U.S. patent application number 11/816280 was filed with the patent office on 2009-01-08 for exhaust purifier for internal combustion engine.
This patent application is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Kazuaki Kameda.
Application Number | 20090007545 11/816280 |
Document ID | / |
Family ID | 37087114 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090007545 |
Kind Code |
A1 |
Kameda; Kazuaki |
January 8, 2009 |
Exhaust Purifier for Internal Combustion Engine
Abstract
An addition valve is provided upstream of an exhaust gas
purifying catalyst (NOx catalyst) in an exhaust pipe and in the
vicinity of a water jacket in a cylinder head, in addition to a
fuel injection valve injecting fuel for combustion in a combustion
chamber. The addition valve injects a reducing agent in an addition
amount in accordance with an operation state of an engine. Using an
engine coolant temperature correlated to a temperature of the
addition valve, a target addition interval of the addition valve is
made shorter with increase in the engine coolant temperature, so
that the addition amount is increased (step 130). The temperature
of the exhaust gas purifying catalyst (catalyst bed temperature) is
raised as a result of increase in the addition amount. Here, a
target injection amount of fuel, representing one of parameters
other than the addition amount that affect the catalyst bed
temperature, is restricted by using an injection amount upper
limit, such that the catalyst bed temperature does not exceed an
upper limit of an allowable range (steps 240, 250).
Inventors: |
Kameda; Kazuaki; (Aichi-ken,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Toyota Jidosha Kabushiki
Kaisha
Toyota-shi
JP
Kabushiki Kaisha Toyota Jidoshokki
Kariya-shi
JP
|
Family ID: |
37087114 |
Appl. No.: |
11/816280 |
Filed: |
April 6, 2006 |
PCT Filed: |
April 6, 2006 |
PCT NO: |
PCT/JP2006/307801 |
371 Date: |
August 15, 2007 |
Current U.S.
Class: |
60/286 ;
60/299 |
Current CPC
Class: |
B01D 53/9431 20130101;
F01N 2900/1811 20130101; F02D 2041/0265 20130101; F01N 3/0814
20130101; F01N 13/009 20140601; Y02T 10/47 20130101; F01N 2560/14
20130101; F01N 2510/06 20130101; Y02T 10/40 20130101; F01N 13/0097
20140603; F01N 3/0821 20130101; F01N 2560/08 20130101; F01N 2610/03
20130101; F01N 2900/08 20130101; F01N 11/002 20130101; F01N 3/0871
20130101; F02D 41/027 20130101; F01N 2610/1493 20130101; F01N 3/106
20130101; F01N 2560/06 20130101; B01D 2251/20 20130101; F01N
2260/024 20130101; F01N 3/0842 20130101; F01N 3/035 20130101 |
Class at
Publication: |
60/286 ;
60/299 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 2005 |
JP |
2005-112847 |
Claims
1. An exhaust gas purifying apparatus of an internal combustion
engine, configured to provide an addition valve upstream of an
exhaust gas purifying catalyst in an exhaust pipe connected to a
combustion chamber in addition to a fuel injection valve injecting
fuel for combustion in the combustion chamber of the internal
combustion engine, and to inject a reducing agent from said
addition valve in an addition amount in accordance with an
operation state of the internal combustion engine, comprising: a
control unit increasing said addition amount with increase in at
least one of a temperature of said addition valve and a value
corresponding thereto and controlling a parameter other than said
addition amount that varies in accordance with an operation of said
internal combustion engine and affects a catalyst temperature of
said exhaust gas purifying catalyst, such that said catalyst
temperature does not exceed an upper limit of an allowable
range.
2. The exhaust gas purifying apparatus of an internal combustion
engine according to claim 1, wherein when at least one of said
temperature of said addition valve and said corresponding value is
high, said control unit makes a degree of increase in said addition
amount greater than when at least one of said temperature of said
addition valve and said corresponding value is low.
3. The exhaust gas purifying apparatus of an internal combustion
engine according to claim 2, wherein said addition valve is
arranged in vicinity of a coolant pipe provided in said internal
combustion engine, and said control unit employs a temperature of a
coolant that flows through said coolant pipe as said corresponding
value.
4. The exhaust gas purifying apparatus of an internal combustion
engine according to claim 1, wherein said control unit restricts an
amount of fuel injection from said fuel injection valve, as control
of said parameter.
5. The exhaust gas purifying apparatus of an internal combustion
engine according to claim 4, wherein when at least one of said
temperature of said addition valve and said corresponding value is
high, said control unit restricts said amount of fuel injection to
a larger extent than when at least one of said temperature of said
addition valve and said corresponding value is low.
6. The exhaust gas purifying apparatus of an internal combustion
engine according to claim 1, wherein said control unit decreases an
amount of fuel injection from said fuel injection valve, as control
of said parameter.
7. The exhaust gas purifying apparatus of an internal combustion
engine according to claim 6, wherein when at least one of said
temperature of said addition valve and said corresponding value is
high, said control unit decreases said amount of fuel injection by
a larger amount than when at least one of said temperature of said
addition valve and said corresponding value is low.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purifying
apparatus of an internal combustion engine, configured to provide
an addition valve upstream of an exhaust gas purifying catalyst in
an exhaust pipe for injection of a reducing agent, in addition to a
fuel injection valve injecting fuel burnt in a combustion chamber
of the internal combustion engine.
BACKGROUND ART
[0002] In an internal combustion engine operating by burning an
air-fuel mixture at a high air-fuel ratio (lean atmosphere) in a
wide operation region, such as a diesel engine, generally, an NOx
catalyst attaining a function to purify a nitrogen oxide NOx in the
exhaust gas is provided in an exhaust pipe thereof. As the NOx
catalyst, for example, a catalyst carrying both an NOx absorbent
capable of absorbing nitrogen oxide NOx in the presence of oxygen
and a precious metal catalyst (precious metal) capable of oxidizing
hydrocarbon HC on a honeycomb structure (carrier) made of porous
ceramics is adopted.
[0003] The NOx catalyst has such a characteristic that it absorbs
nitrogen oxide NOx in a state where oxygen concentration in the
exhaust gas is high, while it emits nitrogen oxide NOx in a state
where oxygen concentration in the exhaust gas is low. In addition,
if hydrocarbon HC or carbon monoxide CO is present in the exhaust
gas when nitrogen oxide NOx is emitted in the exhaust gas, the
precious metal catalyst promotes oxidation reaction of hydrocarbon
HC or carbon monoxide CO, so that oxidation-reduction reaction
using nitrogen oxide NOx as an oxidation component and using
hydrocarbon HC and carbon monoxide CO as a reduction component
occurs therebetween. Namely, hydrocarbon HC and carbon monoxide CO
are oxidized to carbon dioxide CO.sub.2 or water H.sub.2O, and
nitrogen oxide NOx is reduced to nitrogen N.sub.2.
[0004] When the NOx catalyst absorbs a prescribed limit amount of
nitrogen oxide NOx, the NOx catalyst absorbs no more nitrogen oxide
NOx even in a state where oxygen concentration in the exhaust gas
is high. Accordingly, in the internal combustion engine provided
with such an NOx catalyst in the exhaust pipe, an addition valve is
provided upstream of the NOx catalyst in the exhaust pipe,
separately from a fuel injection valve injecting fuel for
combustion in the combustion chamber. By supplying a reducing agent
such as light oil from the addition valve, nitrogen oxide NOx
absorbed in the NOx catalyst is emitted and reduced and purified,
so that NOx absorption capability of the NOx catalyst is recovered
and the NOx absorption amount of the NOx catalyst does not reach
the limit amount (see, for example, Japanese Patent Laying-Open No.
2003-120392).
[0005] If the temperature of the addition valve is raised, however,
a component that is likely to volatilize (hereinafter, referred to
as a volatile component) in the reducing agent that passes the
addition valve evaporates, and a remaining deposit component is
adhered and deposited in an injection hole of the addition valve
and its surroundings. It may be likely that the deposits clog the
injection hole and the reducing agent is not appropriately injected
from the injection hole. In order to address this problem, it is
proposed to increase the amount of addition of the reducing agent
from the addition valve under the condition that the temperature of
the addition valve is high, such as when the temperature of a
coolant in the internal combustion engine is high and when the
engine load is high.
[0006] When the amount of addition of the reducing agent is
increased as above, the reducing agent at a low temperature passes
the addition valve in a large amount, and the reducing agent
removes the heat of the addition valve. The temperature at the tip
end portion of the addition valve including the injection hole is
lowered, and clogging of the injection hole is suppressed. On the
other hand, if the increased reducing agent burns on the exhaust
downstream side of the addition valve, the temperature of the NOx
catalyst (catalyst bed temperature) is raised by the heat generated
during combustion. If the NOx catalyst is excessively heated, the
catalyst bed temperature may exceed the upper limit of a
temperature range (allowable range) where the NOx catalyst
appropriately functions.
DISCLOSURE OF THE INVENTION
[0007] The present invention was made in view of such situations,
and an object of the present invention is to provide an exhaust gas
purifying apparatus of an internal combustion engine capable of
suppressing overheat of an exhaust gas purifying catalyst while
suppressing clogging of an injection hole of an addition valve.
[0008] A configuration for achieving the object above and a
function and effect thereof will be described in the following.
[0009] The present invention is directed to an exhaust gas
purifying apparatus of an internal combustion engine, configured to
provide an addition valve upstream of an exhaust gas purifying
catalyst in an exhaust pipe connected to a combustion chamber in
addition to a fuel injection valve injecting fuel for combustion in
the combustion chamber of the internal combustion engine, and to
inject a reducing agent from the addition valve in an addition
amount in accordance with an operation state of the internal
combustion engine. The apparatus includes a control unit increasing
the addition amount with increase in at least one of a temperature
of the addition valve and a value corresponding thereto and
controlling a parameter other than the addition amount, that varies
in accordance with an operation of the internal combustion engine
and affects a catalyst temperature of the exhaust gas purifying
catalyst, such that the catalyst temperature does not exceed an
upper limit of an allowable range.
[0010] According to this configuration, the control unit increases
the addition amount with the increase in the temperature of the
addition valve and the increased reducing agent passes the addition
valve, so that the heat of the addition valve is removed. The
temperature of the addition valve is lowered, evaporation of the
volatile component in the reducing agent is suppressed, and
clogging of the injection hole in the addition valve is suppressed.
On the other hand, the increased reducing agent burns, and the
temperature of the exhaust gas purifying catalyst is raised by the
heat generated during the combustion. In order to address this, the
control unit controls the parameter other than the addition amount
that varies in accordance with the operation of the internal
combustion engine and affects the temperature of the exhaust gas
purifying catalyst. Therefore, even if the temperature of the
exhaust gas purifying catalyst is raised as a result of increase in
the reducing agent, it is possible, by controlling the parameter,
to suppress such a situation that the catalyst temperature of the
exhaust gas purifying catalyst exceeds the upper limit of the
allowable range.
[0011] Thus, according to the present invention, overheat of the
exhaust gas purifying catalyst can be suppressed by controlling the
parameter, while clogging of the injection hole of the addition
valve is suppressed by increasing the addition amount.
[0012] Preferably, when at least one of the temperature of the
addition valve and the corresponding value is high, the control
unit makes a degree of increase in the addition amount greater than
when at least one of the temperature of the addition valve and the
corresponding value is low.
[0013] Here, as the amount of addition of the reducing agent
injected from the addition valve is greater, an amount of heat
removed from the addition valve by the reducing agent is increased
and a degree of lowering in the temperature of the addition valve
becomes greater. In this regard, in the present invention, when at
least one of the temperature of the addition valve and the value
corresponding thereto is high, the degree of increase in the
addition amount is made larger than when at least one of the
temperature of the addition valve and the value corresponding
thereto is low. In this manner, the degree of increase in the
addition amount is varied in accordance with the temperature of the
addition valve. Therefore, when the temperature of the addition
valve is relatively low, excessive cooling of the addition valve
due to excessive increase in the addition amount can be suppressed.
In addition, when the temperature of the addition valve is high,
such a phenomenon as clogging of the injection hole of the addition
valve due to an insufficient addition amount and resultant
insufficient cooling of the addition valve can be suppressed.
[0014] Further preferably, the addition valve is arranged in the
vicinity of a coolant pipe provided in the internal combustion
engine, and the control unit employs a temperature of a coolant
that flows through the coolant pipe as the corresponding value.
[0015] According to this configuration, as the addition valve is
arranged in the vicinity of the coolant pipe provided in the
internal combustion engine, the temperature of the addition valve
tends to be affected by the heat of the coolant that flows in the
coolant pipe. When the temperature of the coolant is not so high,
such as during a low or intermediate load operation or the like of
the internal combustion engine, the addition valve is cooled as a
result of heat removal by the coolant, and clogging of the
injection hole is unlikely. When the temperature of the coolant is
high, such as during a high load operation or the like of the
internal combustion engine, efficiency in cooling the addition
valve is lowered, and the temperature of the addition valve may
exceed the temperature at which the volatile component in the
reducing agent evaporates. Therefore, according to the present
invention, the effect of the invention described above is reliably
obtained by employing the temperature of the coolant as the value
corresponding to the temperature of the addition valve and by
increasing the addition amount based on the corresponding
value.
[0016] Further preferably, the control unit restricts an amount of
fuel injection from the fuel injection valve, as control of the
parameter.
[0017] In the internal combustion engine and the exhaust gas
purifying apparatus, as the amount of fuel injection from the fuel
injection valve is decreased, the temperature of the exhaust gas
and the catalyst is accordingly lowered. The injection amount is
thus restricted as in the present invention, so that the injection
amount is decreased as compared with a case where the injection
amount is not restricted, and the temperature of the exhaust gas is
lowered. Then, the temperature of the catalyst is lowered, and it
is less likely that the upper limit of the allowable range is
exceeded. In this manner, according to the present invention, the
effect of the invention described above is reliably obtained.
[0018] Further preferably, when at least one of the temperature of
the addition valve and the corresponding value is high, the control
unit restricts the amount of fuel injection to a larger extent than
when at least one of the temperature of the addition valve and the
corresponding value is low.
[0019] Here, as the injection amount is restricted to a larger
extent, an amount of lowering in the temperature of the exhaust gas
and the catalyst as a result of restriction becomes greater. In
this regard, in the present invention, when at least one of the
temperature of the addition valve and the value corresponding
thereto is high (when the degree of increase in the addition amount
is great and an amount of increase in the temperature of the
catalyst is great), the injection amount is restricted to a larger
extent than when at least one of the temperature of the addition
valve and the value corresponding thereto is low. In this manner,
the degree of restriction of the injection amount is varied in
accordance with at least one of the temperature of the addition
valve and the value corresponding thereto, so that undue lowering
in the engine output due to excessive restriction of the injection
amount can be suppressed when at least one of the temperature of
the addition valve and the value corresponding thereto is
relatively low. In addition, when at least one of the temperature
of the addition valve and the value corresponding thereto is high,
such a situation that restriction of the injection amount is
insufficient (the injection amount is great) and the temperature of
the catalyst consequently exceeds the upper limit of the allowable
range can be suppressed.
[0020] Further preferably, the control unit decreases an amount of
fuel injection from the fuel injection valve, as control of the
parameter.
[0021] In the internal combustion engine and the exhaust gas
purifying apparatus, as the amount of fuel injection from the fuel
injection valve is decreased, the temperature of the exhaust gas
and the catalyst is accordingly lowered. The injection amount is
thus decreased as in the present invention, so that the temperature
of the exhaust gas is lowered as compared with a case where the
injection amount is not decreased. Then, the temperature of the
catalyst is lowered, and it is unlikely that the upper limit of the
allowable range is exceeded. In this manner, according to the
present invention, the effect of the invention described above is
reliably obtained.
[0022] Further preferably, when at least one of the temperature of
the addition valve and the corresponding value is high, the control
unit decreases the amount of fuel injection by a larger amount than
when at least one of the temperature of the addition valve and the
corresponding value is low.
[0023] Here, as the amount of fuel injection is decreased to a
larger extent (the injection amount is decreased), an amount of
lowering in the temperature of the exhaust gas and the catalyst as
a result of decrease becomes greater. In this regard, in the
present invention, when at least one of the temperature of the
addition valve and the value corresponding thereto is high (when
the degree of increase in the addition amount is great and the
amount of increase in the temperature of the catalyst is great),
the injection amount is decreased more than when at least one of
the temperature of the addition valve and the value corresponding
thereto is low. In this manner, the degree of decrease in the
injection amount is varied in accordance with at least one of the
temperature of the addition valve and the value corresponding
thereto, so that undue lowering in the engine output due to
excessive decrease in the injection amount can be suppressed when
at least one of the temperature of the addition valve and the value
corresponding thereto is relatively low. In addition, when at least
one of the temperature of the addition valve and the value
corresponding thereto is high, such a situation that decrease in
the injection amount is insufficient (the injection amount is
great) and the temperature of the catalyst exceeds the upper limit
of the allowable range can be suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a diagram showing a configuration of a first
embodiment implementing the present invention.
[0025] FIG. 2 is a schematic plan view showing an addition valve
and a portion around the same in an engine.
[0026] FIG. 3 is a timing chart illustrating a period during which
a reducing agent is added and an interval of addition.
[0027] FIG. 4A is a flowchart showing a procedure for injection
hole clogging suppression processing.
[0028] FIG. 4B is a flowchart showing a procedure for injection
amount restriction processing.
[0029] FIG. 5 is a characteristic diagram showing relation of an
engine coolant temperature, a catalyst bed temperature, an addition
amount, and an injection amount upper limit.
[0030] FIG. 6 is a flowchart showing a procedure for injection
amount decrease processing in a second embodiment of the present
invention.
[0031] FIG. 7 is a characteristic diagram showing relation between
an engine coolant temperature and a correction amount.
[0032] FIG. 8 is a characteristic diagram showing relation of an
engine coolant temperature, a catalyst bed temperature, an addition
amount, and a correction amount.
BEST MODES FOR CARRYING OUT THE INVENTION
First Embodiment
[0033] A first embodiment implementing the present invention will
be described hereinafter with reference to FIGS. 1 to 5. FIG. 1
shows a configuration of a multi-cylinder diesel engine
(hereinafter, simply referred to as an engine) 11 serving as an
internal combustion engine to which the present embodiment is
applied and an exhaust gas purifying apparatus 12. FIG. 2 shows a
schematic plan view of engine 11.
[0034] Engine 11 generally includes an intake pipe 13, a combustion
chamber 14 for each cylinder 10, and an exhaust pipe 15. An air
cleaner 16 purifying air taken in intake pipe 13 is provided in a
most upstream portion of intake pipe 13. In engine 11, an airflow
meter 17 for detecting a flow rate of air in intake pipe 13, a
compressor 18A of a turbo charger 18, an intercooler 19, and an
intake air throttle valve 21 are sequentially arranged toward the
intake air downstream side of air cleaner 16. Intake pipe 13 is
branched at an intake manifold 22 provided on the intake air
downstream side of intake air throttle valve 21, and connected to
combustion chamber 14 for each cylinder 10 through the branch
portion.
[0035] In a cylinder head 23 of engine 11, a fuel injection valve
24 injecting fuel for combustion in combustion chamber 14 is
provided for each cylinder 10. Each fuel injection valve 24 is
supplied with fuel from a fuel tank 26 through a fuel supply pipe
25. In fuel supply pipe 25, a fuel pump 27 suctioning fuel from
fuel tank 26 and pressurizing and delivering the fuel and a common
rail 28 serving as a high-pressure fuel pipe accumulating the
delivered high-pressure fuel are provided. Fuel injection valve 24
for each cylinder 10 is connected to common rail 28.
[0036] Meanwhile, a connection portion of exhaust pipe 15 and each
combustion chamber 14 serves as an exhaust port 29. In exhaust pipe
15, an exhaust manifold 31 for gathering the exhaust gas exhausted
from each combustion chamber 14 through exhaust port 29 and a
turbine 18B of turbo charger 18 are provided.
[0037] In addition, engine 11 adopts an exhaust gas recirculation
(hereinafter, referred to as "EGR") apparatus 32 for recirculating
a part of the exhaust gas in the intake air. EGR apparatus 32
includes an EGR pipe 33 allowing communication between intake pipe
13 and exhaust pipe 15. An upstream side of EGR pipe 33 is
connected to a portion of exhaust pipe 15 between exhaust manifold
31 and turbine 18B. In a midpoint of EGR pipe 33, an EGR cooler
catalyst 34 purifying the recirculated exhaust gas, an EGR cooler
35 cooling the recirculated exhaust gas, and an EGR valve 36
regulating a flow rate of the recirculated exhaust gas are provided
sequentially from the upstream side. The downstream side of EGR
pipe 33 is connected to a portion of intake pipe 13 between intake
air throttle valve 21 and intake manifold 22.
[0038] In such engine 11, the air taken in intake pipe 13 is
purified in air cleaner 16, and thereafter introduced in compressor
18A of turbo charger 18. In compressor 18A, the introduced air is
compressed and delivered to intercooler 19. The air of which
temperature is raised as a result of compression is cooled in
intercooler 19, and thereafter the air passes through intake air
throttle valve 21 and intake manifold 22, and distributed and
supplied to combustion chamber 14 of each cylinder 10. The flow
rate of the air in intake pipe 13 is regulated by controlling a
degree of opening of intake air throttle valve 21. The flow rate of
the air, that is, the amount of intake air, is detected by airflow
meter 17.
[0039] In combustion chamber 14 into which the air is introduced,
fuel is injected from fuel injection valve 24 in the compression
stroke of each cylinder 10. Then, a mixture of the air introduced
through intake pipe 13 and the fuel injected from fuel injection
valve 24 is burnt in combustion chamber 14. The combustion gas at
high temperature and high pressure generated at this time causes a
piston 37 to carry out reciprocating motion, a crankshaft 38
serving as an output shaft rotates, and drive force (output torque)
of engine 11 is obtained. An NE sensor 39 detecting an engine speed
NE indicating a rotation speed of crankshaft 38 is provided in
engine 11.
[0040] The exhaust gas generated as a result of combustion in
combustion chamber 14 of each cylinder 10 is introduced in turbine
18B of turbo charger 18 through exhaust manifold 31. When turbine
18B is driven by the stream of the introduced exhaust gas,
compressor 18A provided in intake pipe 13 is driven in
synchronization, and the air is compressed as described above.
[0041] Meanwhile, a part of the exhaust gas generated as a result
of combustion is introduced in EGR pipe 33. The exhaust gas
introduced in EGR pipe 33 is purified by EGR cooler catalyst 34 and
cooled in EGR cooler 35, and thereafter recirculated in the air on
the intake air downstream side of intake air throttle valve 21 in
intake pipe 13. The flow rate of the exhaust gas thus recirculated
is regulated by controlling the degree of opening of EGR valve
36.
[0042] Engine 11 is configured as described above. Exhaust gas
purifying apparatus 12 for purifying the exhaust gas exhausted from
engine 11 will now be described. Exhaust gas purifying apparatus 12
includes not only an addition valve 41 but also a plurality of
(three) catalytic converters (a first catalytic converter 42, a
second catalytic converter 43, and a third catalytic converter 44)
serving as exhaust gas purifying catalysts.
[0043] First catalytic converter 42 at the most upstream portion is
arranged on the exhaust downstream side of turbine 18B. A
storage-reduction type NOx catalyst is accommodated in first
catalytic converter 42. The NOx catalyst is implemented, for
example, in such a manner that a honeycomb structure is employed as
a carrier and an NOx absorbent capable of absorbing nitrogen oxide
NOx in the presence of oxygen and a precious metal catalyst
(precious metal) capable of oxidizing hydrocarbon HC are carried
thereon.
[0044] The NOx absorbent has such a characteristic that it absorbs
nitrogen oxide NOx in a state where the oxygen concentration in the
exhaust gas is high, while it emits nitrogen oxide NOx in a state
where the oxygen concentration is low. In addition, if hydrocarbon
HC, carbon monoxide CO or the like is present in the exhaust gas
when nitrogen oxide NOx is emitted in the exhaust gas, the precious
metal catalyst promotes oxidation reaction of hydrocarbon HC or
carbon monoxide CO, so that oxidation-reduction reaction employing
nitrogen oxide NOx as an oxidation component and employing
hydrocarbon HC and carbon monoxide CO as a reduction component
occurs therebetween. Namely, hydrocarbon HC and carbon monoxide CO
are oxidized to carbon dioxide CO.sub.2 or water H.sub.2O, and
nitrogen oxide NOx is reduced to nitrogen N.sub.2.
[0045] Second catalytic converter 43 is arranged on the exhaust
downstream side of first catalytic converter 42. A
storage-reduction type NOx catalyst is accommodated in second
catalytic converter 43. The NOx catalyst includes a porous material
allowing passage of a gas component in the exhaust gas and
preventing passage of particulate matter PM in the exhaust gas.
This porous material is employed as the carrier of the NOx
catalyst, and the carrier carries the NOx absorbent and the
precious metal catalyst. Third catalytic converter 44 is arranged
on the exhaust downstream side of second catalytic converter 43. In
third catalytic converter 44, an oxidizing catalyst purifying the
exhaust gas through oxidation of hydrocarbon HC and carbon monoxide
CO in the exhaust gas is carried.
[0046] Addition valve 41 is arranged in exhaust pipe 15, upstream
of first catalytic converter 42. In the present embodiment, in
order to satisfy this condition, addition valve 41 is attached to a
portion in the vicinity of exhaust port 29 in cylinder head 23.
Addition valve 41 is attached to cylinder head 23 in such a manner
that an injection hole 41A at the tip end is exposed in exhaust
port 29. As shown in FIG. 2, this position is located around a
water jacket 45 serving as a coolant pipe provided in cylinder head
23. Addition valve 41 is attached to such a position, so that
addition valve 41 is cooled by the engine coolant that flows
through water jacket 45.
[0047] As shown in FIG. 1, addition valve 41 is connected to fuel
pump 27 through a fuel pipe 46, and injects and adds the fuel
supplied from fuel pump 27 into the exhaust gas as the reducing
agent. The added fuel temporarily turns the exhaust gas to a
reduction atmosphere, so that nitrogen oxide NOx stored in first
catalytic converter 42 and second catalytic converter 43 is reduced
and purified. In addition, second catalytic converter 43
simultaneously purifies particulate matter PM.
[0048] A coolant temperature sensor 47 detecting a temperature of
the engine coolant (engine coolant temperature THW) that flows
through water jacket 45 is attached to cylinder head 23. In
addition, an exhaust gas temperature sensor 48 detecting a
temperature of the exhaust gas (exhaust gas temperature) that
passes a space between first catalytic converter 42 and second
catalytic converter 43 in exhaust pipe 15, that is, the temperature
of the exhaust gas before entering second catalytic converter 43,
is arranged in that space. Moreover, an exhaust gas temperature
sensor 49 detecting a temperature of the exhaust gas that passes
through a space downstream of second catalytic converter 43 in
exhaust pipe 15, that is, the temperature of the exhaust gas
immediately after passing through second catalytic converter 43, is
arranged in that space. Further, a differential pressure sensor 51
detecting a differential pressure between the pressure of the
exhaust gas on the exhaust upstream side of second catalytic
converter 43 and the pressure of the exhaust gas on the exhaust
downstream side thereof is arranged in exhaust pipe 15. In
addition, oxygen sensors 52, 53 detecting a concentration of oxygen
in the exhaust gas are arranged on the exhaust upstream side of
first catalytic converter 42 of exhaust pipe 15, and between second
catalytic converter 43 and third catalytic converter 44,
respectively.
[0049] An electronic control unit 61 serving as control means
controls engine 11 and exhaust gas purifying apparatus 12 described
above. Electronic control unit 61 includes a CPU executing various
types of processing involved with control of engine 11, an ROM
storing a program or data necessary for control, an RAM storing a
result of processing or the like performed by the CPU, an
input/output port for transmitting/receiving information to/from
the outside, and the like.
[0050] In addition to each sensor described above, an accelerator
sensor 54 detecting how far the accelerator is pressed down by a
driver, a common rail sensor 55 detecting an internal pressure
(rail pressure) of common rail 28, a throttle valve sensor 56
detecting a position of intake air throttle valve 21, and the like
are connected to the input port of electronic control unit 61.
[0051] Meanwhile, intake air throttle valve 21, fuel injection
valve 24, fuel pump 27, addition valve 41, EGR valve 36, and the
like are connected to the output port of electronic control unit
61. Electronic control unit 61 controls these components connected
to the output port based on the result of detection of each sensor,
so as to control the operation of engine 11, exhaust gas
purification, and the like.
[0052] Electronic control unit 61 controls fuel injection, which
represents one type of control involved with the operation of
engine 11. In fuel injection control, a basic injection amount
optimal for an operation state of engine 11 is calculated based on
an accelerator pressing-down degree detected by accelerator sensor
54 and engine speed NE detected by NE sensor 39. In addition, a
maximum injection amount is determined by correcting, based on
signals from various sensors, the basic maximum injection amount
determined by engine speed NE (the theoretically possible injection
amount). Comparing the basic injection amount and the maximum
injection amount with each other, the smaller amount is set as a
target injection amount. In addition, basic target injection timing
is calculated based on the accelerator pressing-down degree and
engine speed NE, that are corrected based on a signal from various
sensors, and target injection timing optimal for the operation
state of engine 11 at that time is calculated. Then, current supply
to fuel injection valve 24 is controlled based on the target
injection amount and the target injection timing, so as to
open/close fuel injection valve 24.
[0053] In addition, electronic control unit 61 controls the exhaust
gas purifying catalyst, which represents one type of control
involved with purification of the exhaust gas. In order to control
the exhaust gas purifying catalyst, four catalyst control modes,
i.e., a catalyst regeneration control mode, a sulfur poisoning
recovery control mode, an NOx reduction control mode, and a normal
control mode, are set, and electronic control unit 61 selects the
catalyst control mode in accordance with a state of catalytic
converters 42 to 44 and executes that mode.
[0054] The catalyst regeneration control mode refers to a mode of
control such that particulate matter PM deposited particularly in
second catalytic converter 43 is burnt and exhausted as carbon
dioxide CO.sub.2 and water H.sub.2O. The sulfur poisoning recovery
control mode refers to a mode of control such that, when the NOx
catalyst in first catalytic converter 42 and second catalytic
converter 43 is poisoned with sulfur oxide SOx and storage
capability of nitrogen oxide NOx is lowered, sulfur oxide SOx is
released.
[0055] The NOx reduction control mode refers to such a mode that
nitrogen oxide NOx absorbed in the NOx catalyst is released and
reduced and purified so as to recover the NOx absorption capability
of the NOx absorbent, by adding and supplying the reducing agent to
upstream of first catalytic converter 42 in exhaust pipe 15 by
means of addition valve 41 before the NOx absorption amount of the
NOx absorbent in the NOx catalyst reaches the limit.
[0056] For example, as shown in FIG. 3, in this mode, an open/close
cycle consisting of valve-opening and valve-closing of addition
valve 41 is repeated. The reducing agent is supplied from addition
valve 41 as a result of valve-opening, while supply is stopped as a
result of valve-closing. By varying a period during which addition
valve 41 is open (addition period) and a time period from the start
of valve-opening until the start of next valve-opening (addition
interval), the amount of addition of the reducing agent is
adjusted. In other words, as the addition period is longer or as
the addition interval is shorter, the addition amount becomes
greater. In the present embodiment, the addition amount is adjusted
by varying the addition interval.
[0057] The state other than described above corresponds to the
normal control mode, in which the reducing agent is not added from
addition valve 41.
[0058] Here, as injection hole 41A of addition valve 41 is exposed
in exhaust port 29, the tip end portion of addition valve 41
including the portion around injection hole 41A tends to be exposed
to the exhaust gas and the temperature thereof tends to be high. In
addition, when the temperature of the engine coolant (engine
coolant temperature THW) that flows through water jacket 45 is high
in a high-load operation or the like of engine 11, efficiency in
cooling addition valve 41 by the engine coolant is lowered and the
temperature of addition valve 41 tends to be high. As the
temperature is raised, the volatile component contained in the
reducing agent evaporates, and the remaining deposit component is
adhered and deposited in injection hole 41A of addition valve 41
and its surroundings. As the deposits clog injection hole 41A, the
reducing agent may not appropriately be injected through injection
hole 41A and a spray state may be poorer. In order to overcome such
defects, it is effective to moderately cool the tip end portion of
addition valve 41, particularly the portion around injection hole
41A.
[0059] In the present embodiment, NOx reduction is controlled such
that clogging of injection hole 41A of addition valve 41 is
suppressed. FIG. 4A is a flowchart showing a specific procedure for
clogging suppression processing. Electronic control unit 61
executes a series of processing shown in the flowchart as
processing to be performed every prescribed time.
[0060] In the clogging suppression processing, initially in step
110, electronic control unit 61 calculates a target period of
addition of the reducing agent, that is, a target valve-opening
period of addition valve 41, based on engine speed NE. In addition,
in step 120, a target interval of addition of the reducing agent is
calculated. In calculation, a map defining in advance relation, for
example, of engine speed NE and the target injection amount with
the target addition interval is referred to. In the map, it is
defined such that the addition interval is shorter as engine speed
NE is higher or as the target injection amount is greater. Then,
the target addition interval corresponding to engine speed NE and
the target injection amount at that time is found based on the map.
Here, as the target injection amount, the injection amount
separately calculated in fuel injection control described above is
used.
[0061] In succession, in step 130, the target addition interval
calculated in step 120 above is corrected, using the temperature of
addition valve 41 or a value corresponding thereto. Here, engine
coolant temperature THW detected by coolant temperature sensor 47
is employed as the value corresponding to the temperature of
addition valve 41. This is because addition valve 41 is arranged in
the vicinity of water jacket 45 provided in cylinder head 23 as
described above, and the temperature of addition valve 41 tends to
be affected by the heat of the engine coolant that flows through
water jacket 45. In correction, the target addition interval in
step 120 is corrected such that, when engine coolant temperature
THW is high, the target addition interval is made shorter than when
engine coolant temperature THW is low, that is, such that the
number of times of addition per unit time is greater and the
addition amount is increased.
[0062] In step 140, current supply to addition valve 41 is
controlled based on the target addition period in step 110 and the
target addition interval corrected in step 130. As a result of
current supply, addition valve 41 is opened/closed, and the
reducing agent is injected through injection hole 41A into exhaust
port 29. After the processing in step 140, a series of clogging
suppression processing ends.
[0063] As described above, as engine coolant temperature THW is
higher, the efficiency in cooling of addition valve 41 by the
engine coolant is lowered and the temperature of addition valve 41
is raised. Meanwhile, as a result of increase in the addition
amount, a large amount of reducing agent not yet much affected by
the heat of the exhaust gas and at a low temperature passes
addition valve 41. The reducing agent that passes removes a large
amount of heat of addition valve 41, and the temperature of
addition valve 41 becomes lower than the temperature at which the
volatile component in the reducing agent evaporates. Consequently,
formation of deposits in injection hole 41A and the portion around
the same resulting from evaporation of the volatile component is
suppressed.
[0064] In addition, in increasing the addition amount, when engine
coolant temperature THW is high, the degree of increase is made
larger than when engine coolant temperature THW is low. In other
words, the degree of increase in the addition amount is varied in
accordance with engine coolant temperature THW correlated with the
temperature of addition valve 41. Therefore, when the temperature
of addition valve 41 (engine coolant temperature THW) is relatively
low, the degree of increase in the addition amount is small, and
the amount of heat removed from addition valve 41 by the increased
reducing agent is relatively small. Accordingly, excessive cooling
of addition valve 41 by the increased reducing agent is less
likely. Alternatively, when the temperature of addition valve 41
(engine coolant temperature THW) is high, the degree of increase in
the addition amount is great, and the amount of heat removed from
addition valve 41 by the increased reducing agent is greater.
Accordingly, such a phenomenon that increase in the addition amount
is insufficient and addition valve 41 is not sufficiently cooled is
less likely.
[0065] On the other hand, heat generated as a result of combustion
of the reducing agent increased in the above-described manner
causes increase in the temperature of the NOx catalyst (catalyst
bed temperature) in first and second catalytic converters 42, 43.
If the NOx catalyst is excessively heated, the upper limit of a
temperature range (allowable range) where the NOx catalyst
appropriately functions may be exceeded.
[0066] In the present embodiment, in order for the catalyst bed
temperature not to exceed the upper limit of the allowable range
due to the increase in the amount of addition of the reducing
agent, control for restricting the amount of fuel injection from
fuel injection valve 24 is carried out. FIG. 4B is a flowchart
showing a specific procedure for injection amount restriction
processing. Electronic control unit 61 executes a series of
processing shown in the flowchart as processing to be performed
every prescribed time.
[0067] In the injection amount restriction processing, initially in
step 210, electronic control unit 61 reads engine coolant
temperature THW detected by coolant temperature sensor 47 at that
time. Then, in step 220, electronic control unit 61 determines
whether engine coolant temperature THW in step 210 is higher than a
threshold value a that has been set in advance. Threshold value ax
represents an upper limit, or a value close thereto, of a range of
temperature that engine coolant temperature THW may take on the
condition that the catalyst bed temperature does not exceed the
upper limit of the allowable range even if the reducing agent in an
amount increased in accordance with the increase in engine coolant
temperature THW is added.
[0068] Here, it is assumed that a determination condition in step
220 is satisfied (THW>.alpha.). Then, if the fuel is injected in
accordance with the target injection amount, the catalyst bed
temperature may exceed the upper limit of the allowable range due
to the heat of the exhaust gas, the heat generated during
combustion of the reducing agent, and the like. Accordingly, in
order to lower the temperature of the exhaust gas such that the
catalyst bed temperature does not exceed the upper limit, in step
230, the injection amount upper limit in accordance with engine
coolant temperature THW in step 210 is set. When engine coolant
temperature THW is high, the injection amount upper limit is set to
a value lower than when engine coolant temperature THW is low. That
is, when deviation from threshold value .alpha. of engine coolant
temperature THW is great, the injection amount upper limit is set
to a value lower than when deviation is small.
[0069] In succession, in step 240, whether the target injection
amount calculated separately in fuel injection control described
above is greater than the injection amount upper limit in step 230
is determined. If this determination condition is satisfied (target
injection amount>injection amount upper limit), the injection
amount upper limit is set in step 250 as the final target injection
amount to be instructed to addition valve 41. That is, the target
injection amount is restricted by the injection amount upper limit.
After the processing in step 250, a series of injection amount
restriction processing ends.
[0070] In contrast, if the determination condition in step 220
above is not satisfied (THW.ltoreq..alpha.) and if the
determination condition in step 240 is not satisfied (target
injection amount.ltoreq.injection amount upper limit), in both
cases, it is considered that there is no possibility that the
catalyst bed temperature exceeds the upper limit of the allowable
range. Therefore, a series of injection amount restriction
processing ends without performing the processing in steps 230 to
250 in the former case, or without performing the processing in
step 250 in the latter case. In these cases, the target injection
amount is not restricted but used as it is as the final target
injection amount.
[0071] Therefore, as engine coolant temperature THW is higher,
through the clogging suppression processing described above, the
amount of addition of the reducing agent from addition valve 41 is
increased in order to suppress clogging of injection hole 41A, and
in addition, when engine coolant temperature THW is high, the
degree of increase is made larger than when engine coolant
temperature THW is low, whereby the catalyst bed temperature is
raised. On the other hand, if engine coolant temperature THW
exceeds threshold value a and if the catalyst bed temperature is
likely to exceed the upper limit of the allowable range, the
injection amount upper limit is set based on engine coolant
temperature THW. The target injection amount is restricted so as
not to exceed the injection amount upper limit. Namely, when the
target injection amount exceeds the injection amount upper limit,
the target injection amount is substantially decreased. As a result
of restriction of the target injection amount, the target injection
amount is made smaller than when it is not restricted, and the fuel
in an amount in accordance with that target injection amount is
injected and burnt, whereby the temperature of the exhaust gas is
lowered. As the increase in the catalyst bed temperature due to the
exhaust gas is made smaller, the catalyst bed temperature is less
likely to exceed the upper limit of the allowable range.
[0072] In restricting the target injection amount, when engine
coolant temperature THW is high, the injection amount upper limit
is set to a value lower than when engine coolant temperature THW is
low, and the target injection amount is restricted to a larger
extent. In other words, the degree of restriction of the target
injection amount is varied in accordance with engine coolant
temperature THW used for correcting the target addition interval
(increase in the addition amount). Therefore, when engine coolant
temperature THW is relatively low, that is, when the degree of
increase in the addition amount is relatively small and when the
increase in the catalyst bed temperature due to the increase in the
addition amount is small, the degree of restriction of the target
injection amount is small. Accordingly, such a phenomenon that the
target injection amount is excessively restricted and output of
engine 11 is unduly lowered is less likely. In addition, when
engine coolant temperature THW is high, that is, when the degree of
increase in the addition amount is great and when the increase in
the catalyst bed temperature due to the increase in the addition
amount is great, the degree of restriction of the target injection
amount is great. Accordingly, such a phenomenon that restriction of
the target injection amount is insufficient (the target injection
amount is greater than an appropriate value) and the catalyst bed
temperature exceeds the upper limit of the allowable range is less
likely.
[0073] FIG. 5 shows relation of engine coolant temperature THW, the
catalyst temperature, the amount of addition of the reducing agent,
and the injection amount upper limit. Here, in a temperature region
where engine coolant temperature THW is lower than threshold value
.alpha., restriction of the target injection amount by the
injection amount upper limit is not carried out. In addition, in
this temperature region, in order to suppress clogging of injection
hole 41A of addition valve 41, when engine coolant temperature THW
is high, the amount of addition of the reducing agent is made
larger than when engine coolant temperature THW is low. With the
increase in the addition amount, that is, with the increase in
engine coolant temperature THW, the catalyst bed temperature is
also raised and approaches to the upper limit of the allowable
range.
[0074] When engine coolant temperature THW attains to threshold
value .alpha. or higher, the injection amount upper limit in
accordance with engine coolant temperature THW is set. When the
target injection amount exceeds the injection amount upper limit,
the injection amount upper limit is set as the target injection
amount, so that the target injection amount is made smaller, the
temperature of the exhaust gas is lowered, and the catalyst bed
temperature is accordingly lowered. As a result of such lowering,
such a situation that the catalyst bed temperature exceeds the
upper limit of the allowable range is suppressed. As deviation from
the upper limit of the allowable range of the catalyst bed
temperature is greater, further increase in the addition amount is
permitted.
[0075] On the other hand, due to the increase in engine coolant
temperature THW, clogging of injection hole 41A is more likely. If
the addition amount is increased after engine coolant temperature
THW attains to a prescribed value .beta. (>.alpha.), clogging of
injection hole 41A is suppressed. In contrast, the catalyst bed
temperature is raised with the increase in the addition amount and
approaches the upper limit of the allowable range. In order to
address this, the injection amount upper limit is set to a lower
value with the increase in engine coolant temperature THW. As a
result of restriction based on the injection amount upper limit,
the target injection amount is made smaller, and the temperature of
the exhaust gas and the catalyst bed temperature are lowered.
[0076] According to the present embodiment described in detail
above, the following effects can be obtained.
[0077] (1) With the increase in the temperature of addition valve
41, the amount of addition of the reducing agent is increased, and
the target injection amount representing a parameter other than the
addition amount that affects the catalyst bed temperature is
controlled such that the catalyst bed temperature does not exceed
the upper limit of the allowable range. Therefore, while
suppressing clogging of injection hole 41A by increasing the
addition amount, overheat of the NOx catalyst can be suppressed by
controlling the target injection amount.
[0078] (2) Engine coolant temperature THW is employed as the value
corresponding to the temperature of addition valve 41. When engine
coolant temperature THW is high, the degree of increase in the
addition amount is made larger than when engine coolant temperature
THW is low. By thus varying the degree of increase in the addition
amount in accordance with engine coolant temperature THW, when the
temperature of addition valve 41 is relatively low, excessive
cooling of addition valve 41 due to excessive increase in the
addition amount can be suppressed. In addition, when the
temperature of addition valve 41 is high, clogging of injection
hole 41A due to insufficient addition amount and resultant
insufficient cooling of addition valve 41 can be suppressed.
[0079] (3) The injection amount upper limit in accordance with
engine coolant temperature THW is set. When the target injection
amount exceeds the injection amount upper limit, the injection
amount upper limit is set as the final target injection amount. As
a result of such restriction using the injection amount upper
limit, the target injection amount is made smaller than when no
restriction is imposed, and the temperature of the exhaust gas is
lowered. Accordingly, the catalyst bed temperature is lowered and
less likely to exceed the upper limit of the allowable range.
Therefore, the effect described in (1) above is reliably
attained.
[0080] In addition, as a result of restriction above, the target
injection amount can be decreased only when the catalyst bed
temperature is likely to exceed the upper limit of the allowable
range, that is, only when necessary. Therefore, unnecessary
decrease in the target injection amount can be suppressed, as
compared with a case where the target injection amount is decreased
without exception when engine coolant temperature THW exceeds
threshold value .alpha..
[0081] (4) When engine coolant temperature THW is high, the
injection amount upper limit is set to a value lower than when
engine coolant temperature THW is low, whereby the target injection
amount is restricted to a larger extent. By thus varying the degree
of restriction of the target injection amount in accordance with
engine coolant temperature THW, undue lowering in the output of
engine 11 due to excessive restriction of the target injection
amount can be suppressed when engine coolant temperature THW is
relatively low. In addition, when engine coolant temperature THW is
high, such a situation that restriction of the target injection
amount is insufficient (the target injection amount is great) and
the catalyst bed temperature exceeds the upper limit of the
allowable range can be suppressed.
[0082] (5) The temperature of addition valve 41 is affected not
only by engine coolant temperature THW but also by the heat of the
exhaust gas. The temperature of the exhaust gas tends to vary in
accordance with the fuel injection amount and tends to lower as the
fuel injection amount is smaller. Here, in the first embodiment,
with the increase in engine coolant temperature THW, the target
injection amount is restricted and decreased. Therefore, not only
by increasing the addition amount but also by decreasing the target
injection amount, the temperature of addition valve 41 is lowered
and the clogging phenomenon of injection hole 41A can further
effectively be suppressed.
Second Embodiment
[0083] A second embodiment implementing the present invention will
now be described with reference to FIGS. 6 to 8. The second
embodiment is different from the first embodiment in that the
target injection amount is decreased, instead of restricting the
same, for control such that the catalyst bed temperature does not
exceed the upper limit of the allowable range with the increase in
the addition amount of the reducing agent. As the configuration of
engine 11 and exhaust gas purifying apparatus 12 is the same as in
the first embodiment, description thereof will not be repeated.
[0084] FIG. 6 is a flowchart showing a specific procedure for
injection amount decrease processing. Electronic control unit 61
executes a series of processing shown in the flowchart as
processing to be performed every prescribed time.
[0085] In the injection amount decrease processing, initially in
step 310, electronic control unit 61 reads engine coolant
temperature THW detected by coolant temperature sensor 47 at that
time. Then, in step 320, electronic control unit 61 determines
whether engine coolant temperature THW in step 310 is higher than
threshold value .alpha.. Threshold value .alpha. is the same as
described in the first embodiment.
[0086] Here, it is assumed that a determination condition in step
320 above is satisfied (THW>.alpha.). Then, if the fuel is
injected in accordance with the target injection amount as it is,
the catalyst bed temperature may exceed the upper limit of the
allowable range due to the heat of the exhaust gas, the heat
generated during combustion of the reducing agent, and the like.
Accordingly, the processing for lowering the temperature of the
exhaust gas such that the catalyst bed temperature does not exceed
the upper limit is performed. Specifically, in step 330, a
correction amount (>0) for decreasing the target injection
amount is calculated based on engine coolant temperature THW in
step 310. For example, as shown in FIG. 7, the correction amount
can be set such that the correction amount is smaller when engine
coolant temperature THW is low, while the correction amount is
greater as engine coolant temperature THW is higher, that is, as
deviation from threshold value .alpha. of engine coolant
temperature THW is greater.
[0087] In succession, in step 340, in controlling fuel injection
described above, the correction amount in step 330 is subtracted
from the target injection amount calculated in a different routine,
and the subtraction result is set as the final target injection
amount to be instructed to fuel injection valve 24. After the
processing in step 340, a series of injection amount decrease
processing ends.
[0088] In contrast, if the determination condition in step 320
above is not satisfied (THW.ltoreq..alpha.), it is considered that
there is no possibility that the catalyst bed temperature exceeds
the upper limit of the allowable range even though the addition
amount is increased with the increase in engine coolant temperature
THW. Therefore, in this case, a series of injection amount decrease
processing ends without performing the processing in steps 330 and
340. Here, the target injection amount is not corrected but used as
it is as the final target injection amount.
[0089] Therefore, as engine coolant temperature THW is higher,
through the clogging suppression processing described above, the
amount of addition of the reducing agent from addition valve 41 is
increased in order to suppress clogging of injection hole 41A, and
in addition, when engine coolant temperature THW is high, the
degree of increase is made larger than when engine coolant
temperature THW is low, whereby the catalyst bed temperature
becomes higher. On the other hand, if engine coolant temperature
THW exceeds threshold value .alpha. and if the catalyst bed
temperature is likely to exceed the upper limit of the allowable
range, the correction amount is calculated based on engine coolant
temperature THW. As a result of correction using the correction
amount, the target injection amount is decreased. As the fuel in
the decreased target injection amount is injected and burnt, the
temperature of the exhaust gas is lowered. As the increase in the
catalyst bed temperature due to the exhaust gas is made smaller, it
is less likely that the catalyst bed temperature exceeds the upper
limit of the allowable range.
[0090] In decreasing the target injection amount, when engine
coolant temperature THW is high, the correction amount is set to a
value greater than when engine coolant temperature THW is low, and
the target injection amount is corrected to a lower value. In other
words, the degree of decrease in the target injection amount is
varied in accordance with engine coolant temperature THW used for
correcting the target addition interval (correction for increase in
the addition amount). Therefore, when engine coolant temperature
THW is relatively low, that is, when the degree of increase in the
addition amount is relatively small and when the increase in the
catalyst bed temperature due to the increase in the addition amount
is small, the degree of decrease in the target injection amount is
small. Accordingly, such a phenomenon that the target injection
amount is excessively decreased and the output of engine 11 is
unduly lowered is less likely. In addition, when engine coolant
temperature THW is high, that is, when the degree of increase in
the addition amount is great and when the increase in the catalyst
bed temperature due to the increase in the addition amount is
great, the degree of decrease in the target injection amount is
great. Accordingly, such a phenomenon that decrease in the target
injection amount is insufficient (the target injection amount is
greater than an appropriate value) and the catalyst bed temperature
exceeds the upper limit of the allowable range is less likely.
[0091] FIG. 8 shows relation of engine coolant temperature THW, the
catalyst bed temperature, the amount of addition of the reducing
agent, and the correction amount for the target injection amount.
Here, in a temperature region where engine coolant temperature THW
is lower than threshold value .alpha., the target injection amount
is not decreased. In addition, in this temperature region, in order
to suppress clogging of injection hole 41A of addition valve 41,
when engine coolant temperature THW is high, the degree of increase
is made larger than when engine coolant temperature THW is low, so
that the amount of addition of the reducing agent becomes greater.
With the increase in the addition amount, that is, with the
increase in engine coolant temperature THW, the catalyst bed
temperature is also raised and approaches to the upper limit of the
allowable range.
[0092] When engine coolant temperature THW attains to threshold
value .alpha. or greater, the correction amount (>0) in
accordance with engine coolant temperature THW is calculated, and
the target injection amount is decreased by that correction amount.
When fuel in an amount in accordance with the decreased target
injection amount is injected from fuel injection valve 24 and
burnt, the temperature of the exhaust gas is lowered, and the
catalyst bed temperature is accordingly lowered. As a result of
such temperature lowering, such a situation that the catalyst bed
temperature exceeds the upper limit of the allowable range is
suppressed. As deviation between the catalyst bed temperature and
the upper limit of the allowable range becomes greater, further
increase in the addition amount is permitted.
[0093] On the other hand, due to the increase in engine coolant
temperature THW, clogging of injection hole 41A is more likely. If
the addition amount is increased when engine coolant temperature
THW attains to a prescribed value .beta. (>.alpha.), clogging of
injection hole 41A is suppressed, whereas the catalyst bed
temperature is raised with the increase in the addition amount and
approaches the upper limit of the allowable range. In order to
address this, a larger value for the correction amount is set, and
the target injection amount is decreased to a larger extent using
this larger correction amount. When fuel in an amount in accordance
with the decreased target injection amount is injected from fuel
injection valve 24 and burnt, the temperature of the exhaust gas
and the catalyst bed temperature are lowered.
[0094] According to the second embodiment described above as well,
in addition to the effects similar to (1), (2) and (5) described
above as in the first embodiment, the following effect can be
obtained.
[0095] (6) The correction amount in accordance with engine coolant
temperature THW is set, so that the target injection amount is
decreased using that correction amount. As a result of injection
and combustion of the fuel in the decreased target injection
amount, the temperature of the exhaust gas is lowered. Therefore,
such a situation that the catalyst bed temperature exceeds the
upper limit of the allowable range can reliably be suppressed by
lowering the catalyst bed temperature.
[0096] (7) When engine coolant temperature THW is high, the
correction amount is greater than when engine coolant temperature
THW is low. By thus varying the correction amount used for
decreasing the target injection amount in accordance with engine
coolant temperature THW, when engine coolant temperature THW is
relatively low, undue lowering in the output of engine 11 due to
excessive decrease in the target injection amount can be
suppressed. In addition, when engine coolant temperature THW is
high, such a situation that decrease in the target injection amount
is insufficient (the target injection amount is great) and the
catalyst bed temperature exceeds the upper limit of the allowable
range can be suppressed.
[0097] The present invention can be implemented in another
embodiment shown below. [0098] A substance other than the fuel may
be injected from addition valve 41 as the reducing agent. [0099]
The amount of addition of the reducing agent from addition valve 41
may be adjusted by varying a period during which the reducing agent
is added, instead of or in addition to the addition interval above.
[0100] In the embodiments, the addition amount is corrected by
correcting the target addition interval based on engine coolant
temperature THW, because the temperature of addition valve 41 is
affected by the engine coolant. Instead or in addition, the
addition amount (addition interval, addition period) may be
corrected based on a parameter other than engine coolant
temperature THW that affects the temperature of addition valve 41.
For example, the temperature of the exhaust gas or the engine load
may be employed as the parameter. In this case, the addition amount
is increased as the temperature of the exhaust gas is higher or as
the engine load is higher. Alternatively, the addition amount
(addition interval, addition period) may be corrected based on the
detected temperature of addition valve 41 itself. [0101] The
parameter that is varied along with the operation of engine 11 and
affects the catalyst bed temperature may include supercharge
pressure, injection timing, rail pressure, and the like, in
addition to the above-mentioned fuel injection amount. Therefore,
any of these parameters (including the target injection amount) or
combination thereof may be used to carry out control such that the
catalyst bed temperature does not exceed the upper limit of the
allowable range with the increase in the addition amount. For
example, an amount of air supplied to combustion chamber 14 is
increased by raising the supercharge pressure, so that the
temperature of the exhaust gas and hence the catalyst bed
temperature can be lowered. In addition, earlier injection timing
is set (injection timing is advanced) so that the exhaust gas is
emitted while the combustion pressure is low, and the temperature
of the exhaust gas (catalyst bed temperature) can be lowered.
Moreover, the end of an injection period is moved ahead by raising
the rail pressure, so that the temperature of the exhaust gas
(catalyst bed temperature) can be lowered. [0102] Exhaust gas
purifying apparatus 12 of the present invention is applicable to an
engine in which addition valve 41 is arranged at a position distant
from water jacket 45 of cylinder head 23, for example, on the
exhaust downstream side of exhaust port 29. In this case, as
addition valve 41 is less likely to be affected by the heat of the
engine coolant, whether or not the temperature of addition valve 41
exceeds the temperature at which the volatile component in the
reducing agent evaporates is monitored based on a parameter other
than engine coolant temperature THW, and the amount of addition of
the reducing agent is increased based on the result of monitoring.
Examples of such parameters include the temperature of the exhaust
gas, the engine load, and the like as described above.
[0103] It should be understood that the embodiments disclosed
herein are illustrative and non-restrictive in every respect. The
scope of the present invention is defined by the terms of the
claims, rather than the description above, and is intended to
include any modifications within the scope and meaning equivalent
to the terms of the claims.
* * * * *