U.S. patent application number 14/648814 was filed with the patent office on 2015-10-22 for exhaust emission control system for internal combustion engine (as amended).
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Satoshi KOBAYAKAWA.
Application Number | 20150300229 14/648814 |
Document ID | / |
Family ID | 50883370 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150300229 |
Kind Code |
A1 |
KOBAYAKAWA; Satoshi |
October 22, 2015 |
EXHAUST EMISSION CONTROL SYSTEM FOR INTERNAL COMBUSTION ENGINE (AS
AMENDED)
Abstract
An object of the present invention is to increase the NO.sub.x
purification rate of an unactivated SCR catalyst in an exhaust
emission control system for an internal combustion engine that has
an oxidation catalyst and the SCR catalyst disposed downstream of
the oxidation catalyst. In order to achieve this object, the
present invention provides an exhaust emission control system for
an internal combustion engine, including an oxidation catalyst
disposed in an exhaust passage of the internal combustion engine,
an SCR catalyst that is disposed downstream of the oxidation
catalyst in the exhaust passage, and a supply device for supplying
unburnt fuel to the oxidation catalyst when the SCR catalyst is not
yet activated, wherein the amount of NO.sub.2 that flows into the
SCR catalyst is increased by reducing the amount of unburnt fuel
supplied, when a low-load operation of the internal combustion
engine is performed.
Inventors: |
KOBAYAKAWA; Satoshi;
(Sunto-gun, Shizuoka-ken, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi, Aichi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi
JP
|
Family ID: |
50883370 |
Appl. No.: |
14/648814 |
Filed: |
December 2, 2013 |
PCT Filed: |
December 2, 2013 |
PCT NO: |
PCT/JP2013/082314 |
371 Date: |
June 1, 2015 |
Current U.S.
Class: |
60/286 |
Current CPC
Class: |
F02D 2200/602 20130101;
Y02T 10/24 20130101; F02D 41/0255 20130101; F02D 41/1475 20130101;
Y02T 10/42 20130101; B01D 2255/1021 20130101; F02D 41/025 20130101;
Y02T 10/26 20130101; B01D 2257/404 20130101; B01D 2255/1023
20130101; F01N 3/208 20130101; F01N 2900/1621 20130101; Y02T 10/40
20130101; B01D 2255/50 20130101; F01N 2900/1602 20130101; F01N
2900/1404 20130101; B01D 2255/2092 20130101; F02D 41/0235 20130101;
Y02T 10/12 20130101; B01D 53/9477 20130101; B01D 53/9495 20130101;
F01N 2900/1812 20130101; F02D 2200/0802 20130101; F02D 41/0002
20130101; F02D 41/405 20130101; B01D 2251/2062 20130101; B01D
2258/012 20130101; F01N 2610/03 20130101; B01D 2251/2067 20130101;
F01N 3/106 20130101 |
International
Class: |
F01N 3/20 20060101
F01N003/20 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2012 |
JP |
2012-264124 |
Claims
1. An exhaust emission control system for an internal combustion
engine, comprising: an oxidation catalyst disposed in an exhaust
passage of the internal combustion engine; a selective reduction
catalyst that is disposed downstream of the oxidation catalyst in
the exhaust passage; temperature increasing unit for increasing a
temperature of exhaust flowing out of the oxidation catalyst by
supplying unburnt fuel to the oxidation catalyst, when the
oxidation catalyst is activated but the selective reduction
catalyst is not activated; and control unit for reducing the amount
of unburnt fuel to be supplied to the oxidation catalyst by the
temperature increasing unit, when a low-load operation of the
internal combustion engine is performed during a period in which
the unburnt fuel is supplied to the oxidation catalyst by the
temperature increasing unit.
2. The exhaust emission control system for an internal combustion
engine according to claim 1, wherein when a temperature of the
selective reduction catalyst becomes equal to or higher than a
predetermined temperature, the control unit executes either an
increasing process for increasing an intake air amount of the
internal combustion engine or a reducing process for reducing the
intake air amount, whichever process that increases a NO.sub.x
purification rate of the selective reduction catalyst higher.
3. The exhaust emission control system for an internal combustion
engine according to claim 2, wherein the predetermined temperature
is a lowest temperature at which the NO.sub.x purification rate of
the selective reduction catalyst increases as a result of
increasing the amount of nitrogen dioxide contained in the
exhaust.
4. The exhaust emission control system for an internal combustion
engine according to claim 2, wherein when the temperature of the
selective reduction catalyst drops from the predetermined
temperature or higher to less than the predetermined temperature,
the control unit reduces the amount of unburnt fuel to be supplied
to the oxidation catalyst by the temperature increasing unit.
5. The exhaust emission control system for an internal combustion
engine according to claim 3, wherein when the temperature of the
selective reduction catalyst drops from the predetermined
temperature or higher to less than the predetermined temperature,
the control unit reduces the amount of unburnt fuel to be supplied
to the oxidation catalyst by the temperature increasing unit.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust emission control
system that has an oxidation catalyst disposed in an exhaust
passage of an internal combustion engine and a selective reduction
catalyst (also described hereinafter as selective catalytic
reduction (SCR) catalyst) disposed downstream of the oxidation
catalyst in the exhaust passage.
BACKGROUND ART
[0002] An exhaust emission control system having an oxidation
catalyst and SCR catalyst disposed in an exhaust passage of an
internal combustion engine has conventionally been known. A
technique for supplying unburnt fuel (hydrocarbon (HC)) to the
oxidation catalyst for the purpose of increasing the temperatures
of the oxidation catalyst and the SCR catalyst has been proposed as
the exhaust emission control system (see Patent Literature 1).
[0003] Patent Literature 2 discloses a technique pertaining to an
exhaust emission control system with an oxidation catalyst, an SCR
catalyst, and a bypass passage bypassing the oxidation catalyst, in
which the amount of exhaust passing through the bypass passage is
increased when nitrogen dioxide (NO.sub.2) is generated excessively
by the oxidation catalyst.
[0004] Patent Literature 3 discloses a technique for increasing the
temperature of the SCR to a predetermined temperature by supplying
unburnt fuel to the oxidation catalyst when the ratio of NO.sub.2
in NO.sub.x flowing out of the oxidation catalyst is 50%.
PRIOR APT DOCUMENTS
Patent Documents
[0005] Patent Literature 1: Japanese Patent Application
Publication. No. 2002-295277
[0006] Patent Literature 2: Japanese Patent Application Publication
No. 2005-023921
[0007] Patent Literature 3: Japanese Patent Application Publication
No. 2012-007557
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0008] Incidentally, continuous supply of unburnt fuel during a
period between the completion of the activation of the oxidation
catalyst and the start of the activation of the SCR catalyst is
likely to lower the NO.sub.x purification rate of the SCR catalyst.
For instance, when low-load operation of the internal combustion
engine is started during the period between the completion of the
activation of the oxidation catalyst and the start of the
activation of the SCR catalyst, the amount of increase in the
temperature of the SCR catalyst in relation to the amount of
unburnt fuel supplied becomes low, rising the risk of lowering the
NO.sub.x purification rate prior to the activation of the SCR
catalyst.
[0009] The present invention was contrived in view of the foregoing
circumstances, and an object thereof is to provide a technique for
an exhaust emission control system for an internal combustion
engine that has an oxidation catalyst and an SCR catalyst disposed
downstream of the oxidation catalyst, the technique being capable
of increasing the NO.sub.x purification rate when an SCR catalyst
is not yet activated.
Means for Solving the Problem
[0010] In order to solve the foregoing problem, the present
invention accomplishes the following: in an exhaust emission
control system for an internal combustion engine, which has an
oxidation catalyst disposed in an exhaust passage of the internal
combustion engine, a selective reduction catalyst (SCR catalyst)
that is disposed downstream of the oxidation catalyst in the
exhaust passage, and a supply device for supplying unburnt fuel to
the oxidation catalyst when the SCR catalyst is not yet activated,
the amount of NO.sub.2 that flows into the SCR catalyst is
increased by reducing the amount of unburnt fuel supplied, when a
low-load operation of the internal combustion engine is
performed.
[0011] Specifically, the exhaust emission control system for an
internal combustion engine according to the present invention
has:
[0012] an oxidation catalyst disposed in an exhaust passage of the
internal combustion engine;
[0013] a selective reduction catalyst (SCR catalyst) that is
disposed downstream of the oxidation catalyst in the exhaust
passage;
[0014] temperature increasing means for increasing the temperature
of exhaust flowing out of the oxidation catalyst by supplying
unburnt fuel to the oxidation catalyst, when the oxidation catalyst
is activated but the selective reduction catalyst (SCR catalyst) is
not activated; and
[0015] control means for reducing the amount of unburnt fuel to be
supplied from the temperature increasing means, when a low-load
operation of the internal combustion engine is performed during a
period in which the unburnt fuel is supplied to the oxidation
catalyst by the temperature increasing means.
[0016] As a method for activating the oxidation catalyst and the
SCR catalyst, the following method is used in general: a method for
supplying a small amount of unburnt fuel to the oxidation catalyst
prior to the activation of the oxidation catalyst and then
supplying the oxidation catalyst with unburnt fuel in an amount
greater than the amount supplied prior to the activation of the
oxidation catalyst during a period between the activation of the
oxidation catalyst and the activation of the SCR catalyst.
[0017] Incidentally, when the low-load operation of the internal
combustion engine is performed, the temperature of exhaust to be
emitted from the interned combustion engine drops. Therefore, even
when oxidation reaction heat of unburnt fuel is applied to the
exhaust in the oxidation catalyst, the amount of heat transmitted
from the exhaust to the SCR catalyst becomes low. In addition, when
a large amount of unburnt fuel is supplied to the oxidation
catalyst, most of carbon monoxide (NO) of the exhaust is not
oxidized in the oxidation catalyst, resulting in a reduction in the
amount of NO.sub.2 that flows into the SCR catalyst. Moreover,
prior to the activation of the SCR catalyst, in some cases the
amount of increase in the NO.sub.x purification rate becomes lower
than the amount of increase in the temperature of the SCR catalyst.
Therefore, when the amount of unburnt fuel to be supplied to the
oxidation catalyst increases when the low-load operation of the
internal combustion engine is started during the period between the
activation of the oxidation catalyst and the activation of the SCR
catalyst, the NO.sub.x purification rate of the SCR catalyst is
likely to drop.
[0018] When the low-load operation of the internal combustion
engine is performed, a method is considered in which the amount of
NO.sub.x generated upon combustion of the fuel in the cylinders of
an internal combustion engine is reduced by supplying some of the
exhaust into the cylinders in the form of exhaust gas recirculation
(EGR) gas. However, when the SCR catalyst is not activated, there
is a possibility that the internal combustion engine enters a cold
state, making it difficult to supply a sufficient amount of EGR gas
to reduce the amount of NO.sub.x generated.
[0019] On the other hand, the exhaust emission control system for
an internal combustion engine according to the present invention,
when (during the period in which) the low-load operation of the
internal combustion engine is performed during the period between
the activation of the oxidation catalyst and the activation of the
SCR catalyst, the amount of unburnt fuel to be supplied to the
oxidation catalyst is reduced. In this case, because the amount of
unburnt fuel to be oxidized in the oxidation catalyst is reduced,
the amount of NO to be oxidized in the oxidation catalyst
increases. As a result, the amount of NO.sub.2 (NO.sub.2 ratio)
contained in the exhaust flowing into the SCR catalyst increases.
When the exhaust with a high NO.sub.2 ratio flows into the SCR
catalyst, the NO.sub.x purification rate of the SCR catalyst
becomes higher than when the exhaust gas that does not include much
NO.sub.2 flows into the SCR catalyst.
[0020] Therefore, the exhaust emission control system for an
internal combustion engine according to the present invention can
increase the NO.sub.x purification rate of the SCR catalyst when
low-load operation of the internal combustion engine is performed
during the period between the activation of the oxidation catalyst
and the activation of the SCR catalyst. In other words, the exhaust
emission control system for an internal combustion engine according
to the present invention can increase, as much as possible, the
NO.sub.x purification rate of the SCR catalyst when it is difficult
to supply a sufficient amount of EGR gas.
[0021] Note that such expression as "reducing the amount of unburnt
fuel" means not only to reduce she amount of unburnt fuel by values
greater than zero but also to reduce the amount of unburnt fuel to
zero (to stop supplying the unburnt fuel).
[0022] In the exhaust emission control system for an internal
combustion engine according to the present invention, when the
temperature of the SCR catalyst becomes equal or higher than a
predetermined temperature, the control means may execute either an
increasing process for increasing an intake air amount of the
internal combustion engine or a reducing process for reducing the
intake air amount, whichever process that increases the NO.sub.x
purification rate of the SCR catalyst higher. The "predetermined
temperature" here means, for example, the lowest temperature at
which the NO.sub.x purification rate of the SCR catalyst increases
as the NO.sub.2 ratio increases.
[0023] When the reducing process for reducing the intake air amount
is executed when the temperature of the SCR catalyst is equal to or
higher than the predetermined temperature, the NO.sub.2 ratio of
the exhaust flowing into the SCR catalyst increases. As a result,
the NO.sub.x purification rate of the SCR catalyst rises.
[0024] In a case where the temperature of the exhaust is high,
there is a possibility that the NO.sub.x purification rate of the
SCR catalyst becomes higher when the increasing process for
increasing the intake air amount is executed as compared to when
the reducing process for reducing the intake air amount is
executed. In other words, when the increasing process for
increasing the intake air amount is executed when the temperature
of the exhaust is high, the speed at which the temperature of the
SCR catalyst increases accelerates, leading to an acceleration of
the speed at which the NO.sub.x purification rate increases.
[0025] On the other hand, the implementation of the increasing
process for increasing the intake air amount of the internal
combustion engine or the reducing process for reducing the intake
air amount, whichever increases the NO.sub.x purification rate of
the SCR catalyst, can further increase the NO.sub.x purification
rate of the SCR catalyst.
[0026] Note that the implementation of the increasing process for
increasing the intake air amount when the temperature of the SCR
catalyst is high enough is likely to increase the temperature of
the SCR catalyst excessively. Thus, when the temperature of the SCR
catalyst is high enough, the reducing process may be executed prior
to the increasing process for increasing the intake air amount.
[0027] Also, in the exhaust emission control system for an internal
combustion engine according to the present invention, when the
temperature of the SCR catalyst drops from the predetermined
temperature or higher to less than the predetermined temperature,
the control means may reduce the amount of unburnt fuel to be
supplied to the oxidation catalyst by the temperature increasing
means.
[0028] When low-load operation of the internal combustion engine
(e.g., idling) is continued while the temperature of the SCR
catalyst is equal to or higher than the predetermined temperature,
there is a possibility that the temperature of the SCR catalyst
drops to less than the predetermined temperature. In a case where
the temperature of the SCR catalyst decreases from the
predetermined temperature or higher to less than the predetermined
temperature, the temperature increasing means supplies unburnt fuel
to the oxidation catalyst. However, even when unburnt fuel is
supplied to the oxidation catalyst while the low-load operation of
the internal combustion engine is continued as described above, the
speed at which the temperature of the SCR catalyst increases does
not rise enough, and the NO.sub.x purification rate of the SCR
catalyst drops.
[0029] On the other hand, when the amount of unburnt fuel to be
supplied to the oxidation catalyst by the temperature increasing
means is reduced when the temperature of the SCR catalyst drops
from the predetermined temperature or higher to less than the
predetermined temperature, the amount of decrease in the NO.sub.x
purification rate of the SCR catalyst can be kept low.
Effects of the Invention
[0030] According to the present invention, the exhaust emission
control system for an internal combustion engine that has an
oxidation catalyst and an SCR catalyst disposed downstream of the
oxidation catalyst can increase the NO.sub.x purification rate as
much as possible when the SCR catalyst is not yet activated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a diagram showing a schematic configuration of an
internal combustion engine to which the present invention is
applied, and a pumping system of the internal combustion
engine;
[0032] FIG. 2 is a diagram schematically showing a map defining an
operation region where unburnt fuel is supplied and an operation
region where the supply of the unburnt fuel is stopped;
[0033] FIG. 3 is a diagram for explaining a low-load operation
region where the supply of the unburnt fuel is stopped;
[0034] FIG. 4 is a diagram for explaining a high-load operation
region where the unburnt fuel is supplied;
[0035] FIG. 5 is a timing chart that shows chronological changes in
the amount of unburnt fuel supplied to am oxidation catalyst, the
temperature of an SCR catalyst, the NO.sub.2 ratio of exhaust
emitted from the oxidation catalyst, and the NO.sub.x purification
rate of the SCR catalyst, the changes being caused when a
temperature increasing process is executed;
[0036] FIG. 6 is a diagram showing changes in the NO.sub.x
purification rate during a period in which the temperature of the
SCR catalyst increases from a lower limit value to an activating
temperature;
[0037] FIG. 7 is a flowchart showing a processing routine that is
executed by an ECU at the time of the execution of the temperature
increasing process; and
[0038] FIG. 8 is a flowchart showing a processing routine that is
executed by the ECU when the temperature of the SCR catalyst
increases to the activating temperature or higher.
THE BEST MODE FOR CARRYING OUT THE INVENTION
[0039] Specific embodiments of the present invention are described
hereinafter based on the drawings. The sizes, materials, shapes,
relative arrangements and the like of the components described in
the present embodiments are not intended to limit the technical
scope of the present invention thereto, unless otherwise
specifically stated.
Embodiment 1
[0040] First of all, the first embodiment of the present invention
is described based on FIGS. 1 to 7. FIG. 1 is a diagram showing a
schematic configuration of an internal combustion engine to which
the present invention is applied, and a pumping system of the
internal combustion engine. The internal combustion engine 1 shown
in FIG. 1 is a compression ignition internal combustion engine
(diesel engine) with a plurality of cylinders. The internal
combustion engine to which the present invention is applied may not
only be a compression ignition internal combustion engine but also
a spark ignition internal combustion engine (gasoline engine)
operated by lean combustion.
[0041] The internal combustion engine 1 has a fuel injection valve
1a for injecting fuel into the cylinders. An intake passage 2 and
an exhaust passage 3 are connected to the internal combustion
engine 1. The intake passage 2 is a passage that guides fresh air
(air) in the atmosphere to the cylinders of the internal combustion
engine 1. The exhaust passage 3 is a passage that circulates the
burnt gas (exhaust) that is emitted from the cylinders of the
internal combustion engine 1.
[0042] An intake throttle valve (a throttle valve) 4 is disposed in
the middle of the intake passage 2. The throttle valve 4 is a valve
mechanism for regulating the amount of air taken into the cylinders
of the internal combustion engine 1 by changing the cross-sectional
area of the intake passage 2. Note that the throttle valve 4 has a
valve element and an electric motor for opening/closing the valve
element, in which the electric motor is controlled by an ECU 10
described hereinafter.
[0043] A first catalyst casing 5 and a second catalyst casing 6 are
disposed in series in the middle of the exhaust passage 3 at the
upstream side. The first catalyst casing 5 is a tubular casing
having an oxidation catalyst and a particulate filter installed
therein. The oxidation catalyst may be supported by a catalyst
support disposed upstream of the particulate filter or by the
particulate filter. The oxidation catalyst and the particulate
filter may be contained in casings that are independent from each
other.
[0044] The second catalyst casing 6 is a tabular casing having a
catalyst support supporting a selective reduction catalyst (SCR
catalyst) thereon. The catalyst support is obtained by, for
example, coating a monolith substrate with an alumina-based or
zeolite-based active substance (support), the monolith substrate
being composed of a cordierite or Fe--Cr--Al-based heat-resistant
steel and having a honeycomb cross section. Furthermore, a precious
metal catalyst (e.g., platinum (Pt), palladium (Pd), etc.) with an
oxidative capacity is supported on the catalyst support.
[0045] Note that the catalyst support that supports the oxidation
catalyst may be disposed downstream of the SCR catalyst in the
second catalyst casing 6. The oxidation catalyst in this case is to
oxidize a reducing agent that passes through the SCR catalyst out
of the reducing agent supplied to the SCR catalyst.
[0046] An adding valve 7 for adding (injecting) an additive, a
NH.sub.3 or a precursor of NH.sub.3, is attached to the exhaust
passage 3 between the first catalyst casing 5 and the second
catalyst casing 6. The adding valve 7 is a valve unit that has an
injection hole opened and closed by the motion of a needle. The
adding valve 7 is connected to a tank 71 by a pump 70. The pump 70
suctions the additive stored in the tank 71 and pneumatically feeds
the suctioned additive to the adding valve 7. The adding valve 7
injects into the exhaust passage 3 the additive that is
pneumatically fed from the pump 70. Note that the adding valve 7
and pump 70 are an aspect of a reducing agent supply device
according to the present invention.
[0047] A solution such as urea or ammonium carbamate, or NH.sub.3
gas can be used as the additive stored in the tank 71. The present
embodiment describes an example of using a urea aqueous solution as
the additive.
[0048] Once injected from the adding valve 7, the urea aqueous
solution flows into the second catalyst casing 6 along with
exhaust. At this moment, the urea aqueous solution undergoes
pyrolysis or hydrolysis under the heat of exhaust gas and the
second catalyst casing 6. Pyrolysis or hydrolysis of the urea
aqueous solution generates ammonia (NH.sub.3). The generated
ammonia (NH.sub.3) is absorbed by (or stored in) the SCR catalyst.
The ammonia (NH.sub.3) absorbed by the SCR catalyst reacts with
nitrogen oxide (NO.sub.x) contained in the exhaust and generates
nitrogen (N.sub.2) and water (H.sub.2O). In other words, the
ammonia (NH.sub.3) functions as a reducing agent for the nitrogen
oxide (NO.sub.x).
[0049] The internal combustion engine 1 also has an EGR device that
includes an EGR passage 100 communicating the intake passage 2 to
the exhaust passage 3 and an EGR valve 101 for changing the
cross-sectional area of the EGR passage 100. The EGR passage 100 is
a passage for guiding some of the exhaust of the exhaust passage 3
as EGR gas to the intake passage 2 at the downstream of the
throttle valve 4. The EGR valve 101 is a valve mechanism for
changing the cross-sectional area of the EGR passage 100 and
thereby regulating the amount of EGR gas supplied from the exhaust
passage 3 to the intake passage 2. Note that the EGR valve 101 has
a valve element and an electric motor for opening/closing the valve
element, in which the electric motor is controlled by the ECU
10.
[0050] The internal combustion engine 1 with such a configuration
is installed side-by-side with the ECU 10. The ECU 10 is an
electronic control unit with a CPU, a ROM, a RAM, a backup RAM, and
the like. The ECU 10 is electrically connected to a first exhaust
temperature sensor 8, a second exhaust temperature sensor 9, a
crank position sensor 11, an accelerator position sensor 12, an air
flow meter 13, an A/F sensor 14, and various other sensors.
[0051] The first exhaust temperature sensor 6 is disposed in the
exhaust passage 3 between the first catalyst casing 5 and the
second catalyst casing 6, and outputs an electric signal
correlating with the temperature of the exhaust flowing out of the
first catalyst casing 5, that is, the temperature of the oxidation
catalyst stored in the first catalyst casing 5. The second exhaust
temperature sensor 9 is disposed downstream of the second catalyst
casing 6 in the exhaust passage 3, and outputs an electric signal
correlating with the temperature of the exhaust flowing out of the
second catalyst casing 6, that is, the temperature of the SCR
catalyst stored in the second catalyst casing 6. The crank position
sensor 11 outputs an electric signal correlating with the
rotational position of the output shaft (crank shaft) of the
internal combustion engine 1. The accelerator position sensor 12
outputs an electric signal correlating with the amount of operation
(accelerator position) of the accelerator pedal. The air flow meter
13 outputs an electric signal correlating with the amount of air
taken into the internal combustion engine 1 (intake air amount).
The A/F sensor 14 is disposed upstream of the first catalyst casing
5 in the exhaust passage 3 and outputs an electric signal
correlating with the fuel-air ratio of the exhaust.
[0052] The ECU 10 is electrically connected to the fuel injection
valve 1a, throttle valve 4, adding valve 7, pump 70, EGR valve 101,
and various other devices. The ECU 10 electrically controls these
various devices based on the foregoing output signals of the
various sensors. For instance, in addition to the known control
such as fuel injection control of the internal combustion engine 1
and addition control for intermittently injecting the additive from
the adding valve 7, the ECU 10 executes a temperature increasing
process for increasing the temperatures of the oxidation catalyst
stored in the first catalyst casing 5 and the SCR catalyst stored
in the second catalyst casing 6 when these temperatures are low. A
method for executing the temperature increasing process according
to the present embodiment is described hereinafter.
[0053] Cold start of the internal combustion engine 1 brings about
a state in which the oxidation catalyst and the selective reduction
catalyst are not activated, that is, a state in which the oxidation
catalyst cannot oxidize the unburnt fuel components (HC, CO, etc.)
of the exhaust and the selective reduction catalyst cannot reduce
the nitrogen oxide (NO.sub.x) of the exhaust. For this reason, the
oxidation catalyst and selective redaction catalyst need to be
activated promptly.
[0054] In contrast, there is considered a method for supplying a
small amount of unburnt fuel to the oxidation catalyst prior to the
activation of the oxidation catalyst and SCR catalyst and supplying
the oxidation catalyst with unburnt fuel in an amount larger than
the amount supplied prior to the activation of the oxidation
catalyst, during a period between the activation of the oxidation
catalyst and the activation of the SCR catalyst. Note that a method
for adding fuel to the exhaust passage 3 at the upstream of the
oxidation catalyst or performing post injection from the fuel
injection valve into an expansion stroke cylinder or an exhaust
stroke cylinder can be used as the method for supplying unburnt
fuel to the oxidation catalyst.
[0055] Incidentally, when low-load operation of the internal
combustion engine 1 is performed during the period between the
activation of the oxidation catalyst and the activation of the SCR
catalyst, the temperature of the exhaust to be emitted from the
internal combustion engine 1 falls. Therefore, even when oxidation
reaction heat of the unburnt fuel is generated in the oxidation
catalyst, the amount of heat transmitted from the exhaust to the
SCR catalyst becomes low. In addition, when a large amount of
unburnt fuel is supplied to the oxidation catalyst, most of the
carbon monoxide (NO) of the exhaust is not oxidized in the
oxidation catalyst, resulting in a reduction in the amount of
NO.sub.2 flowing into the SCR catalyst. Moreover, when the
temperature of the SCR catalyst is lower than a lower limit value,
in some cases the amount of increase in the NO.sub.x purification
rate becomes lower than the amount of increase in temperature of
the SCR catalyst. Therefore, when the amount of unburnt fuel to be
supplied to the oxidation catalyst increases at the time of the
low-load operation of the internal combustion engine 1 during the
period between the activation of the oxidation catalyst and the
activation of the SCR catalyst, the NO.sub.x purification rate of
the SCR catalyst is likely to drop.
[0056] On the other hand, there is also considered a method for
supplying some of the exhaust into the cylinders of the internal
combustion engine 1 as EGR gas and thereby reducing the amount
NO.sub.x generated when low-load operation of the internal
combustion engine 1 is performed. However, when the SCR catalyst is
not activated, a high possibility that the internal combustion
engine 1 is in a cold state makes it difficult to supply the EGR
gas enough to reduce the amount of amount of NO.sub.x
generated.
[0057] In the temperature increasing process according to the
present embodiment, therefore, during the period between the
activation of the oxidation catalyst and the activation of the SCR
catalyst, the ECU 10 stops the supply of the unburnt fuel to the
oxidation catalyst when the temperature of the SCR catalyst is
equal to or higher than the lower limit value and low-load
operation of the internal combustion engine 1 is performed. The
"lower limit value" is the lowest temperature at which the SCR
catalyst can purify the NO.sub.x under the condition that, for
example, NO.sub.2 is present in the exhaust.
[0058] More specifically, in accordance with the map shown in FIG.
2, the ECU 10 switches between supplying unburnt fuel and stopping
the supply thereof. FIG. 2 is a schematic diagram of the map in
which a load and rotational speed of the internal combustion engine
1 are parameters, the map being defined beforehand by a matching
process that uses the experiments and the like. As shown in FIG. 2,
in a low-load operation region in which the temperature of the
exhaust is low, unburnt fuel is not supplied to the oxidation
catalyst. In a high-load operation region in which the temperature
of the exhaust is high, unburnt fuel is supplied to the oxidation
catalyst. The "low-load operation region" is an operation region in
which, under the assumption that unburnt fuel is supplied to the
oxidation catalyst, a temperature increasing effect of the
oxidation catalyst that is obtained by the reaction heat of the
unburnt fuel and a temperature lowering effect of the oxidation
catalyst that is obtained by the heat transmitted from the
oxidation catalyst to the exhaust cancel each other out.
Specifically, the low-load operation region is an operation region
in which the temperature of the oxidation catalyst exceeds the
temperature of the exhaust when the temperature of the oxidation
catalyst is increased by the supply of unburnt fuel, as shown in
FIG. 3. The "high-load operation region," on the other hand, is an
operation region in which can be attained, under the assumption
that unburnt fuel is supplied to the oxidation catalyst, a synergy
between the temperature increasing effect of the oxidation catalyst
that is obtained by the reaction heat of the unburnt fuel and the
temperature increasing effect of the oxidation catalyst that is
obtained by the heat transmitted from the exhaust to the oxidation
catalyst. Specifically, the high-load operation region is an
operation region in which the temperature of the exhaust becomes
higher than the temperature of the oxidation catalyst when the
temperature of the oxidation catalyst is increased by the supply of
unburnt fuel, as shown in FIG. 4. The alphabet "X" shown in FIGS. 3
and 4 represents a period of time required for the reaction heat of
the unburnt fuel to be reflected in the temperature of the
oxidation catalyst since the start of the supply of the unburnt
fuel.
[0059] FIG. 5 is a timing chart that snows chronological changes in
the amount of unburnt fuel supplied to the oxidation catalyst, the
temperature of the SCR catalyst, the NO.sub.2 ratio of the exhaust
emitted from the oxidation catalyst, and the NO.sub.x purification
rate of the SCR catalyst, the changes being caused when supplying
unburnt fuel and stopping the supply thereof are switched in
accordance with the map shown in FIG. 2. The solid lines shown in
FIG. 5 each show chronological changes that occur when switching
between supplying unburnt fuel and stopping the supply thereof in
accordance with the operating state of the internal combustion
engine 1, while the dashed lines shown in FIG. 5 each show
chronological changes that occur when the unburnt fuel is supplied
regardless of the operating state of the internal combustion engine
1.
[0060] In FIG. 5, during the period in which the temperature of the
SCR catalyst is equal to or higher than a lower limit value Ts0 and
low-load operation of the internal combustion engine 1 is performed
(the period between t1 and t2 shown in FIG. 5), the amount of NO
that is oxidized in the oxidation catalyst becomes higher when the
supply of the unburnt fuel is stopped, as compared to wren the
supply of the unburnt fuel is not stopped. As a result, although
the speed at which the temperature of the SCR catalyst increases
becomes lower when the supply of the unburnt fuel is stopped as
compared to when the supply of the unburnt fuel is not stopped, the
NO.sub.2 ratio of the exhaust increases. As a result, the NO.sub.x
purification rate of the SCR catalyst becomes higher when the
unburnt fuel is not supplied, as compared to when the unburnt fuel
is supplied. Note that the process for switching between supplying
unburnt fuel and stopping the supply thereof is executed until the
temperature of the SCR catalyst reaches an activating temperature.
The "activating temperature" here is a temperature corresponding to
"predetermined temperature" according to the present invention, the
lowest temperature at which the NO.sub.x purification rate of the
SCR catalyst increases as a result of increasing the NO.sub.2 ratio
of the exhaust.
[0061] Therefore, the NO.sub.x purification rate of the SCR
catalyst in the period between the activation of the oxidation
catalyst and the activation of the SCR catalyst (the period in
which the temperature of the SCR catalyst increases from the lower
limit value to the activating temperature) becomes higher when
switching between supplying unburnt fuel and stopping the supply
thereof in accordance with the operating state of the internal
combustion engine 1, as compared to when the unburnt fuel is
supplied regardless of the operating state of the internal
combustion engine 1 (see FIG. 6). Note that the solid line shown in
FIG. 6 indicates the NO.sub.x purification rate obtained when
switching between supplying unburnt fuel and stopping the supply
thereof in accordance with the operating state of the internal
combustion engine 1, while the dashed line indicates the NO.sub.x
purification rate obtained when unburnt fuel is supplied regardless
of the operating state of the internal combustion engine 1. The
term Ts0 shown in FIG. 6 represents the lower limit value, and the
term Ts1 represents the activating temperature of the SCR
catalyst.
[0062] A procedure for executing the temperature increasing process
according to the present embodiment is described hereinafter with
reference to FIG. 7. FIG. 7 is a flowchart showing a processing
routine that is executed by the ECU 10 at the time of the execution
of the temperature increasing process. The processing routine of
FIG. 7 is stored in the ROM or the like of the ECU 10 in advance
and executed periodically by the ECU 10 (CPU).
[0063] In the processing routine of FIG. 7, first, the ECU 10
determines in the process of S101 whether the temperature of the
oxidation catalyst is equal to or higher than an activating
temperature Tdoc. The "activating temperature Tdoc" here means the
lowest temperature expressed by the oxidation ability of the
oxidation catalyst. It is assumed that the output signal of the
first exhaust temperature sensor 8 is used as the temperature of
the oxidation catalyst.
[0064] When the result of the determination of the process of S101
is negative, the ECU 10 executes the process of S101 again. When,
on the other hand, the result of the determination of the process
of S101 is positive, the ECU 10 proceeds to the process of
S102.
[0065] In the process of S102, the ECU 10 supplies unburnt fuel to
the oxidation catalyst. In so doing, the ECU 10 supplies the
unburnt fuel to the oxidation catalyst by causing the fuel
injection valve 1a of the expansion stroke cylinder or exhaust
stroke cylinder to inject fuel (post injection). Note that the
temperature increasing means according to the present invention is
realized by causing the ECU 10 to execute the process of S102.
[0066] In S103, the ECU 10 determines whether the temperature of
the SCR catalyst is equal to or higher than the lower limit value
Ts0. It is assumed that the output signal of the second exhaust
temperature sensor 9 is used as the temperature of the SCR
catalyst. When the result of the determination of the process of
S103 is negative, the ECU 10 returns to the process of S101.
However, when the result of the determination of the process of
S103 is positive, the ECU 10 proceeds to the process of S104.
[0067] In the process of S104, the ECU 10 determines whether the
internal combustion engine 1 is in the low-load operation state. In
other words, the ECU 10 determines whether the operating state of
the internal combustion engine 1 defined from the engine load and
engine speed belongs to the "stop the supply of unburnt fuel"
section shown in FIG. 2. In so doing, it is assumed that the ECU 10
uses the output signal of the accelerator position sensor 12
(accelerator position) as the engine load.
[0068] When the result of the determination of the process of S104
is positive, the ECU 10 proceeds to the process of S105. In the
process of S105, the ECU 10 stops the supply of the unburnt fuel to
the oxidation catalyst. In this case, the NO.sub.2 ratio of the
exhaust flowing out of the oxidation catalyst increases, resulting
in an increase in the NO.sub.x purification rate of the SCR
catalyst. Note that the control means according to the present
invention is realized by causing the ECU 10 to execute the process
of S105.
[0069] On the other hand, when the result of the determination of
the process of S104 was negative, the ECU 10 proceeds to the
process of S106. In the process of S106, the ECU 10 continues to
supply the unburnt fuel to the oxidation catalyst. This increases
the amount of heat that is contained in the exhaust flowing out of
the oxidation catalyst, thereby rapidly increasing the temperature
of the SCR catalyst. Such rapid increase in the temperature of the
SCR catalyst leads to an increase in the NO.sub.x purification rate
of the SCR catalyst.
[0070] Subsequent to the execution of the process of S105 or S106,
the ECU 10 executes the process of S107. In other words, in the
process of S107, the ECU 10 determines whether the temperature of
the SCR catalyst has increased to the activating temperature Ts1 or
higher.
[0071] When the result of the determination of the process S107 is
negative, the ECU 10 returns to the process of S104. When the
result of the determination of the process S107 is positive, the
ECU 10 proceeds to the process of S108 to end the supply of the
unburnt fuel to the oxidation catalyst.
[0072] The NO.sub.x purification rate of the SCR catalyst can be
increased as much as possible during the period between the
activation of the oxidation catalyst and the activation of the SCR
catalyst by causing the ECU 10 to execute the temperature
increasing process in accordance with the processing routine of
FIG. 7, as described above.
Embodiment 2
[0073] The second embodiment of the present invention is described
next. The second embodiment provides the configurations different
from those of the first embodiment, in which the descriptions of
the same configurations are omitted.
[0074] The first embodiment has described an example in which the
NO.sub.x purification rate is increased during the period in which
the temperature of the SCR catalyst increases from the lower limit
value to the activating temperature. However, the present
embodiment describes an example in which the NO.sub.x purification
rate is increased when the temperature of the SCR catalyst is equal
to or higher than the activating temperature.
[0075] When the temperature of the SCR catalyst reaches the
activating temperature and the intake air amount of the internal
combustion engine 1 is reduced, the NO.sub.2 ratio of the exhaust
flowing into the SCR catalyst increases. As a result, the NO.sub.x
purification rate of the SCR catalyst rises.
[0076] Incidentally, under the condition that the temperature of
the exhaust increases, there is a possibility that the NO.sub.x
purification rate of the SCR catalyst becomes higher when the
intake air amount is increased, as compared to when the intake air
amount is reduced. In other words, increasing the intake air amount
of the internal combustion engine 1 when the temperature of the
exhaust is high rapidly accelerates the speed at which the
temperature of the SCR catalyst increases, leading to an
acceleration of the speed at which the NO.sub.x purification rate
increases.
[0077] Of the process for increasing the intake air amount of the
internal combustion engine 1 (the increasing process) and the
process tor reducing the intake air amount of the internal
combustion engine 1 (the reducing process), the exhaust emission
control system for an internal combustion engine according to the
present embodiment executes the one that leads to an increase in
the NO.sub.x purification rate of the SCR catalyst, when the
temperature of the SCR catalyst becomes equal to or higher than the
activating temperature.
[0078] Specifically, the ECU 10 first computes the amount of
increase in the temperature of the SCR catalyst, with the amount of
increase in the temperature of the oxidation catalyst and the
amount of increase in the intake air amount as parameters.
Subsequently, the ECU 10 computes the amount of increase in the
NO.sub.x purification rate, with the amount of increase in the
temperature of the SCR catalyst as a parameter (referred to as
"first increase amount," hereinafter). The ECU 10 also computes the
amount of increase in the NO.sub.x purification rate, with the
amount of decrease in the temperature of the SCR catalyst, the
amount of decrease in the intake air amount, and the NO.sub.2 ratio
as parameters (referred to as "second increase amount,"
hereinafter). The ECU 10 executes the increasing process when the
first increase amount is greater than the second increase amount,
but executes the reducing process when the second increase amount
is greater than the first increase amount.
[0079] When such a method switches between the increasing process
for increasing the intake air amount and the reducing process for
reducing the intake air amount, the NO.sub.x purification rate that
is obtained when the temperature of the SCR catalyst is equal to or
greater than the activating temperature can be increased as much as
possible.
[0080] However, when the increasing process for increasing the
intake air amount is executed when the temperature of the SCR
catalyst is already nigh enough, there is a possibility that the
temperature of the SCR catalyst becomes excessively high.
Therefore, when the temperature of the SCR catalyst is sufficiently
high, the execution of the reducing process for reducing the intake
air amount may be prioritized over the increasing process for
increasing the intake air amount.
[0081] Because the exhaust has a great amount of neat when the
internal combustion engine 1 is in the high-load operation state,
the NO.sub.x purification rate obtained when the increasing process
is executed is higher than the NO.sub.x purification rate obtained
when the reducing process is executed. On the other hand, because
the exhaust has a small amount of heat when the internal combustion
engine 1 is in the low-load operation state, the NO.sub.x
purification rate obtained when the reducing process is executed is
higher than the NO.sub.x purification rate obtained when the
increasing process is executed. The ECU 10, therefore, may execute
the reducing process when the internal combustion engine 1 is in
the low-load operation state, and execute the increasing process
when the internal combustion engine 1 is in the high-load operation
state.
Embodiment 3
[0082] The third embodiment of the present invention is described
next based on FIG. 8. This embodiment provides the configurations
different from those of the first embodiment, in which the
descriptions of the same configurations are omitted.
[0083] The first embodiment has described an example in which the
NO.sub.x purification rate is increased during the period in which
the temperature of the SCR catalyst increases from the lower limit
value to the activating temperature. The present embodiment, on the
other hand, describes an example in which the NO.sub.x purification
rate is increased when the temperature of the SCR catalyst drops
from the temperature range of equal to or higher than the
activating temperature to a temperature range of less than the
activating temperature.
[0084] When the low-load operation of the internal combustion
engine 1 (e.g., idling) is continued while the temperature of the
SCR catalyst is equal to or higher than the activating temperature,
there is a possibility that the temperature of the SCR catalyst
drops to less than the activating temperature. In other words, when
the temperature of the SCR catalyst drops from the activating
temperature or higher to less than the activating temperature, it
can be assumed that the low-load operation state of the internal
combustion engine 1 is continued. When unburnt fuel is supplied to
the oxidation catalyst during the low-load operation of the
internal combustion engine 1, there is a possibility that not only
is the temperature of the SCR catalyst not increased adequately,
but also that the NO.sub.x purification rate of the SCR catalyst
drops due to the decrease in the NO.sub.2 ratio of the exhaust.
[0085] In the exhaust emission control system for an internal
combustion engine according to the present embodiment, when the
temperature of the SCR catalyst drops from the activating
temperature or higher to less than the activating temperature, the
ECU 10 does not supply unburnt fuel to the oxidation catalyst. In
other words, when the temperature of the SCR catalyst drops from
the activating temperature or higher to less than the activating
temperature, the ECU 10 does not supply unburnt fuel to the
oxidation catalyst, as in the case where the temperature of the SCR
catalyst is equal to or higher than the lower limit value but less
than the activating temperature and the low-load operation of the
internal combustion engine 1 is performed. This method can minimize
the amount of decrease in the NO.sub.x purification rate of the SCR
catalyst when the temperature of the SCR catalyst drops from the
activating temperature or higher to less than the activating
temperature.
[0086] A procedure for controlling the supply of unburnt fuel
according to the present embodiment is described hereinafter with
reference to FIG. 8. FIG. 8 is a flowchart showing a processing
routine that is executed by the ECU 10 when the temperature of the
SCR catalyst increases to the activating temperature or higher.
This processing routine is stored in advance in the ROM or the like
of the ECU 10.
[0087] In the processing routine shown in FIG. 8, the ECU 10 first
determines in the process of S201 whether the temperature of the
SCR catalyst is lowered to less than the activating temperature
Ts1. When the result of the determination of the process of S201 is
negative, the ECU 10 executes the process of S201 again. However,
when the result of the determination of the process of S201 is
positive, the ECU 10 proceeds to the process of S202, to limit the
supply of unburnt fuel to the oxidation catalyst. In other words,
the ECU 10 prohibits the supply of unburnt fuel to the oxidation
catalyst. Note that the ECU 10 may execute the processing routine
of the first embodiment (see FIG. 7) after executing the process of
S202.
[0088] The foregoing embodiments can prevent a decrease in the
NO.sub.x purification rate of the SCR catalyst while preventing an
increase in the fuel consumption accompanied by the supply of
unburnt fuel, when the temperature of the SCR catalyst drops from
the activating temperature or higher to less than the activating
temperature.
EXPLANATION OF THE REFERENCE NUMERALS
[0089] 1 Internal combustion engine
[0090] 1a Fuel injection valve
[0091] 2 Intake passage
[0092] 3 Exhaust passage
[0093] 4 Throttle valve
[0094] 5 First catalyst casing
[0095] 6 Second catalyst casing
[0096] 7 Adding valve
[0097] 8 First exhaust temperature sensor
[0098] 9 Second exhaust temperature sensor
[0099] 10 ECU
[0100] 14 A/F sensor
[0101] 70 Pump
[0102] 71 Tank
* * * * *