U.S. patent application number 09/779631 was filed with the patent office on 2001-09-06 for control apparatus for direct-injection engine.
Invention is credited to Araki, Keiji, Kuji, Youichi, Kuroki, Masayuki, Taga, Junichi, Yokota, Kazuya.
Application Number | 20010018825 09/779631 |
Document ID | / |
Family ID | 18579512 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010018825 |
Kind Code |
A1 |
Kuji, Youichi ; et
al. |
September 6, 2001 |
Control apparatus for direct-injection engine
Abstract
In steps S25 to S42, in order to activate an inactive catalyst
earlier, the air-fuel ratio in a combustion chamber is set to be
.lambda..apprxeq.1 during a period T1 after engine start until
catalyst light-off at around 50% HC purifying ratio immediately
after the HC purifying ratio begins to rapidly rise, before the
catalyst warms up, divisional injection is made at least during the
period T1 required until light-off is reached, and a swirl is
weakened during the period T1 compared to a latter period which
follows this period T1. The divisional injection is made and the
swirl is strengthened even during the latter period.
Inventors: |
Kuji, Youichi;
(Hiroshima-ken, JP) ; Kuroki, Masayuki;
(Hiroshima-ken, JP) ; Taga, Junichi;
(Hiroshima-ken, JP) ; Yokota, Kazuya;
(Hiroshima-ken, JP) ; Araki, Keiji;
(Hiroshima-ken, JP) |
Correspondence
Address: |
STAAS & HALSEY
Suite 500
700 Eleventh Street, N.W.
Washington
DC
20001
US
|
Family ID: |
18579512 |
Appl. No.: |
09/779631 |
Filed: |
February 9, 2001 |
Current U.S.
Class: |
60/286 ; 60/274;
60/285; 60/295 |
Current CPC
Class: |
F02D 2041/389 20130101;
Y02T 10/123 20130101; Y02T 10/12 20130101; Y02T 10/40 20130101;
Y02T 10/44 20130101; Y02T 10/26 20130101; F02D 41/402 20130101;
F02D 41/0255 20130101; F02B 2075/125 20130101 |
Class at
Publication: |
60/286 ; 60/285;
60/274; 60/295 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2000 |
JP |
2000-059000 |
Claims
What is claimed is:
1. A control apparatus for a direct-injection engine, which
comprises a fuel injection valve for directly injecting fuel into a
combustion chamber, and an exhaust gas purifying catalyst inserted
in an exhaust path, and divisionally injects fuel in at least two
injections including trailing injection that begins to inject after
a middle period of a compression stroke, and leading injection
earlier than the trailing injection within a period from an intake
stroke to an ignition timing before the catalyst warms up,
characterized by comprising: varying means for forcibly changing an
intake flow strength in the combustion chamber, and in that an
air-fuel ratio in the combustion chamber is set to be
.lambda..apprxeq.1 before the catalyst warms up, and said varying
means operates to divisionally inject fuel at least during a former
period from engine start until the catalyst, which is halfway
through catalyst temperature rise, is partially activated, before
the catalyst warms up, and to set the intake flow strength in the
former period to be lower than that in a latter period halfway
through the catalyst temperature rise after the former period.
2. The apparatus according to claim 1, characterized in that said
varying means divisionally injects fuel and increases the intake
flow strength even during the latter period.
3. The apparatus according to claim 1, characterized in that the
former period is a period required until an activation state of the
catalyst reaches an HC purifying ratio substantially half a maximum
HC purifying ratio of the catalyst itself.
4. The apparatus according to claim 1, characterized in that said
varying means operates when the engine runs in a low-speed range
and before the catalyst warms up.
5. The apparatus according to claim 1, characterized in that a
trailing injection timing during the former period with the low
intake flow strength is retarded with respect to the latter period
with the high intake flow strength.
6. The apparatus according to claim 1, characterized in that when
fuel is divisionally injected in two injections during the former
period, a fuel injection amount in leading injection is set to be
smaller than a fuel injection amount in trailing injection.
7. The apparatus according to claim 1, characterized in that the
fuel injection amount in leading injection is set to be not less
than 1/4 a total injection amount.
8. The apparatus according to claim 6, characterized in that fuel
is two-divisionally injected in an intake stroke and compression
stroke.
9. The apparatus according to claim 1, characterized in that an
air-fuel ratio in each cylinder during the former period is set to
be leaner than the latter period within a range of
.lambda..apprxeq.1.
10. The apparatus according to claim 1, characterized in that an
air-fuel ratio in each cylinder during the former period is set to
be close to .lambda.=1 but leaner than .lambda.=1 before the
beginning of oxygen feedback, and is set to be .lambda.=1 after the
beginning of oxygen feedback.
11. The apparatus according to claim 1, characterized in that a
feedback reference value upon oxygen feedback during the former
period is set to be leaner than a feedback reference value upon
oxygen feedback during the latter period.
12. The apparatus according to claim 1, characterized in that
before the catalyst warms up, an ignition timing is retarded with
respect to an identical load and identical engine speed after the
catalyst warms up.
13. The apparatus according to claim 1, characterized in that a
swirl is generated in each cylinder with a local air-fuel ratio
around a spark plug being richer than a mean air-fuel ratio of the
entire cylinder by trailing injection, and said varying means
changes a swirl ratio given by (swirl flow angular velocity/engine
rotation angular velocity) in the cylinder.
14. The apparatus according to claim 1, characterized in that a
spark plug is disposed at a central upper end portion of each
cylinder, a fuel injection valve is disposed at a peripheral upper
end portion of the cylinder, and a stratification cavity is formed
on a top of a piston near the fuel injection valve.
15. A control apparatus for a direct-injection engine, which
comprises a fuel injection valve for directly injecting fuel into a
combustion chamber, and an exhaust gas purifying catalyst inserted
in an exhaust path, and divisionally injects fuel in at least two
injections including trailing injection that begins to inject after
a middle period of a compression stroke, and leading injection
earlier than the trailing injection within a period from an intake
stroke to an ignition timing before the catalyst warms up,
comprising: an intake flow control valve for forcibly changing an
intake flow strength in the combustion chamber; and a controller
for setting an air-fuel ratio in the combustion chamber to be
.lambda..apprxeq.1 before the catalyst warms up, and operating said
intake flow control valve to divisionally inject fuel at least
during a former period from engine start until the catalyst, which
is halfway through catalyst temperature rise, is partially
activated, before the catalyst warms up, and to set the intake flow
strength in the former period to be lower than that in a latter
period halfway through the catalyst temperature rise after the
former period.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a control apparatus for a
direct-injection engine that reduces HC exhausted immediately after
a direct-injection engine is started.
BACKGROUND OF THE INVENTION
[0002] HC emission while a catalyst is still inactive immediately
after cold start of an engine accounts for a very large portion of
total emissions. In order to reduce HC emissions at this timing,
the following method is used conventionally; the ignition timing is
retarded after the top dead center of compression to raise the
exhaust gas temperature so as to activate a three-way catalyst
earlier.
[0003] However, this method suffers the following two problems.
[0004] (1) Since the ignition timing is retarded considerably, fuel
economy suffers, and also combustion stability suffers due to a
cold period.
[0005] (2) Although the period until the catalyst begins to be
partially activated and an HC purifying ratio begins to rise
rapidly (to be referred to as partial light-off hereinafter) is
shortened, HC (RawHC) emitted by the engine during this period is
exhausted unpurified, and a scheme for reducing RawHC emitted by
the engine during the period until partial light-off is
required.
[0006] Also, the following technique has been proposed. That is,
fuel is injected in two injections, i.e., an intake stroke and
compression stroke while the catalyst remains inactive, and
stratified air-fuel mixtures are formed, i.e., an air-fuel mixture
richer than the stoichiometric air-fuel ratio is formed around a
spark plug and a leaner air-fuel mixture is formed around the
former air-fuel mixture, thereby promoting warming up of the
catalyst, and assuring high combustion stability. More
specifically, this reference describes that the air-fuel ratio is
set at .lambda..apprxeq.1, the injection amount of the intake
stroke is set to be equal to or larger than that of the compression
stroke, the ignition timing is retarded, and swirl is generated
(Japanese Patent Laid-Open No. 10-212987). U.S. Pat. No. 6,044,642
discloses that fuel injection is divided to a trailing injection on
a compression stroke and a leading injection earlier than the
trailing injection, and a swirl is strengthened so as to activate
the catalyst earlier.
[0007] However, when a swirl is strengthened before the catalyst
warms up, since it has an effect of raising the burning rate, high
combustion stability can be assured upon retarding the ignition
timing, but an afterburning effect suffers, resulting in an
insufficient RawHC reduction effect during the period until partial
light-off.
SUMMARY OF THE INVENTION
[0008] The present invention has been made in consideration of the
aforementioned problems, and has as its object to provide a control
apparatus for a direct-injection engine, which can shorten the
period required until light-off at a predetermined HC purifying
ratio at which at least the HC purifying ratio begins to rise
before a catalyst warms up, reduces RawHC emitted by the engine
before light-off to the predetermined HC purifying ratio, and can
minimize deterioration of fuel economy.
[0009] In order to solve the above problems and to achieve the
above object, according to the first aspect of a control apparatus
for a direct-injection engine of the present invention, the
air-fuel ratio in a combustion chamber is set to be
.lambda..apprxeq.1 before the catalyst warms up, and a varying
means operates to divisionally inject fuel during at least a former
period from the start of an engine until a catalyst, which is
halfway through the catalyst temperature rise before the catalyst
warms up, is partially activated, and to set an intake flow
strength in the former period to become lower than that in the
latter period halfway through the catalyst temperature rise after
the former period. In this way, combustion becomes slow to promote
afterburning so as to shorten the period required until the
predetermined light-off state in which the HC purifying ratio of
the catalyst begins to rapidly rise, thus suppressing deterioration
of fuel economy and reducing RawHC emitted by the engine before the
predetermined light-off state.
[0010] According to the second aspect, the varying means increases
the intake flow strength while divisionally injecting fuel even
during the latter period. In this manner, deterioration of fuel
economy can be suppressed while promoting warming up of the
catalyst.
[0011] According to the third aspect, the former period is a period
required until the activation state of the catalyst reaches an HC
purifying ratio approximately half a maximum HC purifying ratio of
the catalyst itself. As a result, even when the intake flow
strength is increased later, sufficiently high HC purification is
assured, and warming up of the catalyst can be promoted while
suppressing deterioration of fuel economy and assuring high HC
purification.
[0012] According to the fourth aspect, the varying means operates
before the catalyst warms up in a low engine-speed range. In this
manner, the amount of RawHC emitted by the engine before the
catalyst warms up can be reduced.
[0013] According to the fifth aspect, the former period with a low
intake flow strength takes a trailing injection timing retarded
with respect to that in the latter period with a high intake flow
strength. In this fashion, combustion stability can be assured by
suppressing fuel mist from scattering, and the exhaust gas
temperature can be rapidly raised by slow combustion.
[0014] According to the sixth aspect, when fuel is injected in two
injections in the former period, the fuel injection amount in
leading injection is set to be smaller than that in trailing
injection. In this manner, the exhaust gas temperature rise effect
and HC & NO.sub.x reduction effects are obtained while assuring
combustion stability.
[0015] According to the seventh aspect, the fuel injection amount
in leading injection is set to be 1/4 or more of the total
injection amount. As a result, the exhaust gas temperature rise
effect and HC & NO.sub.x reduction effects are obtained while
assuring combustion stability.
[0016] According to the eighth aspect, fuel is two-divisionally
injected in intake and compression strokes. In this way, the
exhaust gas temperature rise effect and HC & NO.sub.x reduction
effects are obtained while assuring combustion stability.
[0017] According to the ninth aspect, the air-fuel ratio in a
cylinder in the former period is set to be leaner than that in the
latter period within the range of .lambda..apprxeq. 1. As a result,
ignitability drop due to offset of a rich air-fuel mixture around
the spark plug during the former period with a low intake flow
strength can be prevented. In addition, since the air-fuel ratio in
the entire cylinder is slightly leaner than .lambda.=1, the RawHC
emission amount emitted by the engine during the former period with
a low HC purifying ratio can be reduced.
[0018] According to the 10th aspect, the air-fuel ratio in a
cylinder during the former period is set to be close to but leaner
than .lambda.=1 before beginning of o.sub.2 feedback, and is set at
.lambda.=1 after beginning of o.sub.2 feedback. In this manner, the
purification efficiency of a three-way function of the catalyst
(three-way catalyst, NO.sub.x catalyst, or the like) can be
improved by setting the air-fuel ratio at .lambda.=1 after
beginning of o.sub.2 feedback, while reducing RawHC emission from
the engine.
[0019] According to the 11th aspect, a feedback reference value
upon o.sub.2 feedback during the former period is set to be leaner
than that upon o.sub.2 feedback during the latter period. As a
result, RawHC emission amount reduction and suppression of fuel
economy deterioration can be achieved while maintaining the
three-way function of the catalyst.
[0020] According to the 12th aspect, the ignition timing is
retarded with respect to an identical load and identical engine
speed after the catalyst warms up. In this manner, while
afterburning due to slow combustion can be advanced earlier than
normal setting to promote warming up of the catalyst, reducing
RawHC emission from the engine.
[0021] According to the 13th aspect, a swirl is generated in a
cylinder so that the local air-fuel ratio around a spark plug
becomes rich by trailing injection, and the varying means changes
the swirl ratio in the cylinder. As a result, since a rich air-fuel
mixture can be surely locally present around the spark plug, a
variation factor of an indicated mean effective pressure Pi (Pi
variation factor=Pi standard deviation .sigma./Pi cycle mean
value.times.100 (%)) in a stratified engine can be reduced, thus
suppressing deterioration of combustion stability.
[0022] According to the 14th aspect, the spark plug is disposed on
the central upper end portion of the cylinder, a fuel injection
valve is disposed on the peripheral upper end portion of the
cylinder, and a stratification cavity is formed on the top of a
piston near the fuel injection valve. With this arrangement, since
a rich air-fuel mixture can be surely locally present around the
spark plug, the Pi variation factor can be reduced in the
stratified engine, thus suppressing deterioration of combustion
stability.
[0023] According to the 15th aspect, the air-fuel ratio in a
combustion chamber is set to be .lambda..apprxeq.1 before the
catalyst warms up, and an intake flow control valve operates to
divisionally inject fuel during at least a former period from the
start of an engine until a catalyst, which is halfway through the
catalyst temperature rise before the catalyst warms up, is
partially activated, and to set an intake flow strength in the
former period to become lower than that in the latter period
halfway through the catalyst temperature rise after the former
period. In this way, combustion becomes slow to promote
afterburning so as to shorten the period required until the
predetermined light-off state in which the HC purifying ratio of
the catalyst begins to rapidly rise, thus suppressing deterioration
of fuel economy and reducing RawHC emitted by the engine before the
predetermined light-off state.
[0024] Other objects and advantages besides those discussed above
shall be apparent to those skilled in the art from the description
of a preferred embodiment of the invention which follows. In the
description, reference is made to accompanying drawings, which form
a part thereof, and which illustrate an example of the invention.
Such example, however, is not exhaustive of the various embodiments
of the invention, and therefore reference is made to the claims
which follow the description for determining the scope of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic sectional view showing the structure
of a combustion chamber portion of a direct-injection engine
according to an embodiment of the present invention;
[0026] FIG. 2 is a schematic view showing the overall
direct-injection engine;
[0027] FIG. 3 is a table showing various parameters to be input to
an engine control ECU to detect the states of the engine and a
catalyst and to implement engine control;
[0028] FIGS. 4A and 4B are flow charts showing the catalyst
temperature control in a direct-injection gasoline engine in the
embodiment of the present invention;
[0029] FIG. 5 is a flow chart showing the catalyst temperature
control in a direct-injection gasoline engine in the embodiment of
the present invention;
[0030] FIG. 6 is a flow chart showing the catalyst temperature
control in a direct-injection gasoline engine in the embodiment of
the present invention;
[0031] FIG. 7 is a graph showing the relationship between the crank
angle and mass burning ratio, i.e., showing the burning rates of
divisional and simultaneous injections;
[0032] FIG. 8 is a graph showing the relationship between the crank
angle and mass burning ratio, i.e., showing the burning rates of
weak and strong swirls;
[0033] FIG. 9 is a graph showing the relationship among the strong
and weak swirls, RawHC emission amount, HC purifying ratio, and
catalyst temperature before and after a period in which the
catalyst reaches light-off;
[0034] FIG. 10 is a timing chart showing the fuel injection amounts
and injection timings in two-divisional injections;
[0035] FIG. 11 is a graph showing the relationship between the
trailing injection timing and RawHC emission amount;
[0036] FIG. 12 is a graph showing the relationship between the
trailing injection timing and Pi variation factor;
[0037] FIG. 13 is a graph showing the relationship among the
trailing injection ratio, Pi variation factor, exhaust gas
temperature, fuel consumption ratio, HC emission amount, and
NO.sub.x emission amount;
[0038] FIG. 14 is a graph showing the output from an O.sub.2 sensor
upon O.sub.2 feedback control;
[0039] FIG. 15 is a chart showing the relationship between a change
in output from the O.sub.2 sensor and a corresponding change in
feedback correction coefficient upon O.sub.2 feedback control;
and
[0040] FIG. 16 is a graph showing the relationship among the HC
purifying ratio, catalyst temperature, and RawHC emission amount
with respect to the air-fuel ratio.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] An embodiment of the present invention will be described in
detail hereinafter with reference to the accompanying drawings.
Structure of Direct-injection Engine
[0042] FIG. 1 is a schematic sectional view showing the structure
of a combustion chamber portion of a direct-injection engine of
this embodiment.
[0043] As shown in FIG. 1, reference numeral 1 denotes an engine. A
plurality of cylinders are formed in a cylinder block 2, and a
cylinder head 3 is fixed to the top of the cylinder block 2 via a
gasket. In each cylinder, a piston 4 is inserted, and a combustion
chamber 5 is formed between the top surface of the piston 4 and the
lower surface of the cylinder head 3. An intake port 6 and exhaust
port 7 that communicate with the combustion chamber 5, and an
intake valve 8 and exhaust valve 9 that open/close these ports 6
and 7 are disposed, and a spark plug 10 and injector 11 are
disposed to oppose the combustion chamber 5. The injector 11
directly injects fuel into the combustion chamber 5.
[0044] A recess having a substantially trapezoidal section is
formed on the lower surface of the cylinder head 3 to define the
upper portion of the combustion chamber 5. The intake port 6 is
open to the upper surface portion of the combustion chamber 5, and
the exhaust port 7 is open to its slant surface portion. Two each
intake and exhaust ports 6 and 7 are juxtaposed in a direction
perpendicular to the page of FIG. 1, and intake and exhaust valves
8 and 9 are respectively provided to them. The intake and exhaust
valves 8 and 9 are driven by a valve driving mechanism comprising a
cam shaft and the like (not shown) and are opened/closed at
predetermined timings.
[0045] The spark plug 10 is disposed on substantially the central
upper portion of the combustion chamber 5, and is attached to the
cylinder head 3 so that its spark gap looks into the combustion
chamber 5.
[0046] The injector 11 is disposed on the peripheral edge portion
of the combustion chamber 5, and is attached to the cylinder head 3
on one side of the intake ports 6. A nozzle portion of the injector
11 looks into a wall surface 12 between the upper surface portion
of the combustion chamber 5 to which the intake port 6 is open, and
a joint surface with respect to the cylinder block 2, and injects
fuel obliquely downward.
[0047] A stratification cavity 13 is formed on the top portion of
the piston 4 near the injector 11. The position and direction of
the injector 11, the position of the cavity 13, and the position of
the spark plug 10 are set in advance, so that fuel, which is
injected from the injector 11 toward the cavity 13 during the
latter half of a compression stroke in which the piston 4 is
located near the top dead center, is reflected by the cavity 13 and
reaches near the spark plug 10.
[0048] FIG. 2 is a schematic view of the overall direct-injection
engine.
[0049] As shown in FIG. 2, an intake path 15 and exhaust path 16
are connected to the engine 1. The downstream of the intake path 15
branches at an intake manifold into paths in units of cylinders,
two parallel branch paths are formed in each cylinder-dependent
path 15a, and two intake ports 6 are open to the combustion chamber
5 in FIG. 1 at the downstream ends of these branch paths. An intake
flow control valve 17 is provided to one branch path. By
controlling the degree of opening of the intake flow control valve
17, an intake flow (swirl or tumble) is generated in the combustion
chamber 5 by intake air supplied from the other branch path, and
the intake flow strength is controlled. Note that the intake flow
strength can also be changed by controlling the degree of opening
of one of the two intake valves or by variably controlling the
valve timings.
[0050] A throttle valve 18 is provided in the middle of the intake
path 15, and is controlled by an electric actuator 19 such as a
stepping motor or the like, so as to control the intake air
amount.
[0051] An O.sub.2 sensor 21 for detecting the air-fuel ratio in
exhaust gas is provided to the exhaust path 16, and a catalyst
device 22 having catalysts for purifying exhaust gas is provided.
The catalyst device 22 comprises a three-way catalyst 22a which is
disposed on the downstream of the exhaust path 16 and purifies
exhaust gas by removing HC, CO, and NO.sub.x, and an NO.sub.x
catalyst 22b which is disposed on the downstream of the three-way
catalyst 22a and adsorbs NO.sub.x. The NO.sub.x catalyst 22b
adsorbs NO.sub.x at an air-fuel ratio .lambda.>1 leaner than a
stoichiometric air-fuel ratio .lambda.=1 when stratified combustion
is made by setting the air-fuel ratio to fall within a lean range
that satisfies .lambda.>1 after warming up. Also, the NO.sub.x
catalyst 22b exhibits a three-way function near the stoichiometric
air-fuel ratio, and releases NO.sub.x adsorbed at an air-fuel ratio
richer than .lambda.=1 to react it with HC and CO.
[0052] If the catalyst device 22 is disposed immediately downward
an exhaust manifold 16a (directly coupled thereto), the catalyst
temperature rises too fast in a high-speed, high-load state. For
this reason, the catalyst device 22 is inserted in the middle of an
exhaust pipe 16b connected to the exhaust manifold 16a away from
the engine, so as to protect the catalysts.
[0053] An EGR path 43 for recirculating some of exhaust gas is
connected between the exhaust and intake paths 16 and 15, and an
EGR valve 44 is inserted in this EGR path 43.
[0054] A supercharger 40 and a waste gate 41 that bypasses the
supercharger 40 are provided to the exhaust pipe 16b on the
upstream side of the catalyst device 22. The waste gate 41 is
opened/closed by a waste gate valve 42 to suppress an excessive
rise of the supercharging pressure.
[0055] An engine control ECU (electric control unit) 30 receives
signals from the O.sub.2 sensor 31 for detecting the oxygen content
in exhaust gas, a crank angle sensor 23 for detecting the crank
angle of the engine, an accelerator opening degree sensor 24 for
detecting the degree of opening of an accelerator (the depression
amount of an accelerator pedal), an airflow meter 25 for detecting
an intake air amount, a water temperature sensor 26 for detecting
the temperature of engine cooling water, an engine speed sensor 27,
an intake temperature sensor 28, an atmospheric pressure sensor 29,
and the like.
[0056] FIG. 3 is a table showing various parameters input to the
engine control ECU to detect the states of the engine and catalysts
and to implement engine control.
[0057] The engine control ECU 30 includes a temperature state
discrimination module 31, running state detection module 32, fuel
supply control module 33, injection amount arithmetic module 34,
ignition timing control module 35, and engine speed control module
36.
[0058] The temperature state discrimination module 31 estimates the
catalyst temperature on the basis of the histories of the engine
speed detection signal from the engine speed sensor 27, the
accelerator opening degree detection signal from the accelerator
opening degree sensor 24, the intake flow rate detection signal
from the airflow meter 25, the water temperature detection signal
from the water temperature sensor 26, a fuel injection amount Ta,
injection mode, and the like, and discriminates if the catalyst
device is in a cold state lower than the activation temperature.
Note that the catalyst cold state may be determined if the water
temperature is less than a first temperature, and the catalyst
warm-up state may be determined if it is equal to or higher than
the first temperature. Furthermore, the temperature state
discrimination module 31 also estimates the engine temperature, and
determines an engine cold state if the water temperature is less
than a second temperature; or an engine warm-up state if it is
equal to or higher than the second temperature. Note that the
second temperature is higher than the first temperature. Note that
temperature state discrimination for discriminating the catalyst
warm-up state may be implemented by combining water temperature
detection and discrimination of the time elapsed after engine
start, or may be implemented by directly detecting the catalyst
temperature.
[0059] The injection mode has an injection pattern such as intake
stroke injection (uniform combustion range) or (stratified
combustion range), and divisional (split) or simultaneous injection
in these ranges, and is set in advance in units of running ranges.
Therefore, the injection mode is set by discriminating the running
range.
[0060] The running state detection module 32 discriminates an
engine running range such as a lean range, rich range, and the like
on the basis of the engine speed detection signal from the engine
speed sensor 27, the accelerator opening degree detection signal
from the accelerator opening degree sensor 24, the intake flow rate
detection signal from the airflow meter 25, the water temperature
detection signal from the water temperature sensor 26, the intake
temperature detection signal from the intake temperature sensor 28,
and the atmospheric pressure detection signal from the atmospheric
pressure sensor 29. Also, the module 32 discriminates a transient
running state such as abrupt acceleration, high-load running, and
the like of the engine on the basis of the intake flow rate
detection signal. Furthermore, the module 32 discriminates an
engine cold or warm running state of the engine on the basis of the
water temperature detection signal. Moreover, the O.sub.2 detection
signal from the O.sub.2 sensor 21 is output when the O.sub.2 sensor
21 is activated, and is used in O.sub.2 feedback control.
[0061] The fuel injection control module 33 computes a fuel
injection timing Qa on the basis of the engine speed detection
signal from the engine speed sensor 27, the accelerator opening
degree detection signal from the accelerator opening degree sensor
24, the intake flow rate detection signal from the airflow meter
25, the water temperature detection signal from the water
temperature sensor 26, and the O.sub.2 detection signal from the
O.sub.2 sensor 21.
[0062] The injection amount arithmetic module 34 computes a fuel
injection amount Ta on the basis of the engine speed detection
signal from the engine speed sensor 27, the accelerator opening
degree detection signal from the accelerator opening degree sensor
24, the intake flow rate detection signal from the airflow meter
25, the water temperature detection signal from the water
temperature sensor 26, the fuel pressure, and the injection
mode.
[0063] The fuel pressure is an ejection pressure of a high-pressure
fuel pump, which acts on the injector 11, and the injection amount
Ta is corrected based on the differential pressure between the fuel
pressure sensor output and intra-cylinder pressure (estimated
value).
[0064] The fuel injection control module 33 and injection amount
arithmetic module 34 control the fuel injection timing Qa and
injection amount (pulse width) Ta from the injector 11 via an
injector driving circuit 37. In the catalyst cold state, the
air-fuel ratio of the entire combustion chamber 5 is set at a
substantially stoichiometric air-fuel ratio .lambda..apprxeq.1, and
divisional injection for divisionally injecting fuel in at least
two injections, i.e., trailing injection after the middle period of
a compression stroke and leading injection in the former half of an
intake period earlier than the trailing injection is made to form
an air-fuel mixture at a stoichiometric air-fuel ratio (.lambda.=
1) or richer (.lambda.<1) in a region near the spark plug 10 in
the combustion chamber 5, and to form an air-fuel mixture at an
air-fuel ratio .lambda.>1 leaner than the stoichiometric
air-fuel ratio .lambda.=1 around the region near the spark plug 10
during the period ranging from the intake stroke to the ignition
timing.
[0065] The ignition timing control module 35 computes an ignition
timing .theta.ig on the basis of the engine speed detection signal
from the engine speed sensor 27, the accelerator opening degree
detection signal from the accelerator opening degree sensor 24, the
intake flow rate detection signal from the airflow meter 25, the
water temperature detection signal from the water temperature
sensor 26, and the injection mode.
[0066] The ignition timing control module 35 outputs a control
signal to an ignition device 38 to control the ignition timing
.theta.ig in accordance with the engine running state. The module
35 basically controls the ignition timing .theta.ig to match MBT
(around an ignition timing at which the best torque is produced),
but retards the ignition timing when the engine load is very low in
the catalyst cold state as needed, as will be described later.
[0067] The engine control ECU 30 also controls the intake air
amount by outputting a control signal to the actuator 19 that
drives the throttle valve 18. When, for example, stratified
combustion is made by fuel injection in a compression stroke alone
after the engine warms up, the engine control ECU 30 adjusts the
intake air amount to attain a lean air-fuel ratio. A throttle valve
opening degree .theta.tv is computed based on the engine speed
detection signal from the engine speed sensor 27, the accelerator
opening degree detection signal from the accelerator opening degree
sensor 24, the intake flow rate detection signal from the airflow
meter 25, the intake temperature detection signal from the intake
temperature sensor 28, the atmospheric pressure detection signal
from the atmospheric pressure sensor 29, and the injection
mode.
[0068] As will be described later, when the ignition timing is
retarded while the engine load is very low in the catalyst cold
state, the intake air amount and fuel injection amount are
increased. Furthermore, the engine control ECU 30 controls the
intake flow control valve 17 to generate a swirl in the combustion
chamber 5 upon, e.g., divisional injection, and controls the EGR
valve 44 to attain EGR upon, e.g., stratified combustion in which
the air-fuel ratio is leaner than .lambda.= 1.
[0069] The intake flow control valve 17 is controlled to open/close
by the engine speed detection signal from the engine speed sensor
27, the accelerator opening degree detection signal from the
accelerator opening degree sensor 24, the intake flow rate
detection signal from the airflow meter 25, and the injection mode,
thus controlling the swirl ratio (swirl flow angular
velocity/engine rotation angular velocity) in each cylinder.
[0070] An EGR valve opening degree .theta.egr is computed based on
the engine speed detection signal from the engine speed sensor 27,
the accelerator opening degree detection signal from the
accelerator opening degree sensor 24, the intake flow rate
detection signal from the airflow meter 25, the water temperature
detection signal from the water temperature sensor 26, and the
injection mode.
[0071] The engine control ECU 30 discriminates engine start on the
basis of the engine speed detection signal from the engine speed
sensor 27, and a starter signal.
Catalyst Temperature Control
[0072] Catalyst temperature control for activating the cold
catalyst (catalysts 22a and 22b) early, and reducing HC emissions
will be explained below.
[0073] FIGS. 4A to 6 are flow charts showing the catalyst
temperature control in the direct-injection gasoline engine of this
embodiment.
[0074] An outline of the catalyst temperature control will be
explained first.
[0075] In this embodiment, in order to activate the inactive
catalyst early, if the engine is running in a low-speed range
during a period T1 (former period) after the engine start while the
catalyst has not warmed up yet until light-off to a predetermined
HC purifying ratio (around 50% purifying ratio) of the catalyst 22a
immediately after the HC purifying ratio of the catalyst 22a rises
abruptly, and during a period T2 (latter period) from the state of
around 50% purifying ratio state of the catalyst 22a until
light-off at around 100% HC purifying ratio is reached, the
following control is executed:
[0076] (i) Fuel is divisionally injected in at least two
injections, i.e., trailing injection that starts after the middle
period of the compression stroke, and leading injection earlier
than the trailing injection during the period from the intake
stroke to the ignition timing.
[0077] (ii) The air-fuel ratio in the combustion chamber is set to
be .lambda..apprxeq.1, divisional injection is made during at least
a period (T1+T2) required until the catalyst 22a reaches light-off
at around 100% purifying ratio, and a swirl during the former
period T1 of the period until light-off at around 100% purifying
ratio is set to be weaker than the latter period T2.
[0078] (iii) Divisional injection is made even during the latter
period T2, and the swirl is strengthened.
[0079] (iv) During the former period T1 in which the swirl is
weakened, the later injection timing is retarded with respect to
the latter period T2 in which the swirl is strengthened.
[0080] (v) During the period (T1+T2) until the catalyst 22a reaches
light-off at around 100% purifying ratio, the fuel injection amount
of leading injection is set to be smaller than that of trailing
injection and to be 1/4 or more the total injection amount.
[0081] (vi) The air-fuel ratio in the cylinder during the former
period T1 is basically set to be leaner than the latter period T2
within the range of .lambda..apprxeq.1.
[0082] (vii) The air-fuel ratio during the former period T1 is set
to be lean near .lambda.=1 before the beginning of O.sub.2
feedback, and is set to be .lambda.=1 after the beginning of
O.sub.2 feedback. More specifically, a feedback reference value
upon O.sub.2 feedback during the former period T1 is set to be
leaner than that upon O.sub.2 feedback during the latter period
T2.
[0083] (viii) The ignition timing is retarded with respect to an
identical load and identical engine speed after the catalyst warms
up.
[0084] (ix) The swirl ratio (swirl flow angular velocity/engine
rotation angular velocity) in each cylinder is set so that the
local air-fuel ratio around the spark plug becomes rich by trailing
injection.
[0085] The actual flow of the engine control ECU 30 for
implementing control operations (i) to (xi) will be explained below
with reference to FIGS. 4A to 6.
[0086] As shown in FIGS. 4A and 4B, the engine control ECU 30 reads
detection signals from the O.sub.2 sensor 21, the crank angle
sensor 23 for detecting the crank angle of the engine, the
accelerator opening degree sensor 24, the airflow meter 25, the
water temperature sensor 26, the engine speed sensor 27, the intake
temperature sensor 28, the atmospheric pressure sensor 29, the fuel
pressure sensor, a starter, and the like in step S1.
[0087] It is checked in step S2 if a flag F3 is set. The flag F3 is
reset to zero when the engine is cold; it is set at 1 when the
engine is warm.
[0088] If the flag F3 is set in step S2 (YES in step S2), the flow
jumps to step S45 (to be described later with reference to FIG.
6).
[0089] If the flag F3 is reset (NO in step S2), since the catalyst
22 has not warmed up yet, the flow advances to step S3.
[0090] It is checked based on the starter signal in step S3 if the
engine is turned on by the starter. If the starter is ON in step S3
(YES in step S3), since the engine has started, the flow advances
to step S4; if the starter is OFF (NO in step S3), the flow
advances to step S23.
[0091] In step S4, the injection pulse width Ta upon starting the
engine is computed.
[0092] In step S5, an injection timing .theta.ak upon starting the
engine is computed.
[0093] In step S6, the control waits based on the crank angle of
the engine detected by the crank angle sensor 23 until the
injection timing .theta.ak computed in step S5 is reached. When the
injection timing .theta.ak has been reached (YES in step S6), the
flow advances to step S7.
[0094] In step S7, fuel is injected by the injector 11 to have the
injection pulse width Ta computed in step S4.
[0095] It is checked in step S8 if the engine speed has exceeded a
predetermined engine speed (e.g., 500 rpm) at which it can be
determined that the engine has started. The processes in steps S4
to S7 are repeated until the engine speed exceeds the predetermined
engine speed.
[0096] In step S9, the catalyst temperature is estimated based on
the history of the engine speed, accelerator opening degree, intake
flow rate, engine water temperature, and the like.
[0097] It is checked in step S10 if the catalyst 22 has not warmed
up yet, i.e., its temperature is lower than the activation
temperature. If the catalyst 22 has not warmed up yet in step S10
(YES in step S10), a flag F1 is set in step S11; if the catalyst 22
has warmed up (NO in step S10), the flag F3 is set in step S12 and
the flow returns. The flag F1 is set if the catalyst 22 has not
warmed up yet, and is reset to zero if it has warmed up.
[0098] In step S13, a timer is set in the period T1 until the
catalyst reaches light-off at around 50% purifying ratio on the
basis of the engine water temperature, intake temperature, and the
like.
[0099] In step S14, the timer begins to count down the period T1
until light-off is reached.
[0100] It is checked in step S15 if the O.sub.2 sensor is
activated. If the O.sub.2 sensor is inactive in step S15 (NO in
step S15), the flow advances to step S16; if the O.sub.2 sensor is
active (YES in step S15), the flow advances to step S19.
[0101] In step S16, the injection pulse width Ta and ignition
timing .theta.ig are computed on the basis of the engine speed,
accelerator opening degree, intake flow rate, engine water
temperature, and the like.
[0102] In step S17, the injection pulse width Ta is distributed to
.alpha.: 1-.alpha. to compute an leading injection pulse Tak
(=.alpha..times.Ta) and trailing injection pulse Tad
(=(1-.alpha.).times.Ta) in divisional injection.
[0103] In step S18, an leading basic injection timing .theta.akb
and trailing basic injection timing .theta.adb are set, and the
flow advances to step S25 in FIG. 5.
[0104] If the O.sub.2 sensor is active in step S15 (YES in step
S15), the injection pulse width Ta and ignition timing .theta.ig
are computed on the basis of the engine speed, accelerator opening
degree, intake flow rate, engine water temperature, oxygen
concentration, and the like in step S19.
[0105] In step S20, the injection pulse width Ta is distributed to
.alpha.: 1-.alpha. to compute a leading injection pulse Tak
(=.alpha..times.Ta) and trailing injection pulse Tad
(=(1-.alpha.).times.Ta) in divisional injection.
[0106] In step S21, a leading basic injection timing .theta.akb and
trailing basic injection timing .theta.adb are set.
[0107] It is checked in step S22 if a flag F2 is set. The flag F1
is set at 1 when the period T1 required until the catalyst reaches
light-off at around 50% HC purifying ratio has elapsed, and the
intake flow control valve 17 is closed to switch from a weak swirl
to a strong swirl.
[0108] If the flag F2 is set in step S22 (YES in step S22), the
flow jumps to step S36 to be described later with reference to FIG.
5.
[0109] If the flag F2 is reset (NO in step S22), the flow advances
to step S25 in FIG. 5.
[0110] If the starter is OFF in step S3 (NO in step S3), since the
engine is at a halt or is running but has not started, it is
checked in step S23 if the flag F1 is set.
[0111] If the flag F1 is set in step S23 (YES in step S23), since
the catalyst 22 has not warmed up yet, the flow jumps to step S14
mentioned above; if the flag F1 is reset (NO in step S23), since
the catalyst 22 has warmed up, the flow jumps to step S24.
[0112] It is checked in step S24 if the flag F2 is set.
[0113] If the flag F2 is set in step S24 (YES in step S24), since
the period T1 required until the catalyst reaches light-off at
around 50% HC purifying ratio has elapsed, the flow jumps to step
S19 mentioned above.
[0114] If the flag F2 is reset in step S24 (NO in step S24), the
flow returns.
[0115] As shown in FIG. 5, it is checked in step S25 if the timer
has counted down the period T1.
[0116] If the period T1 has not elapsed yet in step S25 (NO in step
S25), the flow advances to step S26; if the period T1 has elapsed
(YES in step S25), the flow advances to step S34.
Process of Former Period T1 Until Catalyst Reaches Light-Off at
Around 50% HC Purifying Ratio
[0117] In step S26, the intake flow control valve is kept open to
set a weak swirl since the period T1 has not elapsed yet.
[0118] In step S27, the leading injection timing .theta.ak is set
to be the leading basic injection timing .theta.ask (.theta.ak=
.theta.akb), and the trailing injection timing .theta.ad is
retarded .theta.k from the trailing basic injection timing
.theta.adb (.theta.ad=.theta.adb-.theta.k- ).
[0119] In step S28, the control waits based on the crank angle of
the engine detected by the crank angle sensor 23 until the leading
injection timing .theta.ak computed in step S27 is reached. If the
leading injection timing .theta.ak has been reached (YES in step
S28), the flow advances to step S29.
[0120] In step S29, fuel is injected from the injector 11 to have
the leading injection pulse width Tak computed in step S17 or
S20.
[0121] In step S30, the control waits based on the crank angle of
the engine detected by the crank angle sensor 23 until the trailing
injection timing .theta.ad (retarded) computed in step S27 is
reached. If the trailing injection timing .theta.ad has been
reached (YES in step S30), the flow advances to step S31.
[0122] In step S31, fuel is injected from the injector 11 to have
the trailing injection pulse width Tad computed in step S17 or
S20.
[0123] In step S32, the control waits until the ignition timing
.theta.ig computed in step S16 or S19 is reached. If the ignition
timing .theta.ig has been reached (YES in step S32), the flow
advances to step S33.
[0124] In step S33, the spark plug 10 is ignited at the ignition
timing .theta.ig computed in step S16 or S19.
Process of Latter Period T2 After Elapse of Former Period T1
[0125] In step S34, the intake flow control valve 17 is closed to
set a strong swirl since the period T1 has elapsed.
[0126] In step S35, the flag F2 is set at 1.
[0127] In step S36, the leading injection timing .theta.ak is set
to be the leading basic injection timing .theta.ask (.theta.ak=
.theta.akb), and the trailing injection timing .theta.ad is set to
be the trailing basic injection timing .theta.adb to restore the
timing from trailing injection retard (.theta.ad= .theta.adb).
[0128] In step S37, the control waits based on the crank angle of
the engine detected by the crank angle sensor 23 until the leading
injection timing .theta.ak computed in step S36 is reached. If the
leading injection timing .theta.ak has been reached (YES in step
S37), the flow advances to step S38.
[0129] In step S38, fuel is injected from the injector 11 to have
the leading injection pulse width Tak computed in step S17 or
S20.
[0130] In step S39, the control waits based on the crank angle of
the engine detected by the crank angle sensor 23 until the trailing
injection timing .theta.ad computed in step S36 is reached. If the
trailing injection timing .theta.ad has been reached (YES in step
S39), the flow advances to step S40.
[0131] In step S40, fuel is injected from the injector 11 to have
the trailing injection pulse width Tad computed in step S17 or
S20.
[0132] In step S41, the control waits until the ignition timing
.theta.ig computed in step S16 or S19 is reached. If the ignition
timing .theta.ig has been reached (YES in step S41), the flow
advances to step S42.
[0133] In step S42, the spark plug 10 is ignited at the ignition
timing .theta.ig computed in step S16 or S19.
[0134] It is checked in step S43 if the catalyst 22 has not warmed
up yet, i.e., its temperature is lower than the activation
temperature. If the catalyst 22 has not warmed up yet in step S43
(YES in step S43), the flow returns; if the catalyst 22 has warmed
up (NO in step S43), the flag F3 is set in step S44, and the flow
then returns.
Control Executed When Engine is Warm
[0135] As shown in FIG. 6, if the flag F3 is set in step S2 (YES in
step S2), since the engine is warm, and the catalyst 22 (catalysts
22a and 22b) has warmed up, the flow advances to step S45 to
execute normal control.
[0136] In step S45, a running range (stratified combustion or
uniform combustion) is determined on the basis of the engine speed,
accelerator opening degree, intake flow rate, engine water
temperature, intake temperature, atmospheric pressure, and the
like.
[0137] In step S46, the injection pulse width Ta, injection timing
Ta, injection timing .theta.a, opening degree .theta.tv of the
throttle valve 18, opening degree .theta.egr of the EGR valve 44,
and ignition timing .theta.ig corresponding to the running range
determined in step S45 are computed.
[0138] It is checked in step S47 if the engine speed is equal to or
higher than a predetermined engine speed N1 (e.g., 2,000 to 2,500
rpm).
[0139] If the engine speed is equal to or higher than the
predetermined engine speed N1 in step S47 (YES in step S47), the
intake flow control valve 17 is opened in step S48, and the flow
advances to step S51.
[0140] If the engine speed is less than the predetermined engine
speed N1 in step S47 (NO in step S47), it is checked in step S49 if
the accelerator opening degree is large.
[0141] If the accelerator opening degree is large in step S49 (YES
in step S49), since the engine is accelerating or is imposed a high
load, the flow advances to step S48 to open the intake flow control
valve 17 so as to weaken a swirl.
[0142] On the other hand, if the accelerator opening degree is not
large in step S49 (NO in step S49), the intake flow control valve
17 is closed in step S50 to strengthen a swirl.
[0143] It is checked in step S51 if the engine key is OFF.
[0144] If the engine key is OFF in step S51 (YES in step S51), the
flags F1 to F3 are reset to zero in step S52, and the flow
returns.
[0145] On the other hand, if the engine key is not OFF in step S51
(NO in step S51), the flow returns.
[0146] FIGS. 7 and 8 show the relationship between the crank angle
and mass burning ratio, and they indicate that the burning rate
becomes higher with increasing mass burning ratio while the crank
angle remains the same. FIG. 7 compares the burning rates in
divisional and simultaneous injections. FIG. 8 compares the burning
rates of weak and strong swirls. FIG. 9 shows the relationship
among the weak and strong swirls, RawHC emission amount, HC
purifying ratio, and catalyst temperature before and after a period
required until the catalyst 22 reaches light-off at around 100%
purifying ratio. FIG. 10 is a timing chart showing the fuel
injection amount and injection timing in two-divisional injections.
FIG. 11 shows the relationship between the trailing injection
timing and RawHC emission amount. FIG. 12 shows the relationship
between the trailing injection timing and Pi variation factor. FIG.
13 shows the relationship among the trailing injection ratio, Pi
variation factor, exhaust gas temperature, fuel consumption ratio,
HC emission amount, and NO.sub.x emission amount.
[0147] In the aforementioned flow, since fuel is divisionally
injected in at least two injections, i.e., trailing injection after
the middle period of the compression stroke and leading injection
earlier than the trailing injection during the period from the
intake stroke to the ignition timing during the period T1 required
until the catalyst 22a reaches light-off at around 50% HC purifying
ratio (steps S26 to S33), the burning rate results in slow
combustion compared to simultaneous injection, as shown in FIG. 7,
so as to promote afterburning and warming up of the catalyst 22,
resulting in an increase in HC purifying ratio. Therefore, the
RawHC emission amount can be reduced.
[0148] Note that the middle period of the compression stroke means
that when the compression stroke is divided into three periods,
i.e., former, middle, and latter periods, as shown in FIG. 10,
i.e., a period ranging from BTDC (before the top dead center)
120.degree. to BTDC 60.degree. of the crank angle. Therefore,
trailing injection starts after BTDC 120.degree.. If the trailing
injection timing is too late, since combustion stability suffers,
as will be described later, trailing injection preferably starts
before an elapse of a 3/4 period of the compression stroke (before
BTDC 45.degree.).
[0149] That is, trailing injection is set to start within the
period ranging from BTDC 120.degree. to BTDC 40.degree., and
leading injection is set to start in an appropriate period before
trailing injection, e.g., within the period of the intake stroke,
as shown in FIG. 10.
[0150] While divisional injection is made at least during the
period T1, a weak swirl is generated during this period T1 (step
S26).
[0151] Since a swirl is weakened during the period T1 required
until the catalyst 22a reaches light-off at around 50% HC purifying
ratio, the burning rate becomes slow compared to a strong swirl, as
shown in FIGS. 8 and 9, thus promoting afterburning to increase the
exhaust gas temperature. As a result, since warming up of the
catalyst is promoted to increase the HC purifying ratio, the
unpurified HC emission amount can be reduced.
[0152] When a weak swirl state continues, fuel economy deteriorates
although the RawHC reduction and catalyst warming up effects
slightly improve. For this reason, deterioration of fuel economy is
suppressed by changing from a weak swirl to a strong swirl while
making divisional injection even in the latter period T2 (step
S34). Note that a swirl may be strengthened by simultaneously
injecting fuel in only the intake period during the latter period
T2.
[0153] Since the engine 1 shown in FIG. 1 has, on the top of each
piston 4, the stratification cavity 12 for trapping fuel injected
from the injector 11 and guiding it toward the spark plug 10, the
swirl ratio (swirl flow angular velocity/engine rotation angular
velocity) in each cylinder is set so that the local air-fuel ratio
around the spark plug 10 becomes rich by trailing injection upon
fuel injection from the injector 11 after the middle period of the
compression stroke 11.
[0154] Furthermore, as shown in FIG. 10, the trailing injection
timing .theta.ad (dotted curve) during the period T1 in which a
swirl is weakened is retarded compared to the trailing injection
timing .theta.ad (solid curve) during the period T2 in which a
swirl is strengthened (step S27). In this way, since combustion
stability can be assured by suppressing fuel mist from spreading,
and the exhaust gas temperature can be rapidly raised by slow
combustion, the catalyst 22 is activated earlier to reduce RawHC
emission, as shown in FIG. 11. Also, since a rich air-fuel mixture
can be locally present around the spark plug 10, the Pi variation
factor can be reduced to suppress combustion stability drop, as
shown in FIG. 12.
[0155] In divisional injection before the catalyst warms up, both
leading and trailing injections are controlled to inject fuel that
contributes main burning within a main burning period. That is, in
general, the mass fuel ratio up to around 10% in the burning
process in the combustion chamber is called an initial burning
period, and that ranging from around 10% to around 90% is called a
main burning period, and initial burning in which trailing
injection fuel ignites and burns ranges from the initial burning
period to the former period of the main burning period. Hence, the
injection amounts are set so that an air-fuel mixture at an
air-fuel ratio at which leading injection fuel can catch fire
(capable of flame propagation) due to burning of trailing injection
fuel is formed to make the leading injection fuel contribute to
main burning together with trailing injection fuel, and a lean
air-fuel mixture formed by the leading injection fuel burns
slowly.
[0156] More specifically, as the air-fuel ratio that allows spread
of flame due to burning of trailing injection fuel, the leading
injection amount during the period T1 is set to be smaller than the
trailing injection amount compared to the latter period T2, and is
set to be 1/4 (25%) or more the total injection amount (conversely,
the trailing injection amount is suppressed to be 3/4 (75%) or less
the total injection amount).
[0157] As shown in FIG. 13, when the trailing injection ratio is
smaller than 20%, the exhaust gas temperature rise effect and
HC/NO.sub.x reduction effect cannot be sufficiently obtained; when
the trailing injection ratio exceeds 20%, the exhaust gas
temperature rise effect and HC/NO.sub.x reduction effect increase
with increasing trailing injection ratio, but the Pi variation
factor and fuel consumption rate increase gradually. When the
trailing injection ratio exceeds 80%, the Pi variation factor
exceeds an allowable range, and combustion stability suffers.
[0158] Therefore, in order to assure high combustion stability
while obtaining a sufficient exhaust gas temperature rise effect
and HC/NO.sub.x reduction effect, the trailing injection ratio
preferably falls within the range from 25% to 75%. Within this
range, the exhaust gas temperature rise effect and HC/NO.sub.x
reduction effect increase with increasing rate injection ratio,
i.e., with decreasing leading injection ratio. If a small leading
injection amount is set so that the air-fuel ratio in the
combustion chamber becomes equal to or higher than a combustible
air-fuel ratio (around 30) by only leading injection, a lean
air-fuel mixture is formed by leading injection fuel, and burns
slowly during the latter period of the burning period. In this way,
a sufficiently high exhaust gas temperature rise effect and
HC/NO.sub.x reduction effect can be obtained while assuring high
combustion stability and suppression deterioration of fuel
economy.
[0159] FIG. 14 shows the output from the O.sub.2 sensor upon
O.sub.2 feedback. FIG. 15 shows the relationship between a change
in output from the O.sub.2 sensor upon O.sub.2 feedback, and a
corresponding change in feedback correction coefficient. FIG. 16
shows the relationship among the HC purifying ratio, catalyst
temperature, and RawHC emission amount with respect to the air-fuel
ratio.
[0160] In this embodiment, in addition to divisional injection,
weak swirl, trailing injection timing retard, and control for
setting the leading injection amount< trailing injection amount
during the period T1 required until the catalyst 22a reaches
light-off at around HC purifying ratio, the air-fuel ratio in the
combustion chamber during this period T1 is set to be lean
(.lambda..apprxeq.1) before the beginning of O.sub.2 feedback (the
O.sub.2 sensor is activated) (steps S19 to S22) to reduce the
NO.sub.x emission amount by the lean NO.sub.x catalyst, and is set
at .lambda.=1 after the beginning of O.sub.2 feedback (steps S16 to
S18) to improve the purification efficiency implemented by the
three-way function of the catalyst 22.
[0161] More specifically, a feedback reference value upon O.sub.2
feedback during the period T1 is set to be leaner than that upon
O.sub.2 feedback during the latter period T2 (e.g., AF
14.7.fwdarw.15.5). After the O.sub.2 sensor is activated, feedback
control starts to set the stoichiometric air-fuel ratio having the
detection signal of the O.sub.2 sensor as a reference value (e.g.,
0.55 V).
[0162] The output from the O.sub.2 sensor abruptly changes at the
stoichiometric air-fuel ratio (.lambda.=1), as shown in FIG. 14. In
the feedback control based on the output from the O.sub.2 sensor,
the feedback correction coefficient of the fuel injection amount
can be changed between a proportional gain as a P value and an
integral action rate as an I value, as shown in FIG. 15. When the
output from the O.sub.2 sensor is rich, the proportional gain is
changed in a direction to decrease the fuel injection amount by the
P or I value; when the output from the O.sub.2 sensor is lean, the
proportional gain is changed in a direction to increase the fuel
injection amount by the P or I value. Also, delay times T.sub.RL
and T.sub.LR are respectively set for inversion from rich to lean
of the O.sub.2 sensor and vice versa.
[0163] When the air-fuel ratio is controlled to be leaner than the
stoichiometric air-fuel ratio, the delay time T.sub.RL is adjusted
to be larger than T.sub.LR to shift the average value of the
feedback correction coefficient in the direction to decrease the
fuel injection amount, thus adjusting to shift the air-fuel ratio
toward the leaner side than the stoichiometric air-fuel ratio.
Also, the same adjustment can be achieved by using different P and
I values depending on whether the output from the O.sub.2 sensor is
rich or lean.
[0164] In O.sub.2 feedback control, normal control is made to set
the air-fuel ratio at the stoichiometric air-fuel ratio by setting
equal delay times T.sub.RL and T.sub.LR on the rich and lean
sides.
[0165] As shown in FIG. 16, when a lean air-fuel ratio is set
(.lambda..apprxeq.1) before the beginning of O.sub.2 feedback
during the period T1, warming up of the catalyst can be promoted to
increase the HC purifying ratio, thus reducing emission of
unpurified HC.
[0166] When the catalyst has not warmed up, the ignition timing
.theta.ig is retarded with respect to an identical load and
identical engine speed after the catalyst warms up, and
afterburning by slow combustion is promoted to effect warming up of
the catalyst, thus reducing emission of unpurified HC.
[0167] In this embodiment, when the three-way catalyst 22a has
reached light-off at around 100% purifying ratio, completion of
warming up of the NO.sub.x catalyst 22b is determined.
Alternatively, the period T2 may be set based on the state of the
NO.sub.x catalyst 22b.
[0168] Note that the present invention can also be applied to an
engine which as an NO.sub.x catalyst alone, and can be applied to
changes and modifications of the above embodiment within the scope
of the invention.
[0169] The present invention is not limited to the above
embodiments and various changes and modifications can be made
within the spirit and scope of the present invention. Therefore, to
apprise the public of the scope of the present invention the
following claims are made.
* * * * *