U.S. patent application number 13/256563 was filed with the patent office on 2012-02-23 for exhaust purification system of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Takamitsu Asanuma, Daichi Imai, Yuka Nakata, Hiromasa Nishioka, Kazuhiro Umemoto.
Application Number | 20120042636 13/256563 |
Document ID | / |
Family ID | 42739361 |
Filed Date | 2012-02-23 |
United States Patent
Application |
20120042636 |
Kind Code |
A1 |
Asanuma; Takamitsu ; et
al. |
February 23, 2012 |
EXHAUST PURIFICATION SYSTEM OF INTERNAL COMBUSTION ENGINE
Abstract
The exhaust purification system of an internal combustion engine
is provided with an NO.sub.X selective reduction catalyst which is
arranged in an engine exhaust passage and which has the function of
absorbing NO.sub.X which is contained in exhaust gas when an
air-fuel ratio of inflowing exhaust gas is lean, releasing the
absorbed NO.sub.X when the air-fuel ratio of the inflowing exhaust
gas becomes a stoichiometric air-fuel ratio or rich, and
selectively reducing the NO.sub.X and with a fuel addition valve
which feeds fuel to the NO.sub.X selective reduction catalyst. In
the case of the region near the stoichiometric air-fuel ratio in
the region where the air-fuel ratio of the exhaust gas flowing into
the NO.sub.X selective reduction catalyst is lean, the fuel
addition valve is used to feed fuel to the NO.sub.X selective
reduction catalyst to selectively reduce the NO.sub.X.
Inventors: |
Asanuma; Takamitsu;
(Mishima-shi, JP) ; Nishioka; Hiromasa;
(Susono-shi, JP) ; Imai; Daichi; (Susono-shi,
JP) ; Nakata; Yuka; (Susono-shi, JP) ;
Umemoto; Kazuhiro; (Susono-shi, JP) |
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi
JP
|
Family ID: |
42739361 |
Appl. No.: |
13/256563 |
Filed: |
March 19, 2009 |
PCT Filed: |
March 19, 2009 |
PCT NO: |
PCT/JP2009/056209 |
371 Date: |
October 31, 2011 |
Current U.S.
Class: |
60/287 |
Current CPC
Class: |
F01N 3/2066 20130101;
F01N 2900/1404 20130101; F01N 3/0814 20130101; B01D 2258/014
20130101; Y02T 10/24 20130101; B01D 53/9422 20130101; F01N 3/0842
20130101; Y02A 50/20 20180101; F01N 2610/03 20130101; Y02T 10/40
20130101; Y02T 10/47 20130101; B01D 2255/91 20130101; B01D 2258/012
20130101; B01D 2251/208 20130101; Y02T 10/12 20130101; F01N 9/00
20130101; F01N 2900/1402 20130101; Y02A 50/2344 20180101; F01N
2570/14 20130101; Y02T 10/22 20130101; F01N 13/02 20130101; B01D
53/9477 20130101; F01N 3/101 20130101 |
Class at
Publication: |
60/287 |
International
Class: |
F01N 3/18 20060101
F01N003/18 |
Claims
1. An exhaust purification system of an internal combustion engine
which is provided with an NO.sub.X reduction catalyst which is
arranged in an engine exhaust passage and which has the function of
absorbing NO.sub.X which is contained in exhaust gas when an
air-fuel ratio of inflowing exhaust gas is lean, releasing the
absorbed NO.sub.X when the air-fuel ratio of the inflowing exhaust
gas becomes a stoichiometric air-fuel ratio or rich, and
selectively reducing the NO.sub.X and a reducing agent feed device
which feeds a reducing agent to the NO.sub.X reduction catalyst,
wherein, in the case of the region near the stoichiometric air-fuel
ratio in the region where the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X reduction catalyst is lean, the reducing
agent feed device is used to feed reducing agent to the NO.sub.X
reduction catalyst to selectively reduce the NO.sub.X.
2. An exhaust purification system of an internal combustion engine
as set forth in claim 1, further provided with a three-way catalyst
which is arranged downstream of the NO.sub.X reduction catalyst in
the engine exhaust passage, wherein the reducing agent feed device
includes a fuel addition valve which feeds fuel to the engine
exhaust passage at the upstream side of the NO.sub.X reduction
catalyst, and, if increasing the amount of fuel which is injected
at the combustion chambers of the engine body so as to lower the
air-fuel ratio of the exhaust gas flowing into the three-way
catalyst to the stoichiometric air-fuel ratio or less, the system
feeds fuel from the fuel addition valve to selectively reduce the
NO.sub.X at the NO.sub.X reduction catalyst when the air-fuel ratio
of the exhaust gas flowing into the NO.sub.X reduction catalyst is
inside the region near the stoichiometric air-fuel ratio.
3. An exhaust purification system of an internal combustion engine
which is provided with an NO.sub.X reduction catalyst which is
arranged in an engine exhaust passage and which has the function of
absorbing NO.sub.X which is contained in exhaust gas when an
air-fuel ratio of inflowing exhaust gas is lean, releasing the
absorbed NO.sub.X when the air-fuel ratio of the inflowing exhaust
gas becomes a stoichiometric air-fuel ratio or rich, and
selectively reducing the NO.sub.X, a three-way catalyst which is
arranged downstream of the NO.sub.X reduction catalyst, a reducing
agent feed device which feeds a reducing agent to the NO.sub.X
reduction catalyst, and an air-fuel ratio reducing device which
reduces the air-fuel ratio of the exhaust gas flowing into the
three-way catalyst, wherein, in the operating region of the
internal combustion engine, there is a specific operating region in
which the air-fuel ratio of the exhaust gas flowing into the
NO.sub.X reduction catalyst is lean, at the NO.sub.X reduction
catalyst, the NO.sub.X purification rate due to the selective
reduction gradually decreases along with a temperature rise and, at
the three-way catalyst, the NO.sub.X purification rate gradually
increases along with a temperature rise, and, in this specific
operating region, when the NO.sub.X purification rate of the
NO.sub.X reduction catalyst becomes smaller than a predetermined
judgment value, the reducing agent feed device feeds the reducing
agent to the NO.sub.X reduction catalyst for selective reduction of
NO.sub.X and the air-fuel ratio reducing device makes the air-fuel
ratio of the exhaust gas flowing into the three-way catalyst the
stoichiometric air-fuel ratio or rich for reduction of
NO.sub.X.
4. An exhaust purification system of an internal combustion engine
as set forth in claim 3, further provided with a temperature
raising device which raises a temperature of an NO.sub.X reduction
catalyst and an absorption amount detection device which detects an
absorption amount of NO.sub.X of an NO.sub.X reduction catalyst,
wherein, if the air-fuel ratio of the exhaust gas flowing into the
NO.sub.X reduction catalyst is lean, when a temperature of an
NO.sub.X reduction catalyst is lower than a judgment value of a low
temperature side for selective reduction and an absorption amount
of NO.sub.X of an NO.sub.X reduction catalyst is an allowable value
or more, the temperature raising device is used to raise the
NO.sub.X reduction catalyst, then the reducing agent feed device
feeds the reducing agent to the NO.sub.X reduction catalyst for
selective reduction of the NO.sub.X.
5. An exhaust purification system of an internal combustion engine
as set forth in claim 3, wherein, if the air-fuel ratio of the
exhaust gas flowing into the NO.sub.X reduction catalyst is lean,
when the temperature of an NO.sub.X reduction catalyst is higher
than a judgment value of a high temperature side for selective
reduction, the system makes the air-fuel ratio of the exhaust gas
flowing into the three-way catalyst the stoichiometric air-fuel
ratio or rich to reduce the NO.sub.X at the three-way catalyst.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purification
system of an internal combustion engine.
BACKGROUND ART
[0002] The exhaust gas of a diesel engine, gasoline engine, or
other internal combustion engine, for example, contains carbon
monoxide (CO), unburned fuel (BC), nitrogen oxides (NO.sub.X),
particulate matter (PM), or other ingredients. The internal
combustion engine has an exhaust purification system attached to it
to purify these ingredients.
[0003] Japanese Patent No 278304 discloses an exhaust purification
system provided with a device for removing nitrogen oxides
constituted by an NO.sub.X absorption and release material which
absorbs NO.sub.X when the air-fuel ratio of the exhaust gas is lean
and releases the absorbed NO.sub.X when the air-fuel ratio of the
exhaust gas is the stoichiometric air-fuel ratio or less. In this
system, it is disclosed, at the time of releasing and treating the
NO.sub.X, to make the air-fuel ratio of the exhaust gas the
stoichiometric air-fuel ratio or less and to further make the
temperature of the NO.sub.X absorption and release material rise so
as to improve the purification rate of NO.sub.X.
[0004] Japanese Patent Publication (A) No 2007-154764 discloses an
exhaust purification system of an internal combustion engine
arranging in an engine exhaust passage a post treatment system
which is comprised of an SO.sub.X trapping catalyst, a particulate
filter carrying an NO.sub.X storage reduction catalyst, and an
NO.sub.X storage reduction catalyst and a fuel feed valve for
feeding fuel for post treatment use to the post treatment system.
In this system, it is disclosed, when the degree of deterioration
of any catalyst exceeds a predetermined degree of deterioration, to
cause the catalyst with the lowest degree of deterioration to
perform the purification action of the exhaust gas.
[0005] The NO.sub.X which is contained in the exhaust gas can be
purified by reduction. To purify the NO.sub.X which is contained in
the exhaust gas of the engine body, sometimes a selective reduction
catalyst which can selectively reduce the NO.sub.X is arranged in
the engine exhaust passage. The exhaust purification system in
which the selective reduction catalyst is arranged can temporarily
store the NO.sub.X which is exhausted from the engine body in the
selective reduction catalyst when the selective reduction catalyst
is a low temperature. Further, by feeding the reducing agent to the
selective reduction catalyst in a predetermined temperature region,
it is possible to selectively reduce the NO.sub.X which is
exhausted from the engine body.
[0006] Further, to purify the NO.sub.X which is contained in the
exhaust gas of the engine body, sometimes a three-way catalyst is
arranged in the engine exhaust passage. When the temperature of the
three-way catalyst is the activation temperature or more and the
air-fuel ratio of the exhaust gas flowing into the three-way
catalyst is the stoichiometric air-fuel ratio or rich, a three-way
catalyst may be used to reduce the NO.sub.X.
[0007] In this regard, the selective reduction catalyst has less of
an ability to selectively reduce the NO.sub.X when the air-fuel
ratio of the inflowing exhaust gas is lean if the temperature is
higher than a predetermined temperature region. That is, sometimes
the purification rate of NO.sub.X of the selective reduction
catalyst becomes lower. Furthermore, the selective reduction
catalyst sometimes becomes lower in purification rate of NO.sub.X
in the region of an air-fuel ratio near the stoichiometric air-fuel
ratio in the region of a lean air-fuel ratio of the inflowing
exhaust gas. In this way, there was the problem that the selective
reduction catalyst becomes smaller in purification rate of NO.sub.X
in a predetermined operating region.
DISCLOSURE OF INVENTION
[0008] The present invention has as its object to provide an
exhaust purification system of an internal combustion engine which
suppresses a fall in purification ability of nitrogen oxides.
[0009] The first exhaust purification system of an internal
combustion engine of the present invention is provided with an
NO.sub.X reduction catalyst which is arranged in an engine exhaust
passage and which has the function of absorbing NO.sub.X which is
contained in exhaust gas when an air-fuel ratio of inflowing
exhaust gas is lean, releasing the absorbed NO.sub.X when the
air-fuel ratio of the inflowing exhaust gas becomes a
stoichiometric air-fuel ratio or rich, and selectively reducing the
NO.sub.X and with a reducing agent feed device which feeds a
reducing agent to the NO.sub.X reduction catalyst. In the case of
the region near the stoichiometric air-fuel ratio in the region
where the air-fuel ratio of the exhaust gas flowing into the
NO.sub.X reduction catalyst is lean, the reducing agent feed device
is used to feed a reducing agent to the NC reduction catalyst to
selectively reduce the NO.sub.X. Due to this configuration, ins the
region near the stoichiometric air-fuel ratio, it is possible to
suppress a drop in the purification ability of nitrogen oxides.
[0010] In this invention, a three-way catalyst is provided arranged
downstream of the NO.sub.X reduction catalyst in the engine exhaust
passage, and the reducing agent feed device includes a fuel
addition valve which feeds fuel to the engine exhaust passage at
the upstream side of the NO.sub.X reduction catalyst. If increasing
the amount of fuel which is injected at the combustion chambers of
the engine body so as to lower the air-fuel ratio of the exhaust
gas flowing into the three-way catalyst to the stoichiometric
air-fuel ratio or less, it is possible to feed fuel from the fuel
addition valve to selectively reduce the NO.sub.X at the NO.sub.X
reduction catalyst when the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X reduction catalyst is inside the region
near the stoichiometric air-fuel ratio.
[0011] The second exhaust purification system of an internal
combustion engine of the present invention is provided with an
NO.sub.X reduction catalyst which is arranged in an engine exhaust
passage and which has the function of absorbing NO.sub.X which is
contained in exhaust gas when an air-fuel ratio of inflowing
exhaust gas is lean, releasing the absorbed NO.sub.X when the
air-fuel ratio of the inflowing exhaust gas becomes a
stoichiometric air-fuel ratio or rich, and selectively reducing the
NO.sub.X, a three-way catalyst which is arranged downstream of the
NO.sub.X reduction catalyst, a reducing agent feed device which
feeds a reducing agent to the NO.sub.X reduction catalyst, and an
air-fuel ratio reducing device which reduces the air-fuel ratio of
the exhaust gas flowing into the three-way catalyst. In the
operating region of the internal combustion engine, there is a
specific operating region in which the air-fuel ratio of the
exhaust gas flowing into the NO.sub.X reduction catalyst is lean,
at the NO.sub.X reduction catalyst, the NO.sub.X purification rate
due to the selective reduction gradually decreases along with a
temperature rise and, at the three-way catalyst, the NO.sub.X
purification rate gradually increases along with a temperature
rise. In this specific operating region, when the NO.sub.X
purification rate of the NO.sub.X reduction catalyst becomes
smaller than a predetermined judgment value, the reducing agent
feed device feeds the reducing agent to the NO.sub.X reduction
catalyst for selective reduction of NO.sub.X, and the air-fuel
ratio reducing device makes the air-fuel ratio of the exhaust gas
flowing into the three-way catalyst the stoichiometric air-fuel
ratio or rich for reduction of NO.sub.X. Due to this constitution,
it is possible to suppress a drop in the purification ability of
nitrogen oxides.
[0012] In this invention, the system is further provided with a
temperature raising device which raises a temperature of an
NO.sub.X reduction catalyst and with an absorption amount detection
device which detects an absorption amount of NO.sub.X of an
NO.sub.X reduction catalyst. If the air-fuel ratio of the exhaust
gas flowing into the NO.sub.X reduction catalyst is lean, when a
temperature of an NO.sub.X reduction catalyst is lower than a
judgment value of a low temperature side for selective reduction
and an absorption amount of NO.sub.X of an NO.sub.X reduction
catalyst is an allowable value or more, the temperature raising
device may be used to raise the NO.sub.X reduction catalyst, then
the reducing agent feed device may feed the reducing agent to the
NO.sub.X reduction catalyst for selective reduction of the
NO.sub.X.
[0013] In this invention, if the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X reduction catalyst is lean, when the
temperature of an NO.sub.X reduction catalyst is higher than a
judgment value of a high temperature side for selective reduction,
it is possible to make the air-fuel ratio of the exhaust gas
flowing into the three-way catalyst the stoichiometric air-fuel
ratio or rich to reduce the NO.sub.X at the three-way catalyst.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 is a schematic view of an internal combustion engine
of an embodiment.
[0015] FIG. 2 is an enlarged schematic cross-sectional view of an
NO.sub.X selective reduction catalyst.
[0016] FIG. 3 shows graphs for explaining the properties of an
NO.sub.X selective reduction catalyst and a three-way catalyst in a
first embodiment.
[0017] FIG. 4 is a graph for explaining an operating region of an
exhaust purification system in a first embodiment.
[0018] FIG. 5 is a first flow chart for explaining control of an
exhaust purification system in a first embodiment.
[0019] FIG. 6 is a second flow chart for explaining control of an
exhaust purification system in a first embodiment.
[0020] FIG. 7 is a graph for explaining the relationship between an
NO.sub.X adsorption amount and an adsorption speed in an NO.sub.X
selective reduction catalyst.
[0021] FIG. 8 is a map of an NO.sub.X amount which is exhausted
from an engine body per unit time as a function of the engine speed
and amount of injection of fuel in a combustion chamber.
[0022] FIG. 9 is a map of an NO.sub.X amount which is adsorbed per
unit time as a function of a bed temperature of an NO.sub.X
selective reduction catalyst and an NO.sub.X amount which flows
into the NO.sub.X selective reduction catalyst.
[0023] FIG. 10 is an explanatory view of an injection pattern at a
time of normal operation.
[0024] FIG. 11 is an explanatory view of an injection pattern at
the time of after injection in a combustion chamber.
[0025] FIG. 12 is a graph for explaining an NO.sub.X purification
rate of an exhaust purification system in a first embodiment.
[0026] FIG. 13 is an enlarged schematic cross-sectional view of an
NO.sub.X storage reduction catalyst.
[0027] FIG. 14 is an explanatory view of an injection pattern at
the time of post injection in a combustion chamber.
[0028] FIG. 15 is a graph for explaining the relationship between
an air-fuel ratio of the exhaust gas and desorption of NO.sub.X in
an NO.sub.X selective reduction catalyst.
[0029] FIG. 16 is a graph for explaining a first operating example
in an exhaust purification system of an internal combustion engine
in a second embodiment.
[0030] FIG. 17 is a flow chart for explaining control of a first
operating engine of an exhaust purification system in a second
embodiment.
[0031] FIG. 18 is a time chart for explaining a second operating
example of an exhaust purification system in a second
embodiment.
BEST MODE FOR CARRYING OUT INVENTION
First Embodiment
[0032] Referring to FIG. 1 to FIG. 14, an exhaust purification
system of an internal, combustion engine in a first embodiment will
be explained.
[0033] FIG. 1 is an overall view of an internal combustion engine
in the present embodiment. In the present embodiment, the
explanation will be made taking as an example a compression
ignition type diesel, engine. The internal combustion engine is
provided with an engine body 1. Further, the internal combustion
engine is provided with an exhaust purification system. The engine
body 1 includes cylinders constituted by combustion chambers 2,
electronically controlled fuel injectors 3 for injecting fuel to
the respective combustion chambers 2, an intake manifold 4, and an
exhaust manifold 5.
[0034] The intake manifold 4 is connected through an intake duct 6
to an outlet of a compressor 7a of an exhaust turbocharger 7. An
inlet of the compressor 7a is connected through an intake air
detector 8 to an air cleaner 9. Inside the intake duct 6, a
throttle valve 10 which is driven by a step motor is arranged.
Furthermore, around the intake duct 6, a cooling device 11 is
arranged for cooling the intake air which flows through the inside
of the intake duct 6. In the embodiment shown in FIG. 1, the engine
cooling water is guided to the cooling device 11. The engine
cooling water is used to cool the intake air.
[0035] On the other hand, the exhaust manifold 5 is connected to an
inlet of an exhaust turbine 7b of the exhaust turbocharger 7. An
outlet of the exhaust turbine 7b is connected to an exhaust
purification system. The exhaust purification system is a system
which can purify exhaust gas which is exhausted from the engine
body 1.
[0036] The exhaust purification system in the present embodiment
includes an NO.sub.X reduction catalyst constituted by an NO.sub.X
selective reduction catalyst (SCR) 17. The NO.sub.X selective
reduction catalyst 17 can selectively reduce the NO.sub.X by
feeding a reducing agent. The NO.sub.X selective reduction catalyst
17 is connected through an exhaust pipe 12 to the outlet of the
exhaust turbine 7b. Further, the exhaust purification system in the
present embodiment includes a three-way catalyst 18. The three-way
catalyst 18 is arranged in the engine exhaust passage at the
downstream side of the NO.sub.X selective reduction catalyst 17.
The three-way catalyst 18 can oxidize CO and HC and, furthermore,
reduce the NO.sub.R.
[0037] In the engine exhaust passage upstream of the NO.sub.X
selective reduction catalyst 17, that is, in the exhaust pipe 12, a
fuel addition valve 13 is arranged as a reducing agent feed device
for feeding reducing agent to the NO.sub.X selective reduction
catalyst 17. In the present embodiment, the fuel of the engine body
1 is used as a reducing agent. The fuel addition valve 13 is formed
so as to have a fuel feed action which feeds or stops the feed of
fuel. The fuel addition valve 13 in the present embodiment is
formed so as to inject fuel.
[0038] In the engine exhaust passage at the upstream side of the
three-way catalyst 18, an air-fuel ratio reducing device
constituted by a fuel addition valve 14 is arranged. Here, in the
present invention, the ratio of the air and fuel (hydrocarbons) of
the exhaust gas which was fed to the engine intake passage,
combustion chambers, or engine exhaust passage is called the
air-fuel ratio of the exhaust gas (A/F). The fuel addition valve 14
can feed fuel to the engine exhaust passage so as to reduce the
air-fuel ratio of the exhaust gas flowing to the three-way catalyst
18. The fuel addition valve 14 in the present embodiment is formed
so as to inject fuel of the engine body 1.
[0039] Between the exhaust manifold 5 and the intake manifold 4, an
EGR passage 18 is arranged for exhaust gas recirculation (EGR).
Inside the EGR passage 18, an electronically controlled EGR control
valve 19 is arranged. Further, around the EGR passage 18, a cooling
device 20 is arranged for cooling the EGR gas which flows through
the inside of the EGR passage 18. In the embodiment shown in FIG.
1, the engine cooling water is guided to the inside of the cooling
device 20. The engine cooling water is used to cool the EGR
gas.
[0040] These fuel injectors 3 are connected through fuel feed tubes
21 to the common rail 22. The common rail 22 is connected through
an electronically controlled variable discharge fuel pump 23 to the
fuel tank 24. The fuel which is stored in the fuel tank 24 is fed
by the fuel pump 23 to the common rail 22. The fuel which was fed
to the inside of the common rail 22 is fed through these fuel feed
tubes 21 to the fuel injectors 3.
[0041] The electronic control unit 30 is comprised of a digital
computer. The electronic control unit 30 in the present embodiment
functions as a control device of the exhaust purification system.
The electronic control unit 30 includes components mutually
connected by a bidirectional bus 31 such as a ROM (read only
memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34,
input port 35, and output port 36.
[0042] Downstream of the NO.sub.X selective reduction catalyst 17,
a temperature sensor 26 is arranged as a temperature detection
device for detecting the temperature of the NO.sub.X selective
reduction catalyst 17. Downstream of the three-way catalyst 18, a
temperature sensor 27 is arranged as the temperature detection
device for detecting the temperature of the three-way catalyst 18.
Upstream of the NO.sub.X selective reduction catalyst 17, an
air-fuel ratio sensor 28 is arranged for detecting the air-fuel
ratio of the exhaust gas flowing into the NO.sub.X selective
reduction catalyst 17. The output signals of these temperature
sensors 26 and 27 and air-fuel ratio sensor 28 are input through
corresponding AD converters 37 to the input port 35.
[0043] An output signal of the intake air detector 8 is input
through a corresponding AD converter 37 to the input port 35. The
accelerator pedal 40 has a load sensor 41 connected to it to
generate output voltage which is proportional to an amount of
depression of the accelerator pedal 40. An output voltage of the
load sensor 41 is input through a corresponding AD converter to the
input port 35. Furthermore, the input port 35 has a crank angle
sensor 32 connected to it for generating an output pulse every time
the crankshaft rotates by for example 15.degree.. The output of the
crank angle sensor 42 can be used to detect the rotational speed of
the engine body.
[0044] On the other hand, the output port 36 is connected through
corresponding drive circuits 38 to the fuel injectors 3, a step
motor for driving the throttle valve 10, EGR control valve 19, and
fuel pump 23. Furthermore, the output port 36 is connected through
corresponding drive circuits 38 to the fuel addition valves 13 and
14. The fuel addition valves 13 and 14 in the present embodiment
are controlled by the electronic control unit 30.
[0045] FIG. 2 is an enlarged schematic cross-sectional view of an
NO.sub.X selective reduction catalyst in the present embodiment.
The NO.sub.X selective reduction catalyst 17 of the present
embodiment selectively reduces the NO.sub.X by feeding a reducing
agent constituted by HC. The NO.sub.X selective reduction catalyst
17 includes a catalyst metal 48 for promoting a reduction reaction
of NO.sub.X. The catalyst metal 48 in the present embodiment is
formed from silver (Ag). The catalyst metal is not limited to this
embodiment. It may be any metal enabling selective reduction of
NO.sub.X. As the catalyst metal, for example, platinum (Pt),
palladium (Pd), rhodium (Rh), iridium (Ir), or another precious
metal or copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), or
another base metal may be used.
[0046] The NO.sub.X selective reduction catalyst includes a
catalyst carrier 49 for holding the catalyst metal 48. The catalyst
carrier 49 in the present embodiment is formed on the surface of a
substrate. The catalyst carrier 49 is, for example, formed from
zeolite or aluminum oxide (Al.sub.2O.sub.3) or other porous
substance.
[0047] The NO.sub.X selective reduction catalyst has the function,
in a predetermined temperature region, of selectively reducing the
NO.sub.X in the presence of a suitable amount of HC or other
reducing agent. The NO.sub.X is broken down into N.sub.2 and
O.sub.2 by reduction. Furthermore, when the air-fuel ratio of the
exhaust gas is lean, in a predetermined temperature region, the
catalyst metal 48 of the selective reduction catalyst adsorbs the
NO.sub.X. The NO.sub.X is, for example, adsorbed at the catalyst
metal in the form of silver nitrate. The temperature region at
which the NO.sub.X is adsorbed is generally a region lower than the
temperature region at which the NO.sub.X is selectively reduced.
Further, the NO.sub.X selective reduction catalyst releases the
adsorbed NO.sub.X when the air-fuel ratio of the inflowing exhaust
gas becomes the stoichiometric air-fuel ratio or rich.
[0048] The three-way catalyst includes, as the catalyst metal,
platinum (Pt), palladium (Pd), rhodium (Rh), or other precious
metal. The precious metal is supported on aluminum oxide or another
catalyst carrier. The catalyst carrier is, for example, formed on
the surface of a honeycomb-shaped cordierite or other substrate.
The three-way catalyst purifies the three components of HC, CO, and
NO.sub.X with a high efficiency by the air-fuel ratio of the
inflowing exhaust gas being made about the stoichiometric air-fuel
ratio. The three-way catalyst falls in NO.sub.X reduction ability
when the air-fuel ratio of the inflowing exhaust becomes higher
than the stoichiometric air-fuel ratio. That is, when the air-fuel
ratio of the inflowing exhaust gas becomes lean, the NO.sub.X
purification rate falls.
[0049] FIG. 3 shows graphs for explaining the purification rate of
NO.sub.X of the NO.sub.X selective reduction catalyst and three-way
catalyst in the present embodiment. The top two graphs show the
properties of the NO.sub.X selective reduction catalyst. The bottom
most graph shows the property of the three-way catalyst. The graph
of the NO.sub.X selective reduction catalyst shows the property at
the time of the state where the air-fuel ratio of the inflowing
exhaust gas is lean. The graph of the three-way catalyst shows the
property at the time when the air-fuel ratio of the inflowing
exhaust gas is the stoichiometric air-fuel ratio or rich.
[0050] Here, in the present invention, "purification of NO.sub.X"
shows the removal of NO.sub.X from the inside of the exhaust gas
and includes the meanings of both absorption of NO.sub.X and
reduction of NO.sub.X. Further, in the present invention,
"absorption" includes physical adsorption, chemical adsorption,
storage, and deposition.
[0051] The NO.sub.X selective reduction catalyst can adsorb
NO.sub.X by the catalyst metal when the bed temperature is low. In
the operating region, the region A is the region where adsorption
is used to purify the NO.sub.X. In the present embodiment, in the
region where the purification rate of NO.sub.X by adsorption is
higher than the purification rate of NO.sub.X by selective
reduction, adsorption is used to purify the NO.sub.X. The
temperature T.sub.A is the bed temperature when the purification
rate of NO.sub.X by selective reduction and the purification rate
of NO.sub.X by adsorption become the same. The region A is the
operating region where the bed temperature of the NO.sub.X
selective reduction catalyst is less than the temperature
T.sub.A.
[0052] In the operating region, the region B is the region where
the NO.sub.X selective reduction catalyst selectively reduces the
NO.sub.X. In the present embodiment, as the region B, a region
where the purification rate of NO.sub.X by selective reduction
becomes the purification rate of NO.sub.X by adsorption or more is
selected. The region B is the region where the bed temperature of
the NO.sub.X selective reduction catalyst is the temperature
T.sub.A to the temperature T.sub.B. The judgment value of the
temperature of the low temperature side of the region B is the
temperature T.sub.A, while the judgment value of the temperature of
the high temperature side is the temperature T.sub.B. The
purification rate of NO.sub.X by selective reduction gradually
falls as the temperature T.sub.B is approached. Referring to FIG.
1, in the present embodiment, it is possible for the fuel addition
valve 13 to feed a reducing agent constituted by fuel so as to
selectively reduce the NO.sub.X.
[0053] The range of the region B for selective reduction is not
limited to the above. It is possible to select any temperature
range. For example, as the temperature of the low temperature side
T.sub.A used as the judgment value, it is possible to employ the
temperature where the purification rate of NO.sub.X by adsorption
falls to a predetermined value.
[0054] In this regard, if the bed temperature of the NO.sub.X
selective reduction catalyst rises, the NO.sub.X which was adsorbed
at the catalyst metal is desorbed. The bed temperature at which the
NO.sub.X is desorbed from the NO.sub.X selective reduction catalyst
is within the region where the purification rate of NO.sub.X by
selective reduction becomes smaller. In this temperature region,
the selective reduction function of the NO.sub.X selective
reduction catalyst falls and the desorption rate of the NO.sub.X
becomes larger.
[0055] On the other hand, the three-way catalyst has an activation
temperature for purifying the NO.sub.X. As the temperature becomes
higher from the activation temperature, the purification rate of
NO.sub.X gradually becomes higher. At a predetermined temperature,
the purification rate becomes constant. In the present embodiment,
the operating region above a temperature T.sub.C, which is higher
than the activation temperature and which can achieve a
predetermined NO.sub.X purification rate, is made the region C. In
the region C, it is possible to efficiently reduce the NO.sub.X by
the three-way catalyst. The selection of the region C, that is, the
selection of the temperature T.sub.C, is not limited to this mode.
It is possible to select any region where a three-way catalyst can
be used to reduce the NO.sub.X.
[0056] FIG. 4 shows a graph for explaining the operating region in
the exhaust purification system in the present embodiment. FIG. 4
shows a graph which schematically shows the relationship between
the bed temperatures of the catalysts of the NO.sub.X selective
reduction catalyst or three-way catalyst and the air-fuel ratios of
the exhaust gas flowing to the respective catalysts. The abscissa
shows the bed temperatures of these catalysts. The ordinate shows
the air-fuel ratios of the exhaust gas flowing to the respective
catalysts.
[0057] The operating region is divided into the region A, region B,
and region C plus the region D and region E. The region D is the
region where the air-fuel ratio of the exhaust gas flowing into the
three-way catalyst is lean and the bed temperature of the three-way
catalyst is the temperature T.sub.C or more. The region E is the
region where the air-fuel ratio of the exhaust gas flowing into the
three-way catalyst is the stoichiometric air-fuel ratio or rich and
the temperature is lower than the temperature T.sub.C.
[0058] Referring to FIG. 1, in the internal combustion engine of
the present embodiment, at the time of normal operation, the
air-fuel ratio of the exhaust gas exhausted from the engine body 1
is lean. Further, in the present embodiment, the bed temperature of
the NO.sub.X selective reduction catalyst 17 and the bed
temperature of the three-way catalyst 18 are substantially the
same. When the operating region of the internal combustion engine
is the region A and region B, mainly the NO.sub.X selective
reduction catalyst is used to purify the NO.sub.X. In the region C,
region D, and region E, mainly the three-way catalyst is used to
purify the NO.sub.X.
[0059] FIG. 5 and FIG. 6 show flow charts for explaining control in
the exhaust purification system in the present embodiment. FIG. 6
is the flow chart for explaining control at the time when the
air-fuel ratio of the exhaust gas flowing into the NO.sub.X
selective reduction catalyst is the stoichiometric air-fuel ratio
or rich.
[0060] First, at step 100, it is judged if the air-fuel ratio of
the exhaust gas which is exhausted from the engine body 1 is larger
than the stoichiometric air-fuel ratio. That is, it is judged if
the air-fuel ratio of the exhaust gas flowing into the NO.sub.X
selective reduction catalyst 17 is lean. In the present embodiment,
the air-fuel ratio sensor 28 is used to detect the air-fuel ratio
of the exhaust gas flowing into the NO.sub.X selective reduction
catalyst 17.
[0061] When, at step 100, the air-fuel ratio of the exhaust gas is
lean, the routine proceeds to step 101. At step 101, it is judged
if the bed temperature of the NO.sub.X selective reduction catalyst
is within a range from the temperature T.sub.A to the temperature
T.sub.B. That is, it is judged if the operating state of the
exhaust purification system is in the range of the region B. In the
present embodiment, the temperature sensor 26 which is arranged
downstream of the NO.sub.X selective reduction catalyst is used to
detect the bed temperature of the NO.sub.X selective reduction
catalyst 17.
[0062] When, at step 101, the bed temperature of the NO.sub.X
selective reduction catalyst is not within the range from the
temperature T.sub.A to the temperature T.sub.B, the routine
proceeds to step 105. At step 105, it is judged if the bed
temperature of the three-way catalyst 18 is the temperature T.sub.C
or more. That is, it is judged if the operating state of the
exhaust purification system is in the range of the region D. In the
present embodiment, the temperature sensor 27 which is arranged
downstream of the three-way catalyst 18 is used to detect the bed
temperature of the three-way catalyst 18.
[0063] If the bed temperature of the three-way catalyst is less
than the temperature T.sub.C, it is judged that the operating state
of the exhaust purification system is in the region A and the
routine proceeds to step 107. In this way, in the present
embodiment, when the air-fuel ratio of the exhaust gas flowing into
the NO.sub.X selective reduction catalyst is lean, it is judged
that the operating state is any region of the region A, region or
region C.
[0064] When, at step 101, it is judged if the bed temperature of
the NO.sub.X selective reduction catalyst is within the range from
the temperature T.sub.A, to the temperature T.sub.B, the routine
proceeds to step 102. The operating state is in the range of the
region B. At step 102, fuel is fed from the fuel addition valve 13
to enable the NO.sub.X selective reduction catalyst 17 to
selectively reduce the NO.sub.X.
[0065] Next, at step 103, it is judged if the purification rate of
the NO.sub.X selective reduction catalyst is less than a
predetermined judgment value RX. Referring to FIG. 3, in the
present embodiment, as this judgment value, the purification rate
RX is employed. The temperature T.sub.D is the bed temperature when
the NO.sub.X purification rate becomes RX. In the present
embodiment, it is judged if the bed temperature of the NO.sub.X
selective reduction catalyst is larger than the temperature
T.sub.D. When, at step 103, the purification rate of NO.sub.X is RX
or more, it is judged that purification by selective reduction is
sufficient and this control is ended. When the purification rate of
NO.sub.X is less than RX, the routine proceeds to step 104. That
is, when the bed temperature of the NO.sub.X selective reduction
catalyst is larger than the temperature T.sub.D, the routine
proceeds to step 104.
[0066] At step 104, it is judged if the bed temperature of the
three-way catalyst is the temperature T.sub.C or more. When the bed
temperature of the three-way catalyst is less than the temperature
T.sub.C, this control is ended. In this way, referring to FIG. 4,
when the exhaust purification system is operating in the region in
the region B which does not overlap with the region D, the NO.sub.X
is selectively reduced to purify the NO.sub.X.
[0067] Referring to FIG. 5, when, at step 104, it is judged that
the bed temperature of the three-way catalyst is the temperature
T.sub.C or more, the routine proceeds to step 106. Referring to
FIG. 4, when the exhaust purification system is operating in the
region where the region B and the region D overlap, the routine
proceeds to step 106. At step 106, control is performed to lower
the air-fuel ratio of the exhaust gas flowing into the three-way
catalyst.
[0068] In the present embodiment, referring to FIG. 1, fuel is fed
from the fuel addition valve 14 which is arranged at the upstream
side of the three-way catalyst 18 to control the air-fuel ratio of
the exhaust gas flowing into the three-way catalyst to the
stoichiometric air-fuel ratio. As shown by the arrow 91 in FIG. 4,
the operating state shifts from the region where the region B and
the region P overlap to the region C. By controlling the air-fuel
ratio of the exhaust gas flowing into the three-way catalyst to the
stoichiometric air-fuel ratio, it is possible to purify the
NO.sub.X at the three-way catalyst as well.
[0069] Referring to FIG. 3, in the region in the region B where the
temperature is high, the purification rate of NO.sub.X by selective
reduction falls. Furthermore, desorption of NO.sub.X from the
NO.sub.X selective reduction catalyst can occur. For this reason,
with purification of NO.sub.X by only the upstream side NO.sub.X
selective reduction catalyst, the purification rate becomes low.
However, when the bed temperature of the three-way catalyst is the
temperature T.sub.C or more, it is possible to make the air-fuel
ratio of the exhaust gas the stoichiometric air-fuel ratio to
enable the three-way catalyst to also purify the NO.sub.X. In this
way, in the region where the region B and the region D overlap,
both the upstream side NO.sub.X selective reduction catalyst and
the downstream side three-way catalyst can be used to reduce the
NO.sub.X to raise the purification rate of NO.sub.X.
[0070] Next, referring to FIG. 5, when, at step 105, the bed
temperature of the three-way catalyst is the temperature T.sub.C or
more, the routine proceeds to step 106. When, at step 106,
controlling the air-fuel ratio of the exhaust gas flowing into the
three-way catalyst to the stoichiometric air-fuel ratio, the
NO.sub.X is purified by the three-way catalyst. Referring to FIG.
4, when the exhaust purification system is operating in the region
in the region P which does not overlap with the region B, the
three-way catalyst is used to purify the NO.sub.X.
[0071] At step 106 of the present embodiment, control is performed
so that the air-fuel ratio of the exhaust gas flowing into the
three-way catalyst becomes the stoichiometric air-fuel ratio, but
the invention is not limited to this embodiment. Control may also
be performed so that the air-fuel ratio of the exhaust gas flowing
into the three-way catalyst becomes rich. However, the three-way
catalyst is preferably controlled to the stoichiometric air-fuel
ratio so that the purification rates of CO, HC, and NO.sub.X become
higher under conditions of an air-fuel ratio of the inflowing
exhaust gas of the stoichiometric air-fuel ratio.
[0072] When, at step 105, the bed temperature of the three-way
catalyst is less than the temperature T.sub.C, the routine proceeds
to step 107. When the operating state shown in FIG. 4 is the region
A, the NO.sub.X selective reduction catalyst adsorbs NO.sub.X so as
to purify the NO.sub.X. In this regard, in the NO.sub.X selective
reduction catalyst, sometimes the NO.sub.X adsorption amount is
large, so the NO.sub.X adsorption rate becomes small. At step 107,
it is judged if the NO.sub.X adsorption amount which is adsorbed at
the NO.sub.X selective reduction catalyst is a predetermined
allowable value or more.
[0073] FIG. 7 shows a graph for explaining the relationship between
the NO.sub.X adsorption amount and adsorption rate in the NO.sub.X
selective reduction catalyst. It is learned that if the NO.sub.X
adsorption amount increases, the adsorption rate of NO.sub.X
decreases. In the present embodiment, the point of the adsorption
rate RY where the adsorption speed of the NO.sub.X becomes slowed
to a predetermined value is made the allowable value of the
NO.sub.X adsorption amount at step 107. The exhaust purification
system in the present embodiment is provided with an absorption
amount detection device which detects the NO.sub.X absorption
amount of the NO.sub.X selective reduction catalyst. Next, the
absorption amount detection device in the present embodiment will
be explained.
[0074] FIG. 8 shows a map of the NO.sub.X amount which is exhausted
from the engine body per unit time in the present embodiment. For
example, a map of the released amount NOXA of NO.sub.X per unit
time is prepared in advance as a function of the engine speed N and
the injection amount TAQ of fuel which is injected into the
combustion chambers 2. This map is, for example, built in the ROM
32 of the electronic control unit 30. In the present embodiment,
the NO.sub.X amount which is exhausted from the engine body 1 and
the NO.sub.X amount which flows into the NO.sub.X selective
reduction catalyst become equal. Using this map, it is possible to
calculate the NO.sub.X amount flowing into the NO.sub.X selective
reduction catalyst per unit time which is calculated in accordance
with the operating state. Next, the NO.sub.X adsorption amount is
calculated from the NO.sub.X amount flowing into the NO.sub.X
selective reduction catalyst.
[0075] FIG. 9 shows a map of the NO.sub.X adsorption amount which
is adsorbed in the NO.sub.X selective reduction catalyst per unit
time in the present embodiment. For example, a map of the NO.sub.X
adsorption amount NOKB per unit time is prepared in advance as a
function of the bed temperature TSCR of the NO.sub.X selective
reduction catalyst and the NO.sub.X amount NOXA flowing into the
NO.sub.X selective reduction catalyst. This map is, for example,
built in the ROM 32 of the electronic control unit 30. Using this
map, it is possible to calculate the NO.sub.X adsorption amount per
unit time which is calculated in accordance with the operating
state. By cumulatively adding the NO.sub.X adsorption amount per
unit time, it is possible to calculate the NO.sub.X amount which is
adsorbed at the NO.sub.X selective reduction catalyst at any
time.
[0076] The absorption amount detection device which detects the
absorption amount of NO.sub.X is not limited to this mode. Any
configuration may be used to detect the absorption amount of
NO.sub.X. For example, NO.sub.X sensors are arranged at the
upstream side and the downstream side of the NO.sub.X selective
reduction catalyst. These detect the NO.sub.X amount flowing into
the NO.sub.X selective reduction catalyst per unit time and the
NO.sub.X amount flowing out from the NO.sub.X selective reduction
catalyst per unit time. It is possible to use the difference in
outputs of these NO.sub.X sensors to calculate the NO.sub.X amount
which is absorbed in the NO.sub.X selective reduction catalyst per
unit time.
[0077] Referring to FIG. 5, when, at step 107, the NO.sub.X
adsorption amount is less than the allowable value, this control is
ended. That is, in the region A of FIG. 4, when the adsorbable
amount of NO.sub.X is larger than a predetermined value, adsorption
by the NO.sub.X selective reduction catalyst is used to purify the
NO.sub.X. When, at step 107, the NO.sub.X adsorption amount is the
allowable value or more, the routine proceeds to step 108.
[0078] At step 108, the NO.sub.X selective reduction catalyst is
raised in temperature from the region A to the region B. That is,
the NO.sub.X selective reduction catalyst is raised in temperature
in the temperature region where it is possible to obtain a
sufficient purification rate of NO.sub.X by selective reduction of
NO.sub.X. Referring to FIG. 3, for example, the bed temperature of
the NO.sub.X selective reduction catalyst is raised to within a
range from the temperature T.sub.A to the temperature T.sub.D.
[0079] The exhaust purification system in the present embodiment
include a temperature raising device for raising the temperature of
the NO.sub.X selective reduction catalyst. The temperature raising
device in the present embodiment includes the fuel injectors 3 and
the electronic control unit 30 of the engine body 1. The injection
pattern in the combustion chambers 2 of the engine body 1 is
changed to raise the temperature of the exhaust gas exhausted from
the engine body 1. By raising the temperature of the exhaust gas,
NO.sub.X selective reduction catalyst 17 is also raised in
temperature. Here, the change of the injection pattern in the
combustion chambers will be explained.
[0080] FIG. 10 shows the injection pattern of fuel at the time of
normal operation of the internal combustion engine in the present
embodiment. The injection pattern A is an injection pattern of fuel
at the time of normal operation. At the time of normal operation,
the main injection FM is performed at about compression top dead
center TDC. The main injection FM is performed at a crank angle of
about 0.degree.. Further, to stabilize the combustion in the main
injection FM, pilot injection FP is performed before the main
injection FM. The pilot injection FP, for example, is performed in
a range of crank angle of from about 10.degree. to about 40.degree.
before compression top dead center TAG. At the time of normal
operation, as shown by the injection pattern B, it is also possible
not to perform pilot injection FP and to perform only main
injection FM. In the present embodiment, the explanation is given
with reference to the example of an injection pattern at which
pilot injection FP is performed. At the time of normal operation,
when being operated by the injection pattern A, the air-fuel ratio
of the exhaust gas which is exhausted from the engine body is
lean.
[0081] FIG. 11 shows the injection pattern when raising the
temperature of the exhaust gas which is exhausted from the engine
body. In the injection pattern C, the injection timing of the main
injection FM is retarded from compression top dead center TDC. That
is, the injection timing of the main injection FM is retarded.
Along with the retardation of the injection timing of the main
injection FM, the injection timing of the pilot injection FP is
also retarded. By retarding the injection timing of the main
injection FM, it is possible to raise the temperature of the
exhaust gas.
[0082] Furthermore, after the main injection FM, as auxiliary
injection, after injection FA is performed. The after injection FA
is performed in a combustible period of time after the main
injection. The after injection FA is, for example, performed in the
range of a crack angle after compression top dead center up to
about 40.degree.. For example, this is performed in the range of a
crank angle after compression top dead center of about 20.degree.
to about 30.degree.. By performing after injection FA, the
afterburn period becomes longer, so the temperature of the exhaust
gas can be raised. By changing the injection pattern in the
combustion chamber in this way, it is possible to raise the
temperature of the exhaust gas which is exhausted from the engine
body. The temperature raising device for raising the temperature of
the NO.sub.X selective reduction catalyst is not limited to this
mode. It is possible to employ any device which can raise the
temperature of an NO.sub.X reduction catalyst.
[0083] Referring to FIG. 5, at step 108, the NO.sub.X selective
reduction catalyst is raised in temperature to the region B, then
the routine proceeds to step 109. At step 109, the fuel addition
valve 13 of the upstream side of the NO.sub.X Selective reduction
catalyst is used to add fuel to thereby selectively reduce the
NO.sub.X. In this way, the NO.sub.X is purified by selective
reduction at the region B shown in FIG. 4.
[0084] In the present embodiment, if the exhaust purification
system is operating in the region A, the NO.sub.X selective
reduction catalyst reduces the NO.sub.X when the NO.sub.X
adsorption amount becomes the allowable value or more, but the
invention is not limited to this embodiment. A three-way catalyst
may also be used for reduction. For example, the temperature of the
three-way catalyst is raised to the activation temperature or more.
Furthermore, it is possible to make the air-fuel ratio of the
exhaust gas flowing into the three-way catalyst the stoichiometric
air-fuel ratio or rich so as to reduce the NO.sub.X at the
three-way catalyst. At this time, when the temperature of the
NO.sub.X selective reduction catalyst becomes the NO.sub.X release
temperature or more, the NO.sub.X which is adsorbed at the NO.sub.X
selective reduction catalyst is released. The released NO.sub.X can
be reduced together with the NO.sub.X which is exhausted from the
engine body at the three-way catalyst.
[0085] At step 100 of FIG. 5, when the air-fuel ratio of the
exhaust gas flowing into the NO.sub.X selective reduction catalyst
is the stoichiometric air-fuel ratio or rich, the routine proceeds
to step 111 of FIG. 6. At step 111, it is judged if the bed
temperature of the three-way catalyst is the temperature T.sub.C or
more. If the bed temperature of the three-way catalyst is the
temperature T.sub.C or more, the NO.sub.X can already be reduced by
the three-way catalyst, so this control is ended. Referring to FIG.
4, in the case where the exhaust purification system is operating
in the region C, the three-way catalyst can purify the NO.sub.X.
For this reason, the operation in the region C is continued. At
this time, the bed temperature of the three-way catalyst has not
sufficiently risen, so when not sufficiently activated, it is also
possible to raise the temperature of the three-way catalyst. The
temperature of the three-way catalyst can be raised, for example,
in the same way as the temperature of the NO.sub.X selective
reduction catalyst is raised, by changing the injection pattern in
the combustion chambers.
[0086] When, at step 111, the bed temperature of the three-way
catalyst is less than the temperature T.sub.C, the routine proceeds
to step 112. At step 112, the three-way catalyst is raised to the
temperature T.sub.C or more. Referring to FIG. 4, when the exhaust
purification system is operating in the region E, the system is
shifted to the region C. The temperature of the three-way catalyst
is raised to reduce the NO.sub.X at the three-way catalyst.
[0087] FIG. 12 shows a graph for explaining the purification rate
of NO.sub.X in the exhaust purification system in the present
embodiment. The abscissa shows the bed temperature of the
catalysts. The ordinate show the purification rate of the exhaust
purification system as a whole. The exhaust purification system in
the present embodiment purifies NO.sub.X in the NO.sub.X selective
reduction catalyst and additionally purifies NO.sub.X in the
three-way catalyst in the region where the region B and the region
C overlap, so can obtain a high purification rate of NO.sub.X.
[0088] In the present embodiment, the explanation was given with
reference to an NO.sub.X selective reduction catalyst, which can
selectively reduce the NO.sub.X by feeding HC, as the NO.sub.X
reduction catalyst, but the invention is not limited to this
embodiment. The NO.sub.X reduction catalyst may be any catalyst
which has the function of absorbing the NO.sub.X which is contained
in exhaust gas when the air-fuel ratio of the inflowing exhaust gas
is lean, releasing the absorbed NO.sub.X when the air-fuel ratio of
the inflowing exhaust gas is the stoichiometric air-fuel ratio or
rich, and further selectively reducing the NO.sub.X. For example,
the NO.sub.X reduction catalyst may include an NO.sub.X storage
reduction catalyst.
[0089] FIG. 13 is an enlarged schematic cross-sectional view of an
NO.sub.X storage reduction catalyst. The NO.sub.X storage reduction
catalyst is comprised of a substrate on which for example a
catalyst carrier 45 comprised of alumina is formed. On the surface
of the catalyst carrier 45, a precious metal 46 is carried
dispersed. On the surface of the catalyst carrier 45, a layer of an
NO.sub.X absorbent 47 is formed. The precious metal 46, for
example, includes platinum (Pt). As the ingredients forming the
NO.sub.X absorbent 47, for example, at least one ingredient
selected from potassium (K), sodium (Na), cesium (Cs), or other
such alkali metal, barium (Ba), calcium (Ca), or other such alkali
earth, or lanthanum (La), yttrium (Y), or other such rare earth is
used.
[0090] In the NO.sub.X storage reduction catalyst, when the
air-fuel ratio of the exhaust gas is lean, the NO which is
contained in the exhaust gas is oxidized on the precious metal 46
and becomes NO.sub.2. NO.sub.2 is stored in the form of nitrate
ions NO.sub.3.sup.- inside of the NO.sub.X absorbent 47. As opposed
to this, when the air-fuel ratio of the exhaust gas is rich or the
stoichiometric air-fuel ratio, the nitrate ions NO.sub.3.sup.-
inside of the NO.sub.X absorbent 47 are released in the form of
NO.sub.2 from the NO.sub.X absorbent 47. The released NO.sub.X is
reduced to N.sub.2 by the unburned hydrocarbons and carbon monoxide
which are contained in the exhaust gas.
[0091] The NO.sub.X storage reduction catalyst carries a precious
metal for causing selective reduction, so can selectively reduce
the NO.sub.X by the feed of a reducing agent. In particular, even
when the NO.sub.X storage reduction catalyst has deteriorated, the
function of selectively reducing the NO.sub.X remains. In this way,
the NO.sub.X storage reduction catalyst has the function of
selectively reducing the NO.sub.X, so the present invention can be
applied even to a system where the NO.sub.X reduction catalyst
includes an NO.sub.X storage reduction catalyst.
[0092] In the present embodiment, as the reducing agent feed device
for feeding a reducing agent to the NO.sub.X selective reduction
catalyst, a fuel addition valve is arranged. The reducing agent
feed device is not limited to this mode and may feed a reducing
agent to the NO.sub.X selective reduction catalyst. For example,
the reducing agent feeding means includes the fuel injectors 3 of
the engine body 1. It is also possible to change the injection
pattern in the combustion chambers so as to feed unburned fuel to
the NO.sub.X selective reduction catalyst.
[0093] FIG. 14 shows the injection pattern when feeding unburned
fuel to the NO.sub.X selective reduction catalyst. The injection
pattern D comprises main injection FM, then post injection FPO. The
post injection FPO is injection in which fuel is not burned in the
combustion chambers. The post injection FPO is auxiliary injection
in the same way as after injection. The after injection has an
effect on the engine output, while the post injection does not
contribute to the engine output. The post injection FPO is
performed, for example, in the range of a crank angle after
compression top dead center of about 90.degree. to about
120.degree.. By performing post injection in the combustion
chambers, it is possible to feed unburned fuel to the NO.sub.X
selective reduction catalyst.
[0094] Further, in the present embodiment, as an air-fuel ratio
reducing device which makes the air-fuel ratio of the exhaust gas
flowing into the three-way catalyst smaller, a fuel addition valve
is arranged. The air-fuel ratio reducing device is not limited to
this mode. It may also be configured to lower the air-fuel ratio of
the mixture flowing into the three-way catalyst.
[0095] For example, the air-fuel ratio reducing device may include
a fuel addition valve arranged at the upstream side of the NO.sub.X
selective reduction catalyst. By feeding unburned fuel from the
upstream side of the NO.sub.X selective reduction catalyst, the
air-fuel ratio of the exhaust gas flowing into the three-way
catalyst can be made smaller. Alternatively, by performing post
injection at the combustion chambers, the air-fuel ratio of the
exhaust gas flowing into the three-way catalyst can be made
smaller. However, if feeding a large amount of unburned fuel to the
NO.sub.X selective reduction catalyst, sometimes HC poisoning
occurs and the NO.sub.X selective reduction catalyst falls in
purification rate. For this reason, a system for feeding fuel is
preferably arranged at the downstream side of the NO.sub.X
selective reduction catalyst.
[0096] Further, an oxygen additive valve for feeding oxygen may be
arranged in the engine exhaust passage at the upstream side of the
three-way catalyst. For example, an air feed valve may be arranged
for feeding air into the engine exhaust passage. The three-way
catalyst is designed so as to exhibit a superior oxidation
performance and reduction performance near the stoichiometric
air-fuel ratio. For this reason, when the air-fuel ratio of the
exhaust gas flowing out from the NO.sub.x selective reduction
catalyst is deeply rich, it is possible to feed air from the air
feed valve to make the air-fuel ratio approach the stoichiometric
air-fuel ratio. As a result, it is possible to perform superior
purification at the three-way catalyst.
Second Embodiment
[0097] Referring to FIG. 1 and FIG. 15 to FIG. 18, an exhaust
purification system of an internal combustion engine in a second
embodiment can be explained. The configuration of the exhaust
purification system of an internal combustion engine in the present
embodiment is similar to the exhaust purification system in the
first embodiment. Referring to FIG. 1, the NO.sub.X selective
reduction catalyst 17 is arranged in the engine exhaust passage,
while the three-way catalyst 18 is arranged downstream of the
NO.sub.X selective reduction catalyst 17.
[0098] FIG. 15 shows a graph explaining the relationship between
the air-fuel ratio of the exhaust gas flowing into the NO.sub.X
selective reduction catalyst and the NO.sub.X amount which is
desorbed from the NO.sub.X selective reduction catalyst. The
inventors discovered that in the slightly lean region near the
stoichiometric air-fuel ratio in the region where the air-fuel
ratio of the inflowing exhaust gas is lean, the absorbed NO.sub.X
is desorbed, so the NO.sub.X purification rate falls. In the region
near the stoichiometric air-fuel ratio, the NO.sub.X is gradually
desorbed along with the fall of the oxygen concentration. In the
present invention, the lean region adjoining the stoichiometric
air-fuel ratio where the absorbed NO.sub.X is desorbed is called
the "desorption region". In the desorption region, for example, the
air-fuel ratio of the exhaust gas flowing into the catalyst is
about 18 or less, that is, larger than 14.7 (stoichiometric
air-fuel ratio).
[0099] Referring to FIG. 1, in the present embodiment, when the
NO.sub.X selective reduction catalyst is operating in the
desorption region, the fuel addition valve 13 which is arranged
upstream of the NO.sub.X selective reduction catalyst 17 is used to
feed a reducing agent to the NO.sub.X selective reduction catalyst
so as to selectively reduce the desorbed NO.sub.X.
[0100] FIG. 16 shows a graph for explaining a first operating
example in the present embodiment. In the first operating example,
when operating in the region A, the NO.sub.X amount which is
adsorbed at the NO.sub.X selective reduction catalyst exceeds a
predetermined allowable value. At this time, control is performed
to make the NO.sub.X selective reduction catalyst release the
NO.sub.X and to reduce the released NO.sub.X. In the present
embodiment, as shown by the arrow 92, control is performed to make
the operating state of the exhaust purification system shift from
the region A to the region C.
[0101] By making the air-fuel ratio of the exhaust gas flowing into
the NO.sub.X selective catalyst the stoichiometric air-fuel ratio
or rich, the NO.sub.X which is absorbed at the NO.sub.X selective
reduction, catalyst is exhausted. In the present embodiment, the
NO.sub.X is released by shifting the air-fuel ratio of the exhaust
gas flowing into the NO.sub.X selective reduction catalyst from
lean to rich. Further, by making the temperature of the three-way
catalyst 18 the activation temperature or more, the released
NO.sub.X is purified by the three-way catalyst. That is, by
shifting from the region A to the region C, NO.sub.X is released
from the NO.sub.X selective reduction catalyst and NO.sub.X is
reduced at the three-way catalyst.
[0102] Referring to FIG. 1, in the present embodiment, after
injection is performed in the combustion chamber 2 in the engine
body 1 (see FIG. 11). Furthermore, the throttle valve 10 of the
engine intake passage is throttled back to reduce the amount of air
flowing into the combustion chamber 2 for rich combustion control.
By reducing the opening degree of the throttle valve 10, it is
possible to make the air-fuel ratio of the exhaust gas which is
exhausted from the combustion chamber 2 the stoichiometric air-fuel
ratio or rich. By making the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X selective reduction catalyst the
stoichiometric air-fuel ratio or rich, it is possible to release
the NO.sub.X. In the present embodiment, the air-fuel ratio of the
exhaust gas which is exhausted from the combustion chamber is made
rich.
[0103] Further, by performing after injection in the combustion
chambers, the temperature of the exhaust gas rises and the
three-way catalyst can be made the activation temperature or more.
For this reason, the three-way catalyst can reduce the NO.sub.X
which is released from the NO.sub.X selective reduction
catalyst.
[0104] At least part of the fuel of the after injection is burned
in the combustion chambers. By having at least part of the fuel
burned, the light unburned hydrocarbons (HC) and CO etc. which are
contained in the exhaust gas are increased. It is possible to feed
the engine exhaust passage light unburned hydrocarbons (HC), CO,
etc. as a reducing agent. Light unburned hydrocarbons, CO, etc. are
superior in reducibility, so are preferable as a reducing
agent.
[0105] Referring to FIG. 16, when performing after injection in the
combustion chambers and, furthermore, throttling back the throttle
valve to adjust the air-fuel ratio of the exhaust gas for rich
combustion control, the air-fuel ratio of the exhaust gas gradually
becomes smaller. For this reason, there is a time period during
which the air-fuel ratio of the exhaust gas flowing into the
NO.sub.X selective reduction catalyst is within the desorption
region. In the present embodiment, by feeding a reducing agent to
the NO.sub.X selective reduction catalyst in the desorption region,
the NO.sub.X which is desorbed from the NO.sub.X selective
reduction catalyst is selectively reduced.
[0106] FIG. 17 is a flow chart of control of the exhaust
purification system of an internal combustion engine in the present
embodiment. At step 201, it is judged if rich combustion control is
being performed in the combustion chambers. At step 201, in the
case of a period during the rich combustion control, the routine
proceeds to step 202. At step 202, it is judged if the air-fuel
ratio of the exhaust gas is inside the desorption region. At step
202, it is judged if the air-fuel ratio of the exhaust gas flowing
into the NO.sub.X selective reduction catalyst 17 is larger than
the stoichiometric air-fuel ratio. Furthermore, it is judged if the
air-fuel ratio of the exhaust gas flowing into the NO.sub.X
selective reduction catalyst 17 is a predetermined judgment value
or less. As this judgment value, for example, a value at the end of
the desorption region where the air-fuel ratio is large can be
used.
[0107] When, at step 202, the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X selective reduction catalyst 17 is within
the range of the desorption region, this control is ended. When the
air-fuel ratio of the exhaust gas flowing into the NO.sub.X
selective reduction catalyst 17 is within the range of the
desorption region, the routine proceeds to step 203.
[0108] At step 203, a reducing agent is fed to the NO.sub.X
selective reduction catalyst 17. In the present embodiment, fuel is
injected from the fuel addition valve 13. In the present
embodiment, after injection is performed in the combustion chamber
2, the temperature of the exhaust gas which is exhausted from the
engine body 1 rises. For this reason, the temperature of the
NO.sub.X selective reduction catalyst 17 can be raised to a
temperature where selective reduction is possible. By injecting
fuel from the fuel addition valve 13, it is possible to reduce the
NO.sub.X which is desorbed from the NO.sub.X selective reduction
catalyst.
[0109] By feeding fuel from the fuel addition valve, it is possible
to shift to the operating state enabling reduction of NO.sub.X in a
short period of time. For example, as the change of the combustion
pattern etc., the air-fuel ratio of the exhaust gas changes by a
relatively slow speed. As opposed to this, the response in
injection from the fuel addition valve is high. Even when the time
period during which the air-fuel ratio of the exhaust gas is in the
desorption region or the desorbed NO.sub.X is slight, it is
possible to reliably reduce the NO.sub.X.
[0110] After step 203 ends, the routine proceeds to step 201. At
step 201, this control is ended when not during rich combustion
control.
[0111] In the present embodiment, the explanation was given with
reference to the example of operation passing through the
desorption region during the period when rich combustion control is
being performed, but the invention is not limited to this
embodiment. The present invention can be applied to the time, if
performing any operation, the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X reduction catalyst becomes within the
desorption region. In particular, the present invention exhibits a
remarkable effect when the time during which the exhaust
purification system operates in the desorption region is long.
[0112] In the present embodiment, to lower the air-fuel ratio of
the exhaust gas flowing into the NO.sub.X selective reduction
catalyst, after injection is performed in a combustion chamber, but
the invention is not limited to this embodiment. Post injection may
also be performed. In this way, by increasing the amount of fuel
which is injected into the combustion chamber, it is possible to
lower the air-fuel, ratio of the exhaust gas flowing into the
NO.sub.X selective reduction catalyst.
[0113] FIG. 18 shows the time chart for explaining the second
operating example in the present embodiment. The second operating
example is the operating example when the air-fuel ratio of the
exhaust gas flowing into the NO.sub.X selective reduction catalyst
becomes within a range of the desorption region for a while. The
second operating example is an example where the vehicle in which
the internal combustion engine is arranged is accelerating.
[0114] The vehicle is driven by a certain speed until the time
t.sub.1. The vehicle is accelerated from the time t.sub.1 to the
time t.sub.4. The air-fuel ratio of the exhaust gas flowing into
the NO.sub.X selective reduction catalyst becomes smaller from the
time t.sub.1 to the time t.sub.1. At this time, in the period from
the time t.sub.2 to the time t.sub.3, the air-fuel ratio of the
exhaust gas is a value in the desorption region.
[0115] Referring to FIG. 1, the air-fuel ratio of the exhaust gas
flowing into the NO.sub.X selective reduction catalyst 17 can be
detected by the air-fuel ratio sensor 28. The exhaust purification
system detects when the air-fuel ratio of the exhaust gas flowing
into the NO.sub.X selective reduction catalyst 17 becomes a value
in the desorption region. In this case, it is possible to feed fuel
from the fuel addition valve 13 at the upstream side of the
NO.sub.X selective reduction catalyst 17 so as to reduce the
desorbed NO.sub.X. In the present embodiment, the NO.sub.X
selective reduction catalyst is continuously fed fuel from the fuel
addition valve during the period in which the air-fuel ratio of the
inflowing exhaust gas is a value in the desorption region. When
feeding fuel from the fuel addition valve to the NO.sub.X selective
reduction catalyst, it is also possible to intermittently feed the
fuel.
[0116] The comparative example is an example where the NO.sub.X
selective reduction catalyst is not fed a reducing agent. In the
comparative example, when the air-fuel ratio of the exhaust gas is
inside of the desorption region, the NO.sub.X amount which is
exhausted from the NO.sub.X selective reduction catalyst becomes
greater. As opposed to this, when the air-fuel ratio of the exhaust
gas is inside the desorption region, by feeding the NO.sub.X
selective reduction catalyst a reducing agent, it is possible to
reduce the NO.sub.X, amount which is exhausted from the NO.sub.X
selective reduction catalyst.
[0117] The present embodiment selectively reduces the NO.sub.X
which is exhausted from the desorption region inside of the
NO.sub.X selective reduction catalyst so as to purify the NO.sub.x,
but the invention is not limited to this embodiment. It is also
possible to arrange a system which purifies the NO.sub.X which is
exhausted from the NO.sub.X selective reduction catalyst at the
downstream side of the NO.sub.X selective reduction catalyst.
[0118] Further, in the present embodiment, as the NO.sub.X
reduction catalyst, an NO.sub.X selective reduction catalyst is
arranged, but the invention is not limited to this embodiment. The
NO.sub.X reduction catalyst may be any catalyst which has the
function of absorbing the NO.sub.X which is contained in the
exhaust gas when the air-fuel ratio of the inflowing exhaust gas is
lean, releasing the absorbed NO.sub.X when the air-fuel ratio of
the inflowing exhaust gas is the stoichiometric air-fuel ratio or
rich, and, furthermore, selectively reducing the NO.sub.X.
[0119] For example, the NO.sub.X reduction catalyst can include en
NO.sub.X storage reduction catalyst. In the NO.sub.X storage
reduction catalyst as well, NO.sub.X is stored in the NO.sub.X
absorbent and is adsorbed at the catalyst metal. For this reason,
when the air-fuel ratio of the inflowing exhaust gas is in the
desorption region, desorption of NO.sub.X occurs. At this time, by
using the reducing agent feed device to feed a reducing agent, it
is possible to selectively reduce the NO.sub.X.
[0120] In the NO.sub.X storage reduction catalyst, the NO.sub.X is
mainly stored in the NO.sub.X absorbent. For this reason, the
desorbed amount of NO.sub.X in the desorption region becomes
greater at the NO.sub.X selective reduction catalyst than the
NO.sub.X storage reduction catalyst. For this reason, when the
exhaust purification system is provided with an NO.sub.X selective
reduction catalyst, the advantageous effect of the present
invention becomes remarkable. That is, in the NO.sub.X selective
reduction catalyst, the NO.sub.X is easily desorbed at the
desorption region, so the effect of suppressing the exhaust of
NO.sub.X of the present invention becomes remarkable.
[0121] The rest of the configuration, the action, and the effects
are similar to the first embodiment, so here the explanations will
not be repeated.
[0122] The above embodiments may be suitably combined. In the above
figures, the same or corresponding parts are assigned the same
reference notations. Note that the above embodiments are
illustrations and do not limit the invention. Further, the
embodiments include changes shown in the claims.
* * * * *