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Description  |
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FIELD OF THE INVENTION
This invention generally relates to an exhaust gas recirculation system for
an internal combustion engine. More particularly, this invention relates
to an electronic control circuit for controlling the recirculation rate of
the exhaust gas.
BACKGROUND OF THE INVENTION
In some conventional exhaust gas recirculation systems for an internal
combustion engine, the rate of exhaust gas to be recirculated is
controlled by an electromagnetic valve responsive to a driving pulse
signal, where the duty cycle of the driving pulse signal is controlled in
accordance with various engine parameters such as the flow rate of the
intake air of the engine, and the rotational speed of the crankshaft of
the engine.
As the power source of the driving pulse signal, a battery is usually used.
If the voltage of the battery is constant, the flow rate of the gas is
proportionally controlled via the electromagnetic valve in accordance with
the calculated duty cycle of the driving pulse signal. However, the
voltage of the battery is apt to vary in a considerably wide range. When
the voltage of the driving pulse signal of the electromagnetic valve
varies, the response of the electromagnetic valve varies accordingly. In
other words, the actual duty cycle of the operation (the period of time
for which the valve opens with respect to the duration of one cycle of
open and close states) of the electromagnetic valve does not correspond
with the duty cycle of the driving pulse signal. This means that, when the
voltage of the power supply is different from the standard voltage
thereof, the recirculation rate of the exhaust gas differs from a desired
rate which is expected, although the duty cycle of the driving pulse
signal is correctly controlled.
SUMMARY OF THE INVENTION
This invention has been achieved to overcome the above described drawback
of the conventional exhaust gas recirculation system.
It is, therefore, an object of the present invention to provide an exhaust
gas recirculation system for an internal combustion engine, in which the
recirculation rate of the exhaust gas is optimally controlled
irrespectively of the variation of the voltage of the power supply.
Another object of the present invention is to provide such a system in
which the opening duration of the valve head of an electromagnetic valve
is compensated for by controlling the duty cycle of the driving pulse
signal in accordance with the difference between a predetermined voltage
and the actual voltage of the power supply.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and features of the present invention will become
more readily apparent from the following detailed description of the
preferred embodiments taken in conjunction with the accompanying drawings
in which:
FIG. 1 shows a schematic view of the exhaust gas recirculation system
according to the present invention;
FIG. 2 is a graphical representation showing the relationship between the
duty cycle of the driving pulse signal and the opening duration of the
electromagnetic valve shown in FIG. 1;
FIG. 3A to FIG. 3D are graphical representations useful for understanding
the idea of compensation of duty cycle of the driving pulse signal applied
to the electromagnetic valve shown in FIG. 1;
FIG. 4 shows a schematic block diagram of a first preferred embodiment of
the control signal generator shown in FIG. 1; and
FIG. 5 shows a schematic block diagram of a second preferred embodiment of
the control signal generator shown in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Prior to the description of the preferred embodiments, a general idea of an
exhaust gas recirculation system for an internal combustion engine is
discussed. In FIG. 1, an exhaust gas recirculation (which will be referred
to as EGR hereinafter) system is shown. It is to be noted that FIG. 1 is
intended to show both of a known EGR system and a novel EGR system
according to the present invention since the novel feature of the present
invention resides in the control signal generator 9 which is shown as a
black box.
An internal combustion engine 2 has an intake passage 1, and an exhaust
passage 3. A throttle flap 7 is disposed in the intake passage 1 to
control the flow rate of the intake air. A recirculation passage 4 is
provided to fluidly connect the exhaust passage 3 to the intake passage 1.
An EGR control valve assembly 5 is interposed in the EGR passage 4 to
control the recirculation rate of the exhaust gas. The EGR control valve
assembly 5 has first and second chambers 5-1 and 5-2 arranged in opposite
sides with respect to a diaphragm 5d. The diaphragm 5d is normally biased
downwardly by the force of a spring 5s. The second chamber 5-2 of the EGR
control valve assembly 5 is communicated with the atmosphere via an
opening 5o. A valve head 5h is fixedly connected via a rod to the
diaphragm 5d and is arranged to abut on a valve seat 5b when biased
downwardly so that the valve head 5h controls the amount of recirculation
gas. The first chamber 5-1 is communicated with the intake passage 1 via a
conduit 30 and a constant pressure valve 8. An orifice 32 is provided in
the conduit 30. The constant pressure valve 8 is provided to produce a
constant vacuum pressure from the vacuum prevailing in the intake passage
1 downstream of the throttle flap 7. The orifice 32 is provided to reduce
the vacuum pressure obtained by the constant pressure valve 8 in a manner
that the EGR control valve assembly 5 actuates in the best working range
in view of the control vacuum pressure to flow rate characteristic of the
EGR control valve assembly 5. The conduit 30 is communicated via an
electromagnetic (solenoid) valve 6 with the atmosphere. The
electromagnetic valve 6 has a valve head 6a disposed on a movable member
which is arranged to move up and down in response to a driving pulse
signal S.sub.1 applied to the winding of the electromagnetic valve 6. The
valve head 6a is arranged to abut on a valve seat 6b arranged at the end
of a conduit 6c communicating with the atmosphere. In this shown example,
the conduit 6c extending from the electromagnetic valve 6 is communicated
with the intake passage 1 upstream of the throttle flap 7 to induce the
atmospheric pressure. The electromagnetic valve 6 has a chamber
communicating with the conduit 30. With this arrangement, the vacuum in
the conduit 30 is diluted by the atmospheric pressure to an extent defined
by the ON/OFF ratio of the electromagnetic valve 6. The ON/OFF ratio of
the operation of the electromagnetic valve 6 is controlled by the duty
cycle of the driving pulse signal S.sub.1. Accordingly, the pressure in
the first chamber 5-1 of the EGR control valve assembly 5 is controlled in
turn.
The control signal generator 9 is responsive to two signals S.sub.2 and
S.sub.3 respectively indicative of the flow rate of the intake air and the
engine rotational speed. The control signal generator 9 may be a
microcomputer and is arranged to produce the driving pulse signal S.sub.1
in accordance with these input signals S.sub.2 and S.sub.3. The
microcomputer calculates an optimal recirculation rate of the exhaust gas
in view of these engine parameters. The duty cycle of the dirving pulse
signal S.sub.1 is controlled by the microcomputer so as to result in the
optimal recirculation rate.
FIG. 2 shows the opening duration characteristics of the electromagnetic
valve 6 with respect to the voltage of the driving pulse signal S.sub.1.
Since the driving current of the electromagnetic valve 6 is derived from a
suitable power source, such as a battery, the voltage of the driving pulse
signal sometimes varies undesirably. As shown in FIG. 2, the duration of
opening of the electromagnetic valve 6 varies as the voltage of the
driving pulse signal S.sub.1 varies even though the duty cycle of the
driving pulse signal S.sub.1 is maintained constant.
The reason for this variation in opening duration of the electromagnetic
valve 6 is that there is a time delay in the operation of the movable
member in the electromagnetic valve 6. If the electromagnetic valve 6 is
of the type that the valve head thereof opens upon energization of the
winding and closes by the force of spring upon deenergization, the time
delay at the initial stage of the opening of the valve head increases as
the voltage of the driving pulse signal S.sub.1 decreases (the illustrated
electromagnetic valve 6 is of this type). On the other hand, if the
electromagnetic valve 6 is of the type that the valve head closes upon
energization of the winding and opens by the force of spring upon
deenergization, the time delay at the initial stage of the closing of the
valve head increases as the voltage of the driving pulse signal decreases.
In the former case, therefore, the duration for which the valve head opens
becomes shorter as the voltage decreases, and in the latter case, the
duration for which the valve head opens becomes longer as the voltage
decreases.
Since the power supply used for various electronical equipment in a motor
vehicle is a battery, the voltage of the driving pulse signal is apt to
vary in a considerably wide range. For instance, it is possible for the
voltage of the battery to vary from 10 to 16 volts when normally used. As
shown in FIG. 2, the opening duration of the electromagnetic valve 6
varies as the voltage of the driving pulse signal S.sub.1, i.e. the
voltage of the battery, varies as 10-13-16 volts.
In order to eliminate the disadvantages due to the variation in opening
duration in accordance with the variation of the voltage of the power
supply, it is possible to maintain the voltage of the driving pulse signal
S.sub.1 constant by means of a constant voltage circuit. However, this
method is not practical for the following reason. The reason is that since
the required current by the electromagnetic valve 6 is relatively large,
such as 0.5 ampere, while the variation range of the voltage of the power
supply is wide, the loss in power in a constant voltage circuit is great
and accordingly much heat is generated therein.
Referring to FIG. 3A, a graphical representation of the relationship
between the duty cycle and the flow rate of the recirculation gas is
shown. It is assumed that the standard voltage of the power supply is 13
volts. As shown when the duty cycle of the driving pulse signal S.sub.1 is
50 percent, the flow rate is at "a". However, if the voltage of the power
supply falls to 10 volts, the flow rate becomes "b" which is below the
flow rate "a" (see points A and B on the two lines). In order to obtain
the flow rate "a", without changing the voltage of the power supply, the
duty cycle of the driving pulse signal has to be increased as much as
W.sub.2 (see point C). The change in duty cycle will be referred to as
compensation of duty cycle hereinafter. In FIG. 3A, the amounts of duty
cycle to be compensated for at various values of duty cycle to obtain a
required flow rate of the recirculation gas are indicated by references
W.sub.1 to W.sub.4.
In FIG. 3B, the relationship between the duty cycle and the amount of duty
cycle to be compensated for is shown. The line "m" shows the difference in
duty cycle for obtaining the same flow rate by the two voltages (13 volts
and 10 volts) of the power supply.
In FIG. 3C, the relationships between the duty cycle and the amount of duty
cycle to be compensated for are shown with respect to various voltages of
the power supply. Since it is assumed that 13 volts is the standard
voltage of the power supply, the abscissa of the graph corresponds to 13
volts. From FIG. 3C it is possible to find a general formula to find the
amount of duty cycle to be compensated for as a function of the duty
cycle.
It is assumed that the slope of the line showing the relationship between
the duty cycle and the amount of duty cycle to be compensated for is
.alpha., when the voltage difference between the standard voltage V.sub.BO
and the actual voltage V.sub.B of the power supply is one volt. If the
amount of duty cycle to be compensated for when the duty cycle is 100
percent, is expressed in terms .beta., the amount Y of duty cycle to be
compensated for is generally expressed by the following equation, wherein
X is the duty cycle of the driving pulse signal S.sub.1.
Y=[.alpha.(100-X)+.beta.].multidot.(V.sub.BO -V.sub.B) (1)
Since the value of V.sub.B can be measured, and the values of .alpha. and
.beta. are respectively obtained from an experiment, the value of Y is
obtained as a function of duty cycle X and the voltage V.sub.B of the
power supply. In other words, the values of .alpha., .beta., and V.sub.BO
are treated as constant values, while the values of X and U.sub.B are
variables in the above formula.
The above equation (1) is applied when the electromagnetic valve 6 is of
the type that the valve head opens upon energization. If the
electromagnetic valve 6 is of the type that the valve head closes upon
energization, the amount of duty cycle to be compensated for will be
expressed by the following equation (2), since the relationship between
the duty cycle and the flow rate of the recirculation gas is shown by the
graphical representation of FIG. 3D.
Y=-(.alpha.X+.beta.).multidot.(V.sub.BO -V.sub.B) (2)
Reference is now made to FIG. 4 which shows a schematic block diagram of a
control signal generator used in a first preferred embodiment of the
exhaust gas recirculation system according to the present invention. The
control signal generator shown in FIG. 4 is used in place of the control
signal generator 9 shown in FIG. 1. The control signal generator comprises
a computing circuit 10 which is of a conventional type, a first subtractor
11a, a first multiplier 11b, a first adder 12, a second subtractor 13, a
second multiplier 14, a switching circuit 16, a second adder 17, a pulse
generator 18, first and second comparators 15a and 15b, and an OR gate 20.
The computing circuit 10 is responsive to at least two input signals
S.sub.2 and S.sub.3 respectively indicative of the flow rate of the intake
air and the rotational speed of the crankshaft of the engine 2. The flow
rate of the intake air may be derived from a conventional airflow meter
(not shown), while the rotational speed of the engine may be derived from
a suitable tacho generator (not shown). The computing circuit 10 produces
an output analogue signal S.sub.4 indicative of the duty cycle of a
driving pulse signal which will be fed to the electromagnetic valve 6
shown in FIG. 1.
It is to be noted that if the control signal generator 9 had only the
computing circuit 10 and the pulse generator 18, and thus the pulse
generator 18 produced the driving pulse signal S.sub.1 in accordance with
the output signal S.sub.4 of the computing circuit 10, the control signal
generator 9 would be the same as the conventional one. In other words,
according to the present invention, the output signal S.sub.4 of the
computing circuit 10 is not directly fed to the pulse generator 18, but
via the second adder 17, in which the magnitude of the output signal
S.sub.4 of the computing circuit 10 is modified by a suitable compensation
signal. In order to perform such compensation, the control signal
generator 9 is provided with the circuitry 9a enclosed by a chain line in
addition to the conventional computing circuit 10 and the also
conventional pulse generator 18. The circuitry 9a enclosed by the chain
line may be called a compensation circuit.
The output of the computing circuit 10 is connected to a first input of the
second adder 17 and to a first input of the first subtractor 11a. The
first subtractor 11a has a second input for receiving a first reference
signal V.sub.100 the voltage of which corresponds to a duty cycle of 100
percent. The first subtractor 11a produces a signal indicative of the
difference between the two input signals by subtracting the value of the
signal V.sub.100 from the value of the other signal S.sub.4. The output
of the first subtractor 11a is connected to an input of the first
multiplier 11b in which the difference obtained by the first subtractor
11a is multiplied by a predetermined (constant) value which corresponds to
the before mentioned .alpha.. Since the value of .alpha. is constant, this
step of multiplication may be performed by simply amplifying the
difference by a predetermined amplification degree .alpha.. In other
words, the first multiplier 11b may be an amplifier. The first multiplier
11b therefore, produces an output signal S.sub.5 which will be expressed
in the following equation:
S.sub.5 =.alpha.(100-X)
The output of the first multiplier 11b is connected to a first input of the
first adder 12 which has a second input. The second input of the first
adder 12 is responsive to a second reference voltage V.sub..beta. which
corresponds to the before mentioned .beta.. The first adder 12 produces an
output signal S.sub.6 which will be expressed by the following equation.
S.sub.6 =.alpha.(100-X)+.beta.
The above mentioned two values .alpha. and .beta., which are constant, are
obtained by experiments. Once these values are obtained, these values are
respectively preset by adjusting the amplification degree of the amplifier
used in place of the first multiplier 11b or the multiplication factor of
the first multiplier 11b and the voltage of the second reference voltage
V.sub..beta..
The second subtractor 13 has first and second inputs respectively
responsive to a third reference voltage V.sub.BO, while the second input
is responsive to the voltage V.sub.B of the power supply such as the
battery (not shown). It will be readily understood that the above
mentioned first, second and third reference voltages V.sub.100,
V.sub..beta., and V.sub.BO may be produced by suitable voltage dividers
(not shown). The second subtractor 13 produces an output signal S.sub.7
indicative of the difference between the two input voltages V.sub.BO and
V.sub.B by subtracting the voltage V.sub.B of the power supply from the
predetermined voltage V.sub.BO. The voltage of the output signal S.sub.7
of the second subtractor 13 will be expressed by the following equation.
S.sub.7 =V.sub.BO -V.sub.B
The second multiplier 14 is responsive to the output signals S.sub.6 and
S.sub.7 of the first adder 12 and the second subtractor 13 to produce an
output signal S.sub.8 by multiplying S.sub.6 by S.sub.7. The output signal
S.sub.8 of the second multiplier 14 will be expressed by the following
equation.
S.sub.8 =[.alpha.(100-X)+.beta.].multidot.(V.sub.BO =V.sub.B)
It will be seen that the value indicated by the signal S.sub.8 corresponds
to the value expressed by the equation (1). This signal S.sub.8 is fed via
the switching circuit 16 to the second input of the second adder 17 in
which the signal S.sub.8 is added to the analogue signal S.sub.4 from the
computing circuit 10. Since two voltages S.sub.4 and S.sub.8 are added to
each other, if the voltage of the signal S.sub.8 is positive, the voltage
of the signal S.sub.4 increases. On the other hand, if the voltage of the
signal S.sub.8 is negative, the voltage of the signal S.sub.4 decreases.
The second adder 17 produces an output signal S.sub.10 indicative of the
sum of the two voltages of the signals S.sub.4 and S.sub.8. This output
signal S.sub.10 is supplied to the pulse generating circuit 18 in which a
driving pulse signal S.sub.1, the voltage of which equals the voltage
V.sub.B of the power supply, is produced. The duty cycle of the driving
pulse signal S.sub.1 is controlled in accordance with the voltage of the
signal S.sub.10. It will be understood that since the voltage of the
signal S.sub.4 is modified by the voltage of the signal S.sub.8, the duty
cycle of the driving pulse signal S.sub.1 is corrected or compensated for.
In the above it is assumed that the switching circuit 16 is closed to
transmit the output signal S.sub.8 to the second adder 17. However,
actually the switching circuit 16 is controlled by a signal S.sub.9
applied from the OR gate 20. As shown in FIG. 2, although the linearity of
the flow rate characteristic of the electromagnetic valve 6 is high in a
duty cycle range between about 20 percent and about 95 percent, both in
the range above 95 percent and in the range below 20 percent the
electromagnetic valve 6 has poor linearity. Therefore, compensation of the
duty cycle may result in undesirable excursion of the flow rate of the
exhaust gas. For this reason it is advantageous to disable the function of
compensation when the duty cycle indicated by the analogue signals S.sub.4
is off a predetermined range. The signal S.sub.9 is arranged to assume
high and low voltages in accordance with the duty cycle indicated by the
analogue signal S.sub.4 so that the transmission of the output signal
S.sub.8 of the second multiplier 14 is controlled in turn.
It will be described hereinbelow how this signal S.sub.9 is produced. The
output of the computing circuit 10 is connected to a second input of the
first comparator 15a and to a first input of the second comparator 15b.
The first comparator 15a has a first input responsive to a predetermined
voltage V.sub.95 corresponding to a duty cycle of 95 percent, while the
second comparator 15b has a second input responsive to a predetermined
voltage V.sub.20 corresponding to a duty cycle of 20 percent. The first
comparator 15a produces a high level output signal when the voltage of the
signal S.sub.4 is above the predetermined voltage V.sub.95, while the
second comparator 15b produces a high level output signal when the voltage
of the signal S.sub.4 is below the predetermined voltage V.sub.20. These
output signals of the first and second comparators 15a and 15b are fed via
the OR gate 20 to the switching circuit 16, which may be a relay or a gate
circuit. The switching circuit 16 is arranged to become off (open) when
the signal S.sub.8 of the multiplier 14 is supplied to the second adder 17
only when the duty cycle indicated by the output signal S.sub.4 of the
computing circuit 10 resides between 20 percent and 95 percent. In other
words, the modification (compensation) of duty cycle is performed only
when the duty cycle is between 20 percent and 95 percent.
In the above described first embodiment of the EGR system according to the
present invention, the duty cycle of the driving pulse signal S.sub.1 is
compensated for in accordance with the difference between the voltage
V.sub.B of the power supply and the predetermined voltage V.sub.BO. In
other words, the amount of duty cycle to be compensated for is in
proportion at a given rate to the difference in voltage irrespectively of
whether the actual voltage V.sub.B is greater or smaller than the
predetermined voltage V.sub.BO.
However, according to experiments, it has been recognized that the amount
of change in flow rate caused by the voltage difference is larger in case
that the voltage V.sub.B of the power supply is below the predetermined
voltage V.sub.BO than that in case that the voltage V.sub.B of the power
supply is greater than the predetermined voltage V.sub.BO. This means that
it is advantageous to change the amount of duty cycle to be compensated
for in accordance with the fact that the voltage V.sub.B of the power
supply is greater to smaller than the predetermined voltage V.sub.BO.
Hence, the following second embodiment. FIG. 5 illustrates a schematic
block diagram of the control signal generator used in the second preferred
embodiment of the EGR system according to the present invention. The same
elements and circuits also used in the first embodiment shown in FIG. 1
are designated by the like references. The circuit arrangement of the
control signal generator of the second embodiment includes all of the
elements and circuits that are included in the first embodiment circuit
shown in FIG. 1. The circuit arrangement of the second embodiment further
comprises a comparator 21, a multiplier 22 (this multiplier will be
referred to as a third multiplier hereinbelow), and a switching circuit
23. The comparator 21 is responsive to the predetermined voltage V.sub.BO
and the actual voltage V.sub.B of the power supply to produce a high level
output signal when the voltage V.sub.B of the power supply is below the
predetermined voltage V.sub.BO. The output of the switching circuit 16 is
connected to an input of the third multiplier 22 the output of which is
connected to a first stationary contact 23a of the switching circuit 23.
The output of the switching circuit 16 is further connected to a second
stationary contact 23b of the switching circuit 23. The switching circuit
23 has a movable contact 23c arranged to contact with the first or second
stationary contact 23a or 23b in response to a signal from the comparator
21. Although the switching circuit 23 is shown to be a mechanical switch,
such as a relay, an electronic gate circuit may be used instead. The
movable contact 23c of the switching circuit 23 is connected to the second
input of the second adder 17.
The switching circuit 23 is responsive to the output signal of the
comparator 21 and is arranged to transmit the output of the switching
circuit 16 directly to the adder 17 by contacting the movable contact 23c
to the second stationary contact 23b when the output signal of the
comparator 21 assumes a low level. On the other hand, if the voltage
V.sub.B of the power supply is smaller than the predetermined voltage
V.sub.BO, the movable contact 23c is in contact with the first stationary
contact 23a to transmit the output signal of the third multiplier 22 to
the adder 17.
As described in connection with the first embodiment, the output signal
S.sub.8 of the second multiplier 14 is expressed by the equation (1) or
(2) depending on the type of the electromagnetic valve 6. This signal
S.sub.8 is applied to the third multiplier 22 in which the voltage of the
signal S.sub.8 is multiplied by a predetermined value K. A suitable
amplifier may be used in place of the third multiplier 22 in the same
manner as in the first multiplier 11b and the predetermined value K may be
set by adjusting the amplification degree of the amplifier 22. From the
foregoing, it will be understood that the output signal of the amplifier
22 may be expressed by the following equation (3) in case of the reception
of the input signal S.sub.8 expressed by the equation (1) and by the other
equation (4) in case of reception of the input signal S.sub.8 expressed by
the equation (2).
Y=K[.alpha.(100-X)+.beta.)].multidot.(V.sub.BO -V.sub.B) (3)
Y=-K(.alpha.X+.beta.).multidot.(V.sub.BO -V.sub.B) (4)
The value of K which may be set by adjusting the amplification degree of
the amplifier 22 may be obtained by means of experiments. It will be
understood from the foregoing, that the amount of duty cycle to be
compensated for is changed in accordance with the fact the voltage V.sub.B
of the power supply is above or below the predetermined voltage V.sub.BO.
Consequently, the correction of the duty cycle in the second embodiment is
more accurate than in the first embodiment.
Although the preferred embodiments of the EGR system according to the
present invention are described that the control signal generator 9 is
constructed by discrete elements as shown in FIG. 4 and FIG. 5, if a
microcomputer is so programed that it functions as the circuit arrangement
shown in FIG. 4 and/or FIG. 5, the control signal generator 9 may be
substituted with a microcomputer. Furthermore, the EGR system shown in
FIG. 1 is an example for the explanation of the function of the control
signal generator 9 according to the present invention, and therefore,
other arrangements of an EGR system, in which the recirculation rate of
the exhaust gas is controlled by an electromagnetic valve, may be used.
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