U.S. patent application number 11/596975 was filed with the patent office on 2008-03-20 for exhaust gas purifying device of an internal combustion engine.
Invention is credited to Nobuhiro Kondo, Minehiro Murata, Yoshinori Takahashi, Yoshinaka Takeda, Yasuhiro Tsutsui.
Application Number | 20080066449 11/596975 |
Document ID | / |
Family ID | 35394215 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080066449 |
Kind Code |
A1 |
Murata; Minehiro ; et
al. |
March 20, 2008 |
Exhaust Gas Purifying Device Of An Internal Combustion Engine
Abstract
There is provided an oxidizing catalyst located in the upstream
of the exhaust gas purifying catalyst. The catalyst capacity of the
oxidizing catalyst is set so that, in the relationship between a
value obtained by dividing exhaust flow velocity by the catalyst
capacity (exhaust flow velocity/catalyst capacity) and catalyst
activatibility, when the exhaust flow velocity is exhaust flow
velocity at the time of idle operation of the internal combustion
engine, the catalyst activatibility is maximum (prescribed maximum
activation range) and the value obtained by dividing the exhaust
flow velocity by the catalyst capacity is maximum or close to the
maximum.
Inventors: |
Murata; Minehiro; (Tokyo,
JP) ; Takeda; Yoshinaka; (Tokyo, JP) ;
Tsutsui; Yasuhiro; (Tokyo, JP) ; Kondo; Nobuhiro;
(Tokyo, JP) ; Takahashi; Yoshinori; (Tokyo,
JP) |
Correspondence
Address: |
JACOBSON HOLMAN PLLC
400 SEVENTH STREET N.W.
SUITE 600
WASHINGTON
DC
20004
US
|
Family ID: |
35394215 |
Appl. No.: |
11/596975 |
Filed: |
May 12, 2005 |
PCT Filed: |
May 12, 2005 |
PCT NO: |
PCT/JP05/08706 |
371 Date: |
November 20, 2006 |
Current U.S.
Class: |
60/285 ;
60/299 |
Current CPC
Class: |
Y02T 10/12 20130101;
F02D 41/401 20130101; Y02T 10/26 20130101; Y02T 10/44 20130101;
F02B 29/0406 20130101; Y02T 10/40 20130101; F02D 41/403 20130101;
F01N 3/0814 20130101; F02D 41/0245 20130101; Y02T 10/42 20130101;
F02M 26/05 20160201; F02M 26/23 20160201; F01N 13/009 20140601;
F02B 37/00 20130101; B01D 53/9495 20130101; F02D 41/08 20130101;
F01N 3/2006 20130101; F02D 41/0275 20130101; F01N 3/0842 20130101;
F02M 26/54 20160201; F02D 41/0002 20130101 |
Class at
Publication: |
060/285 ;
060/299 |
International
Class: |
F01N 3/20 20060101
F01N003/20 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 2004 |
JP |
2004-149202 |
Claims
1. An exhaust gas purifying device of an internal combustion
engine, comprising: an exhaust gas purifying catalyst interposed in
an exhaust passage of an internal combustion engine for purifying a
harmful component contained in exhaust gas; and an oxidizing
catalyst disposed in an upstream of the exhaust gas purifying
catalyst, wherein: catalyst capacity of the oxidizing catalyst is
set so that, in relationship between a value obtained by dividing
exhaust flow velocity by the catalyst capacity and catalyst
activatibility, when the exhaust flow velocity is exhaust flow
velocity at the time of idle operation of the internal combustion
engine, the catalyst activatibility is maximum and the value
obtained by dividing the exhaust flow velocity by the catalyst
capacity is maximum or close to the maximum.
2. The exhaust gas purifying device of an internal combustion
engine according to claim 1, wherein the exhaust gas purifying
catalyst includes a NOx catalyst.
3. The exhaust gas purifying device of an internal combustion
engine according to claim 1, wherein: the internal combustion
engine is a diesel engine having a throttle valve in an intake
system, and includes engine control means that has throttle control
means arranged for performing an opening/closing operation of the
throttle valve and controls operation of the diesel engine; the
throttle control means closes the throttle valve down to prescribed
opening when the internal combustion engine is in the idle
operation; and the catalyst capacity of the oxidizing catalyst is
set so that, when the internal combustion engine is in the idle
operation and the exhaust flow velocity is exhaust flow velocity in
a state where the throttle valve is closed down to the prescribed
opening by the engine control means, the catalyst activatibility is
maximum and the value obtained by dividing the exhaust flow
velocity by the catalyst capacity is maximum or close to the
maximum.
4. The exhaust gas purifying device of an internal combustion
engine according to claim 3, wherein: the engine control means
further includes fuel injection control means arranged for
controlling a fuel injection amount and fuel injection timing; and
when the internal combustion engine is in the idle operation, the
fuel injection control means retards the fuel injection timing of
main injection to be later than normal timing, and carries out
pilot injection in which the fuel injection amount is smaller than
that of the main injection immediately before the main injection at
least once or more.
5. The exhaust gas purifying device of an internal combustion
engine according to claim 4, wherein the fuel injection control
means retards the fuel injection timing of the main injection to
the vicinity of a misfire limit when the internal combustion engine
is in the idle operation.
6. The exhaust gas purifying device of an internal combustion
engine according to claim 1, wherein: the internal combustion
engine is a diesel engine, and includes engine control means that
has fuel injection control means arranged for controlling a fuel
injection amount and fuel injection timing, and that controls
operation of the diesel engine; and when the internal combustion
engine is in the idle operation, the fuel injection control means
retards fuel injection timing of main injection to be later than
normal timing and carries out pilot injection in which the fuel
injection amount is smaller than that of the main injection
immediately before the main injection at least once or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purifying
device of an internal combustion engine, and more specifically to
technology of improving exhaust gas purifying performance of a
diesel engine at low idle operation.
BACKGROUND ART
[0002] In a vehicle equipped with a diesel engine, the air-fuel
ratio in the diesel engine during normal drive is lean, so that NOx
is easily produced. Therefore, for the purpose of NOx purification,
it has been considered to dispose a NOx catalyst as an exhaust gas
purifying catalyst in an exhaust system. Examples of the NOx
catalyst include an absorption-type NOx catalyst, a selective
catalytic reduction system (SCR system), etc.
[0003] The absorption-type NOx catalyst serves functions in
absorbing NOx in an oxidizing atmosphere and in reducing the
absorbed NOx in a reducing atmosphere. The NOx-absorbing function
and the NOx-reducing function (NOx purification rate) have a
characteristic that these functions are not activated (light-off)
unless catalyst temperature is a certain temperature or higher
temperature as illustrated in FIG. 6.
[0004] Therefore, for example, during low idle (normal idle
operation) of the diesel engine, a fuel injection amount is
extremely small, so that exhaust gas temperature, or catalyst
temperature, is liable to be lower than light-off temperature. As a
result, the NOx catalyst does not fully carries out its functions
during low idle, leading to the problem that the NOx-purifying
performance is degraded.
[0005] In light of such problem, there is technology that has been
developed. According to the technology, an oxidizing catalyst is
separately provided in the upstream of the catalyst to be disposed
in a position where the exhaust gas temperature is relatively high
(for example, near an exhaust manifold or a turbocharger). By using
the oxidative reaction heat of the oxidizing catalyst that is
easily activated because of a high-temperature atmosphere, the
temperature of the exhaust gas purifying catalyst located in the
downstream is raised (for example, Unexamined Japanese Patent
Application Publication No. 10-159545).
[0006] As illustrated in FIG. 7, the oxidizing catalyst has a
characteristic that it is not activated (light-off) and it does not
provide a sufficient HC and CO purification rate unless the
catalyst temperature is equal to or higher than a certain
temperature, as in the case of the NOx catalyst. To be specific,
the oxidizing catalyst has a characteristic that HC and CO are
trapped by absorption till light-off temperature is reached, and
oxidative reaction of the trapped HC and CO gradually occurs once
the light-off temperature is reached, and then the oxidative
reaction of the HC and CO is accelerated as the catalyst
temperature is increased.
[0007] Such characteristic of the oxidizing catalyst is not a
matter, for example, if low idle is maintained and temperature is
moderately increased till the oxidizing catalyst is brought into a
fully activated state. However, for example, if a state in which
the temperature of the oxidizing catalyst is equal to or less than
the light-off temperature lasts for a long time, and thereafter the
diesel engine is accelerated to increase load and exit the low idle
state, the temperature of the oxidizing catalyst is rapidly
increased to exceed the light-off temperature along with the load
increase. This creates the problem that the oxidative reaction of
the trapped HC and CO occurs all at once to generate a large amount
of reaction heat, which induces an excessive temperature rise of
the oxidizing catalyst.
[0008] The greater the catalyst capacity of the oxidizing catalyst
is, or the more time it takes before the light-off temperature is
reached, the larger the amount of the HC and CO trapped in the
oxidizing catalyst is, and the more distinct the above-mentioned
problem becomes.
DISCLOSURE OF THE INVENTION
[0009] The present invention has been made to solve the
above-mentioned problem. It is an object of the invention to
provide an exhaust gas purifying device of an internal combustion
engine, which is capable of securely maintaining an exhaust gas
purifying catalyst at temperature equal to or higher than light-off
temperature by using oxidative reaction heat of an oxidizing
catalyst disposed in upstream during idle operation, and of
preventing an excessive temperature rise of the oxidizing
catalyst.
[0010] In order to accomplish the above-mentioned object, an
exhaust gas purifying device of the present invention comprises an
exhaust gas purifying catalyst interposed in an exhaust passage of
an internal combustion engine for purifying a harmful component
contained in exhaust gas, and an oxidizing catalyst disposed in the
upstream of the exhaust gas purifying catalyst, wherein catalyst
capacity of the oxidizing catalyst is set so that, in relationship
between a value obtained by dividing exhaust flow velocity by the
catalyst capacity and catalyst activatibility, when the exhaust
flow velocity is exhaust flow velocity at the time of idle
operation of the internal combustion engine, the catalyst
activatibility is maximum and the value obtained by dividing the
exhaust flow velocity by the catalyst capacity is maximum or close
to the maximum.
[0011] FIG. 8 shows the relationship between the value obtained by
dividing the exhaust flow velocity in the oxidizing catalyst by the
catalyst capacity (exhaust flow velocity/catalyst capacity) and the
catalyst activatibility (CO and HC purification rate). It is
apparent from FIG. 8 that the oxidizing catalyst has a
characteristic that the catalyst activatibility is great to promote
oxidative reaction in a range where the exhaust flow
velocity/catalyst capacity is small, but when the exhaust flow
velocity/catalyst capacity grows larger and exceeds a certain
value, the catalyst activatibility is sharply deteriorated to
attenuate the oxidative reaction. According to FIG. 8, the greater
the catalyst capacity of the oxidizing catalyst is in relation to
the exhaust flow velocity at the time of idle operation (low idle),
the smaller the exhaust flow velocity/catalyst capacity at the time
of the idle operation becomes.
[0012] However, if the exhaust flow velocity/catalyst capacity at
the time of the idle operation is small as described, even if the
load of the internal combustion engine is increased (increase in
exhaust flow velocity), the catalyst activatibility is great, and a
state in which the oxidative reaction is promoted is maintained for
a while.
[0013] One of the reasons for the above-mentioned problem of the
excessive temperature rise of the oxidizing catalyst is because the
state in which the oxidative reaction is promoted continues even
after the load of the internal combustion engine is increased due
to the above-described relationship between the exhaust flow
velocity/catalyst capacity and the catalyst activatibility.
[0014] The present invention has been made in light of such
knowledge. Therefore, according to the invention claimed in claim
1, the catalyst capacity of the oxidizing catalyst disposed in the
upstream of the exhaust gas purifying catalyst is set so that, when
the exhaust flow velocity is exhaust flow velocity at the time of
idle operation of the internal combustion engine, the catalyst
activatibility is maximum (for example, prescribed maximum
activation range) and the value obtained by dividing the exhaust
flow velocity by the catalyst capacity is maximum or close to the
maximum.
[0015] In other words, as illustrated in FIG. 2, the exhaust flow
velocity/catalyst capacity is regulated, and the catalyst capacity
of the oxidizing catalyst is set so that the exhaust flow
velocity/catalyst capacity at the time of the idle operation (low
idle) is maximum or close to the maximum within the prescribed
maximum activation range of the catalyst activatibility, and more
specifically so that the exhaust flow velocity/catalyst capacity is
close to a value immediately before it begins to decrease
(activation limit).
[0016] If the exhaust flow velocity/catalyst capacity is regulated,
and the catalyst capacity of the oxidizing catalyst is set as
described above, the exhaust gas purifying catalyst is smoothly
raised in temperature due to reaction heat of the oxidative
reaction of the oxidizing catalyst since the catalyst
activatibility is great during the idle operation. On the other
hand, when the load of the internal combustion engine grows larger
(increase in exhaust flow velocity), the catalyst activatibility is
drastically deteriorated, and the oxidative reaction markedly
subsides.
[0017] As a consequence, the oxidizing catalyst functions almost
only during idle operation, and the exhaust gas purifying catalyst
is securely maintained at temperature equal to or higher than the
light-off temperature during the idle operation. At the same time,
if the internal combustion engine is accelerated to increase the
load when the temperature of the oxidizing catalyst is equal to or
less than the light-off temperature, and exits the idle operation
state, for example, as in case that the vehicle is started
immediately after an engine is started or as at the time of
acceleration after deceleration, the catalyst temperature is raised
along with the load increase, while the oxidative reaction in the
oxidizing catalyst notably subsides. Accordingly, the HC and the
CO, which is trapped by the oxidizing catalyst before the light-off
temperature is reached, do not cause oxidative reaction all at
once. This suppresses generation of a large amount of reaction heat
in the oxidizing catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic structure view of an exhaust gas
purifying device of an internal combustion engine installed in a
vehicle according to the present invention;
[0019] FIG. 2 is a view showing exhaust flow velocity/catalyst
capacity at low idle according to the present invention in
relationship between the exhaust flow velocity/catalyst capacity
and catalyst activatibility;
[0020] FIG. 3 is a flowchart showing a control routine of low idle
exhaust gas temperature rise control according to the present
invention;
[0021] FIG. 4 is a view showing oxidizing catalyst temperature (a)
and HC and CO concentration (b) when a throttle valve is
closed;
[0022] FIG. 5 is a view showing main injection timing and pilot
injection timing when fuel injection timing is retarded;
[0023] FIG. 6 is a view showing relationship between temperature of
a NOx catalyst and a NOx purification rate;
[0024] FIG. 7 is a view showing relationship between the
temperature of the oxidizing catalyst and a CO and HC purification
rate; and
[0025] FIG. 8 is a view showing conventional exhaust flow
velocity/catalyst capacity at the time of low idle in the
relationship between the exhaust flow velocity/catalyst capacity
and the catalyst activatibility.
BEST MODE OF CARRYING OUT THE INVENTION
[0026] An embodiment of the present invention will be explained
below with reference to accompanying drawings.
[0027] FIG. 1 is a schematic structure view of an exhaust gas
purifying device of an internal combustion engine installed in a
vehicle according to the present invention. A structure of the
exhaust gas purifying device of an internal combustion engine
according to the present invention will be explained below with
reference to FIG. 1.
[0028] As illustrated in FIG. 1, an engine 1 that is an internal
combustion engine is, for example, a common rail-type in-line
four-cylinder diesel engine. In the common rail-type diesel engine
1, an electromagnetic fuel injection nozzle 4 is disposed in each
cylinder to face a corresponding combustion chamber 2. Each of the
fuel injection nozzles 4 is connected to a common rail 6 through a
high-pressure pipe. Although not shown, the common rail 6 is
connected to a high-pressure pump through a high-pressure pipe and
is further connected to a fuel tank through a low-pressure pipe. As
the engine 1 is a diesel engine, light oil is used as fuel.
[0029] An electromagnetic (DC motor-type) throttle valve 12 is
interposed in an intake passage 10 of the engine 1. Disposed
upstream from the throttle valve 12 is an airflow sensor (AFS) 18
for detecting an intake air amount Qa through an intercooler 14 and
a compressor of a turbocharger 16. The throttle valve 12 is formed,
for example, of a butterfly valve. As the airflow sensor 18, for
example, a hot-wire airflow sensor is employed here, but a Karman
Vortex airflow sensor or the like may be employed.
[0030] An absorption-type NOx catalyst (exhaust gas purifying
catalyst) 22 is interposed in an exhaust passage 20. The
absorption-type NOx catalyst 22 contains alkali metal such as
potassium (K) or alkali earth metal such as Barium (Ba) serving as
a NOx-absorbing agent in addition to a noble metal such as, for
example, platinum (Pt). The absorption-type NOx catalyst 22 absorbs
NOx (harmful component) contained in exhaust gas in an oxidizing
atmosphere where an excess air ratio .lamda. of exhaust gas is high
(that is, lean air-fuel ratio). On the other hand, the
absorption-type NOx catalyst 22 discharges the absorbed NOx in a
reducing atmosphere where the excess air ratio .lamda. of exhaust
gas is low (that is, rich air-fuel ratio), and reduces the NOx
(hereinafter referred to as NOx purge), to thereby purify the NOx.
Because of the noble metal, the absorption-type NOx catalyst 22 has
an oxidizing function at the same time and is capable of purifying
HC and CO as well.
[0031] Disposed immediately upstream of the absorption-type NOx
catalyst 22 in the exhaust passage 20 is a reducing agent injector
24 that mixes a reducing agent (light oil or the like) into exhaust
gas in order to lower the excess air ratio .lamda. of the exhaust
gas during NOx purge.
[0032] Interposed upstream from the absorption-type NOx catalyst 22
in the exhaust passage 20 is a miniature oxidizing catalyst 26 so
as to be positioned immediately downstream of a turbine of the
turbocharger 16. The turbine of the turbocharger 16 is disposed to
be positioned immediately downstream of an exhaust manifold, not
shown, of the engine 1.
[0033] The oxidizing catalyst 26 is a widely used oxidizing
catalyst and is constructed to include a noble metal such as
platinum (Pt). In this application, the oxidizing catalyst 26 is
provided for the purpose of oxidizing treatment of HC, CO and the
like contained in exhaust gas and a temperature rise of the
absorption-type NOx catalyst 22 located in a downstream due to
reaction heat of the oxidative reaction. In other words, the
oxidizing catalyst 26 is located near the exhaust manifold, so that
it is maintained in an activated (light-off) state due to
relatively high exhaust heat even during low idle (idle operation)
in which exhaust gas temperature is low. Therefore, the oxidizing
catalyst 26 is capable of performing the oxidizing treatment of HC
and CO contained in exhaust gas any time. On the other hand, the
absorption-type NOx catalyst 22 is located away from the engine 1,
so that temperature of the absorption-type NOx catalyst tends to
decrease and the absorption-type NOx catalyst tends to be brought
out of the light-off state during low idle. In this application,
the oxidizing catalyst 26 is disposed in the upstream of the
absorption-type NOx catalyst 22 to enable a temperature rise and
temperature maintenance of the absorption-type NOx catalyst 22
located in the downstream by using the reaction heat produced by
the oxidative reaction of the oxidizing catalyst 26 that is in the
light-off state.
[0034] There is the following problem in respect of the oxidizing
catalyst 26. As stated above, for example, as in case that a
vehicle is started right after start of the engine 1 or the like,
or as in the case of acceleration after deceleration, if the engine
1 is accelerated to increase load and exits the low idle state when
the temperature of the oxidizing catalyst 26 is equal to or less
than light-off temperature, the temperature of the oxidizing
catalyst 26 rapidly moves up along with the load increase to exceed
the light-off temperature. Then, the HC and the CO, which are
trapped by the oxidizing catalyst 26 before the light-off
temperature is reached, cause oxidative reaction all at once.
Consequently, a large amount of reaction heat is suddenly produced,
which leads to an excessive temperature rise of the oxidizing
catalyst 26.
[0035] One of the reasons for such a problem is because, as shown
in FIG. 8, the larger catalyst capacity of the oxidizing catalyst
is in relation to exhaust flow velocity at low idle, the smaller
exhaust flow velocity/catalyst capacity (value obtained by dividing
the exhaust flow velocity by the catalyst capacity) at the time of
idle operation, and a state in which the oxidative reaction is
promoted continues even after the load of the engine 1 is increased
due to relationship between the exhaust flow velocity/catalyst
capacity and catalyst activatibility (CO and HC purification rate)
shown in FIG. 8.
[0036] For that reason, in the case of the oxidizing catalyst 26,
based upon the relationship between the exhaust flow
velocity/catalyst capacity and the catalyst activatibility, the
catalyst capacity is set so that, during low idle, the catalyst
activatibility is great and the oxidative reaction is promoted, and
so that the state in which the oxidative reaction is promoted does
not continue when the load of the engine 1 is increased.
[0037] More specifically, as illustrated in FIG. 2, the exhaust
flow velocity/catalyst capacity is regulated, and the catalyst
capacity of the oxidizing catalyst 26 is set so that the exhaust
flow velocity/catalyst capacity at low idle is maximum or close to
the maximum within a prescribed maximum activation range of the
catalyst activatibility (CO and HC purification rate), that is to
say, so that the exhaust flow velocity/catalyst capacity is close
to a value immediately before it begins to decrease (activation
limit).
[0038] To be more specific, as described later, the throttle valve
12 is closed in order to cause the oxidative reaction of as much HC
and CO as possible by using the oxidizing catalyst 26 during low
idle. If the throttle valve 12 is closed in this way, an exhaust
flow rate changes to lower the exhaust flow velocity. In this
application, therefore, based upon the exhaust flow velocity in a
state where the throttle valve 12 is closed down to prescribed
opening, the catalyst capacity of the oxidizing catalyst 26 is set
so that the exhaust flow velocity/catalyst capacity at low idle is
maximum or close to the maximum within the prescribed maximum
activation range of the catalyst activatibility.
[0039] If the exhaust flow velocity/catalyst capacity is regulated,
and the catalyst capacity of the oxidizing catalyst 26 is set in
the aforementioned manner, because the catalyst activatibility is
great during low idle as shown in FIG. 2, the absorption-type NOx
catalyst 22 located in the downstream is smoothly increased in
temperature due to the reaction heat of the oxidative reaction of
the oxidizing catalyst 26. As a result, even during low idle, the
absorption-type NOx catalyst 22 is maintained at temperature equal
to or higher than light-off temperature, and the absorption and
discharge/reduction of NOx are properly carried out in the
absorption-type NOx catalyst 22, which upgrades NOx-purifying
performance. If the engine 1 is increased in load (load increase)
and exits the low idle state, the catalyst activatibility is
drastically reduced, and the oxidative reaction greatly subsides as
shown in FIG. 2. This suppresses the generation of a large amount
of reaction heat that is produced by the oxidative reaction of the
HC and the CO, which are trapped by the oxidizing catalyst 26
before the light-off temperature is reached, and then prevents the
excessive temperature rise of the oxidizing catalyst 26.
Consequently, the oxidizing catalyst 26 is extended in
duration.
[0040] Moreover, if the catalyst capacity of the oxidizing catalyst
26 is set according to the exhaust flow velocity in the state where
the throttle valve 12 is closed down to the prescribed opening, the
oxidizing catalyst 26 can be miniaturized by setting the catalyst
capacity of the oxidizing catalyst 26 to be extremely small. It is
then possible to raise the temperature of the oxidizing catalyst 26
up to the light-off temperature in a short period of time even with
relatively low-temperature exhaust heat that is produced during low
idle. This makes it possible to reduce the amount of the HC and CO
trapped by the oxidizing catalyst 26 before the light-off
temperature is reached. As a consequence, the generation of the
reaction heat in case that the load of the engine 1 is increased is
satisfactorily suppressed correlatively with the subdual of the
oxidative reaction.
[0041] In the aforementioned case, a carrier of the oxidizing
catalyst 26 is preferably formed of a metal carrier having a high
heat conduction rate. By so doing, the oxidizing catalyst 26 can be
raised in temperature to reach the light-off temperature in a
shorter period of time, and the generation of the reaction heat in
case that the load of the engine 1 is increased can be further
suppressed.
[0042] Referring back to FIG. 1, an EGR passage 30 for causing part
of exhaust gas to reflow to an intake system as EGR gas stretches
from the exhaust manifold. The terminal end of the EGR passage 30
is connected to the intake passage 10 downstream from the throttle
valve 12. Interposed in the EGR passage 30 is an electromagnetic
(DC motor type) EGR valve 32, opening of which is adjustable to
arbitrary opening in order to regulate an EGR gas flow rate.
Furthermore, an EGR cooler 34 is also interposed in the EGR passage
30 closely to the exhaust manifold.
[0043] An electronic control unit (ECU) 40 is a control device for
performing various kinds of control of the vehicle, including fuel
injection control of the engine 1 (engine control means), and is
made up of an input/output interface, a CPU, a memory, etc. In
addition to the airflow sensor 18, various sensors are connected to
an input side of the ECU 40, including, for example, a Ne sensor 42
for detecting engine revolution speed Ne from a crank angle of the
engine 1, an accelerator sensor 43 for detecting a driver's
accelerator operation, a temperature sensor 44 for detecting the
temperature of the absorption-type NOx catalyst 22, etc.
[0044] Connected to an output side of the ECU 40 are various
devices including the fuel injection nozzles 4, the throttle valve
12, the reducing agent injector 24, the EGR valve 32, etc.
[0045] Consequently, operation of the various devices is controlled
according to input information from the various sensors, and the
engine 1 and the vehicle are properly controlled.
[0046] Exhaust gas temperature rise control at low idle according
to the present invention, which is performed in the exhaust gas
purifying device of an internal combustion engine thus constructed,
will be explained below.
[0047] FIG. 3 shows a control routine of the low idle exhaust gas
temperature rise control according to the present invention in the
form of a flowchart. The description will be provided below with
reference to the flowchart.
[0048] First, in Step S10, it is determined whether the engine 1 is
currently at low idle. To be concrete, based upon the information
from the Ne sensor 42, it is determined whether the detected engine
revolution speed Ne is revolution speed Ni corresponding to low
idle, and whether accelerator opening detected by the accelerator
sensor 43 is 0%. If the determination result is false (NO), and it
is determined that the engine is not at low idle, the routine is
ended. If the determination result is true (YES), and it is
determined that the engine is at low idle, the routine advances to
Step S12.
[0049] In Step S12, it is determined whether temperature T.sub.cat
of the absorption-type NOx catalyst 22 is lower than light-off
temperature T1 of the absorption-type NOx catalyst 22. If the
determination result is false (NO), and it is determined that the
temperature T.sub.cat of the absorption-type NOx catalyst 22 is
equal to or higher than the light-off temperature T1, it can be
considered that the absorption-type NOx catalyst 22 functions well,
and the routine is ended. If the determination result is true
(YES), and it is determined that the temperature T.sub.cat is lower
than the light-off temperature T1, it is judged that the
temperature of the absorption-type NOx catalyst 22 needs to be
increased, and the routine advances to Step S14.
[0050] In Step S14, the EGR valve 32 is closed to prohibit the
reflow of the EGR gas to the intake system.
[0051] In Step S16, the throttle valve 12 is closed down to the
prescribed opening to reduce the intake air amount Qa (throttle
control means).
[0052] In this manner, when the intake air amount Qa is reduced by
the closing operation of the throttle valve 12, pumping loss is
increased in the engine 1 since there is no reflow of the EGR gas
to the intake system due to the closing of the EGR valve 32. As
against this, a fuel injection amount is increased to maintain the
low idle state. Furthermore, in the diesel engine 1, there
generates a phenomenon that fuel ignition timing is delayed due to
a drop in in-cylinder pressure. Because of this phenomenon, the
exhaust gas temperature of the engine 1 is increased, and the
temperature of the oxidizing catalyst 26 is maintained high.
[0053] Such delay of the ignition timing slows down combustion. As
shown in FIG. 4(b), correlatively with the increase of the fuel
injection amount, HC and CO concentration in the exhaust gas is
increased, and a reaction heat amount produced by the oxidative
reaction of HC and CO in the oxidizing catalyst 26 is increased. As
shown by a solid line in FIG. 4(a), as the temperature of the
oxidizing catalyst 26 is raised higher than outlet temperature
(broken line) of the turbine of the turbocharger 16, the
temperature of the absorption-type NOx catalyst 22 rises without
fail.
[0054] In Step S18, main injection timing of fuel is retarded to be
injected by the fuel injection nozzle 4 so that the main injection
timing is later than fuel injection timing at normal operation
other than low idle (fuel injection control means) Concretely, the
fuel injection timing of main injection is retarded from the fuel
injection timing at normal operation (for example, 5 degrees after
top dead center) to the vicinity of a misfire limit, and is made to
be fuel injection timing that is exclusive to low idle (for
example, 20 degrees after top dead center).
[0055] If the fuel injection timing of the main injection is
retarded to the vicinity of the misfire limit in this manner,
combustion becomes incomplete, and the HC and CO concentration in
the exhaust gas is increased. Then, the reaction heat amount
produced by the oxidative reaction of HC and CO in the oxidizing
catalyst 26 is further increased, which raises the temperature of
the absorption-type NOx catalyst 22 more reliably.
[0056] If the fuel injection timing of the main injection is
retarded to the vicinity of the misfire limit, fuel ignition is
destabilized. This causes a deterioration of combustion or a
misfire in the worst case. Therefore, pilot injection is performed
right before the main injection at this time (fuel injection
control means).
[0057] The pilot injection carries out a function as kindling of
fuel that is injected by the main injection so that the fuel of the
main injection is stably ignited. Accordingly, the pilot injection
injects an extremely small amount of fuel as compared to a fuel
amount of the main injection. The pilot injection may be performed
only once. Considering the function as kindling, however, it is
preferable that the pilot injection be carried out more than once.
In this embodiment, the pilot injection is performed twice, for
example. To be specific, as illustrated in FIG. 5, first pilot
injection is performed near the top dead center at which the
in-cylinder pressure (solid line) becomes maximum, and second pilot
injection in the vicinity of 10 degrees after top dead center
(double pilot injection).
[0058] Consequently, even if the fuel injection timing of the main
injection is retarded to the vicinity of the misfire limit, the
fuel injected by the main injection is stably and surely combusted,
which prevents a misfire and a deterioration of combustion.
[0059] As described above, in the exhaust gas purifying device of
an internal combustion engine according to the present invention,
based upon the exhaust flow velocity in the state where the
throttle valve 12 is closed down to the prescribed opening, the
catalyst capacity of the oxidizing catalyst 26 is set to be as
small as possible so that the exhaust flow velocity/catalyst
capacity at low idle is the maximum or close to the maximum within
the prescribed maximum activation range of the catalyst
activatibility (see FIG. 2). By conducting the low idle exhaust gas
temperature rise control, during low idle, the throttle valve 12 is
closed down to the prescribed opening, and the fuel injection
timing is retarded. By so doing, the temperature rise of the
oxidizing catalyst 26 and the oxidative reaction in the oxidizing
catalyst 26 are promoted, and the temperature rise of the
absorption-type NOx catalyst 22 is also accelerated (see FIG.
3).
[0060] In the exhaust gas purifying device of an internal
combustion engine according to the present invention, it is
possible to make the oxidizing catalyst 26 function effectively
only when the engine 1 is at low idle, to increase the temperature
of the absorption-type NOx catalyst 22 using the reaction heat of
the oxidative reaction of the oxidizing catalyst 26, and to upgrade
the NOx-purifying performance by satisfactorily maintaining the
temperature of the absorption-type NOx catalyst 22 at temperature
equal to or higher than the light-off temperature. It is also
possible to greatly attenuate the oxidative reaction in the
oxidizing catalyst 26, to suppress the sudden generation of a large
amount of reaction heat produced by the oxidative reaction of the
HC and the CO, which are trapped by the oxidizing catalyst 26
before the light-off temperature is reached, and to prevent the
excessive temperature rise of the oxidizing catalyst 26, in case
that the engine 1 is accelerated to increase the load when the
temperature of the oxidizing catalyst 26 is equal to or lower than
the light-off temperature.
[0061] Although the description of the embodiment of the present
invention is finished here, an aspect of the invention is not
limited to the embodiment.
[0062] For example, in the embodiment, the catalyst capacity of the
oxidizing catalyst 26 is set on the basis of the exhaust flow
velocity in the state where the throttle valve 12 is closed down to
the prescribed opening. The throttle valve 12 is closed down to the
prescribed opening during low idle by the low idle exhaust gas
temperature rise control. However, in a system without the throttle
valve 12 or even if the throttle valve 12 is provided to the
system, the catalyst capacity of the oxidizing catalyst 26 may be
set on the basis of the exhaust flow velocity at normal low idle in
which the throttle valve 12 is not closed, and the low idle exhaust
gas temperature rise control, namely, the closing operation of the
throttle valve 12 and the retard of the fuel injection timing may
be omitted. Doing this way still provides a sufficient
advantage.
[0063] The embodiment has been explained, taking as an example the
case in which the oxidizing catalyst 26 is disposed in the upstream
of the absorption-type NOx catalyst 22. However, it is not limited
to the absorption-type NOx catalyst 22, and any exhaust gas
purifying catalyst, for example, a selective catalytic reduction
system (SCR system) may be placed in the position of the
absorption-type NOx catalyst 22.
[0064] Although in the embodiment, both the closing operation of
the throttle valve 12 and the retard of the fuel injection timing
are performed in the low idle exhaust gas temperature rise control
(Step S16 to Step S18), it is possible to carry out only the
closing operation of the throttle valve 12. Alternatively, only the
retard of the fuel injection timing and the pilot injection may be
performed (Step S16 or S18).
[0065] According to the embodiment, the closing operation of the
throttle valve 12 and the retard of the fuel injection timing are
carried out if the temperature T.sub.cat of the absorption-type NOx
catalyst 22 is lower than the light-off temperature T1 of the
absorption-type NOx catalyst 22. The closing operation of the
throttle valve 12 and the retard of the fuel injection timing,
however, may be performed whenever the engine is at low idle,
regardless of the temperature T.sub.cat of the absorption-type NOx
catalyst 22.
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