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BACKGROUND OF THE INVENTION
The present invention generally relates to an air-fuel mixture control for
an automobile engine of a type having a fuel injection system and, more
particularly, to the air-fuel mixture control system effective to
compensate for variation in air-fuel mixing ratio of a combustible mixture
by properly controlling the rate of recirculation of exhaust gases from an
engine exhaust system into an engine fuel intake system in dependence on
an output signal from a composition sensor for detecting the concentration
of a component of exhaust gases which concentration is a function of the
air-fuel mixing ratio of the combustible mixture burned in the engine.
An automobile engine of a type having a fuel injection system is well
known. In this engine, an air-flow meter is installed on a fuel intake
passage of the engine at a position upstream of a fuel injection nozzle
with respect to the direction of flow of a combustible mixture towards the
engine. The air-flow meter is used to detect, and generate an output
signal indicative of, the rate of flow of incoming air which is
subsequently mixed with an injected fuel to form the combustible mixture,
which output signal is utilized to control the rate of supply of fuel to
be injected into the engine fuel intake passage through the fuel injection
nozzle. The use of the air-flow meter is advantageous in that, since the
rate of supply of the fuel can be controlled in dependence on the output
signal from the air-flow meter, the air-fuel mixing ratio of the
combustible mixture can accurately be controlled. An example of this is
disclosed in the Japanese Laid-open Patent Publication No. 53-32232 and
No. 53-32233, both laid open to public inspection on Mar. 27, 1978, and
corresponding to the U.S. Pat. No. 4,163,435, patented Aug. 7, 1979.
However, it has been fairly recognized that, the engine of the type having
the fuel injection system requires the increased manufacturing cost
because both of the fuel injection nozzle and the air-flow meter are
expensive.
What appears to substantially eliminate the above described disadvantage is
a control system wherein a combination of at least two of easily
detectable parameters including the engine speed (number of revolution of
the engine), the negative pressure inside the fuel intake passage and the
opening of a throttle valve is utilized in a computer to calculate the
rate of supply of incoming air so that the rate of supply of fuel which
subsequently mixed with the incoming air can be controlled in
correspondence with the rate of supply of the incoming air.
Where an exhaust gas recirculation system for recirculating a portion of
the exhaust gases from the exhaust passage back to the intake passage for
suppressing the maximum combustion temperature is provided in the engine
utilizing the fuel injection system and also utilizing the air-fuel ratio
control system wherein at least two of the parameters are used to
compensate for variation in the air-fuel mixing ratio of the combustible
mixture, such as taught by the U.S. Pat. No. 4,163,435 in view of the
Japanese Laid-open Patent Publication No. 50-32327, laid open to public
inspection on Mar. 29, 1975, the combustible mixture tends to be enriched
because a portion of the incoming air flowing through the intake passage
towards the engine is substituted by the exhaust gases recirculated
through the EGR system. Therefore, in this possible arrangement, means is
required to compensate for variation in the air-fuel mixing ratio which
would result from the recirculation of the exhaust gases.
In order to compensate for variation in the air-fuel mixing ratio resulting
from the recirculation of the exhaust gases, the conventional procedure is
such as to calculate according to a control map stored in a computer, the
required amount of the exhaust gases to be recirculated appropriate for a
particular engine operating condition on the basis of the parameters
representative of such engine operating condition, and then to control the
opening of an EGR control valve disposed on an EGR passage so as to
satisfy the required amount of the exhaust gases to be recirculated while
a fuel adjusting means is so adjusted as to inject fuel into the intake
passage at a rate or amount corresponding to the amount of the exhaust
gases to be recirculated in anticipation that the exhaust gas
recirculation would take place as required by the operation of the EGR
control valve.
However, since this conventional method is not based on the practical
amount of the exhaust gases recirculated, this conventional method
involves such a disadvantage that, even though the opening of the EGR
control valve is accurately map-controlled, deviation of the amount of the
exhaust gases being recirculated from the required value by reason of, for
example, clogging of the EGR passage results in variation of the air-fuel
mixing ratio which ought to have been compensated for. Therefore, with
this conventional method, an accurate control of the air-fuel mixing ratio
to a stoichiometric value can not be performed.
This disadvantage may be eliminated if the amount of the exhaust gases
being actually recirculated is detected to provide a reference necessary
to determine the rate of supply of the fuel to be injected into the intake
passage, or necessary to perform a feedback control to cause the actual
amount of the exhaust gases recirculated to approximate to the required
amount of the exhaust gases. However, in order to achieve this, the use of
the expensive air-flow meter and/or an EGR detecting system for detecting
the amount of the exhaust gases actually recirculated is required,
rendering the system as a whole to be costly.
In view of the above, we have previously proposed an improved air-fuel
ratio control system wherein a fuel adjusting means is provided for
determining the amount of fuel to be injected by calculating from a
combination of at least two of such parameters representative of an engine
operating condition as the engine speed, the negative pressure and the
opening of the throttle valve and wherein the air-fuel mixing ratio of the
combustible mixture which has been determined by a combination of the
amount of the incoming air, the amount of the fuel injected and the amount
of the exhaust gases actually recirculated is detected by a composition
sensor disposed on the exhaust passage, an output signal from which sensor
is used to accurately control the amount of the exhaust gases to be
recirculated, that is, to institute a feedback control so as to render the
amount of the exhaust gases to be recirculated to be of a value
approximating to the required amount of the exhaust gases.
For this purpose, the composition sensor effective to detect whether the
combustible mixture burned in the engine has been enriched or leaned is
utilized to detect variation in the air-fuel mixing ratio resulting from
the recirculation of the exhaust gases, an output signal of said
composition sensor being supplied to a computer as information so that the
computer can control the EGR control valve according to such information
so as to increase or decrease the amount of the exhaust gases to be
recirculated in correspondence with the amount of deviation of the actual
air-fuel mixing ratio from a predetermined air-fuel mixing ratio, whereby
the air-fuel mixing ratio of the combustible mixture to be supplied
towards the engine can be controlled to the predetermined or
stoichiometric value.
SUMMARY OF THE INVENTION
The present invention is based on our previously proposed concept and has
been developed with a view to substantially eliminating such disadvantages
and inconveniences as hereinbelow discussed.
Since the air-fuel mixing ratio determined by the fuel adjusting means in
dependence on the detected rate of flow of the incoming air (which
air-fuel mixing ratio is hereinafter referred to as "base A/F ratio,")
approximates to the predetermined or stoichiometric air-fuel mixing ratio
where the base A/F ratio is so selected as to become slightly lower than
the predetermined value during a particular engine operating condition,
including low load and high load operating conditions and the cold start
of the engine, in order to secure a good drivability of the engine during
such particular operating condition, the control system designed for
controlling the actual air-fuel mixing ratio to the predetermined value by
adjusting the flow of the exhaust gases being recirculated involves such a
problem that no exhaust gas recirculation take place actually and,
therefore, the emision of the NO.sub.x component of the exhaust gases
cannot be suppressed by the exhaust gas recirculation during the
particular engine operating condition.
In order to substantially eliminate the above discussed problem, the
present invention is such that a control means for controlling the
electromagnetic valve assembly is constituted by first and second EGR
control means. The first EGR control means is operable during the
particular engine operating condition to determine the opening of the EGR
control valve assembly according to a control value read out from a memory
means wherein such control value corresponding to the particular operating
condition is stored, so that the exhaust gases to be recirculated can be
obtained in an amount necessary to suppress the emission of the NO.sub.x
component independently of the control or the air-fuel mixing ratio by the
EGR system. On the other hand, the second EGR control means is operable
during an engine operating condition other than the particular operating
condition to effect a feedback control to the EGR control valve assembly
in dependence on an output signal from the composition sensor so that the
air-fuel mixing ratio can be controlled to the predetermined value by
controlling the amount of the exhaust gases to be recirculated.
BRIEF DESCRIPTION OF THE DRAWINGS
These other objects and features of the present invention will become clear
from the following description taken in conjunction with a preferred
embodiment of the present invention with reference to the accompanying
drawings, in which:
FIG. 1 is a schematic diagram showing an air-fuel ratio control system
embodying the present invention;
FIG. 2 is a schematic block diagram showing a microcomputer used in the
practice of the present invention;
FIG. 3 is a diagram showing the waveform of an output signal from a
composition sensor;
FIG. 4 is a flow chart showing the sequence of control performed by the
microcomputer;
FIG. 5 is an explanatory diagram used to explain a memory Map 1 for setting
a reference pulse width to be applied to a fuel injection nozzle;
FIG. 6 is a graph showing the relationship between variation in an air-fuel
ratio signal and corresponding variation in the duly cycle of an
electromagnetic valve assembly;
FIG. 7 is an explanatory diagram used to explain a memory Map 2 for the
determination of a correction coefficient relative to the reference pulse
width;
FIG. 8 is an explanatory diagram used to explain a memory Map 3 for setting
EGR and EGR-cut regions of engine operating conditions; and
FIG. 9 is an explanatory diagram used to explain a memory Map 4 for setting
the duty cycle of an electromagnetic valve assembly.
DETAILED DESCRIPTION OF THE INVENTION
Before the description of the invention proceeds it is to be noted that
like parts are designated by like reference numerals throughout the
accompanying drawings.
Referring first to FIG. 1, an automobile power plant comprises an internal
combustion engine 1 having at least one combustion chamber communicated on
the one hand to the atmosphere through an intake passage 3 by way of an
air cleaner 2 and on the other hand to the atmosphere through an exhaust
passage 6 by way of an exhaust gas purifying unit 7 in the form of, for
example, a catalytic converter. The intake passage 3 has a throttle valve
4 supported therein for movement between substantially closed and full
open positions for regulating the flow of a combustible air-fuel mixture
towards the engine 1, and a fuel injection nozzle 5 positioned upstream of
the throttle valve 4 with respect to the direction of flow of the air-fuel
mixture towards the engine 1 for injecting a mist of fuel into the intake
passage 3.
The automobile power plant also comprises an exhaust gas recirculating
(EGR) system including an exhaust gas recirculating passage 8 having one
end communicated to a portion of the exhaust passage 6 between the engine
1 and the catalytic converter 7 and the other end communicated to the
intake passage 3 at a position downstream of the throttle valve 4 with
respect to the direction of flow of the air-fuel mixture towards the
engine 1, and an EGR control valve 9 in the form of a diaphragm valve
assembly installed on the EGR passage 8 for regulating the flow of exhaust
gases from the exhaust passage 6 back into the intake passage 3 through
said EGR passage 8 in dependence on engine operating conditions as will be
described later. The EGR control valve has a working chamber 9a defined in
a valve casing by a disphragm member 9d, said diaphragm member 9d being
displaceable to move a valve member 9c, coupled thereto for movement
together with the diaphragm member 9d, between opened and closed
positions. The EGR control valve 9 also has a biasing spring 9b housed
within the working chamber 9a and biasing the diaphragm member 9d to move
the valve member 9c to the closed position in which the recirculation of
the exhaust gases through the EGR passage 8 is interrupted.
The EGR control valve 9 is controlled by a negative pressure developed
inside the intake passage 3 at a position downstream of the throttle valve
4 and, for this purpose, the working chamber 9a of the EGR control valve 9
is communicated through a pressure transmitting passage 10 to a port 11
opening into the intake passage 3 at a position downstream of the throttle
valve 4. The pressure transmitting passage 10 has a three-way
electromagnetic valve assembly 12 disposed thereon and comprising a valve
casing having three ports 12a, 12b and 12c defined therein. The port 12a
is in communication with a portion of the pressure transmitting passage 10
adjacent the intake passage 3, the port 12b is in communication with the
atmosphere through an air duct 13, and the port 12c is in communication
with a portion of the pressure transmitting passage 10 adjacent the EGR
control valve 9. The valve assembly 12 also comprises a solenoid 12d and a
valve member 12e normally held in position to close the port 12b, but
capable of being moved towards the left as viewed in FIG. 1, to close the
port 12a when the solenoid 12d is electrically energized in a manner as
will be described later. In practice, this three-way electromagnetic valve
assembly 12 is so designed as to adjust the magnitude of the negative
pressure to be introduced into the working chamber 9a of the EGR control
valve 9 by allowing air to enter the pressure transmitting passage 10
during the movement of the valve member 12e from the right towards the
left as viewed in FIG. 1 as a result of the energization of the solenoid
12d. Therefore, it has now become clear that the opening of the EGR
control valve 9, and thus the effective cross-sectional area of the EGR
passage 8, can be adjusted by the magnitude of the negative pressure
introduced into the working chamber 9a of the EGR control valve, which
magnitude is in turn adjusted by the valve assembly 12.
Both the amount of fuel to be injected into the intake passage 3 through
the injection nozzle 5 and the duty cycle of the electromagnetic valve
assembly 12 (which duty cycle represents the ratio, expressed as a
percentage, of the time t during which the port 12a is opened relative to
the time .tau. during which the port 12a is closed, namely
.tau./t.times.100. The first and second times t and .tau. are hereinafter
referred to as on-time and off-time of the valve assembly 12,
respectively.) are controlled by a microcomputer 14.
The air-fuel mixing ratio control system embodying the present invention
includes the following sensors, outputs of all of which are supplied to
the microcomputer so that the air-fuel mixing ratio of the combustible
mixture to be supplied to the engine 1 can be ultimately controlled to a
stoichiometric valve at all engine operating conditions for the purpose of
minimizing both the fuel consumption and the emission of pollutants
contained in the exhaust gases.
(a) A pressure sensor 15 disposed on the intake passage 3 at a position
downstream of the throttle valve 4 for detecting, and generating an output
signal indicative of, the negative pressure V developed inside the intake
passage 3.
(b) A temperature sensor 16 for detecting, and generating an output signal
indicative of, the temperature T of a coolant water used to cool the
engine 1.
(c) An engine speed sensor 17 operatively coupled to a power output shaft
(not shown) of the engine 1 for detecting, and generating an output signal
indicative of, the number N of revolution of the engine which sensor 14
may be a tachnometer.
(d) A composition sensor 18 disposed on the exhaust passage 6 between the
engine 1 and the catalytic converter 7 for detecting, and generating an
output signal indicative of, the concentration of a component, for
example, oxygen, of the exhaust gases, which concentration is a function
of the air-fuel mixing ratio of the combustible mixture which has been
combusted in the engine 1.
While the output signals from the respective sensors 15, 16 and 17 are
directly fed to the microcomputer 14, the output signal A/F from the
composition sensor 18 is fed to a comparison circuit 19 operable to
compare the output signal from the composition sensor 18 with a threshold
valve representative of a predetermined air-fuel mixing ratio which may be
a stoichiometric air-fuel mixing ratio and then to generate a difference
signal or air-fuel ratio signal A/F which is applied to the microcomputer
14.
Referring to FIG. 2, the microcomputer 14 comprises a central processing
unit 22, a memory 23, an analog-to-digital converter 24, an analog
multiplexer 25, an input interface circuit 26, an output interface circuit
27, all of them are connected by control buses 28, the receipt and
delivery of address signals and data signals among these components being
performed through address/data buses 29. This microcomputer 14 is of any
known construction and, therefore, the details thereof will not be herein
reiterated for the sake of brevity.
As shown in FIG. 2, the respective output signals from the pressure sensor
15, the temperature sensor 16 and the speed sensor 17 are fed to the
analog multiplexer 25. These output signals from the sensor 15, 16 and 17
fed to the analog multiplexer 25 are occasionally read out therefrom and
then fed to the central processing unit 22. On the other hand, the
air-fuel ratio signal A/F is fed to the input interface circuit 26 and is
occasionally fed therefrom to the central processing unit 22.
In FIG. 2, reference numeral 30 represents a predetermined voltage
generator for supplying a voltage signal representative of the threshold
value Vth to the comparison circuit 19 whereat the output signal from the
composition sensor 18 is compared with the threshold value Vth. The
comparison circuit 19 so connected with the predetermined voltage
generator 30 serves to determine whether the output signal from the
composition sensor 18 is higher or lower than the threshold value Vth as
shown in FIG. 3 to find whether the combustible mixture burned in the
engine 1 has been enriched or leaned with respect to the stoichiometric
air-fuel mixing ratio of, for example, 14.7, the output signal from said
comparison circuit 19 being supplied to the microcomputer 14 in the form
of the air-fuel ratio signal A/F.
Hereinafter, the manner in which the fuel injection control circuit for the
nozzle 5 and the electromagnetic valve assembly 12 are controlled by the
microcomputer 14 will be described with particular reference to a flow
chart shown in FIG. 4.
Referring to FIG. 4, the microcomputer 14 initialized at the step (1) in
response to the application of a start signal thereto performs the
following control process at a predetermined cycle.
In the first place, at the step (2), the output signal from the speed
sensor 18 is read in and, subsequently at the step (3), the engine speed N
is stored at a predetermined address in a first memory area M.sub.1 of the
memory 23. Thereafter and until the step (7), the output signals from the
respective sensors 16 and 17 are sequentially read in with the negative
pressure V and the water temperature T memorized correspondingly in a
similar manner.
At the step (8), the width Lo of a reference pulse which provides a
reference to a fuel injection control pulse to be applied to the nozzle 5
is calculated from the engine speed N and the negative pressure V on the
strength of a memory Map 1 shown in FIG. 5.
The memory Map 1 is a map having a plurality of grid points (ai, j)
occupying respective points of intersection of eight lines equally
dividing the maximum possible range of variation of the negative pressure
V with an equal number of columns dividing equally the range of engine
speed N and wherein the value of fuel injection control pulse width Lo,
that is, the amount of fuel to be injected into the intake passage 3
through the nozzle 5, determined as appropriate to a particular
coordination of the negative pressure V and the engine speed N assigned by
the associated grid point (ai, j) is stored at each grid point (ai, j), as
shown in FIG. 5. This fuel injection control pulse width Lo, which is
hereinafter referred to as reference pulse width, is so selected as to
cause the base A/F ratio to be higher than the stoichiometric value in
consideration of the compensation of the air-fuel mixing ratio effected by
the recirculation of the exhaust gases.
In this case, the memory Map 1 is, as shown in FIG. 5, composed of a
particular region A of engine operating condition including low load and
high load engine operating conditions and a region B of engine operating
condition including a moderate load engine operating condition. The
reference pulse width Lo falling within the particular operating region A
is so selected that the base A/F ratio is lower than that represented by
the reference pulse width Lo falling within the operating region B,
whereby not only can a good drivability of the engine during the operating
condition within the region A be obtained, but also any possible reduction
in combustibility of the engine can be avoided even during the proper
exhaust gas recirculation being effected.
Where the point P assigned by the coordination of the negative pressure V
and the engine speed N read out from the central processing unit 22 does
not match with the grid point (ai, j) as shown in FIG. 5, the reference
pulse width Lo(P) corresponding to the point P can be calculated by
interpolation on the basis of the four grid points (ai-l, j), (ai-l, j-l),
(ai, J-l) and ai, j) all surrounding the point P.
The reference pulse width Lo(P) so calculated is stored at a predetermined
address in the first memory area M.sub.1 of the memory 23 during the step
(9). At the subsequent step (10), the water temperature T memorized during
the step (7) is read out and a correction coefficient K(T) for the water
temperature T relative to the reference pulse width Lo(P) is calculated
according to a memory Map 2 shown in FIG. 7. The calculated correction
coefficient K(T) is then multiplied by the reference pulse width Lo(P), at
the step (11), to determine the pulse width L(=Lo(P).times.K(T)) of the
injection control pulse to be actually applied to the fuel injection
nozzle 5, which pulse width L is temporarily stored at a predetermined
address in the first memory area M.sub.1 of the memory 23 during the step
(12).
By so doing, the determination of the injection control pulse width L
completes, at the microcomputer starts calculation of the duty cycle D of
the electromagnetic valve assembly 12.
For this purpose, subsequent to the determination of the injection control
pulse width L and at the step (13), data such as the engine speed N, the
negative pressure V and the water temperature T all stored in the first
memory area M.sub.1 are read out to find the engine operating condition
according to a memory Map 3, i.e. to find according to the memory Map 3
whether the engine operating condition falls in a region of high base A/F
ratio or whether the engine operating condition falls in a region of low
base A/F ratio.
The memory Map 3 is, as shown in FIG. 8, composed of separate regions A'
and B' corresponding, respectively, to the regions A and B of the memory
Map 1. Since the injection control pulse width L, unlike the reference
pulse width Lo, has a dependency on the temperature T of the coolant water
and since the actual region A' of low base A/F ratio and the actual region
B' of high base A/F ratio vary depending on the water temperature T, the
boundary between the regions A' and B' has been preset according to the
memory Map 1 and the memory Map 2 so that such variation can be
determined.
At the step (14), check is made as to whether the engine operating
condition then occurring falls within the region A' or the region B' and
if it is found as falling within the region B, the step (14) is followed
by the step (15) from which a second EGR control means performs a control
to feedback the EGR control valve 9. On the contrary thereto, if it is
found at the step (14) as falling in the region B', the step (14) is
followed by the step (25) from which a first EGR control means operates to
control the control valve 9 according to a control value stored in a
memory means.
In the second EGR control means, at the step (15), data such as the engine
speed N, the negative pressure V, the water temperature T, the reference
pulse width Lo, the correction coefficient K, the injection control pulse
width L and others, which have been stored in the first memory area
M.sub.1 of the memory 23 are transferred to the second memory area M.sub.2
which is separately provided in the memory 23.
The air-fuel ratio signal A/F from the comparison circuit 20 is read in at
the step (16) and is subsequently, i.e., at the step (17) stored in the
first memory area M.sub.1. At the step (18), the air-fuel ratio signal so
stored is compared with the previously stored air-fuel ratio signal, which
has been transferred to the second memory area M.sub.2, to determine
whether or not the air-fuel mixing ratio has been reversed. If it is found
that the air-fuel mixing ratio has not been reversed, the step (18) is
followed by the step (19), but if it has been found that the air-fuel
mixing ratio has been reversed, the step (18) is followed by the step
(22). At any one of the steps (19) and (22), the combustible mixture
burned in the engine is checked as to whether it has been enriched.
Specifically, assuming that the air-fuel ratio signal A/F (t.sub.1) read in
at a timing t.sub.1 shown in FIG. 6 as well as the air-fuel ratio signal
A/F (t.sub.2) read in a timing t.sub.2 shows that the combustible mixture
burned in the engine 1 has been enriched, and assuming that the increment
of the previous duty cycle D(t.sub.1) of the electromagnetic valve
assembly 12 during a period from the timing t.sub.1 to the timing t.sub.2
has been, for example, 5%, the succeeding duty cycle D(t.sub.2) is set to
a value increased by the increment of 5% relative to the previous duty
cycle D(t.sub.1) That is, D(t.sub.2)=1.05.times.D(t.sub.1)
It is to be noted that, where both of the previous air-fuel ratio signal
A/F(t.sub.3) and the next succeeding air-fuel ratio signal A/F(t.sub.4) at
respective timings t.sub.3 and t.sub.4 in FIG. 6 show that the combustible
mixture has been leaned, the decrement is assumed to be 5% and the next
succeeding duty cycle D(t.sub.4) is reduced at the step (21) by the
decrement of 5% relative to the previous duty cycle D(t.sub.4). That is,
D(t.sub.4)=0.95.times.D(t.sub.3).
On the other hand, where as shown at respective timing t.sub.2 and t.sub.3
in FIG. 6 the previous air-fuel ratio signal A/F(t.sub.2) and the next
succeeding air-fuel ratio signal A/F(t.sub.3) are in reversed relation to
each other, showing that the combustible mixture has been enriched and
leaned at the respective timings t.sub.2 and t.sub.3, at the step (23),
the decrement incident to the reversion of the air-fuel mixing ratio
during the period from the timing t.sub.2 to the timing t.sub.3 is assumed
to be, for example, 15% and the next succeeding duty cycle D(t.sub.3) is
set to a value reduced by the decrement of 15% relative to the previous
duty cycle D(t.sub.2). That is, D(t.sub.3)=0.85.times.D(t.sub.2)
On the contrary thereto, where the air-fuel ratio signal A/F(t) which has
shown at the step (22) that the combustible mixture has been leaned is
reversed to show that the combustible mixture has been enriched, the next
succeeding duty cycle is set at the step (24) to a value increased by the
increment of 15% relative to the previous duty cycle. That is,
D(t.sub.3)=1.15.times.D(t.sub.2).
Where the engine operating condition has been found falling in the region
A' at the step (14), the first control means supersedes the feedback
control, which is effected in dependence on the output signal from the
composition sensor 18, to determine the duty cycle D of the
electromagnetic valve assembly 12 according to a memory Map 4 shown in
FIG. 9.
The memory Map 4 is similar to the memory Map 1 shown in FIG. 5 and is a
map in which a plurality of predetermined duty cycles D of the
electromagnetic valve assembly 12 appropriate to the engine operating
conditions are preset at respective grid points (bi, j) assigned by
associated coordinations of the negative pressure V and the engine speed
N. It is to be noted that, where a particular engine operating condition
does not coincide with any one of the grid points (bi, j), the duty cycle
of the electromagnetic valve assembly 12 appropriate to such particular
engine operating condition can be calculated by interpolation in a manner
similar to that described with reference to FIG. 5.
Accordingly, in the present invention, even when the engine operating
condition falls within the region A', the exhaust gas recirculation is not
completely interrupted and is effected according to the memory Map 4
separately of the feedback control in dependence on the output signal from
the composition sensor 18. Therefore, even though the air-fuel mixing
ratio of the combustible mixture to be supplied to the engine 1 during the
engine operating condition falling within the region A corresponding to
the region A' is set to be low, the exhaust gases can be recirculated in
an amount necessary to suppress the emission of the NO.sub.x component
irrespective of the setting of the air-fuel mixing ratio.
By so doing, the duty cycle D of the electromagnetic valve assembly 12 so
determined at any one of the steps (20), (21), (23), (24) and (25) is
stored at the step (26) in the first memory area M.sub.1.
Thereafter, the injection control pulse width L stored in the first memory
area M.sub.1 is read out at the step (27) and is, as shown in FIG. 2,
applied to the injection nozzle 5 through the interface circuit 27 to
cause said nozzle to inject the fuel into the intake passage 3 for a
period of time equal to the injection control pulse width L.
Subsequently and at the step (28), an output signal indicative of the
calculated duty cycle D for the electromagnetic valve assembly 12 is
applied through the output interface circuit 27 to the valve assembly 12
to control the duty cycle of the valve assembly 12 in dependence on the
calculated duty cycle D.
As has become clear from the above description, in the case where the
engine operating condition falls in the region B', and if it has been
found that the actual air-fuel mixing ratio of the combustible mixture
monitored by the composition sensor 18 was lower than the stoichiometric
value, the duty cycle of the valve assembly 12 is computer-controlled to
increase to allow much air to mixing ratio, the duty cycle of the
electromagnetic valve assembly 12 is adjusted by the microcomputer 14 to
increase. As shown in FIG. 1, the increased duty cycle of the
electromagnetic valve assembly 12 allows much air to enter through the
port 12b into the EGR passage 10 thereby reducing the negative pressure
which has been introduced into the working chamber 9a of the EGR control
valve 9. Consequently, as the negative pressure in the working chamber 9a
reudces, the biasing spring 9b expands gradually outwardly to displace the
diaphragm member 9d to bring the valve member 9c towards the closed
position whereby the rate of flow or recirculation of the exhaust gases
from the exhaust passage 6 into the intake passage 3 is decreased while
the air entering the EGR passage 10 through the port 12b is introduced
into the intake passage 3 at a rate corresponding to the rate of reduction
of the flow of the exhaust gases through the EGR passage 8. By so doing,
the combustible mixture being supplied towards the engine 1 is leaned to
attain the stoichiometric valve.
On the contrary thereto, should the output signal from the composition
sensor 19 show that the combustible mixture burned in the engine 1 has
been leaned relative to the stoichiometric air-fuel mixing ratio, the duty
cycle of the electromagnetic valve assembly 12 is decreased thereby to
allow much negative pressure to be introduced into the working chamber 9a
of the EGR control valve 9. Therefore, in this condition, the diaphragm
member 9d is displaced to move the valve member 9c towards the opened
position, so that the combustible mixture being supplied towards the
engine 1 is enriched in admixture with the recirculated exhaust gas to
attain the stoichiometric air-fuel mixing ratio.
On the contrary thereto, in the case where the engine operating condition
falls in the region A' corresponding to the particular operating region A,
the duty cycle D of the EGR control valve 9 is determined according to the
memory Map 4 with the opening of said valve 9 adjusted to a value
corresponding to such duty cycle D so that the exhaust gas recirculation
can be effected to such an extent that the drivability of the engine 1
will not be hampered, thereby minimizing the emission of the NO.sub.x
component during the particular operating region A.
From the foregoing description, it has now become clear that the present
invention is such that, when the engine of the type utilizing the fuel
injection system including the fuel adjusting means which is
computer-controlled by the utilization of at least two parameters
including the engine speed, the negative pressure and the opening of the
throttle valve is operated under the operating condition falling within
the particular operating region, the air-fuel mixing ratio is set to be
lower than that during the operating condition falling within the other
region than the particular operating region and, on the other hand, these
is employed the first EGR control means for controlling the EGR control
valve according to the control value, which has been stored in the memory
means, when the engine operating condition falls within the particular
operating region and the second EGR control means for controlling the EGR
control valve according to the feedback scheme in dependence on the output
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