U.S. patent application number 12/669054 was filed with the patent office on 2010-07-29 for exhaust gas purification apparatus for internal combustion engine and method of controlling the same.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Mikio Inoue, Kenichi Tsujimoto.
Application Number | 20100186386 12/669054 |
Document ID | / |
Family ID | 40897485 |
Filed Date | 2010-07-29 |
United States Patent
Application |
20100186386 |
Kind Code |
A1 |
Tsujimoto; Kenichi ; et
al. |
July 29, 2010 |
EXHAUST GAS PURIFICATION APPARATUS FOR INTERNAL COMBUSTION ENGINE
AND METHOD OF CONTROLLING THE SAME
Abstract
A small oxidation catalyst and a fuel supply valve are arranged
in an engine exhaust passage. The small oxidation catalyst has a
cross-sectional area smaller than a cross-sectional area of the
engine exhaust passage, and portion of exhaust gas flowing in the
engine exhaust passage flows through the small oxidation catalyst.
The fuel supply valve supplies fuel to the small oxidation
catalyst. Fuel is intermittently supplied from the fuel supply
valve to the small oxidation catalyst to intermittently generate
flames downstream of the small oxidation catalyst.
Inventors: |
Tsujimoto; Kenichi;
(Susono-shi, JP) ; Inoue; Mikio; (Shizuoka-ken,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
40897485 |
Appl. No.: |
12/669054 |
Filed: |
April 22, 2009 |
PCT Filed: |
April 22, 2009 |
PCT NO: |
PCT/IB09/05319 |
371 Date: |
January 14, 2010 |
Current U.S.
Class: |
60/286 ; 60/297;
60/299; 60/303 |
Current CPC
Class: |
F02B 37/00 20130101;
Y02T 10/47 20130101; F01N 9/002 20130101; F01N 3/0814 20130101;
F01N 3/0842 20130101; F01N 9/00 20130101; F01N 3/106 20130101; F01N
3/36 20130101; Y02T 10/40 20130101; F01N 2900/0416 20130101; Y02T
10/26 20130101; F01N 3/035 20130101; F01N 2610/03 20130101; F01N
2240/12 20130101; F01N 2900/1602 20130101; F01N 2900/1606 20130101;
Y02T 10/12 20130101; F01N 3/30 20130101; F01N 3/2033 20130101; F01N
13/0097 20140603; F01N 13/009 20140601 |
Class at
Publication: |
60/286 ; 60/299;
60/303; 60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10; F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2008 |
JP |
2008-215283 |
Claims
1. An exhaust gas purification apparatus for an internal combustion
engine, comprising: a small oxidation catalyst that is arranged in
an engine exhaust passage and that has a cross-sectional area
smaller than a cross-sectional area of the engine exhaust passage,
wherein portion of exhaust gas flowing in the engine exhaust
passage flows through the small oxidation catalyst; a fuel supply
valve that is arranged in the engine exhaust passage and that
supplies fuel to the small oxidation catalyst; a determination unit
that determines whether flames is generated by supplying fuel from
the fuel supply valve; and a control unit that, when the
determination unit determines that the flames is generated by
supplying fuel from the fuel supply valve, intermittently supplies
fuel from the fuel supply valve to the small oxidation catalyst to
intermittently generate flames downstream of the small oxidation
catalyst.
2. The exhaust gas purification apparatus for an internal
combustion engine according to claim 1, wherein a timing at which
fuel is supplied from the fuel supply valve is set so that flame
masses sequentially intermittently generated downstream of the
small oxidation catalyst do not overlap each other.
3. The exhaust gas purification apparatus for an internal
combustion engine according to claim 2, wherein a timing at which
fuel is supplied from the fuel supply valve is set so that fuel is
intermittently supplied from the small oxidation catalyst.
4. The exhaust gas purification apparatus for an internal
combustion engine according to claim 2, wherein a timing at which
fuel is supplied from the fuel supply valve is set so that exhaust
gas having a rich air-fuel ratio intermittently flows out from the
small oxidation catalyst.
5. The exhaust gas purification apparatus for an internal
combustion engine according to claim 1, wherein the small oxidation
catalyst has a cylindrical shape extending in a direction in which
exhaust gas flows.
6. The exhaust gas purification apparatus for an internal
combustion engine according to claim 5, wherein fuel is supplied
from the fuel supply valve toward an upstream-end face of the small
oxidation catalyst.
7. The exhaust gas purification apparatus for an internal
combustion engine according to claim 1, wherein an interval of
timings, at which supply of fuel from the fuel supply valve is
started, is extended as an intake air amount reduces.
8. The exhaust gas purification apparatus for an internal
combustion engine according to claim 1, further comprising: a
secondary air supply device that supplies secondary air into the
engine exhaust passage, wherein when oxygen is insufficient to
generate flames, secondary air is supplied into the engine exhaust
passage.
9. The exhaust gas purification apparatus for an internal
combustion engine according to claim 1, further comprising: one of
an exhaust gas purification catalyst and a particulate filter
arranged in the engine exhaust passage at a portion downstream of
the small oxidation catalyst, wherein flames are generated when the
temperature of the one of the exhaust gas purification catalyst and
the particulate filter needs to be increased.
10. The exhaust gas purification apparatus for an internal
combustion engine according to claim 1, further comprising: a NOx
occlusion catalyst arranged in the engine exhaust passage at a
portion downstream of the small oxidation catalyst, wherein the NOx
occlusion catalyst occludes NOx contained in exhaust gas when the
air-fuel ratio of exhaust gas flowing into the NOx occlusion
catalyst is lean, and releases the occluded NOx when the air-fuel
ratio of exhaust gas flowing into the NOx occlusion catalyst is
rich, and wherein flames of a rich air-fuel ratio are
intermittently generated when NOx or SOx needs to be released from
the NOx occlusion catalyst.
11. The exhaust gas purification apparatus for an internal
combustion engine according to claim 10, wherein, when NOx or SOx
needs to be released from the NOx occlusion catalyst, it is
determined whether an operating state of an engine is such that
flames of a rich air-fuel ratio can be generated, wherein, when NOx
or SOx needs to be released from the NOx occlusion catalyst, and
when the operating state of the engine is such that flames of a
rich air-fuel ratio can be generated, flames are generated, and
wherein, when NOx or SOx needs to be released from the NOx
occlusion catalyst, and when the operating state of the engine is
not such that flames of a rich air-fuel ratio can be generated, the
air-fuel ratio of exhaust gas that flows into the NOx occlusion
catalyst is made rich without generating flames.
12. The exhaust gas purification apparatus for an internal
combustion engine according to claim 11, wherein, when the air-fuel
ratio of exhaust gas is higher than the predetermined air-fuel
ratio, it is determined that the operating state of the engine is
such that flames of a rich air-fuel ratio can be generated.
13. The exhaust gas purification apparatus for an internal
combustion engine according to claim 12, wherein, when the engine
is operating at a light load or decelerating, it is determined that
the operating state of the engine is such that flames of a rich
air-fuel ratio can be generated.
14. A method of controlling an exhaust gas purification apparatus
for an internal combustion engine, the exhaust gas purification
apparatus including a small oxidation catalyst that is arranged in
an engine exhaust passage and that has a cross-sectional area
smaller than a cross-sectional area of the engine exhaust passage,
wherein portion of exhaust gas flowing in the engine exhaust
passage flows through the small oxidation catalyst; and a fuel
supply valve that is arranged in the engine exhaust passage and
that supplies fuel to the small oxidation catalyst, the method
comprising: determining whether flames is generated by supplying
fuel from the fuel supply valve; and when it is determined that the
flames is generated by supplying fuel from the fuel supply valve,
intermittently supplying fuel from the fuel supply valve to the
small oxidation catalyst to intermittently generate flames
downstream of the small oxidation catalyst.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to an exhaust gas purification
apparatus for an internal combustion engine and a method of
controlling the same.
[0003] 2. Description of the Related Art
[0004] Published Japanese Translation of PCT Application No.
2000-514911 (JP-A-2000-514911) describes a known combustor that
brings an air-fuel mixture gas into contact with a catalyst to
generate flames. The combustor includes a small catalyst that is
able to electrically heat the catalyst and a large catalyst that is
arranged downstream of the small catalyst in a direction in which
the mixture gas flows. A mixture gas having a lean air-fuel ratio
is continuously supplied to the electrically heated small catalyst
to continuously generate flames downstream of the small catalyst.
Then, the air-fuel ratio of the mixture gas is increased and the
amount of the mixture gas is increased to continuously generate
flames in the large catalyst.
[0005] In other words, in the above described combustor, a mixture
gas having a lean air-fuel ratio is supplied to the small catalyst,
so fuel is caused to burn with excessive air. This makes it
possible to continuously generate flames. However, when fuel is
supplied from a fuel supply valve to a small oxidation catalyst,
the air-fuel ratio of exhaust gas that flows out from the small
oxidation catalyst may be lean but is mostly rich. That is, exhaust
gas flowing out from the small oxidation catalyst mostly contains
excessive fuel.
[0006] Thus, as in the case of the above described combustor, when
fuel is continuously caused to flow out from the small oxidation
catalyst in order to continuously generate flames, not all fuel
burns favorably because of insufficient air. Therefore, there is a
problem that not only soot is generated but also a flame
temperature does not sufficiently increase.
SUMMARY OF THE INVENTION
[0007] The inventors have been studied that, in order to increase
the temperature of a catalyst, or the like, a small oxidation
catalyst and a fuel supply valve are arranged in an engine exhaust
passage, the small oxidation catalyst has a cross-sectional area
smaller than the cross-sectional area of the engine exhaust
passage, the fuel supply valve is used to supply fuel to the small
oxidation catalyst, and then fuel is supplied from the fuel supply
valve to the small oxidation catalyst to generate flames downstream
of the small oxidation catalyst. As a result, to generate
high-temperature flames without generating soot in the engine
exhaust passage, it has been found that flames are intermittently
generated by intermittently supplying fuel from the fuel supply
valve.
[0008] A first aspect of the invention provides an exhaust gas
purification apparatus for an internal combustion engine. The
exhaust gas purification apparatus includes: a small oxidation
catalyst that is arranged in an engine exhaust passage and that has
a cross-sectional area smaller than a cross-sectional area of the
engine exhaust passage, wherein portion of exhaust gas flowing in
the engine exhaust passage flows through the small oxidation
catalyst; a fuel supply valve that is arranged in the engine
exhaust passage and that supplies fuel to the small oxidation
catalyst; a determination unit that determines whether flames is
generated by supplying fuel from the fuel supply valve; and a
control unit that, when the determination unit determines that the
flames is generated by supplying fuel from the fuel supply valve,
intermittently supplies fuel from the fuel supply valve to the
small oxidation catalyst to intermittently generate flames
downstream of the small oxidation catalyst.
[0009] A second aspect of the invention provides a method of
controlling an exhaust gas purification apparatus for an internal
combustion engine. The exhaust gas purification apparatus includes
a small oxidation catalyst and a fuel supply valve. The small
oxidation catalyst is arranged in an engine exhaust passage and has
a cross-sectional area smaller than a cross-sectional area of the
engine exhaust passage. Portion of exhaust gas flowing in the
engine exhaust passage flows through the small oxidation catalyst.
The fuel supply valve is arranged in the engine exhaust passage and
supplies fuel to the small oxidation catalyst. The method includes:
determining whether flames is generated by supplying fuel from the
fuel supply valve; and, when it is determined that the flames is
generated by supplying fuel from the fuel supply valve,
intermittently supplying fuel from the fuel supply valve to the
small oxidation catalyst to intermittently generate flames
downstream of the small oxidation catalyst.
[0010] According to the aspects of the invention, it is possible to
generate high-temperature flames while suppressing generation of
soot in the engine exhaust passage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The features, advantages, and technical and industrial
significance of this invention will be described in the following
detailed description of example embodiments of the invention with
reference to the accompanying drawings, in which like numerals
denote like elements, and wherein:
[0012] FIG. 1 is a general view of a compression ignition internal
combustion engine;
[0013] FIG. 2A and FIG. 2B are enlarged views around a small
oxidation catalyst shown in FIG. 1;
[0014] FIG. 3 is a view that illustrates NOx absorbing/releasing
action;
[0015] FIG. 4A and FIG. 4B are views that illustrate generation of
flames;
[0016] FIG. 5 is a view that shows a flame generation region;
[0017] FIG. 6A, FIG. 6B and FIG. 6C are time charts that show a
control for supplying fuel from a fuel supply valve;
[0018] FIG. 7A, FIG. 7B and FIG. 7C are time charts that show a
control for supplying fuel from the fuel supply valve;
[0019] FIG. 8 is a time chart that shows a control for supplying
fuel from the fuel supply valve;
[0020] FIG. 9 is a time chart that shows a control for supplying
fuel from the fuel supply valve;
[0021] FIG. 10 is a time chart that shows a control for supplying
fuel from the fuel supply valve;
[0022] FIG. 11 is a flowchart for carrying out exhaust gas
purification process; and
[0023] FIG. 12 is a view that shows a map of an occluded NOx amount
NOXA.
DETAILED DESCRIPTION OF EMBODIMENTS
[0024] FIG. 1 is a general view of a compression ignition internal
combustion engine. FIG. 1 shows an engine body 1, combustion
chambers 2 of cylinders, electronically controlled fuel injection
valves 3 for injecting fuel respectively to the combustion chambers
2, an intake manifold 4, and an exhaust manifold 5. The intake
manifold 4 is connected to an outlet of a compressor 7a of an
exhaust gas turbocharger 7 via an air intake duct 6. An inlet of
the compressor 7a is connected to an air cleaner 9 via an intake
air amount detector 8. A throttle valve 10 is arranged in the air
intake duct 6 and is driven by a step motor. In addition, a cooling
unit 11 is arranged around the air intake duct 6 to cool intake air
that flows through the air intake duct 6. In an embodiment shown in
FIG. 1, engine coolant is introduced into the cooling unit 11, and
intake air is cooled by the engine coolant.
[0025] On the other hand, the exhaust manifold 5 is connected to an
inlet of an exhaust gas turbine 7b of the exhaust gas turbocharger
7, and an outlet of the exhaust gas turbine 7b is connected to an
exhaust gas purification catalyst 13 via an exhaust pipe 12. The
exhaust gas purification catalyst 13 has an oxidation function. A
small oxidation catalyst 14 is arranged in an engine exhaust
passage, that is, in the exhaust pipe 12, at a portion upstream of
the exhaust gas purification catalyst 13. The small oxidation
catalyst 14 has a volume smaller than that of the exhaust gas
purification catalyst 13. Portion of exhaust gas flowing into the
exhaust gas purification catalyst 13 flows through the small
oxidation catalyst 14. A fuel supply valve 15 is arranged in the
engine exhaust passage, that is, in the exhaust pipe 12, at a
portion upstream of the small oxidation catalyst 14. The fuel
supply valve 15 supplies fuel to the small oxidation catalyst
14.
[0026] In the embodiment shown in FIG. 1, the exhaust gas
purification catalyst 13 is made of an oxidation catalyst, and a
particulate filter 16 is arranged in the engine exhaust passage at
a portion downstream of the exhaust gas purification catalyst 13,
that is, at a portion downstream of the oxidation catalyst 13. The
particulate filter 16 collects particulates contained in exhaust
gas. In addition, in the embodiment shown in FIG. 1, a NOx
occlusion catalyst 17 is arranged in the engine exhaust passage at
a portion downstream of the particulate filter 16.
[0027] The exhaust manifold 5 and the intake manifold 4 are
connected to each other via an exhaust gas recirculation
(hereinafter, referred to as EGR) passage 18. An electronically
controlled EGR control valve 19 is arranged in the EGR passage 18.
In addition, the cooling unit 20 is arranged around the EGR passage
18 to cool EGR gas that flows through the EGR passage 18. In the
embodiment shown in FIG. 1, engine coolant is introduced into the
cooling unit 20, and EGR gas is cooled by the engine coolant. On
the other hand, each fuel injection valve 3 is connected to a
common rail 22 via a fuel supply pipe 21, and the common rail 22 is
connected to a fuel tank 24 via an electronically controlled
variable displacement fuel pump 23. Fuel stored in the fuel tank 24
is supplied into the common rail 22 by the fuel pump 23, and the
supplied fuel in the common rail 22 is supplied to each fuel
injection valve 3 via a corresponding one of the fuel supply pipes
21.
[0028] An electronic control unit 30 is formed of a digital
computer, and includes a read-only memory (ROM) 32, a random access
memory (RAM) 33, a microprocessor (CPU) 34, an input port 35 and an
output port 36, which are connected to one another via a
bidirectional bus 31. A temperature sensor 25 is attached to the
small oxidation catalyst 14. The temperature sensor 25 detects the
temperature of the small oxidation catalyst 14. A differential
pressure sensor 26 is attached to the particulate filter 16. The
differential pressure sensor 26 detects a differential pressure
before and after the particulate filter 16. Output signals of these
temperature sensor 25, differential pressure sensor 26 and intake
air amount detector 8 are input to the input port 35 through
respective AD converters 37. A load sensor 41 is connected to an
accelerator pedal 40. The load sensor 41 generates an output
voltage in proportion to a depression amount L of the accelerator
pedal 40. The output voltage of the load sensor 41 is input to the
input port 35 via a corresponding one of the AD converter 37. In
addition, a crank angle sensor 42 is connected to the input port
35. The crank angle sensor 42 generates an output pulse each time a
crankshaft rotates, for example, by 15 degrees. On the other hand,
the output port 36 is connected to each fuel injection valve 3, a
step motor for driving the throttle valve 10, the EGR control valve
19 and the fuel pump 23 via corresponding driving circuits 38.
[0029] FIG. 2A shows an enlarged view of a portion around the small
oxidation catalyst 14 shown in FIG. 1. FIG. 2B shows a
cross-sectional view that is taken along the line IIB-IIB in FIG.
2A. In the embodiment shown in FIG. 2A and FIG. 2B, the small
oxidation catalyst 14 has a base formed of a laminated structure of
a metal thin flat plate and a metal thin corrugated plate. A layer
of a catalyst carrier made of, for example, alumina is formed on a
surface of the base. A precious metal catalyst, such as platinum
Pt, rhodium Rd, and palladium Pd, is supported on the catalyst
carrier. Note that the base may be made of cordierite.
[0030] As is apparent from FIG. 2A and FIG. 2B, the small oxidation
catalyst 14 has a cross section that is smaller in area than the
cross section of a passage through which exhaust gas flows toward
the exhaust gas purification catalyst 13 (or the oxidation catalyst
13), that is, a cross section that is smaller in area than the
cross section of the exhaust pipe 12, and the small oxidation
catalyst 14 has a cylindrical shape extending in a direction in
which exhaust gas flows at the center of the exhaust pipe 12. Note
that, in the embodiment shown in FIG. 2A and FIG. 2B, the small
oxidation catalyst 14 is arranged inside a cylindrical outer frame
27, and the cylindrical outer frame 27 is supported inside the
exhaust pipe 12 by a plurality of stays 28.
[0031] On the other hand, as shown in FIG. 1 and FIG. 2A, a
secondary air supply device 45 is attached to the exhaust pipe 12
to supply secondary air into the exhaust pipe 12. The secondary air
supply device 45 includes an air pump 46, an introducing pipe 47,
and a plurality of secondary air supply nozzles 49. The air pump 46
is controlled on the basis of an output signal of the electronic
control unit 30. The introducing pipe 47 transfers secondary air,
discharged from the air pump 46, to an annular secondary air
distribution chamber 48 around the exhaust pipe 12. The plurality
of secondary air supply nozzles 49 extend from the secondary air
distribution chamber 48 toward the downstream side of the small
oxidation catalyst 14.
[0032] In the embodiment shown in FIG. 1, the oxidation catalyst 13
is formed of a monolithic catalyst that supports, for example, a
precious metal catalyst, such as platinum Pt. In contrast, in the
embodiment shown in FIG. 1, no precious metal catalyst is supported
on the particulate filter 16. Instead, a precious metal catalyst,
such as platinum Pt, may be supported on the particulate filter 16,
and, in this case, the oxidation catalyst 13 may be omitted.
[0033] On the other hand, a catalyst carrier made of, for example,
alumina is also supported on a base of the NOx occlusion catalyst
17 shown in FIG. 1. FIG. 3 schematically illustrates the cross
section of a surface portion of a catalyst carrier 50. As shown in
FIG. 3, precious metal catalysts 51 are dispersedly supported on a
surface of the catalyst carrier 50, and a layer of a NOx absorbent
52 is formed on a surface of the catalyst carrier 50.
[0034] In the example shown in FIG. 3, platinum Pt is used as the
precious metal catalyst 51, a component that constitutes the NOx
absorbent 52, for example, uses at least one selected from an
alkali metal, such as potassium K, sodium Na and cesium Cs, an
alkaline earth, such as barium Ba and calcium Ca, and a rare earth,
such as lanthanum La and yttrium Y.
[0035] If ratio between air and fuel (hydrocarbon) that are
supplied into an engine intake passage, the combustion chambers 2
and the exhaust passage upstream of the NOx occlusion catalyst 17
is referred to as the air-fuel ratio of exhaust gas, the NOx
absorbent 52 occludes NOx when the air-fuel ratio of exhaust gas is
lean, and releases the occluded NOx when the concentration of
oxygen in exhaust gas decreases, thus carrying out NOx
absorbing/releasing action.
[0036] That is, taking the case where barium Ba is used as a
component that constitutes the NOx absorbent 52 as an example, when
the air-fuel ratio of exhaust gas is lean, that is, when the
concentration of oxygen in exhaust gas is high, NO contained in the
exhaust gas is oxidized to NO.sub.2 on the platinum Pt 51 as shown
in FIG. 3, and, subsequently, the NO.sub.2 is absorbed into the NOx
absorbent 52 to be bonded with barium carbonate BaCO.sub.3 while
being diffused in the NOx absorbent 52 in the form of nitrate ion
NO.sub.3.sup.-. In this manner, NOx is occluded into the NOx
absorbent 52. As long as the concentration of oxygen contained in
exhaust gas is high, NO.sub.2 is generated on the surface of the
platinum Pt 51. Unless the NOx absorption capacity of the NOx
absorbent 52 is saturated, NO.sub.2 is absorbed into the NOx
absorbent 52 to produce nitrate ion NO.sub.3.sup.-.
[0037] In contrast, when the air-fuel ratio of exhaust gas is rich
or stoichiometric, the concentration of oxygen in exhaust gas
decreases. Thus, reaction proceeds in the reverse direction (from
NO.sub.3.sup.- to NO.sub.2) and, as a result, nitrate ions
NO.sub.3.sup.- in the NOx absorbent 52 are released from the NOx
absorbent 52 in the form of NO.sub.2. Subsequently, the released
NOx is reduced by unburned HC or CO contained in exhaust gas.
[0038] In this way, when the air-fuel ratio of exhaust gas is lean,
that is, when combustion takes place at a lean air-fuel ratio, NOx
contained in exhaust gas is occluded into the NOx absorbent 52.
However, when combustion continuously takes place at a lean
air-fuel ratio, the NOx absorption capacity of the NOx absorbent 52
is saturated during then. As a result, NOx cannot be absorbed by
the NOx absorbent 52. Then, in the present embodiment, by supplying
fuel from the fuel supply valve 15 prior to saturation of the
absorption capacity of the NOx absorbent 52, the air-fuel ratio of
exhaust gas is temporarily made rich to release NOx from the NOx
absorbent 52.
[0039] Incidentally, SOx, that is, SO.sub.2, is contained in
exhaust gas. If the SO.sub.2 flows into the NOx occlusion catalyst
17, the SO.sub.2 is oxidized by the platinum Pt 51 to SO.sub.3.
Subsequently, the SO.sub.3 is absorbed into the NOx absorbent 52 to
be bonded with barium carbonate BaCO.sub.3 while being diffused in
the NOx absorbent 52 in the form of sulfate ion SO.sub.4.sup.2- to
produce stable sulfate BaSO.sub.4. However, because the NOx
absorbent 52 is strongly basic, the sulfate BaSO.sub.4 is stable
and is difficult to be decomposed. If only the air-fuel ratio of
exhaust gas is simply made rich, the sulfate BaSO.sub.4 remains as
it is without decomposition. Thus, the NOx absorbent 52 contains
increased sulfate BaSO.sub.4 as time elapses. As a result, as time
elapses, the amount of NOx the NOx absorbent 52 can absorb
decreases. That is, the NOx occlusion catalyst 17 experiences
sulfur poisoning.
[0040] Incidentally, in this case, in a state where the temperature
of the NOx occlusion catalyst 17 is increased to a SOx release
temperature, which is higher than or equal to 600.degree. C., when
the air-fuel ratio of exhaust gas that flows into the NOx occlusion
catalyst 17 is made rich, SOx is released from the NOx absorbent
52. Then, in the present embodiment, when the NOx occlusion
catalyst 17 experiences sulfur poisoning, fuel is supplied from the
fuel supply valve 15 to increase the temperature of the NOx
occlusion catalyst 17 to the SOx release temperature. Thus, the
air-fuel ratio of exhaust gas that flows into the NOx occlusion
catalyst 17 is made rich to release SOx from the NOx occlusion
catalyst 17.
[0041] In the embodiment shown in FIG. 2A, the nozzle opening of
the fuel supply valve 15 is arranged at the center of the cross
section of the exhaust pipe 12, and fuel F, that is, light oil F,
is supplied from the nozzle opening toward the upstream-side end
face of the small oxidation catalyst 14. At this time, when the
small oxidation catalyst 14 is activated, fuel is oxidized in the
small oxidation catalyst 14, and heat of oxidation reaction
generated at this time increases the temperature of the small
oxidation catalyst 14.
[0042] Incidentally, because flow resistance is large inside the
small oxidation catalyst 14, the amount of exhaust gas that flows
through the small oxidation catalyst 14 is small. In addition, as
oxidation reaction occurs in the small oxidation catalyst 14, gas
expands inside the small oxidation catalyst 14. Thus, the amount of
exhaust gas that flows through the small oxidation catalyst 14
further reduces. In addition, as the gas temperature increases due
to oxidation reaction, the viscosity of gas increases. Thus, the
amount of exhaust gas that flows through the small oxidation
catalyst 14 further reduces. Hence, the flow rate of exhaust gas
inside the small oxidation catalyst 14 is considerably lower than
the flow rate of exhaust gas that flows inside the exhaust pipe
12.
[0043] In this way, because the flow rate of exhaust gas inside the
small oxidation catalyst 14 is low, oxidation reaction is activated
in the small oxidation catalyst 14. In addition, because the volume
of the small oxidation catalyst 14 is small, the temperature of the
small oxidation catalyst 14 rapidly increases to a considerably
high temperature. At this time, as the temperature of the small
oxidation catalyst 14 is higher than the ignition temperature of
fuel, fuel flowing out from the small oxidation catalyst 14
ignites, and flames are generated downstream of the small oxidation
catalyst 14 as indicated by H in FIG. 2A.
[0044] That is, as fuel is supplied from the fuel supply valve 15
to the small oxidation catalyst 14, portion of the supplied fuel is
oxidized in the small oxidation catalyst 14. On the other hand, the
remaining supplied fuel is decomposed, that is, reformed, into
hydrocarbons having low molecular weight in the small oxidation
catalyst 14. As a result, the reformed fuel flows out from the
small oxidation catalyst 14. At this time, the air-fuel ratio of
exhaust gas that flows out from the small oxidation catalyst 14 may
be lean but is mostly rich. That is, exhaust gas flowing out from
the small oxidation catalyst 14 at this time mostly contains
excessive fuel.
[0045] At this time, when the temperature of the small oxidation
catalyst 14 is higher than the ignition temperature, flames H are
generated as shown in FIG. 2A. In this case, if fuel is caused to
continuously flow out from the small oxidation catalyst 14 in order
to continuously generate flames H, not all fuel burns favorably
because of insufficient air. Therefore, there is a problem that not
only soot is generated but also a flame temperature does not
sufficiently increase. In contrast, at this time, when fuel is
caused to intermittently flow out from the small oxidation catalyst
14 to intermittently generate flames, fuel flowing out from the
small oxidation catalyst 14 is supplied into the atmosphere in
which air is excessive. Thus, all fuel flowing out from the small
oxidation catalyst 14 can be burned favorably.
[0046] Then, in the present embodiment shown in FIG. 4A, fuel is
intermittently supplied from the fuel supply valve 15 to
intermittently generate flames H as shown in FIG. 4B. Note that
FIG. 4B schematically illustrates masses of flames H, that is,
flame masses H, that are generated intermittently inside the
exhaust passage 55.
[0047] As shown in FIG. 4B, in the present embodiment, flame masses
H are sequentially generated so as not to overlap each other. Thus,
in the present embodiment, the timing at which fuel is supplied
from the fuel supply valve 15 is set so that flame masses that are
sequentially intermittently generated downstream of the small
oxidation catalyst 14 do not overlap each other.
[0048] As shown in FIG. 4B, in order for flame masses H not to
overlap each other, it is necessary that fuel is caused to
intermittently flow out from the small oxidation catalyst 14. Thus,
in other words, in the present embodiment, the timing at which fuel
is supplied from the fuel supply valve 15 is set so that fuel is
intermittently supplied from the small oxidation catalyst 14.
[0049] Note that as described above, when fuel is supplied from the
fuel supply valve 15 to the small oxidation catalyst 14, the
air-fuel ratio of exhaust gas that flows out from the small
oxidation catalyst 14 is rich. Thus, at the time of generating
flames H, exhaust gas having a rich air-fuel ratio intermittently
flows out from the small oxidation catalyst 14.
[0050] Next, a region in which flames H are generated will be
described. In FIG. 5, the region on the upper side with respect to
the solid line HX indicates a flame generating region. As shown in
FIG. 5, the flame generating region is determined on the basis of
the function of a temperature TC of the small oxidation catalyst 14
and the concentration of oxygen contained in exhaust gas. Note that
in FIG. 5, the solid line HX represents the relationship between a
concentration of oxygen contained in exhaust gas and a catalyst
temperature TC when fuel ignites. The catalyst temperature TC at
the time when fuel ignites increases as the concentration of oxygen
contained in exhaust gas decreases.
[0051] FIG. 5 shows a range of the concentration of oxygen
contained in exhaust gas during light load operation, including
idling, and during deceleration operation in which fuel is being
injected from the fuel injection valves 3, and the concentration of
oxygen contained in exhaust gas during deceleration operation when
fuel injection from the fuel injection valves 3 is stopped. Air is
exhaust gas when fuel injection is stopped, so the concentration of
oxygen contained in the exhaust gas is approximately 21 percent at
this time. The concentration of oxygen contained in exhaust gas
during light load or deceleration (with fuel injection) is lower
than the above concentration of oxygen, and the catalyst
temperature TC at which fuel ignites during light load or
deceleration (with fuel injection) is about 800.degree. C.
[0052] When the oxidation catalyst 13, and the like, are not
activated, it is necessary to increase the temperature of the
oxidation catalyst 13, and the like, in order to activate the
oxidation catalyst 13, and the like. In addition, to burn
particulates accumulated on the particulate filter 16, it is
necessary to increase the temperature of the particulate filter 16
to approximately 600.degree. C. When SOx is released from the NOx
occlusion catalyst 17 as well, it is necessary to increase the
temperature of the NOx occlusion catalyst 17 to the SOx release
temperature, which is higher than or equal to 600.degree. C. FIG.
6A and FIG. 6B show amounts of fuel supplied from the fuel supply
valve 15 when flames H are generated to increase the temperatures
of the oxidation catalyst 13, particulate filter 16 and NOx
occlusion catalyst 17.
[0053] Note that FIG. 6A shows the amount of fuel supply when the
flow rate of exhaust gas is low, and FIG. 6B shows the amount of
fuel supply when the flow rate of exhaust gas is high. In both
cases, fuel is supplied at constant time intervals .DELTA.t. In
this case, in order for flame masses H not to overlap each other,
it is necessary that the interval .DELTA.t of the timings at which
fuel supply is started is extended as the flow rate of exhaust gas
decreases. Thus, as shown in FIG. 6C, as the flow rate of exhaust
gas decreases, that is, as the intake air amount Gad reduces, the
interval .DELTA.t of the timings at which fuel supply is started is
extended.
[0054] FIG. 7A and FIG. 7B show amounts of fuel supplied from the
fuel supply valve 15 when flames H are intermittently generated
while the air-fuel ratio of exhaust gas is made rich to release NOx
or SOx from the NOx occlusion catalyst 17. As shown in FIG. 6A and
FIG. 6B, when the temperatures of the oxidation catalyst 13,
particulate filter 16 and NOx occlusion catalyst 17 need to be
increased, fuel is supplied from the fuel supply valve 15 so that
the air-fuel ratio of exhaust gas that flows into these oxidation
catalyst 13, particulate filter 16 and NOx occlusion catalyst 17 is
made lean. In contrast, in the case shown in FIG. 7A and FIG. 7B,
fuel is supplied from the fuel supply valve 15 so that the air-fuel
ratio of exhaust gas that flows into the NOx occlusion catalyst 17
is intermittently made rich. Thus, in comparison with the case
shown in FIG. 6A and FIG. 6B, in the case shown in FIG. 7A and FIG.
7B, the amount of fuel supplied from the fuel supply valve 15 each
time is equal to or more than double.
[0055] For releasing NOx or SOx from the NOx occlusion catalyst 17,
when flames H are intermittently generated while the air-fuel ratio
of exhaust gas is made rich, that is, when flames of a rich
air-fuel ratio are intermittently generated, it is possible to
considerably favorably release NOx or SOx from the NOx occlusion
catalyst 17 and favorably reduce the released NOx or SOx. That is,
because generated flames consume almost entire oxygen contained in
exhaust gas, oxygen contained in the exhaust gas sharply reduces.
Thus, NOx is rapidly released from the NOx occlusion catalyst 17.
In addition, generated flames promote thermal decomposition to
produce strongly active reduction components, such as HC, CO, and
H.sub.2. Thus, NOx released from the NOx occlusion catalyst 17 is
favorably reduced. The same applies to SOx.
[0056] In this way, when NOx or SOx is released from the NOx
occlusion catalyst 17, it is desirable to intermittently generate
flames of a rich air-fuel ratio. Thus, in the present embodiment,
when NOx or SOx needs to be released from the NOx occlusion
catalyst 17, flames of a rich air-fuel ratio are intermittently
generated as much as possible.
[0057] Note that FIG. 7A shows the amount of fuel supply when the
flow rate of exhaust gas is low, and FIG. 7B shows the amount of
fuel supply when the flow rate of exhaust gas is high. In both
cases, fuel is injected at constant time intervals .DELTA.t. Note
that in the case as well, in order for flame masses H not to
overlap each other, it is necessary that the interval .DELTA.t of
the timings at which fuel supply is started is extended as the flow
rate of exhaust gas decreases. Thus, as shown in FIG. 7C, as the
flow rate of exhaust gas decreases, that is, as the intake air
amount Ga reduces, the interval .DELTA.t of the timings at which
fuel supply is started is extended.
[0058] Incidentally, when the small oxidation catalyst 14 is
activated, it is possible to easily increase the temperature of the
small oxidation catalyst 14 to 800.degree. C. or above. Thus, as is
apparent from FIG. 5, flames can be generated during light load,
including idling, or during deceleration. However, as an engine
load increases, the amount of fuel injected from the fuel injection
valves 3 increases to decrease the air-fuel ratio. Thus, the
concentration of oxygen contained in exhaust gas is considerably
lower than the concentration of oxygen during light load or during
deceleration (with fuel injection) shown in FIG. 5. As a result, it
is difficult to generate flames.
[0059] Then, in the present embodiment, when oxygen is insufficient
to generate flames, secondary air is supplied from the secondary
air supply device 45 into the engine exhaust passage. Specifically,
when the engine is operating at an intermediate load or at a high
load, the amount of secondary air necessary for adjusting the
concentration of oxygen contained in exhaust gas to, for example,
the oxygen concentration during light load or deceleration (with
fuel injection) shown in FIG. 5 is calculated. Secondary air of the
calculated amount is supplied from the secondary air supply nozzles
49.
[0060] Supplying secondary air when oxygen is insufficient to
generate flames in this way is carried out when the temperatures of
the oxidation catalyst 13, particulate filter 16 and NOx occlusion
catalyst 17 are increased. When secondary air is supplied at the
time when NOx or SOx needs to be released from the NOx occlusion
catalyst 17, there is a possibility that the air-fuel ratio of
exhaust gas cannot be maintained at a rich air-fuel ratio. Thus,
when oxygen is insufficient to generate flames at the time when NOx
or SOx needs to be released from the NOx occlusion catalyst 17, no
secondary air is supplied. In this case, no flame is generated to
allow a rich air-fuel ratio.
[0061] That is, in the present embodiment, when NOx or SOx needs to
be released from the NOx occlusion catalyst 17, it is determined
whether the operating state of the engine is such that flames of a
rich air-fuel ratio H can be generated. When NOx or SOx needs to be
released from the NOx occlusion catalyst 17, and when the operating
state of the engine is such that flames of a rich air-fuel ratio H
can be generated, flames H are generated. In contrast, when the
operating state of the engine is not such that flames of a rich
air-fuel ratio H can be generated at the time when NOx or SOx needs
to be released from the NOx occlusion catalyst 17, the air-fuel
ratio of exhaust gas that flows into the NOx occlusion catalyst 17
is made rich without generating flames H.
[0062] Note that in this case, in the present embodiment, when the
engine is operating at a light load or decelerating, it is
determined that the operating state of the engine is such that
flames of a rich air-fuel ratio can be generated. Thus, when the
engine is operating at a light load or decelerating at the time
when NOx or SOx needs to be released from the NOx occlusion
catalyst 17, flames of a rich air-fuel ratio are generated.
[0063] Note that, as is apparent from FIG. 5, flames can be
generated when the concentration of oxygen contained in exhaust gas
is, for example, higher than a predetermined oxygen concentration
indicated by Do in FIG. 5, that is, when the air-fuel ratio of
exhaust gas is higher than a predetermined air-fuel ratio. In other
words, in the present embodiment, when the air-fuel ratio of
exhaust gas is higher than the predetermined air-fuel ratio, it is
determined that the operating state of the engine is such that
flames of a rich air-fuel ratio can be generated.
[0064] Next, fuel supply control executed in the present embodiment
will be described with reference to FIG. 8 to FIG. 10. Note that
FIG. 8 to FIG. 10 each show the amount of fuel supplied from the
fuel supply valve 15, the amount of secondary air supply, and a
variation in temperature TC of the small oxidation catalyst 14. In
FIG. 8 to FIG. 10, time t0 represents time at which an instruction
for starting supply of fuel from the fuel supply valve 15 is issued
in order to increase temperature or other purposes. In addition,
FIG. 8 to FIG. 10 illustrate the case where the small oxidation
catalyst 14 is activated at a temperature of 200.degree. C.
[0065] First, the case in which flames are intermittently generated
to increase the temperatures of the oxidation catalyst 13,
particulate filter 16 and NOx occlusion catalyst 17 will be
described with reference to FIG. 8. As is apparent from the
temperature TC of the small oxidation catalyst 14, FIG. 8 shows the
case where the small oxidation catalyst 14 is not activated at time
t0. If fuel is supplied from the fuel supply valve 15 when the
small oxidation catalyst 14 is not activated, the supplied fuel
does not undergo oxidation reaction in the small oxidation catalyst
14 and cannot generate flames.
[0066] Thus, in this case, exhaust gas temperature increasing
control is carried out to increase the temperature of exhaust gas
until the small oxidation catalyst 14 is activated, and, when the
small oxidation catalyst 14 is activated, fuel is intermittently
supplied from the fuel supply valve 15. The exhaust gas temperature
increasing control is, for example, carried out by retarding the
timing at which fuel is injected into each combustion chamber 2.
Note that when oxygen is insufficient to generate flames, secondary
air is supplied while fuel is being intermittently supplied, that
is, while flame generation control is being carried out, as shown
in FIG. 8. Note that generation of flames is started while fuel is
being intermittently supplied, that is, during flame generation
control, and, during the flame generation control, the air-fuel
ratio of exhaust gas that flows into the oxidation catalyst 13,
particulate filter 16 and NOx occlusion catalyst 17 is maintained
at a lean air-fuel ratio.
[0067] FIG. 9 shows the case where flames of a rich air-fuel ratio
are generated to cause the NOx occlusion catalyst 17 to release NOx
during light load or during deceleration. In this case as well,
when the small oxidation catalyst 14 is not activated at time t0,
exhaust gas temperature increasing control is carried out, and,
when the small oxidation catalyst 14 is activated, flame generation
preparation control is carried out to increase the temperature of
the small oxidation catalyst 14 for generating flames. During the
flame generation preparation control, fuel of the same amount as
that during the flame generation control shown in FIG. 8 is
intermittently supplied, and the flame generation preparation
control continues until flames are generated. During the flame
generation preparation control, the air-fuel ratio of exhaust gas
that flows into the oxidation catalyst 13, particulate filter 16
and NOx occlusion catalyst 17 is maintained at a lean air-fuel
ratio.
[0068] As flames are generated, the amount of fuel supply is
increased, and the increased amount of fuel is intermittently
supplied. At this time, that is, during rich flame generation
control, flames of a rich air-fuel ratio are generated, and NOx is
released from the NOx occlusion catalyst 17. Note that, when SOx is
released from the NOx occlusion catalyst 17, the duration of the
rich flame generation control is longer than the case shown in FIG.
9.
[0069] FIG. 10 shows the case where NOx is released from the NOx
occlusion catalyst 17 when the engine is operating at an
intermediate or high load. When the engine is operating at an
intermediate or high load, the temperature of exhaust gas is high.
Thus, at this time, the small oxidation catalyst 14 is mostly
activated. In addition, when the engine is operating at an
intermediate or high load, the concentration of oxygen contained in
exhaust gas is low. Thus, it is difficult to generate flames of a
rich air-fuel ratio. Thus, at this time, as shown in FIG. 10, fuel
of the same amount as that during the rich flame generation control
of FIG. 9 is supplied from the fuel supply valve 15 at time t0, and
the air-fuel ratio of exhaust gas that flows into the NOx occlusion
catalyst 17 is intermittently made rich without generating
flames.
[0070] FIG. 11 shows an exhaust gas purification process routine.
The routine is executed by interrupt at constant time intervals.
Referring to FIG. 11, first, in step 60, an amount NOXA of NOx
occluded into the NOx occlusion catalyst 17 per unit time is
calculated. The NOx amount NOXA is stored beforehand in the ROM 32
in the form of a map shown in FIG. 12 as a function of a required
torque TQ and an engine rotational speed N. Subsequently, in step
61, the calculated NOXA is added to a NOx amount .SIGMA.NOX
occluded in the NOx occlusion catalyst 17. Then, in step 62, it is
determined whether the occluded NOx amount .SIGMA.NOX exceeds an
allowable value NX. When .SIGMA.NOX is smaller than or equal to NX,
the process proceeds to step 66.
[0071] In contrast, in step 62, when it is determined that
.SIGMA.NOX is larger than NX, the process proceeds to step 63. In
step 63, it is determined whether flames of a rich air-fuel ratio
can be generated, that is, the engine is operating at a light load
or decelerating. When it is determined that flames can be
generated, the process proceeds step 64. In step 64, the process of
intermittently generating flames of a rich air-fuel ratio H, that
is, the fuel supply control shown in FIG. 9, is carried out, and
.SIGMA.NOX is cleared. Subsequently, the process proceeds to step
66. In contrast, when it is determined that flames cannot be
generated, the process proceeds to step 65. In step 65, the process
of intermittently making the air-fuel ratio rich without generating
flames, that is, the fuel supply control shown in FIG. 10, is
carried out, and .SIGMA.NOX is cleared. Subsequently, the process
proceeds to step 66.
[0072] In step 66, a differential pressure .DELTA.P before and
after the particulate filter 16 is detected by the differential
pressure sensor 26. After that, in step 67, it is determined
whether the differential pressure .DELTA.P exceeds an allowable
value PX. When .DELTA.P is larger than PX, the process proceeds to
step 68. In step 68, an amount of secondary air necessary for
generating flames is calculated. Subsequently, in step 69, in order
to regenerate the particulate filter 16, the process of
intermittently generating flames H to increase the temperature of
the particulate filter 16, that is, the fuel supply control shown
in FIG. 8, is carried out.
* * * * *