the trapping wall with which the exhaust gas is mainly in contact
is necessarily smaller than that an usual particulate filter, and
thus the particulate filter carrying the catalyst for absorbing and
reducing NO.sub.x cannot purify sufficiently NO.sub.x in the
exhaust gas by itself. Besides, if the catalyst for absorbing and
reducing NO.sub.x carried on the particulate filter is covered with
the trapped particulates, the catalyst cannot absorb sufficiently
NO.sub.x in the exhaust gas. Therefore, the particulate filter
cannot sufficiently purify the NO.sub.x in the exhaust gas by
itself.
SUMMARY OF THE INVENTION
[0007] Therefore, an object of the present invention is to provide
a device for purifying the exhaust gas of an internal combustion
engine, which can purify NO.sub.x in the exhaust gas more
sufficiently than the particulate filter carrying the catalyst for
absorbing and reducing NO.sub.x.
[0008] According to the present invention, there is provided a
first device for purifying the exhaust gas of an internal
combustion engine comprising: a particulate filter, arranged in the
exhaust system, which carries a catalyst for absorbing and reducing
NO.sub.x, the catalyst absorbing NO.sub.x when the air-fuel ratio
in the surrounding atmosphere thereof is lean and releasing the
absorbed NO.sub.x to purify NO.sub.x by reduction when said
air-fuel ratio is the stoichiometric or rich; and a catalytic
apparatus, for purifying NO.sub.x, arranged in the exhaust system
upstream the particulate filter, which has an oxidation
function.
[0009] According to the present invention, there is provided a
second device for purifying the exhaust gas of an internal
combustion engine comprising: a particulate filter, arranged in the
exhaust system, which carries an oxidation catalyst, and a
catalytic apparatus for purifying NO.sub.x arranged in the exhaust
system upstream the particulate filter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] In the drawings:
[0011] FIG. 1 is a schematic vertical sectional view of a diesel
engine with a device for purifying the exhaust gas according to the
present invention;
[0012] FIG. 2 is an enlarged vertical sectional view of a
combustion chamber of the diesel engine of FIG. 1;
[0013] FIG. 3 is a bottom view of a cylinder head of the diesel
engine of FIG. 1;
[0014] FIG. 4 is an enlarged vertical sectional view of the
combustion chamber of FIG. 1;
[0015] FIG. 5 is a view showing the relationship between the
lifting amounts of the intake valve and the exhaust valve and the
fuel injection;
[0016] FIG. 6 is a view showing the amounts of produced smoke,
NO.sub.x, and the like;
[0017] FIGS. 7(A) and 7(B) are views showing the combustion
pressure;
[0018] FIG. 8 is a view showing the fuel molecules;
[0019] FIG. 9 is a view showing the relationship between the amount
of produced smoke and the EGR rate;
[0020] FIG. 10 is a view showing the relationship between the
amount of injected fuel and the amount of mixed gas;
[0021] FIG. 11 is a view showing the first operating region (I) and
the second operating region (II);
[0022] FIG. 12 is a view showing the output of the air-fuel ratio
sensor;
[0023] FIG. 13 is a view showing the opening degree of the throttle
valve and the like;
[0024] FIG. 14 is a view showing the air-fuel ratio in the first
operating region (I);
[0025] FIG. 15(A) is a view showing the target opening degree of
the throttle valve;
[0026] FIG. 15(B) is a view showing the target opening degree of
the EGR control valve;
[0027] FIG. 16 is a view showing the air-fuel ratio in the second
operating region (II);
[0028] FIG. 17(A) is a view showing the target opening degree of
the throttle valve;
[0029] FIG. 17(B) is a view showing the target opening degree of
the EGR control valve;
[0030] FIG. 18 is a plan view showing near the changeover portion
and the particulate filter in the exhaust system;
[0031] FIG. 19 is a side view of FIG. 18;
[0032] FIG. 20 is a view showing the other shut-off position of the
valve body that is different from that in FIG. 18 in the changeover
portion;
[0033] FIG. 21 is a view showing the middle position of the valve
body in the changeover portion;
[0034] FIG. 22(A) is a front view showing the structure of the
particulate filter;
[0035] FIG. 22(B) is a side sectional view showing the structure of
the particulate filter;
[0036] FIGS. 23(A) and 23(B) are views explaining the absorbing and
releasing actions of NO.sub.x;
[0037] FIGS. 24(A) and 24(B) are maps of amounts of absorbed
NO.sub.x per a unit time;
[0038] FIG. 25 is a view explaining the oxidizing action of the
particulates;
[0039] FIG. 26 is a view showing the relationship between the
amount of particulates that can be oxidized and removed and the
temperature of the particulate filter;
[0040] FIGS. 27(A), 27(B), and 27(C) are views explaining the
depositing action of the particulates;
[0041] FIG. 28 is a flowchart for preventing the deposition of a
large amount of particulates on the particulate filter; and
[0042] FIGS. 29(A) and 29(B) are enlarged sectional views of the
partition wall of the particulate filter.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0043] FIG. 1 is a schematic vertical sectional view of a
four-stroke diesel engine with a device for purifying the exhaust
gas according to the present invention. FIG. 2 is an enlarged
vertical sectional view of a combustion chamber of diesel engine of
FIG. 1. FIG. 3 is a bottom view of a cylinder head of diesel engine
of FIG. 1. Referring to FIGS. 1-3, reference numeral 1 designates
an engine body, reference numeral 2 designates a cylinder-block,
reference numeral 3 designates a cylinder-head, reference numeral 4
designates a piston, reference numeral 5a designates a cavity
formed on the top surface of piston 4, reference numeral 5
designates a combustion chamber formed in the cavity 5a, reference
numeral 6 designates an electrically controlled fuel injector,
reference numeral 7 designates a pair of intake valves, reference
numeral 8 designates an intake port, reference numeral 9 designates
a pair of exhaust valves, and reference numeral 10 designates an
exhaust port. The intake port 8 is connected to a surge tank 12 via
a corresponding intake tube 11. The surge tank 12 is connected to
an air-cleaner 14 via an intake duct 13. A throttle valve 16 driven
by an electric motor 15 is arranged in the intake duct 13. On the
other hand, the exhaust port 10 is connected to an exhaust pipe 18
via an exhaust manifold 17.
[0044] As shown in FIG. 1, an air-fuel ratio sensor 21 is arranged
in the exhaust manifold 17. The exhaust manifold 17 and the surge
tank 12 are connected with each other via an EGR passage 22. An
electrically controlled EGR control valve 23 is arranged in the EGR
passage 22. An EGR cooler 24 is arranged around the EGR passage 22
to cool the EGR gas flowing in the EGR passage 22. In the
embodiment of FIG. 1, the engine cooling water is led into the EGR
cooler 24 and thus the EGR gas is cooled by the engine cooling
water.
[0045] On the other hand, each fuel injector 6 is connected to the
fuel reservoir, that is, a common rail 26 via a fuel supply tube
25. Fuel is supplied to the common rail 26 from an electrically
controlled variable discharge fuel pump 27. Fuel supplied to the
common rail 26 is supplied to the fuel injector 6 via each fuel
supply tube 25. A fuel pressure sensor 28 for detecting a fuel
pressure in the common rail 26 is attached to the common rail 26.
The discharge amount of the fuel pump is controlled on the basis of
an output signal of the fuel pressure sensor 28 such that the fuel
pressure in the common rail 26 becomes the target fuel
pressure.
[0046] Reference numeral 30 designates an electronic control unit.
It is comprised of a digital computer and is provided with a ROM
(read only memory) 32, a RAM (random access memory) 33, a CPU
(microprocessor) 34, an input port 35, and an output port 36
connected with each other by a bidirectional bus 31. The output
signals of the air-fuel sensor 21 and the fuel pressure sensor 28
are input to the input port 35 via each A/D converter 37. An engine
load sensor 41 is connected to the accelerator pedal 40, which
generates an output voltage proportional to the amount of
depression (L) of the accelerator pedal 40. The output signal of
the engine load sensor 41 is also input to the input port 35 via a
A/D converter 37. Further, the output signal of a crank angle
sensor 42 for generating an output pulse each time the crankshaft
rotates by, for example, 30 degrees is also input to the input port
35. The fuel injector 6, the electronic motor 15, the EGR control
valve 23, the fuel pump 27, and a valve body 71a in a changeover
portion 71 arranged on the exhaust pipe 18 are connected to the
output port 36 via each drive circuit 38 to be actuated on the
basis of the input signals. The changeover portion 71 and the valve
body 71a will be explained in detail later.
[0047] As shown in FIGS. 2 and 3, in the embodiment of the present
invention, the fuel injector 6 comprises a nozzle having six nozzle
holes. Fuel sprays (F) are injected from the nozzle holes in
slightly downward direction against a horizontal plane with equal
angular intervals. As shown in FIG. 3, two fuel sprays (F) of the
six fuel sprays (F) are scattered along the lower surface of each
exhaust valve 9. FIGS. 2 and 3 show the case where fuel is injected
at the end of the compression stroke. In this case, the fuel sprays
(F) progress toward the inside periphery surface of the cavity 5
and, thereafter, is ignited and burned.
[0048] FIG. 4 shows the case in that additional fuel is injected
from the fuel injector 6 when the lifting amount of the exhaust
valves is the maximum in the exhaust stroke. That is, FIG. 5 shows
the case that the main fuel injection (Qm) is carried out close to
the compression top dead center and thereafter the additional fuel
injection (Qa) is carried out in the middle stage of the exhaust
stroke. In this case, the fuel sprays (F) that progress toward the
exhaust valves 9 are directed between the umbrella-like back
surface of the exhaust valve 9 and the exhaust port 10. In other
words, two nozzle holes, of the six nozzle holes of the fuel
injector 6, are formed such that when the exhaust valves 9 are
opened and the additional fuel injection (Qa) is carried out, the
fuel sprays (F) are directed between the back surface of the
exhaust valve 9 and the exhaust port 10. In the embodiment of FIG.
4, these fuel sprays (F) impinge on the back surface of the exhaust
valve 9 and reflect from the back surface of the exhaust valves 9,
and thus are directed into the exhaust port 10.
[0049] Usually, the additional fuel injection (Qa) is not carried
out, and the main fuel injection (Qm) only is carried out. FIG. 6
indicates an example of an experiment showing the change in the
output torque and the amount of smoke, HC, CO, and NO, exhausted at
that time when changing the air-fuel ratio A/F (abscissa in FIG. 6)
by changing the opening degree of the throttle valve 16 and the EGR
rate at the time of low engine load operation. As will be
understood from FIG. 6, in this experiment, the smaller the
air-fuel ratio A/F becomes, the larger the EGR rate becomes. When
the air-fuel ratio is below the stoichiometric air-fuel ratio
(nearly equal 14.6), the EGR rate becomes over 65 percent.
[0050] As shown in FIG. 6, if the EGR rate is increased to reduce
the air-fuel ratio A/F, when the EGR rate becomes close to 40
percent and the air-fuel ratio A/F becomes about 30, the amount of
produced smoke starts to increase. Next, when the EGR rate is
further increased and the air-fuel ratio A/F is made smaller, the
amount of produced smoke sharply increases and peaks. Next, when
the EGR rate is further increased and the air-fuel ratio A/F is
made smaller, the amount of produced smoke sharply decreases. When
the EGR rate is made over 65 percent and the air-fuel ratio A/F
becomes close to 15.0, the amount of produced smoke is
substantially zero. That is, almost no soot is produced. At this
time, the output torque of the engine falls somewhat and the amount
of produced NO.sub.x becomes considerably lower. On the other hand,
at this time, the amounts of produced HC and CO start to
increase.
[0051] FIG. 7(A) shows the changes in combustion pressure in the
combustion chamber 5 when the amount of produced smoke is the
greatest near an air-fuel ratio A/F of 21. FIG. 7(B) shows the
changes in combustion pressure in the combustion chamber 5 when the
amount of produced smoke is substantially zero near an air-fuel
ratio A/F of 18. As will be understood from a comparison of FIG.
7(A) and FIG. 7(B), the combustion pressure is lower in the case
shown in FIG. 7(B), where the amount of produced smoke is
substantially zero, than the case shown in FIG. 7(A) where the
amount of produced smoke is large.
[0052] The following may be said from the results of the experiment
shown in FIGS. 6 and 7. That is, first, when the air-fuel ratio A/F
is less than 15.0 and the amount of produced smoke is substantially
zero, the amount of produced NO.sub.x decreases considerably as
shown in FIG. 6. The fact that the amount of produced NO.sub.x
decreases means that the combustion temperature in the combustion
chamber 5 falls. Therefore, it can be said that when almost no soot
is produced, the combustion temperature in the combustion chamber 5
becomes lower. The same fact can be said from FIG. 7. That is, in
the state shown in FIG. 7(B) where almost no soot is produced, the
combustion pressure becomes lower, therefore the combustion
temperature in the combustion chamber 5 becomes lower at this
time.
[0053] Second, when the amount of produced smoke, that is, the
amount of produced soot, becomes substantially zero, as shown in
FIG. 6, the amounts of exhausted HC and CO increase. This means
that the hydrocarbons are exhausted without changing into soot.
That is, the straight chain hydrocarbons and aromatic hydrocarbons
contained in the fuel and shown in FIG. 8 decompose when raised in
temperature in an oxygen insufficient state resulting in the
formation of a precursor of soot. Next, soot mainly composed of
solid masses of carbon atoms is produced. In this case, the actual
process of production of soot is complicated. How the precursor of
soot is formed is not clear, but whatever the case, the
hydrocarbons shown in FIG. 8 change to soot through the soot
precursor. Therefore, as explained above, when the amount of
production of soot becomes substantially zero, the amount of
exhaust of HC and CO increases as shown in FIG. 6, but the HC at
this time is a soot precursor or in a state of a hydrocarbon before
that.
[0054] Summarizing these considerations based on the results of the
experiments shown in FIGS. 6 and 7, when the combustion temperature
in the combustion chamber 5 is low, the amount of produced soot
becomes substantially zero. At this time, a soot precursor or a
state of hydrocarbons before that is exhausted from the combustion
chamber 5. More detailed experiments and studies were conducted. As
a result, it was learned that when the temperature of the fuel and
the gas around the fuel in the combustion chamber 5 is below a
certain temperature, the process of growth of soot stops midway,
that is, no soot at all is produced and that when the temperature
of the fuel and the gas around the fuel in the combustion chamber 5
becomes higher than the certain temperature, soot is produced.
[0055] The temperature of the fuel and the gas around the fuel when
the process of growth of hydrocarbons stops in the state of the
soot precursor, that is, the above certain temperature, changes
depending on various factors such as the type of the fuel, the
air-fuel ratio, and the compression ratio, so it cannot be said
exactly what it is, but this certain temperature is deeply related
to the amount of production of NO.sub.x. Therefore, this certain
temperature can be defined to a certain degree from the amount of
production of NO.sub.x. That is, the greater the EGR rate is, the
lower the temperature of the fuel, and the gas around it at the
time of combustion, becomes and the lower the amount of produced
NO.sub.x becomes. At this time, when the amount of produced
NO.sub.x becomes around 10 ppm or less, almost no soot is produced
any more. Therefore, the above certain temperature substantially
corresponds to the temperature when the amount of produced NO.sub.x
becomes around 10 ppm or less.
[0056] Once soot is produced, it is impossible to purify it by
after-treatment using a catalyst having an oxidation function. As
opposed to this, a soot precursor or a state of hydrocarbons before
that can be easily purified by after-treatment using a catalyst
having an oxidation function. Thus, it is extremely effective for
the purifying of the exhaust gas that the hydrocarbons are
exhausted from the combustion chamber 5 in the form of a soot
precursor or a state before that with the reduction of the amount
of produced NO.sub.x.
[0057] Now, to stop the growth of hydrocarbons in the state before
the production of soot, it is necessary to suppress the temperature
of the fuel and the gas around it at the time of combustion in the
combustion chamber 5 to a temperature lower than the temperature
where soot is produced. In this case, it was learned that the heat
absorbing action of the gas around the fuel at the time of
combustion of the fuel has an extremely great effect in suppression
the temperatures of the fuel and the gas around it.
[0058] That is, if only air exists around the fuel, the vaporized
fuel will immediately react with the oxygen in the air and burn. In
this case, the temperature of the air away from the fuel does not
rise so much. Only the temperature around the fuel becomes locally
extremely high. That is, at this time, the air away from the fuel
does not absorb much of the heat of combustion of the fuel at all.
In this case, since the combustion temperature becomes extremely
high locally, the unburned hydrocarbons receiving the heat of
combustion produce soot.
[0059] On the other hand, when fuel exists in a mixed gas of a
large amount of inert gas and a small amount of air, the situation
is somewhat different. In this case, the evaporated fuel disperses
in the surroundings and reacts with the oxygen mixed in the inert
gas to burn. In this case, the heat of combustion is absorbed by
the surrounding inert gas, so the combustion temperature no longer
rises so much. That is, the combustion temperature can be kept low.
That is, the presence of inert gas plays an important role in the
suppression of the combustion temperature. It is possible to keep
the combustion temperature low by the heat absorbing action of the
inert gas.
[0060] In this case, to suppress the temperature of the fuel and
the gas around it to a temperature lower than the temperature at
which soot is produced, an amount of inert gas sufficient to absorb
an amount of heat sufficient to lower the temperature is required.
Therefore, if the amount of fuel increases, the amount of required
inert gas increases. Note that, in this case, the larger the
specific heat of the inert gas is, the stronger the heat absorbing
action becomes. Therefore, a gas with a large specific heat is
preferable as the inert gas. In this regard, since CO.sub.2 and EGR
gas have relatively large specific heats, it may be said to be
preferable to use EGR gas as the inert gas.
[0061] FIG. 9 shows the relationship between the EGR rate and smoke
when using EGR gas as the inert gas and changing the degree of
cooling of the EGR gas. That is, the curve (A) in FIG. 9 shows the
case of strongly cooling the EGR gas and maintaining the
temperature of the EGR gas at about 90 degrees C., the curve (B)
shows the case of cooling the EGR gas by a compact cooling
apparatus, and the curve (C) shows the case of not compulsorily
cooling the EGR gas.
[0062] When strongly cooling the EGR gas as shown by the curve (A)
in FIG. 9, the amount of produced soot peaks when the EGR rate is a
slightly below 50 percent. In this case, if the EGR rate is made
about 55 percent or higher, almost no soot is produced.
[0063] On the other hand, when the EGR gas is slightly cooled as
shown by the curve (B) in FIG. 9, the amount of produced soot peaks
when the EGR rate is slightly higher than 50 percent. In this case,
if the EGR rate is made above about 65 percent, almost no soot is
produced.
[0064] Further, when the EGR gas is not forcibly cooled as shown by
the curve (C) in FIG. 9, the amount of produced soot peaks near an
EGR rate of 55 percent. In this case, if the EGR rate is made over
about 70 percent, almost no soot is produced.
[0065] Note that FIG. 9 shows the amount of produced smoke when the
engine load is relatively high. When the engine load becomes
smaller, the EGR rate at which the amount of produced soot peaks
falls somewhat, and the lower limit of the EGR rate, at which
almost no soot is produced, also falls somewhat. In this way, the
lower limit of the EGR rate at which almost no soot is produced
changes in accordance with the degree of cooling of the EGR gas or
the engine load.
[0066] FIG. 10 shows the amount of mixed EGR gas and air, the ratio
of air in the mixed gas, and the ratio of EGR gas in the mixed gas,
required to make the temperature of the fuel and the gas around it,
at the time of combustion, a temperature lower than the temperature
at which soot is produced in the case of the use of EGR gas as an
inert gas. Note that, in FIG. 10, the ordinate shows the total
amount of suction gas taken into the combustion chamber 5. The
broken line (Y) shows the total amount of suction gas able to be
taken into the combustion chamber 5 when supercharging is not being
performed. Further, the abscissa shows the required load. (Z1)
shows the low engine load operation region.
[0067] Referring to FIG. 10, the ratio of air, that is, the amount
of air in the mixed gas shows the amount of air necessary for
causing the injected fuel to completely burn. That is, in the case
shown in FIG. 10, the ratio of the amount of air and the amount of
injected fuel becomes the stoichiometric air-fuel ratio. On the
other hand, in FIG. 10, the ratio of EGR gas, that is, the amount
of EGR gas in the mixed gas, shows the minimum amount of EGR gas
required for making the temperature of the fuel, and the gas around
it, a temperature lower than the temperature at which soot is
produced when the injected fuel has burned completely. This amount
of EGR gas is, expressed in term of the EGR rate, equal to or
larger than 55 percent and, in the embodiment shown in FIG. 10, it
is equal to or larger than 70 percent. That is, if the total amount
of suction gas taken into the combustion chamber 5 is made the
solid line (X) in FIG. 10 and the ratio between the amount of air
and the amount of EGR gas in the total amount of suction gas (X) is
made the ratio shown in FIG. 10, the temperature of the fuel and
the gas around it becomes a temperature lower than the temperature
at which soot is produced and therefore no soot at all is produced.
Further, the amount of produced NO.sub.x at this time is about 10
ppm or less and therefore the amount of produced NO.sub.x becomes
extremely small.
[0068] If the amount of injected fuel increases, the amount of
generated heat at the time of combustion increases, so, to maintain
the temperature of the fuel and the gas around it at a temperature
lower than the temperature at which soot is produced, the amount of
heat absorbed by the EGR gas must be increased. Therefore, as shown
in FIG. 10, the amount of EGR gas has to be increased with an
increase in the amount of injected fuel. That is, the amount of EGR
gas has to be increased as the required engine load becomes
higher.
[0069] On the other hand, in the engine load region (Z2) of FIG.
10, the total amount of suction gas (X) required for inhibiting the
production of soot exceeds the total amount of suction gas (Y) that
can be taken in. Therefore, in this case, to supply the total
amount of suction gas (X), required for inhibiting the production
of soot, into the combustion chamber 5, it is necessary to
supercharge or pressurize both the EGR gas and the intake air or
just the EGR gas. When not supercharging or pressurizing the EGR
gas etc., in the engine load region (Z2), the total amount of
suction gas (X) corresponds to the total amount of suction gas (Y)
that can be taken in. Therefore, in this case, to inhibit the
production of soot, the amount of air is reduced somewhat to
increase the amount of EGR gas and the fuel is made to burn in a
state where the air-fuel ratio is rich.
[0070] As explained above, FIG. 10 shows the case of combustion of
fuel at the stoichiometric air-fuel ratio. In the low engine load
operating region (Z1) shown in FIG. 10, even if the amount of air
is made smaller than the amount of air shown in FIG. 10, that is,
even if the air-fuel ratio is made rich, it is possible to inhibit
the production of soot and make the amount of produced NO.sub.x
around 10 ppm or less. Further, in the low engine load operating
region (Z1) shown in FIG. 10, even if the amount of air is made
greater than the amount of air shown in FIG. 10, that is, the
average air-fuel ratio is made lean of 17 to 18, it is possible to
inhibit the production of soot and make the amount of produced
NO.sub.x around 10 ppm or less.
[0071] That is, when the air-fuel ratio is made rich, the fuel is
in excess, but since the combustion temperature is suppressed to a
low temperature, the excess fuel does not change into soot and
therefore soot is not produced. Further, at this time, only an
extremely small amount of NO.sub.x is produced. On the other hand,
when the average of air-fuel ratio is lean or when the air-fuel
ratio is the stoichiometric air-fuel ratio, a small amount of soot
is produced if the combustion temperature becomes higher, but the
combustion temperature is suppressed to a low temperature, and thus
no soot at all is produced. Further, only an extremely small amount
of NO.sub.x is produced.
[0072] In this way, in the low engine load operating region (Z1),
despite the air-fuel ratio, that is, whether the air fuel ratio is
rich or the stoichiometric air-fuel ratio, or the average of
air-fuel ratio is lean, no soot is produced and the amount of
produced NO.sub.x becomes extremely small. Therefore, considering
the improvement of the fuel consumption rate, it may be said to be
preferable to make the average air-fuel ratio lean.
[0073] By the way, only when the engine load is relative low and
the amount of generated heat is small, can the temperature of the
fuel and the gas around the fuel in the combustion be suppressed to
below a temperature at which the process of growth of soot stops
midway. Therefore, in the embodiment of the present invention, when
the engine load is relative low, the temperature of the fuel and
the gas around the fuel in the combustion is suppressed to below a
temperature at which the process of growth of soot stops midway and
thus a first combustion, i.e., a low temperature combustion, is
carried out. When the engine load is relative high, a second
combustion, i.e., normal combustion, as usual, is carried out.
Here, as can be understood from the above explanation, the first
combustion, i.e., the low temperature combustion is a combustion in
which the amount of inert gas in the combustion chamber is larger
than the worst amount of inert gas causing the maximum amount of
produced soot and thus no soot at all is produced. The second
combustion, i.e., the normal combustion is a combustion in which
the amount of inert gas in the combustion chamber is smaller than
the worst amount of inert gas.
[0074] FIG. 11 shows a first operating region (I) in which the
first combustion, i.e., the low temperature combustion is carried
out and a second operating region (II) in which the second
combustion, i.e., the normal combustion is carried out. In FIG. 11,
the ordinate (L) shows the amount of depression of the accelerator
pedal 40, i.e., the required engine load. The abscissa (N) shows
the engine speed. Further, in FIG. 11, X(N) shows a first boundary
between the first operating region (I) and the second operating
region (II). Y(N) shows a second boundary between the first
operating region (I) and the second operating region (II). The
decision of changing from the first operating region (I) to the
second operating region (II) is carried out on the basis of the
first boundary X(N). The decision of changing from the second
operating region (II) to the first operating region (I) is carried
out on the basis of the second boundary Y(N).
[0075] That is, when the engine operating condition is in the first
operating region (I) and the low temperature combustion is carried
out, if the required engine load (L) increases beyond the first
boundary X(N) that is a function of the engine speed (N), it is
determined that the engine operating region shifts in the second
operating region (II) and thus the normal combustion is carried
out. Thereafter, if the required engine load (L) decreases below
the second boundary Y(N) that is a function of the engine speed
(N), it is determined that the engine operating region shifts in
the first operating region (I) and thus the low temperature
combustion is carried out again.
[0076] FIG. 12 shows the output of the air-fuel ratio sensor 21. As
shown in FIG. 12, the output current (I) of the air-fuel ratio
sensor 21 changes in accordance with the air-fuel ratio A/F.
Accordingly, the air-fuel ratio can be known from the output
current (I) of the air-fuel ratio sensor 21. Next, referring FIG.
13, the engine operating control in the first operating region (I)
and the second operating region (II) will be explained
schematically.
[0077] FIG. 13 shows the opening degree of the throttle valve 16,
the opening degree of the EGR control valve 23, the EGR rate, the
air-fuel ratio, the fuel injection timing, and the amount of
injected fuel with respect to the required engine load (L). As
shown in FIG. 13, in the first operating region (I) when the
required engine load (L) is low, the throttle valve 16 is gradually
opened from near the fully closed state to near the half opened
state along with the increase of the required engine load (L), and
the EGR control valve 23 is gradually opened from near the fully
closed state to the fully opened state along with the increase in
the required engine load (L). In the embodiment shown in FIG. 13,
the EGR rate in the first operating region (I) is made about 70
percent and the air-fuel ratio therein is made slightly lean.
[0078] In the other words, in the first operating region (I), the
opening degrees of the throttle valve 16 and the EGR control valve
23 are controlled such that the EGR rate becomes about 70 percent
and the air-fuel ratio becomes a slightly lean air-fuel ratio. The
air-fuel ratio at this time is controlled to the target air-fuel
ratio to correct the opening degree of the EGR control valve 23 on
the basis of the output signal of the air-fuel ratio sensor 21. In
the first operating region (I), the fuel is injected before the
compression top dead center TDC. In this case, the starting time
(OS) of fuel injection is delayed along with the increase of the
required engine load (L) and the ending time (OE) of fuel injection
is delayed along with the delay of the starting time (Os) of fuel
injection.
[0079] When in the idle operating mode, the throttle valve 16 is
closed to near the fully closed state. At this time, the EGR
control valve 23 is also closed to near the fully closed state.
When the throttle valve 16 is closed near the fully closed state,
the pressure in the combustion chamber 5 in the initial stage of
the compression stroke is made low and thus the compression
pressure becomes low. When the compression pressure becomes low,
the compression work of the piston 4 becomes small and thus the
vibration of the engine body 1 becomes small. That is, when in the
idle operating mode, the throttle valve 16 is closed near the fully
closed state to restrain the vibration of the engine body 1.
[0080] On the other hand, when the engine operating region changes
from the first operating region (I) to the second operating region
(II), the opening degree of the throttle valve 16 increases by a
step from the half opened state toward the fully opened state. In
this time, in the embodiment shown in FIG. 13, the EGR rate
decreases by a step from about 70 percent to below 40 percent and
the air-fuel ratio increases by a step. That is, the EGR rate jumps
beyond the EGR rate extent (FIG. 9) in which the large amount of
smoke is produced and thus the large amount of smoke is not
produced when the engine operating region changes from the first
operating region (I) to the second operating region (II).
[0081] In the second operating region (II), normal combustion, as
usual, is carried out. This combustion causes some production of
soot and NO.sub.x. However, the thermal efficiency thereof is
higher than that of the low temperature combustion. Thus, when the
engine operating region changes from the first operating region (I)
to the second operating region (II), the amount of injected fuel
decreases by a step as shown in FIG. 13.
[0082] In the second operating region (II), the throttle valve 16
is hold in the fully opened state except in a part thereof. The
opening degree of the EGR control valve 23 decreases gradually
along with the increase of the required engine load (L). In this
operating region (II), the EGR rate decreases along with the
increase of the required engine load (L) and the air-fuel ratio
decreases along with the increase of the required engine load (L).
However, the air-fuel ratio is made a lean air-fuel ratio even if
the required engine load (L) becomes high. Further, in the second
operating region (II), the starting time (OS) of fuel injection is
made near the compression top dead center TDC.
[0083] FIG. 14 shows the air-fuel ratios A/F in the first operating
region (I). In FIG. 14, the curves indicated by A/F=15.5, A/F=16,
A/F=17, and A/F=18 shows respectively the cases where the air-fuel
ratios are 15.5, 16, 17, and 18. The air-fuel ratio between two of
the curves is defined by the proportional allotment. As shown in
FIG. 14, in the first operating region (I), the air-fuel ratio is
lean and the more the air-fuel ratio is lean, the lower the
required engine load (L) becomes.
[0084] That is, the amount of generated heat in the combustion
decreases along with the decrease of the required engine load (L).
Therefore, even if the EGR rate decreases along with the decrease
of the required engine load (L), the low temperature combustion can
be carried out. When the EGR rate decreases, the air-fuel ratio
becomes large. Therefore, as shown in FIG. 14, the air-fuel ratio
A/F increases along with the decrease of the required engine load
(L). The larger the air-fuel ratio becomes, the more the fuel
consumption improves. Accordingly, in the present embodiment, the
air-fuel ratio A/F increases along with the decrease in the
required engine load (L) such that the air-fuel ratio is made lean
as much as possible.
[0085] A target opening degree (ST) of the throttle valve 16
required to make the air-fuel ratio the target air-fuel ratio shown
in FIG. 14 is memorized in ROM 32 of the electronic control unit as
a map in which it is a function of the required engine load (L) and
the engine speed (N) shown in FIG. 15(A). A target opening degree
(SE) of the EGR control valve 23 required to make the air-fuel
ratio the target air-fuel ratio shown in FIG. 14 is memorized in
ROM 32 of the electronic control unit as a map in which it is a
function of the required engine load (L) and the engine speed (N)
shown in FIG. 15(B).
[0086] FIG. 16 shows target air-fuel ratios when the second
combustion, i.e., normal combustion, as usual, is carried out. In
FIG. 16, the curves indicated by A/F=24, A/F=35, A/F=45, and A/F=60
shows respectively the cases where the target air-fuel ratios are
24, 35, 45, and 60. A target opening degree (ST) of the throttle
valve 16 required to make the air-fuel ratio the target air-fuel
ratio is memorized in ROM 32 of the electronic control unit as a
map in which it is a function of the required engine load (L) and
the engine speed (N) shown in FIG. 17(A). A target opening degree
(SE) of the EGR control valve 23 required to make the air-fuel
ratio the target air-fuel ratio is memorized in ROM 32 of the
electronic control unit as a map in which it is a function of the
required engine load (L) and the engine speed (N) shown in FIG.
17(B).
[0087] Thus, in the diesel engine of the present embodiment, the
first combustion, i.e., the low temperature combustion, and the
second combustion, i.e., the normal combustion, are changed over on
the basis of the amount of depression (L) of the accelerator pedal
40 and the engine speed (N). In each combustion, the opening
degrees of the throttle valve 16 and the EGR control valve are
controlled on the basis of the maps shown in FIGS. 15 and 17.
[0088] FIG. 18 is a plan view illustrating a device for purifying
the exhaust gas of an embodiment, and FIG. 19 is a side view
thereof. The device comprises a changeover portion 71 connected to
the downstream of the exhaust manifold 17 via an exhaust pipe 18, a
particulate filter 70, a first connecting portion 72a for
connecting one side of the particulate filter 70 to the changeover
portion 71, a second connecting portion 72b for connecting the
other side of the particulate filter 70 to the changeover portion
71, and an exhaust passage 73 on the downstream of the changeover
portion 71. The changeover portion 71 comprises a valve body 71a
that shuts off the flow of exhaust gas in the changeover portion
71. The valve body 71a is driven by a negative pressure actuator, a
step motor or the like. At one shut-off position of the valve body
71a, the upstream side in the changeover portion 71 is communicated
with the first connecting portion 72a and the downstream side
therein is communicated with the second connecting portion 72b, and
thus the exhaust gas flows from one side of the particulate filter
70 to the other side thereof as shown by arrows in FIG. 18.
[0089] FIG. 20 illustrates another shut-off position of the valve
body 71a. At this shut-off position, the upstream side in the
changeover portion 71 is communicated with the second connecting
portion 72b and the downstream side in the changeover portion 71 is
communicated with the first connecting portion 72a, and thus the
exhaust gas flows from the other side of the particulate filter 70
to the one side thereof as shown by arrows in FIG. 20. Thus, by
changing over the valve body 71a, the direction of the exhaust gas
flowing into the particulate filter 70 can be reversed, i.e., the
exhaust gas upstream side and the exhaust gas downstream side of
the particulate filter 70 can be reversed.
[0090] Further, FIG. 21 shows a middle position of the valve body
71a between the two shut-off positions. At the middle position, the
changeover portion 71 is not shut off. The exhaust gas does not
pass through the particulate filter 70 having a higher resistance.
That is, the exhaust gas bypasses the particulate filter 70 and
flows directly into the exhaust passage 73 as shown by arrows in
FIG. 21.
[0091] FIG. 22 shows the structure of the particulate filter 70,
wherein FIG. 22(A) is a front view of the particulate filter 70 and
FIG. 22(B) is a side sectional view thereof. As shown in these
figures, the particulate filter 70 has an elliptic shape, and is,
for example, the wall-flow type of a honeycomb structure formed of
a porous material such as cordierite, and has many spaces in the
axial direction divided by many partition walls 54 extending in the
axial direction. One of any two neighboring spaces is closed by a
plug 53 on the exhaust gas downstream side, and the other one is
closed by a plug 53 on the exhaust gas upstream side. Thus, one of
the two neighboring spaces serves as an exhaust gas flow-in passage
50 and the other one serves as an exhaust gas flow-out passage 51,
causing the exhaust gas to necessarily pass through the partition
wall 54 as indicated by arrows in FIG. 22(B). The particulates
contained in the exhaust gas are much smaller than the pores of the
partition wall 54, but collide with and are trapped on the exhaust
gas upstream side surface of the partition wall 54 and the pores
surface in the partition wall 54. Thus, each partition wall 54
works as a trapping wall for trapping the particulates. In the
present particulate filter 70, in order to oxidize and remove the
trapped particulates, an NO.sub.x absorbent and a noble metal
catalyst as platinum Pt, which will be explained below, are carried
on both side surfaces of the partition wall 54, and preferably also
on the pore surfaces in the partition wall 54, by using an alumina
or the like.
[0092] In the present embodiment, the NO.sub.x absorbent carried on
the partition wall 54 is at least one selected from alkali metals
such as potassium K, sodium Na, lithium Li, cesium Cs, and rubidium
Rb, alkali earth metals such as barium Ba, calcium Ca, and
strontium Sr, rare earth elements such as lanthanum La and yttrium
Y, and transition metals. The NO.sub.x absorbent absorbs NO.sub.x
when the air-fuel ratio (that is a ratio of the supplied air to the
supplied fuel regardless of an amount of fuel burned by using
oxygen in the supplied air) in the surrounding atmosphere is lean
and releases the absorbed NO.sub.x when the air-fuel ratio becomes
stoichiometric or rich, and thus the NO.sub.x absorbent carries out
the absorbing and releasing actions of NO.sub.x.
[0093] The NO.sub.x absorbent can actually carry out the absorbing
and releasing actions of NO.sub.x, but a part of the mechanism of
the absorbing and releasing actions of NO.sub.x is not clear.
However, it is believed that the absorbing and releasing actions of
NO.sub.x takes place by the mechanism shown in FIGS. 23(A) and
23(B). Next, explained below is the mechanism with reference to the
case where platinum Pt and barium Ba are carried on the partition
wall of the particulate filter. The mechanism is the same as in the
case of using another noble metal and another alkali metal, an
alkali earth metal, or a rare earth element.
[0094] Whether in the low temperature combustion or the normal
combustion, when the air-fuel ratio is lean, oxygen concentration
in the exhaust gas is high. At this time, oxygen O.sub.2 in the
exhaust gas adheres onto the surface of platinum Pt in the form of
O.sub.2.sup.- or O.sup.2- as shown in FIG. 23(A). On the other
hand, NO in the exhaust gas reacts with O.sub.2.sup.- or O.sup.2-
on the surface of platinum Pt to produce NO.sub.2
(2NO+O.sub.2.fwdarw.2NO.sub.2). Next, a part of the produced
NO.sub.2 is absorbed in the NO.sub.x absorbent while being oxidized
on platinum Pt, and diffuses in the NO.sub.x absorbent in the form
of nitric acid ion NO.sub.3.sup.- while being combined with barium
oxide BaO as shown in FIG. 23(A). Thus, in the present embodiment,
NO.sub.x contained in the exhaust gas is absorbed in the NO.sub.x
absorbent. As long as oxygen concentration in the surrounding
atmosphere is high, NO.sub.2 is produced on the surface of platinum
Pt, and as long as the ability for absorbing NO.sub.x of the
NO.sub.x absorbent does not saturate, NO.sub.2 is absorbed in the
NO.sub.x absorbent to produce nitric acid ions NO.sub.3.sup.-. on
the other hand, when the air-fuel ratio in the surrounding
atmosphere is made rich, the oxygen concentration drops. As a
result, an amount of NO.sub.2 produced on the surface of platinum
Pt drops. When the amount of produced NO.sub.2 drops, the reaction
reverses (NO.sub.3.sup.-.fwdarw.NO.sub.2) and thus nitric acid ions
NO.sub.3.sup.- in the N .sub.x absorbent are released from the
NO.sub.x absorbent in the form of NO.sub.2. At this time, NO.sub.x
released from the NO.sub.x absorbent is reduced to react with HC,
CO, or the like contained in the surrounding atmosphere, as shown
in FIG. 23(B). Thus, when NO.sub.2 does not exist on the surface of
platinum Pt, NO.sub.2 is released from NO.sub.x absorbent.
Accordingly, when the air-fuel ratio in the surrounding atmosphere
is made rich, all of the absorbed NO.sub.x is released from the
NO.sub.x absorbent in a short time. The released NO.sub.x is
reduced and thus NO.sub.x is not discharged to the outside of the
exhaust system.
[0095] Even when the air-fuel ratio in the surrounding atmosphere
is made stoichiometric, NO.sub.x is released from NO.sub.x
absorbent. However, in this case, NO.sub.x is released gradually
from the NO.sub.x absorbent and thus a relative long period is
required to release all of the NO.sub.x absorbed in the particulate
filter.
[0096] By the way, the ability for absorbing NO.sub.x in the
NO.sub.x absorbent has a limit. Therefore, before the ability
saturates, NO.sub.x must be released from the NO.sub.x absorbent.
Namely, before a current amount of NO.sub.x absorbed in the
particulate filter 70 reaches the limit amount of NO.sub.x that can
be absorbed therein, NO.sub.x must be released from the particulate
filter and the released NO.sub.x must be reduced and purified. For
the purpose, a current amount of NO.sub.x absorbed in the
particulate filter must be estimated. In the present embodiment, a
map of amounts of NO.sub.x absorbed in the particulate filter per a
unit time (A) in the low temperature combustion is predetermined as
shown in FIG. 24(A). In the map, amounts of NO.sub.x absorbed in
the particulate filter per a unit time (A) are set as functions of
a required engine load (L) and an engine speed (N). A map of
amounts of NO.sub.x absorbed in the particulate filter per a unit
time (B) in the normal combustion is predetermined as shown in FIG.
24(B). In the map, amounts of NO.sub.x absorbed in the particulate
filter per a unit time (B) are set as functions of a required
engine load (L) and an engine speed (N). Therefore, a current
amount of NO.sub.x absorbed in the particulate filter can be
estimated to integrate these amounts of NO.sub.x absorbed in the
particulate filter per a unit time (A) and (B). Here, when the low
temperature combustion takes place in a rich air-fuel ratio, the
absorbed NO.sub.x is released and thus an amount of NO.sub.x
absorbed in the particulate filter per a unit time (A) become a
minus value. In the present embodiment, when the estimated amount
of NO.sub.x absorbed in the particulate filter becomes move than a
predetermined permissible value, the low temperature combustion is
carried out at the stoichiometric air-fuel ratio or a rich air-fuel
ratio, fuel is injected into the cylinder in the exhaust stroke, or
the like, and thus the air-fuel ratio in the surrounding atmosphere
of the particulate filter 70 is made stoichiometric or rich to
regenerate the particulate filter. This condition is maintained
till the regeneration of the particulate filter is finished. The
smaller the air-fuel ratio in the surrounding atmosphere is, the
shorter the period in which this condition is maintained
becomes.
[0097] By the way, the particulate filter carrying the NO.sub.x
absorbent can favorably oxidize and remove the particulates trapped
on the trapping walls. The mechanism is explained by using of FIG.
25. As mentioned above, NO.sub.x is absorbed in the NO.sub.x
absorbent 61 in the form of nitric acid ions NO.sub.3.sup.-. When
the particulate 62 adheres on the surface of the NO.sub.x
absorbent, the oxygen concentration drops on the surface of the
NO.sub.x absorbent 61 with which the particulate 62 is in contact.
As the oxygen concentration drops, there occurs a difference in the
concentration from the NO.sub.x absorbent 61 having a high oxygen
concentration and, thus, oxygen in the NO.sub.x absorbent 61 tends
to migrate toward the surface of the NO.sub.x absorbent 61 with
which the particulate 62 is in contact. As a result, nitric acid
ions NO.sub.3.sup.- in the NO.sub.x absorbent 61 are decomposed
into oxygen O and NO, whereby oxygen O migrates toward the surface
of the NO.sub.x absorbent 61 with which the particulate 62 is in
contact, and NO is emitted to the external side from the NO.sub.x
absorbent 61. NO emitted to the outside is oxidized on platinum Pt
on the downstream side and is absorbed again in the NO.sub.x
absorbent 61.
[0098] On the other hand, oxygen O migrating toward the surface of
the NO.sub.x absorbent 61 with which the particulate 62 is in
contact is the oxygen O decomposed from a compound such as a
nitrate. Oxygen O decomposed from the compound has a high level of
energy and exhibits a very high activity. Therefore, oxygen
migrating toward the surface of the NO.sub.x absorbent 61, with
which the particulate 62 is in contact, is active-oxygen O. Upon
coming into contact with active-oxygen O, the particulate 62 is
oxidized without producing luminous flame in a short time of, for
example, a few minutes or a few tens of minutes. Further,
active-oxygen to oxidize the particulate 62 is also released when
NO is absorbed in the NO.sub.x absorbent 61. That is, it can be
considered that NO.sub.x diffuses in the NO.sub.x absorbent 61 in
the form of nitric acid ions NO.sub.3.sup.- while being combined
with oxygen atoms and to be separated from an oxygen atom, and
during this time, active-oxygen is produced. The particulates 62
are also oxidized by this active-oxygen. Further, the particulates
adhered on the particulate filter 70 are oxidized not only by
active-oxygen, but also by the oxygen contained in the exhaust
gas.
[0099] Thus, if the NO.sub.x absorbent and the noble metal catalyst
(which are referred to a catalyst for absorbing and reducing
NO.sub.x below) are carried on the particulate filter, the
particulate filter is effective to purify NO.sub.x in the exhaust
gas and to prevent blocking of the particulate filter meshes with
oxidizing and removing the trapped particulates.
[0100] However, as mentioned above, the structure of the
particulate filter is the wall-flow type in which the exhaust gas
passes through the pores of the trapping walls. Therefore, in
comparison with a usual catalytic apparatus in which the exhaust
gas flows along the partition walls carrying a catalyst, a
dimension between the trapping walls of the particulate filter must
be larger than a dimension between the partition walls of the
catalytic apparatus so that the same amount of exhaust gas can pass
through the same size particulate filter as the catalytic
apparatus. Thus, a frequency in which the exhaust gas is in contact
with the catalyst for absorbing and reducing NO.sub.x carried on
the surface of the trapping wall of the particulate filter is
smaller than a frequency in which the exhaust gas is in contact
with the catalyst for absorbing and reducing NO.sub.x carried on
the surface of the partition wall of the catalytic apparatus.
Besides, when the exhaust gas can pass through the pores of the
trapping wall, the exhaust gas is in contact with the catalyst for
absorbing and reducing NO.sub.x carried in the pores, but the
exhaust gas is mainly in contact with only the catalyst for
absorbing and reducing NO.sub.x carried on the surface of the
trapping wall. In the particulate filter, an area for carrying the
catalyst for absorbing and reducing NO.sub.x on the surface of the
trapping wall is relative small due to many pores. Thus, even if
the catalyst for absorbing and reducing NO.sub.x is carried on the
particulate filter, NO.sub.x in the exhaust gas cannot be
sufficiently purified.
[0101] To solve this problem, in the present embodiment, as shown
in FIGS. 18 and 19, a catalytic apparatus for absorbing and
reducing NO.sub.x 74 is arranged upstream the changeover portion 71
in the exhaust pipe 18. The catalytic apparatus 74 is to compensate
the NO.sub.x purifying of the particulate filter 70 and thus does
not need a large capacity. Accordingly, the catalytic apparatus for
absorbing and reducing NO.sub.x 74 and the particulate filter 70
carrying the catalyst for absorbing and reducing NO.sub.x can
sufficiently purify NO.sub.x in the exhaust gas together.
[0102] The catalytic apparatus 74 may carry the above-mentioned
catalyst for absorbing and reducing NO.sub.x or a catalyst for
reducing NO.sub.x selectively on a honeycomb structure carrier.
[0103] By the way, a soluble organic fraction SO.sub.x is also
contained in the exhaust gas. SO.sub.x has an adhesion property,
adheres the particulates each other on the particulate filter, and
thus makes the particulates become a large mass. This makes it
difficult to oxidize and remove the particulates on the particulate
filter and to keep open the filter meshes. If the catalytic
apparatus 74 carries a catalyst having an oxidation function such
as the catalyst for absorbing and reducing NO.sub.x, the catalytic
apparatus 74 can burn SO.sub.x in the exhaust gas upstream the
particulate filter 70 and thus can prevent blocking of the filter
meshes.
[0104] By the way, the fuel contains sulfur S and thus SO.sub.x is
produced in the combustion of the fuel. SO.sub.x is absorbed in the
form of sulfate in the catalyst for absorbing and reducing NO.sub.x
carried on the particulate filter 70 due to a mechanism similar to
that of the case of NO.sub.x. Sulfate can release active oxygen due
to a mechanism similar to that of the case of nitrate. However,
sulfate is stable and if the air-fuel ratio in the surrounding
atmosphere is made rich, sulfate is hardly released from the
particulate filter. In fact, sulfate remains on the particulate
filter and thus an amount of absorbed sulfate increase gradually.
An amount of nitrate or sulfate that can be absorbed in the
particulate filter has a limit. If an amount of absorbed sulfate in
the particulate filter increases (this is referred to as S.sub.x
pollution, below), an amount of nitrate that can be absorbed in the
particulate filter decreases. Finally, the particulate filter
cannot absorb NOR.
[0105] In the present embodiment, the catalytic apparatus 74
carries the catalyst for absorbing and reducing NO.sub.x and thus
the catalytic apparatus 74 positively absorbs SO.sub.x upstream of
the particulate filter 70. Therefore, the SO.sub.x pollution of the
particulate filter 70 can be prevented. However, the SO.sub.x
pollution occurs in the catalytic apparatus 74. The SO.sub.x
pollution of the catalytic apparatus 74 is avoided as follows.
[0106] First, it is determined if the recovery of the SO.sub.x
pollution is required. In this determination, when an amount of
fuel consumed until now reaches a predetermined amount, it can be
determined that the recovery of the SO.sub.x pollution is required.
Besides, also in the catalytic apparatus, the regeneration process
for releasing NO.sub.x and purifying the released NO.sub.x is
needed similarly to the particulate filter. In the regeneration
process, the air-fuel ratio of the exhaust gas in the upstream side
of the catalytic apparatus is made rich and reducing materials such
as HC in the exhaust gas is used to reduce the released NO.sub.x.
Therefore, the air-fuel ratio of the exhaust gas in the downstream
side of the catalytic apparatus becomes about stoichiometric. On
the other hand, when the regeneration finishes, the air-fuel ratio
of the exhaust gas in the downstream side of the catalytic
apparatus becomes rich similar to that in the upstream side
thereof. By using this, if a regeneration period is detected, it
can be determined that the recovery of the SO.sub.x pollution is
required because, when the SO.sub.x pollution progresses enough to
require the recovery, an amount of absorbed NO.sub.x at the
regeneration time is actually small so that the regeneration period
is shortened.
[0107] When it is determined that the recovery of the SO.sub.x
pollution is required, the combustion air-fuel ratio is made lean
and thus a large amount of oxygen is contained in the exhaust gas.
Simultaneously, fuel is injected into the cylinder in the exhaust
stroke or fuel injected into the exhaust system upstream the
catalytic apparatus 74 or the like. Thus, a sufficient amount of
oxygen and a reducing material such as un-burned fuel are supplied
to the catalytic apparatus. Therefore, the reducing material burns
favorably due to the oxidation function of the catalytic
apparatus.
[0108] Thus, the temperature of the catalytic apparatus rises about
600 degrees C. and thus the stable sulfate can be released as
SO.sub.x when the air-fuel ratio in the surrounding atmosphere is
made stoichiometric or rich and the oxygen concentration drops. If
the temperature of the catalytic apparatus rises over 700 degrees
C., the oxidation catalyst such as platinum Pt sinters and thus the
oxidation function thereof drops. Therefore, the temperature of the
exhaust gas immediately downstream of the catalytic apparatus is
monitored and it is preferred to prevent the sintering of the
oxidation catalyst. In the recovery process of the SO.sub.x
pollution of the catalytic apparatus, the valve body 71a of the
changeover portion 71 is set the middle position. Therefore, the
released SO.sub.x from the catalytic apparatus bypasses the
particulate filter 70 and the released SO.sub.x from the catalytic
apparatus is not absorbed in the particulate filter again. When the
air-fuel ratio in the surrounding atmosphere is made rich for a
predetermined time after the temperature of the catalytic apparatus
is made high, it can be determined that the recovery process of the
SO.sub.x pollution finishes and the combustion air-fuel ratio is
returned to the normal air-fuel ratio.
[0109] By the way, the higher the temperature of the particulate
filter becomes, the more the platinum Pt and the NO.sub.x absorbent
61 are activated. Therefore, the higher the temperature of the
particulate filter becomes, the larger the amount of active-oxygen
O released from the NO.sub.x absorbent 61 per unit time becomes.
Further, naturally, the higher the temperature of particulates is,
the easier the particulates are oxidized. Therefore, the amount of
particulates that can be oxidized and removed without producing
luminous flame on the particulate filter per unit time increases
along with an increase in the temperature of the particulate
filter. Therefore, if the valve body 71a of the changeover portion
71 is set to one of the two shut-off positions simultaneously with
or immediately after the finishing of the recovery process of the
SO.sub.x pollution of the catalytic apparatus 74, the high
temperature (600 degrees C.) of the exhaust gas is led to the
particulate filter. Therefore, the temperature of the particulate
filter rises and the particulates on the particulate filter are
oxidized and removed easily.
[0110] The solid line in FIG. 26 shows the amount of particulates
(G) that can be oxidized and removed without producing luminous
flame per unit time. In FIG. 26, the abscissa represents the
temperature (TF) of the particulate filter. Here, FIG. 26 shows the
case that the unit time is 1 second, that is, the amount of
particulates (G) that can be oxidized and removed per 1 second.
However, any time such as 1 minute, 10 minutes, or the like can be
selected as a unit time. For example, in the case that 10 minutes
is used as unit time, the amount of particulates (G) that can be
oxidized and removed per unit time represents the amount of
particulates (G) that can be oxidized and removed per 10 minutes.
In this case also, the amount of particulates (G) that can be
oxidized and removed without producing luminous flame increases
along with an increase in the temperature of particulate filter 70
as shown in FIG. 26. The amount of particulates emitted from the
combustion chamber per unit time is referred to as an amount of
emitted particulates (M). When the amount of emitted particulates
(M) is smaller than the amount of particulates (G) that can be
oxidized and removed, for example, the amount of emitted
particulates (M) per 1 second is smaller than the amount of
particulates (G) that can be oxidized and removed per 1 second or
the amount of emitted particulates (M) per 10 minutes is smaller
than the amount of particulates (G) that can be oxidized and
removed per 10 minutes, that is, in the area (I) of FIG. 26, the
particulates emitted from the combustion chamber are all oxidized
and removed without producing luminous flame successively on the
particulate filter 70 for the above mentioned short time.
[0111] On the other hand, when the amount of emitted particulates
(M) is larger than the amount of particulates that can be oxidized
and removed (G), that is, in the area (II) of FIG. 26, the amount
of active-oxygen is not sufficient for all particulates to be
oxidized and removed successively. FIGS. 27(A) to (C) illustrate
the manner of oxidation of the particulates in such a case.
[0112] That is, in the case that the amount of active-oxygen is
lacking for oxidizing all particulates, when the particulates 62
adheres on the NO.sub.x absorbent 61, only a part of the
particulates is oxidized as shown in FIG. 27(A), and the other part
of the particulates that was not oxidized sufficiently remains on
the exhaust gas upstream surface of the particulate filter. When
the state where the amount of active-oxygen is lacking continues, a
part of the particulates that was not oxidized remains on the
exhaust gas upstream surface of the particulate filter
successively. As a result, the exhaust gas upstream surface of the
particulate filter is covered with the residual particulates 63 as
shown in FIG. 27(B).
[0113] The residual particulates 63 are gradually transformed into
carbonaceous matter that can hardly be oxidized. Further, when the
exhaust gas upstream surface is covered with the residual
particulates 63, the action of platinum Pt for oxidizing NO and
SO.sub.2, and the action of the NO.sub.x absorbent 61 for releasing
active-oxygen are suppressed. The residual particulates 63 can be
gradually oxidized over a relative long period. However, as shown
in FIG. 28(C), other particulates 64 deposit on the residual
particulates 63 one after the other, and when the particulates are
deposited so as to laminate, even if they are the easily oxidized
particulates, these particulates may not be oxidized since these
particulates are separated away from platinum Pt or from the
NO.sub.x absorbent. Accordingly, other particulates deposit
successively on these particulates 64. That is, when the state
where the amount of emitted particulates (M) is larger than the
amount of particulates that can be oxidized and removed (G)
continues, the particulates deposit to laminate on the particulate
filter.
[0114] Thus, in the area (I) of FIG. 26, the particulates are
oxidized and removed without producing luminous flame for the short
time and in the area (II) of FIG. 26, the particulates are
deposited to laminate on the particulate filter. Therefore, the
deposition of the particulates on the particulate filter can be
prevented if the relationship between the amount of emitted
particulates (M) and the amount of particulates that can be
oxidized and removed (G) is in the area (I). As a result, a
pressure loss of the exhaust gas in the particulate filter hardly
changes and is maintained at a minimum pressure loss value that is
nearly constant. Thus, the decrease of the engine output can be
maintained as low as possible. However, this is not always
realized, and the particulates may deposit on the particulate
filter if nothing is done.
[0115] In the present embodiment, to prevent the deposition of
particulates on the particulate filter, the above electronic
control unit 30 controls the valve body 71a according to a
flowchart shown in FIG. 28. The present flowchart is repeated every
a predetermined time. At step 101, a running distance (A) is
calculated, and at step 102, it is determined if the running
distance reaches a predetermined running distance (As). When the
result is negative, the routine is stopped. However, when the
result is positive, the routine goes to step 103 and the running
distance is reset to 0. Thereafter, at step 104, the valve body 71a
is changed over from one of the two shut-off positions to the other
shut-off position and thus the upstream side and the downstream
side of the particulate filter are reversed.
[0116] FIG. 29 is an enlarged sectional view of the partition wall
54 of the particulate filter. While the vehicle travels over the
predetermined running distance (As), the engine operation in the
area (II) of the FIG. 26 can be carried out. Thus, the particulates
collide with and are trapped by the exhaust gas upstream surface of
the partition wall 54 and the exhaust gas opposing surface in the
pores therein, i.e., one of the trapping surfaces of the partition
wall 54, and are oxidized and removed by active-oxygen released
from the NO.sub.x absorbent, but the particulates can remain for
the insufficient oxidization as shown in FIG. 29(A). At this stage,
the exhaust resistance of the particulate filter does not have a
bad influence on the traveling of the vehicle. However, if the
particulates deposit more, problems in which the engine output
drops considerably and the like occur. At this stage, the present
flowchart reverses the upstream side and the downstream side of the
particulate filter. Therefore, no particulates deposit again on the
residual particulates on one of the trapping surfaces of the
partition wall and thus the residual particulates can be gradually
oxidized and removed by active-oxygen released from the one of the
trapping surfaces. Further, in particular, the residual
particulates in the pores in the partition wall are easily smashed
into fine pieces by the exhaust gas flow in the reverse direction
as shown in FIG. 29(B), and they mainly move through the pores
toward the downstream side.
[0117] Accordingly, many of the particulates smashed into fine
pieces diffuse in the pore in the partition wall, and they contact
directly the NO.sub.x absorbent carried on the pores surface and
are oxidized and removed. Thus, if the NO.sub.x absorbent is also
carried on the pores surface in the partition wall, the residual
particulates can be very easily oxidized and removed. On the other
trapping surface that is now on the upstream side, as the flow of
the exhaust gas is reversed, i.e., the exhaust gas upstream surface
of the partition wall 54 and the exhaust gas opposing surface in
the pores therein to which the exhaust gas mainly impinges (of the
oppose side of one of the trapping surfaces), the particulates in
the exhaust gas adhere newly thereto and are oxidized and removed
by active-oxygen released from the NO.sub.x absorbent. In this
oxidization, a part of the active-oxygen released from the NO.sub.x
absorbent on the other trapping surface moves to the downstream
side with the exhaust gas, and it is made to oxidize and remove the
particulates that still remain on one of the trapping surfaces
despite of the reversed flow of the exhaust gas.
[0118] That is, the residual particulates on one of the trapping
surfaces are exposed to not only active-oxygen released from this
trapping surface but also the remainder of the active-oxygen used
for oxidizing and removing the particulates on the other trapping
surface by reversing the flow of the exhaust gas. Therefore, even
if some particulates deposit laminate on one of the trapping
surfaces of the partition wall of the particulate filter when the
exhaust gas flow is reversed, active-oxygen arrives at the
deposited particulates and no particulates deposit again on the
deposited particulates due to the reversed flow of the exhaust gas
and thus the deposited particulates are gradually oxidized and
removed and it can be oxidized and removed sufficiently for some
period till the next reversal of the exhaust gas. Of course, by
alternately using the one trapping surface and the other trapping
surface of the partition wall, the amount of trapped particulates
on each trapping surface is smaller than that of a particulate
filter in which the single trapping surface always traps the
particulates. This facilitates oxidizing and removal of the trapped
particulates on the trapping surface.
[0119] In the present flowchart, the valve body is changed over
every predetermined running distance. Thus, the valve body is
changed over before the deposited particulates can transform into
carbonaceous matter that can hardly be oxidized. Further, this can
prevent problems in which the large amount of deposited
particulates ignites and burns at once to melt the particulate
filter by the burned heat thereof and the like. Further, when the
valve body is reversed, even if the large amount of particulates
has deposited on one of the trapping surfaces of the partition wall
of the particulate filter, the deposited particulates are easily
smashed into fine pieces by the reversed flow of the exhaust gas. A
part of the particulates that cannot be oxidized and removed in the
pores in the partition wall is discharged from the particulate
filter. However, therefore, it is prevented that the exhaust
resistance of the particulate filter increased more to have a bad
influence on the operation of the vehicle. Further, the other
trapping surface of the partition wall of the particulate filter
can newly trap the particulates.
[0120] Thus, if the valve body is changed over every predetermined
running distance, a large deposition of the particulates on the
particulate filter can be prevented. The valve body does not limit
to be changed over every predetermined running distance. For
example, the valve body may be changed over every predetermined
time or may be irregularly changed over.
[0121] In the present flowchart, utilizing an increase of the
difference pressure between the exhaust gas upstream side and the
exhaust gas downstream side of the particulate filter in accordance
with an amount of particulates deposited on the particulate filter
70, when the difference pressure is larger than a predetermined
pressure, it is determined that some particulates deposit on the
particulate filter and thus the valve body may be changed over.
Concretely, an exhaust pressure on the first connecting portion 72a
(referring to FIG. 18) is detected by a pressure sensor arranged
therein and an exhaust pressure on the second connecting portion
72b (referring to FIG. 18) is detected by another pressure arranged
therein, and it is determined if the absolute value of the
difference between the two pressures is larger than a predetermined
value. Here, the absolute value is used in the determination.
Therefore, even if the exhaust gas upstream side is either the
first connecting portion 72a or the second connecting portion 72b,
the increase in the difference pressure can be determined. Strictly
speaking, the difference pressure between the both sides of the
particulate filter changes in accordance with the pressure of the
exhaust gas discharged from the combustion chamber every engine
operating condition. Accordingly, in the determination of the
deposition of the particulates, it is preferable to specify the
engine operating condition.
[0122] Other than the difference pressure, in the determination for
changing over the valve body, observing the change of electric
resistance on a predetermined partition wall of the particulate
filter, the fact that the electric resistance decreases along with
the deposition of the particulates thereon can be utilized, or the
fact that a transmissivity or reflectivity of light on a
predetermined partition wall of the particulate filter drops along
with the deposition of the particulate thereon can be utilized.
Thus, if the valve body is changed over by the direct determination
of the deposition of the particulates, it will certainly prevent
the engine output dropping considerably.
[0123] Besides, to prevent the large deposition of the
particulates, if the valve body is set from one shut-off position
to the middle position in the recovery process of the SO.sub.x
pollution of the catalytic apparatus, the valve body may be set the
other shut-off position immediately after the finishing of the
recovery process.
[0124] The present device for purifying the exhaust gas can reverse
the exhaust gas upstream side and the exhaust gas downstream side
of the particulate filter by a very simple structure. Further, the
particulate filter requires a large opening area to facilitate the
introduction of the exhaust gas. In the device, the particulate
filter having a large opening area can be used without making it
difficult to mount it on the vehicle as shown in FIGS. 18 and
19.
[0125] Further, when the air-fuel ratio in the surrounding
atmosphere of the particulate filter is made rich, i.e., when the
oxygen concentration therein is decreased, active-oxygen O is
released at once time from the NO.sub.x absorbent 61 to the
outside. Therefore, the deposited particulates become these that
are easily oxidized by the large amount of active-oxygen released
at one time, and can be oxidized and removed thereby without a
luminous flame.
[0126] On the other hand, when the air-fuel ratio is maintained
lean in the surrounding atmosphere of the particulate filter, the
surface of platinum Pt is covered with oxygen, that is, oxygen
contamination is caused. When such oxygen contamination is caused,
the oxidization action to NO.sub.x of platinum Pt drops and thus
the absorbing efficiency of NO.sub.x drops. Therefore, the amount
of active-oxygen released from the NO.sub.x absorbent 61 decreases.
However, when the air-fuel ratio is made rich, oxygen on the
surface of Platinum Pt is consumed and thus the oxygen
contamination is cancelled. Accordingly, when the air-fuel ratio is
changed over from rich to lean again, the oxidization action to
NO.sub.x becomes strong and thus the absorbing efficiency rises.
Therefore, the amount of active-oxygen released from the NO.sub.x
absorbent 61 increases.
[0127] Thus, when the air-fuel ratio is maintained lean, if the
air-fuel ratio is changed over from lean to rich once in a while,
the oxygen contamination of platinum Pt is cancelled every this
time and thus the amount of released active-oxygen increases when
the air-fuel ratio is lean. Therefore, the oxidization action of
the particulates on the particulate filter 70 can be promoted.
[0128] Further, the cancellation of the oxygen contamination causes
the reducing agent to burn and thus the burned heat thereof raises
the temperature of the particulate filter. Therefore, in the
particulate filter, the amount of particulates that can be oxidized
and removed increases and thus the deposited particulates are
oxidized and removed more easily. If the air-fuel ratio is made
rich immediately after the upstream side and the downstream side of
the particulate filter is reversed by the valve body 71a, the other
trapping surface on which the particulates do not remain or deposit
can release the large amount of active-oxygen. Thus, the large
amount of released active-oxygen can oxidize and remove the
deposited particulates more certainly. Of course, regardless of the
reversal of the valve body 71a, the air-fuel ratio may be made rich
once in a while. Therefore, the particulates hardly deposit on the
particulate filter.
[0129] As a method to make the air-fuel ratio rich, for example,
the above-mentioned low temperature combustion may be carried out.
Of course, when changing over from the normal combustion to the low
temperature combustion or before this, the exhaust gas upstream
side and the exhaust gas downstream side of the particulate filter
may be reversed. Further, to make the air-fuel ratio of the
surrounding atmosphere rich, the combustion air-fuel ratio may be
merely made rich. Further, in addition to the main fuel injection
in the compression stroke, the fuel injector may inject fuel into
the cylinder in the exhaust stroke or the expansion stroke
(post-injection) or may injected fuel into the cylinder in the
intake stroke (pre-injection). Of course, an interval between the
post-injection or the pre-injection and the main fuel injection may
not be provided. Further, fuel may be supplied to the exhaust
system. Besides, in the catalytic apparatus and the particulate
filter, the air-fuel ratio in the surrounding atmosphere must be at
least temporarily made rich to release NO.sub.x from the NO.sub.x
absorbent carried thereon. It is preferable to carry out the
air-fuel ratio rich control immediately after the reversing of the
upstream side and the downstream side of the particulate
filter.
[0130] By the way, when SO.sub.3 exists, calcium Ca in the exhaust
gas forms calcium sulfate CaSO.sub.4. Calcium sulfate CaSO.sub.4 is
hardly oxidized and remains on the particulate filter as ash. To
prevent of blocking the meshes of the particulate filter caused by
calcium sulfate CaSO.sub.4, an alkali metal or an alkali earth
metal having an ionization tendency stronger than that of calcium
Ca, such as potassium K may be used as the NO.sub.x absorbent 61.
Therefore, SO.sub.3 diffused in the NO.sub.x absorbent 61 is
combined with potassium K to form potassium sulfate K.sub.2SO.sub.4
and thus calcium Ca is not combined with SO.sub.3 but passes
through the partition walls of the particulate filter. Accordingly,
the meshes of the particulate filter are not blocked by the ash.
Thus, it is desired to use, as the NO.sub.x absorbent 61, an alkali
metal or an alkali earth metal having an ionization tendency
stronger than calcium Ca, such as potassium K, lithium Li, cesium
Cs, rubidium Rb, barium Ba or strontium Sr.
[0131] In the above-mentioned embodiment, the particulate filter 70
carries the catalyst for absorbing and reducing NO.sub.x. However,
in another embodiment, the particulate filter may carry cerium Ce.
Cerium Ce absorbs oxygen when the oxygen concentration in the
surrounding atmosphere is high (Ce.sub.2O.sub.3.fwdarw.2CeO.sub.2)
and releases oxygen when the oxygen concentration decreases
(2CeO.sub.2.fwdarw.CeO.sub- .3). Thus, cerium Ce functions an
oxygen absorbing agent. Iron Fe or tin Sn can be used as the oxygen
absorbing agent. In the present embodiment, the particulate filter
does not carry the catalyst for absorbing and reducing NO.sub.x.
However, the catalytic apparatus is arranged upstream the
particulate filter, the catalytic apparatus can carry a large
amount of catalyst for purifying NO.sub.x, the catalysts do not
become covered with the particulates due to the structure of the
catalytic apparatus, and thus the catalytic apparatus can purify
sufficiently NO.sub.x in the exhaust gas. In the present
embodiment, the trapped particulates on the particulate filter
ignite and burn with producing luminous flame at a high
temperature. At this time, the catalyst for absorbing and reducing
NO.sub.x is not carried on the particulate filter and thus the
catalyst does not deteriorate. Further, when the air-fuel ratio in
the exhaust gas is made rich to, for example, regenerate the
catalytic apparatus, a part of unburned fuel (HC) included in the
exhaust gas is not oxidized on the catalytic apparatus and can flow
into the particulate filter. The unburned fuel has an adhesion
property as same as the above-mentioned SO.sub.x and adheres the
particulates each other on the particulate filter, and thus makes
the particulates become a large mass. In the present embodiment,
when the air-fuel ratio in the exhaust gas is rich, i.e., the
oxygen concentration is low, cerium Ce releases oxygen as mentioned
above and thus the released oxygen oxidizes the unburned fuel on
the particulate filter. Therefore, a large mass of the particulates
is not formed and thus the filter meshes is not blocked thereby.
The oxygen absorbing as cerium Ce agent functions as an oxidation
catalyst. In the present embodiment, if the particulate filter also
carries a noble metal catalyst as platinum Pt, the unburned fuel
can be oxidized more favorably. SO.sub.x in the exhaust gas adheres
the oxygen absorbing agent on the particulate filter and the oxygen
absorbing function thereof drops. However, in the present
embodiment, the catalytic apparatus arranged upstream the
particulate filter can treat SO.sub.x as mentioned above and thus
SO.sub.x does not adhere the oxygen absorbing agent on the
particulate filter. In the present embodiment, a noble metal such
as platinum Pt can release active-oxygen from the held NO.sub.2 and
SO.sub.3 on the surface thereof. The released active-oxygen can
oxidize the particulates on the particulate filter. Further, oxygen
released from the oxygen absorbing agent such as cerium Ce, iron Fe
or tin Sn is also active-oxygen and thus the released active-oxygen
can oxidize the particulates on the particulate filter. Thus, at
least part of the trapped particulates is oxidized and removed and
thus the particulates deposits hardly on the particulate filter of
the present embodiment. In the above two embodiments, the catalytic
apparatus functions to make uniform the temperature distribution in
the exhaust gas before the exhaust gas flows into the particulate
filter. Thus, it is prevented that only a part of the particulate
filter reaches a low temperature. Accordingly, the trapped
particulates are generally oxidized or burned and thus it is
prevented that the particulates deposits on only the part of the
particulate filter and the deposited particulates transform into
carbonaceous matter that can hardly be oxidized or burned.
[0132] Although the invention has been described with reference to
specific embodiments thereof, it should be apparent that numerous
modifications can be made thereto, by those skilled in the art,
without departing from the basic concept and scope of the
invention.
* * * * *