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Claims  |
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I claim:
1. An exhaust gas recirculation control system for an engine comprising:
a rotational speed sensor for sensing rotational speed of the engine;
a pressure sensor for sensing pressure in an inlet pipe of the engine;
an exhaust gas recirculation control valve for controlling an amount of
exhaust gas recirculated from an exhaust pipe of the engine to the inlet
pipe;
an oxygen sensing means including an oxygen sensor and an oxygen
concentration detecting device for sensing oxygen concentration of a gas
mixture of inlet air flowing in the inlet pipe and said recirculated
exhaust gas, said oxygen sensor having a solid electrolyte oxygen pump
cell and a solid electrolyte oxygen sensor cell, and said detecting device
having comparing means for comparing an electromotive force developed
across said oxygen sensor cell with a variable reference voltage to
provide an error output therebetween, and pump current supply means
responsive to said error output for supplying a pump current to said
oxygen pump cell, and means for providing as an output of said detecting
device a pump current signal corresponding to said pump current;
said control means including a memory for storing a plurality of exhaust
gas recirculation rates corresponding to various engine speeds and engine
pressures and predetermined pump currents corresponding to various exhaust
gas recirculation rates and various values of said reference voltage, said
control means including means for determining from said memory a desired
exhaust gas recirculation rate corresponding to a particular engine speed
and pressure detected by said speed and pressure sensors, for providing
from said memory a desired pump current signal corresponding to said
desired exhaust gas recirculation rate, for comparing the output pump
current signal from said detection device with said desired pump current
signal provided from said memory, and for adjusting opening of said
control valve to vary the recirculation rate depending on the result of
said comparing.
2. An exhaust gas recirculation control system for an engine comprising:
a rotational speed sensor for sensing rotational speed of the engine;
a pressure sensor for sensing pressure in an inlet pipe of the engine;
an exhaust gas recirculation control valve for controlling an amount of
exhaust gas recirculated from an exhaust pipe of the engine to the inlet
pipe;
an oxygen sensing means including an oxygen sensor and an oxygen
concentration detecting device for sensing oxygen concentration of a gas
mixture of inlet air flowing in the inlet pipe and said recirculated
exhaust gas, said oxygen sensor including an oxygen pump cell, pump
current supply means, and means providing an output pump current signal
corresponsing to said pump current; and
control means responsive to said oxygen sensing means for controlling
opening of said exhaust gas recirculation control valve to recirculate
said exhaust gas at a desired recirculation rate determined by said engine
speed and said inlet pipe pressure;
said control means including a memory for storing a plurality of exhaust
gas recirculation rates corresponding to various engine speeds and inlet
pipe pressures and predetermined pump currents corresponding to various
exhaust gas recirculation rates and various values of said reference
voltage, said control means including means for determining from said
memory a desired exhaust gas recirculation rate corresponding to a
particular engine speed and pressure detected by said speed and pressure
sensors, for providing from said memory a desired pump current signal
corresponding to a a preset reference voltage for said detecting device,
for comparing the output pump current signal with said desired pump
current signal provided from said memory, and for adjusting opening of
said control valve depending on the result of said comparing.
3. An exhaust gas recirculation control system for an engine comprising:
a rotational speed sensor for sensing rotational speed of the engine;
a pressure sensor for sensing pressure in an inlet pipe of the engine;
an exhaust gas recirculation control valve for controlling an amount of
exhaust gas recirculated from an exhaust pipe of the engine to the inlet
pipe;
an oxygen sensing means including an oxygen sensor and an oxygen
concentration detecting device for sensing oxygen concentration of a gas
mixture of inlet air flowing in the inlet pipe and said recirculated
exhaust gas, said inlet pipe being divided into first and second inlet
paths parallel to each other, said first inlet path having mounted thereon
said oxygen sensing means;
said oxygen sensor having a solid electrolyte oxygen pump cell and a solid
electrolyte oxygen sensor cell, and said detecting device having comparing
means for comparing an electromotive force developed across said oxygen
sensor cell with a variable reference voltage to provide an error output
therebetween, pump current supply means responsive to said error output
for supplying a pump current to said oxygen pump cell, and means providing
as an output of said oxygen concentration detecting device an output pump
current signal corresponding to said pump current; and
control means responsive to said oxygen sensing means for controlling
opening of said exhaust gas recirculation control valve to recirculate
said exhaust gas at a desired recirculation rate determined by said engine
speed and said inlet pipe pressure;
said control means including a memory for storing a plurality of exhaust
gas recirculation rates corresponding to various engine speeds and engine
pressures and predetermined pump currents corresponding to various exhaust
gas recirculation rates and various values of said reference voltage, said
control means including means for determining from said memory a desired
exhaust gas recirculation rate corresponding to a particular engine speed
and pressure detected by said speed and pressure sensors, for providing
from said memory a desired pump current signal corresponding to a
predetermined reference voltage for comparing the output pump current
signal with said desired pump current signal from said memory, and for
adjusting opening of said control valve depending on the result of said
comparing.
4. An exhaust gas recirculation control system for an engine comprising:
a rotational speed sensor for sensing rotational speed of the engine;
a pressure sensor for sensing pressure in an inlet pipe of the engine;
an exhaust gas recirculation control valve for controlling an amount of
exhaust gas recirculated from an exhaust pipe of the engine to the inlet
pipe;
an oxygen sensing means including an oxygen sensor and an oxygen
concentration detecting device for sensing oxygen concentration of a gas
mixture of inlet air flowing in the inlet pipe and said recirculated
exhaust gas, said inlet pipe being divided into first and second inlet
paths parallel to each other, said first inlet path having mounted thereon
said oxygen sensing means;
said oxygen sensor having a solid electrolyte oxygen pump cell and a solid
electrolyte oxygen sensor cell, and said detecting device having comparing
means for comparing an electromotive force developed across said oxygen
sensor cell with a fixed reference voltage to provide an error output
therebetween, and pump current supply means responsive to said error
output for supplying a pump current to said oxygen pump cell to provide as
an output of said oxygen concentration detecting device an output pump
current signal corresponding to said pump current; and
control means responsive to said oxygen sensing means for controlling
opening of said exhaust gas recirculation valve to recirculate said
exhaust gas at a desired recirculation rate determined by said engine
speed and inlet pipe pressure;
said control means including a memory for storing a plurality of exhaust
gas recirculation rates corresponding to various engine speeds and engine
pressures and predetermined pump currents corresponding to various exhaust
gas recirculation rates and various values of said reference voltage, said
control means including means for determining from said memory a desired
exhaust gas recirculation rate corresponding to a particular engine speed
and pressure detected by said speed and pressure sensors, for providing a
desired pump current signal corresponding to a preset reference voltage
for said detecting device, for comparing the output pump current signal
with said desired pump current signal from said memory, and for adjusting
the opening of said control valve depending on the result of said
comparing.
5. An exhaust gas recirculation control system for an engine comprising:
a rotational speed sensor for sensing rotational speed of the engine;
a pressure sensor for sensing pressure in an inlet pipe of the engine;
an exhaust gas recirculation control valve for controlling an amount of
exhaust gas recirculated from an exhaust pipe of the engine to the inlet
pipe;
an oxygen sensing means for sensing oxygen concentration of a gas mixture
of inlet air flowing in the inlet pipe and said recirculated exhaust gas;
and
control means responsive to said oxygen sensing means for controlling
opening of said exhaust gas recirculation control valve to recirculate
said exhaust gas at a recirculation rate predetermined according to the
engine speed and inlet pipe pressure sensed by said sensors.
6. An exhaust gas recirculation control system for an engine according to
claim 5 further including memory means for storing predetermined
recirculation rates corresponding to various engine speeds and inlet
pressures.
7. An exhaust gas recirculation control system for an engine according to
claim 5 wherein said oxygen sensing means comprises an oxygen sensor and
an oxygen concentration detecting device.
8. An exhaust gas recirculation control system for an engine according to
claim 7 wherein said oxygen sensor includes a solid electrolyte oxygen
pump cell and a solid electrolyte oxygen sensor cell, and said detecting
device includes comparing means for comparing an electromotive force
developed across said oxygen sensor cell with a variable reference voltage
to provide an error output therebetween, and pump current supply means,
responsive to said error output, for supplying a pump current to said
oxygen pump cell and for providing as the output of said detecting device
a pump current signal corresponding to said pump current.
9. An exhaust gas recirculation control system for an engine according to
claim 7 wherein said oxygen sensor includes a solid electrolyte oxygen
pump cell and a solid electrolyte oxygen sensor cell, and said detecting
device includes comparing means for comparing an electromotive force
developed across said oxygen sensor cell with a fixed reference voltage to
provide an error output therebetween, and pump current supply means,
responsive to said error output, for supplying a pump current to said
oxygen pump cell and for providing as the output of said detecting device
a pump current signal corresponding to said pump current.
10. An exhaust gas recirculation control system for an engine according to
claim 7, said engine having a inlet pipe divided into first and second
inlet paths parallel to each other, said first inlet path having mounted
thereon said oxygen sensing means and having introduced thereinto a part
of said exhaust gas through said control valve.
11. An exhaust gas recirculation control system for an engine according to
claim 10 wherein said oxygen sensor includes a solid electrolyte oxygen
pump cell and a solid electrolyte oxygen sensor cell, and said detecting
device includes comparing means for comparing an electromotive force
developed across said oxygen sensor cell with a variable reference voltage
to provide an error output therebetween, and pump current supply means,
responsive to said error output, for supplying a pump current to said
oxygen pump cell and for providing as the output of said oxygen
concentration detecting device a pump current signal corresponding to said
pump current.
12. An exhaust gas recirculation control system for an engine according to
claim 10 wherein said oxygen sensor includes a solid electrolyte oxygen
pump cell and a solid electrolyte oxygen sensor cell, and said detecting
device includes comparing means for comparing an electromotive force
developed across said oxygen sensor cell with a fixed reference voltage to
provide an error output therebetween, and pump current supply means,
responsive to said error output, for supplying a pump current to said
oxygen pump cell and for providing as the output of said oxygen
concentration detecting device a pump current signal corresponding to said
pump current.
13. An exhaust gas recirculation control system for an engine according to
claim 10 wherein said second inlet path has mounted thereon a fuel
injection supply device. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to an exhaust gas recirculation control system for
an internal combustion engine, and in particular to an exhaust gas
recirculation feedback control system for an engine comprising an oxygen
sensor for sensing the oxygen concentration in the inlet air for the
engine in which a recirculated exhaust gas is mixed whereby the mixing
rate (exhaust gas recirculation rate) of the recirculated gas is
accurately controlled according to the output signal of the oxygen sensor.
It is well known in the art to perform so-called "exhaust gas
recirculation" (hereinafter abbreviated as EGR) where a part of the
exhaust gas is introduced onto the inlet side of an engine in order to
reduce nitrogen oxides (hereinafter abbreviated as NOx) which are harmful
components contained in the exhaust gas of an internal combustion engine.
Since the flow rate of the recirculated exhaust gas affects the
performance and fuel consumption, etc., of the engine in addition to the
reduction rate of NOx, it is desired that the exhaust gas be accurately
controlled according to the operating condition of the engine.
However, when an EGR control valve for controlling the flow rate of the
exhaust gas is used for a long time, a large amount of a liquid material
such as carbon contained in the exhaust gas becomes attached to the
recirculation control valve so that the flow rate of the exhaust gas at
the initial stage corresponding to the opening of the recirculation
controlling valve is changed, resulting in inaccurate control.
U.S. Pat. No. 4,168,683 issued to Hata et al. on Sept. 25, 1979 discloses
an EGR control system for an internal combustion engine in which a
feedback control signal representing the amount of actually recirculated
exhaust gas is produced by detecting the concentration of either CO.sub.2
or H.sub.2 O in the recirculated exhaust gas.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an EGR control
system for an engine with accurate control performed according to the
operating condition of the engine without deterioration with age by making
use of the concept that in the inlet air, the oxygen concentration is
proportional to the mixing rate of the exhaust gas, i.e., the EGR rate.
In order to accomplish this object, the flow rate of the recirculated
exhaust gas is controlled by an EGR control valve disposed between the
exhaust pipe of the engine and the inlet pipe of the engine so that the
EGR rate may assume a desired value determined by the above operating
condition of the engine. The EGR control valve is controlled by an
electronic control unit on the basis of the outputs of a rotational speed
sensor for sensing the rotation of the engine and of a pressure sensor for
sensing the pressure in the inlet pipe of the engine as well as the output
of an oxygen concentration detecting device which is connected to an
oxygen sensor mounted on the inlet pipe.
The electronic control unit preferably includes a memory in which EGR rates
corresponding to various engine speeds and inlet gas pressures and
predetermined pump currents corresponding to various EGR rates and various
values of the reference voltage for the oxygen concentration detecting
device as a parameter are previously stored. The electronic control unit
determines from the memory a desired EGR rate corresponding to the present
engine speed and inlet pipe pressure, further determines from the
determined desired EGR rate a desired reference voltage for the detecting
device corresponding to a preset pump current, sets the determined
reference voltage in the detecting device, compares the output pump
current with the preset pump current, and adjusts the opening of the
control valve depending on the compared result. Alternatively, the
reference voltage in the oxygen concentration detecting device may be
fixed so that the pump current as a parameter assumes a desired value.
Furthermore, the inlet pipe may be divided into two inlet paths for better
detection of oxygen concentration, one path of which has mounted thereon
the oxygen sensor and has introduced thereinto a part of the exhaust gas
while the other path of which has mounted thereon a fuel injection supply
device.
BRIEF DESCRIPTION OF THE DRAWINGS
An EGR control system for an engine according to this invention will be
more readily apparent from the preferred embodiments shown in the
accompanying drawings in which:
FIG. 1 shows the arrangement of a preferred embodiment of an EGR control
system for an engine according to this invention;
FIG. 2 shows the arrangement of the combination of an oxygen sensor and an
oxygen concentration detecting device used in FIG. 1;
FIG. 3 shows a sectional view of the oxygen sensor taken along Line
III--III in FIG. 2;
FIG. 4 shows a characteristic curve of oxygen concentration as a function
of EGR rate;
FIG. 5 shows characteristic curves of pump current as a function of EGR
rate for various reference voltage levels of the oxygen concentration
detecting device being a parameter;
FIG. 6 shows a flow chart of processes executed by an electronic control
unit used in FIG. 1 according to a first embodiment of this invention
applied to the arrangement of FIG. 1;
FIG. 7 shows a flow chart of processes executed by an electronic control
unit used in FIG. 1 according to a second embodiment of this invention
applied to the arrangement of FIG. 1;
FIG. 8 shows the arrangement of another preferred embodiment of an EGR
control system for an engine according to this invention; and,
FIG. 9 shows characteristic curves of oxygen concentration as a function of
EGR rate for the arrangement of FIG. 8.
Throughout the figures, the same reference numerals indicate identical or
corresponding portions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinbelow will be described in detail the preferred embodiments of an EGR
control system for an engine according to this invention, with reference
to the drawings.
In FIG. 1, which shows the arrangement of an EGR control system for an
engine according to a first embodiment of this invention, an internal
combustion engine 10 is equipped with an inlet pipe 12 which has included
inside it a throttle valve 14. The inlet pipe 12 has mounted thereon a
pressure sensor 16 for detecting the pressure within the inlet pipe 12, an
oxygen sensor 18 for sensing the oxygen concentration in the inlet air for
the engine 10 which flows through the inlet pipe 12, and a fuel injection
valve 20 for supplying fuel into the inlet pipe 12. The oxygen sensor 18
is disposed on the upstream side of the fuel injection valve 20 in the
inlet pipe 12 so that the fuel injected by the fuel injection valve 20
will not adversely affect the sensing performance of the oxygen sensor 18.
The engine 10 is also equipped with an exhaust pipe 22. An EGR control
valve 24 is disposed between pipes 26a and 26b interconnecting the inlet
pipe 12 and the exhaust pipe 22, the opening of which is
electromagnetically controlled so that the flow rate of an exhaust gas
recirculated from the exhaust pipe 22 to the inlet pipe may assume a
desired value. The oxygen sensor 18 is electrically connected to an oxygen
concentration detecting device 28 which is further electrically connected
to an electronic control unit 30. The electronic control unit 30 is also
electrically connected to a rotational speed sensor 32 for sensing the
rotational speed of the engine 10 and to the pressure sensor 16.
The electronic control unit 30 receives as inputs electrical signals from
the pressure sensor 16, the oxygen concentration detecting device 28, and
the rotational speed sensor 32. A control means included in the unit 30
controls the opening of the control valve 24 and a correction means also
included in the unit 30 corrects the degree of opening according to the
operating condition of the engine 10 determined by the above input signals
so that the flow rate of the recirculated exhaust gas is controlled to the
above-mentioned desired level. The electronic control unit 30 also has a
function of changing a reference voltage, which will be described later,
of the oxygen concentration detecting device 28 according to the above
input information.
FIG. 2 shows the arrangements of the oxygen sensor 18 and the oxygen
concentration detecting device 28 as described in U.S. patent application
Ser. No. 606,910 filing date May 4, 1984, and FIG. 3 shows a sectional
view of the oxygen sensor 18 taken along Line III--III in FIG. 2. In FIGS.
2 and 3, an oxygen sensor 18 is projecting into the inlet pipe 12 of the
engine 10. This sensor 18 is formed of a solid electrolyte oxygen pump
cell 40, a solid electrolyte oxygen sensor cell 42, and a supporting base
44. The solid electrolyte oxygen pump cell 40 includes an ion conductive
solid electrolyte (stabilized zirconia) 46 in the form of a plate, with a
thickness of about 0.5 mm, having platinum electrodes 48 and 50 disposed
on opposite sides thereof. The solid electrolyte oxygen sensor cell 42,
like the pump cell 40, includes an ion conductive solid electrolyte 52 in
the form of a plate having platinum electrodes 54 and 56 disposed on
opposite sides thereof. The supporting base 44 supports the oxygen pump
cell 40 and the oxygen sensor cell 42 so that they are oppositely disposed
with a minute gap "d" of about 0.1 mm therebetween.
The oxygen concentration detecting device 28 is electrically coupled to the
pump cell 40 and the sensor cell 42. More specifically, the electrode 54
is connected through a resistor R1 to the inverting input of an
operational amplifier A, the non-inverting input of which is grounded
through a variable DC reference voltage source Vr. The electrode 48 is
connected through a resistor Ro to the emitter of an NPN transistor Tr
whose collector is grounded through a DC power source B and whose base is
connected to the output of the operational amplifier A and the inverting
input of the operational amplifier A through a capacitor C. The electrodes
50 and 56 are grounded. It is to be noted that the DC reference voltage
source Vr may be of a switch-over type which can select one of a number of
voltage sources. This switch-over type oxygen sensor is described in U.S.
patent application Ser. No. 606,926 filed on May 4, 1984 and assigned to
the same assignee as this patent application. It is also to be noted that
the voltage of the variable voltage source Vr is controlled by an
electrical signal from the electronic control unit 30. A different type of
oxygen sensor which may be applied to this invention is disclosed in U.S.
Pat. No. 4,272,329 issued to Hetrick.
The oxygen concentration detecting device 28 receives as an input an
electromotive force "e" developed across the electrodes 54 and 56 of the
oxygen sensor cell 42. The operational amplifier A provides as an output
an electrical signal proportional to the difference between the reference
voltage Vr and the voltage at input point I which is the inverting input
terminal of the operational amplifier A. The output of the operational
amplifier A drives the transistor Tr to control a pump current Ip flowing
through the electrodes 48 and 50. The reference voltage Vr can be set to a
predetermined value by the electronic control unit 30 according to the
operating condition of the engine 10. The resistor Ro serves to develop
thereacross an output voltage corresponding to the pump current Ip which
is proportional to the oxygen concentration in the inlet air for the
engine 10. Ro is preset to a desired resistance so that the pump current
Ip will not be excessive in relation to the DC power source B.
FIG. 4 shows a characteristic curve of the oxygen concentration (percent)
in the inlet air mixed with the recirculated exhaust gas as a function of
the exhaust gas mixing rate, i.e., the EGR rate (percent). It is seen from
FIG. 4 that the oxygen concentration is inversely proportional to the EGR
rate.
FIG. 5 shows characteristic curves of pump current Ip (mA) as a function of
EGR rate (percent). Since it is known from, e.g., the above noted U.S.
Pat. No. 4,272,329 that the pump current provided as an output from the
oxygen sensor is proportional to the oxygen concentration in the inlet
pipe, it can be said that in FIG. 5 the oxygen concentration can be
substituted for the pump current. These 3 characteristic curves indicate
the variation of the pump current Ip in response to the variation of the
EGR rate of the engine 10 with the reference voltage Vr being held at 50
mV, 100 mV, and 300 mV respectively. It is seen from FIG. 5 that the pump
current Ip is inversely proportional to the EGR rate and that the
reference voltage Vr is directly proportional to the EGR rate, supposing
that the pump current Ip be fixed at a constant value, such as 50 mA.
It is to be noted that although FIG. 5 merely shows the reference voltage
Vr of 50 mV, 100 mV, and 300 mV, any values between 20 to 290 mV may be
used. Also, since the oxygen pump cell 40 may break down due to an
excessive pump current Ip flowing therethrough, the pump current Ip is
limited to 60 mA or less by the presence of the DC power source B and the
resistor Ro.
The operation of the EGR control system for an engine according to this
invention will now be described with reference to FIG. 6 which is a flow
chart of the operations performed by the electronic control unit 30.
When the engine 10 is started, the inlet air sucked into the engine 10 is
introduced from the atmosphere through an air cleaner (not shown) and the
throttle valve 14 to the inlet pipe 12. On the other hand, the electronic
control unit 30 determines the operating condition of the engine 10 on the
basis of the electrical signals from the pressure sensor 16 and the
rotational speed sensor 32 and calculates an EGR rate according to the
determined operating condition. Methods of determination and calculation
are well known in the art and so a detailed description thereof is omitted
here.
At the initial stage, the electronic control unit 30 causes the EGR control
valve 24 to open by a certain degree corresponding to the initial EGR rate
as calculated above. The exhaust gas recirculated by operation of the
control valve 24 is introduced into the inlet pipe 12 through the pipes
26a and 26b and is mixed with the inlet air. As the inlet air thus mixed
with a part of the exhaust gas is introduced through the inlet pipe 12
into the engine 10, the oxygen sensor 18 senses the oxygen concentration
in the inlet air. Namely, some of the inlet air is introduced into the
minute gap "d" between the oxygen pump cell 40 and the oxygen sensor cell
42 of the oxygen sensor 18. Then, the oxygen in the gap is pumped out of
the gap by the operation of the oxygen pump cell 40, which is operated by
the pump current Ip. Consequently, there arises a difference in oxygen
concentration between the outside and the inside of the gap, and an
electromotive force "e" develops across the oxygen sensor cell 42 in
proportion to the difference in oxygen concentration. This electromotive
force "e" is input to the inverting input terminal of the operational
amplifier A through the resistor R1. The operational amplifier A provides
as an output an electrical signal corresponding to the difference between
the reference voltage Vr and the voltage at the inverting input of the
amplifier A. This causes the pump current Ip to flow through the resistor
Ro to the oxygen pump cell 40 in proportion to the output of the
operational amplifier A.
On the other hand, when the electrical signals from the speed sensor 32 and
the pressure sensor 16 are provided to the electronic control unit 30 as
shown in Steps S1 and S2 in FIG. 6, a desired EGR rate is determined from
data previously stored in a memory (not shown) included in the unit 30 on
the basis of the operating condition of the engine 10 determined by these
electrical signals, and then a desired reference voltage Vr is determined
from the characteristic curves shown in FIG. 5 which are also previously
stored in the memory. For example, supposing that the pump current Ip is
required to be set at 50 mA, when it is found from the operating condition
determined by the speed signal and the pressure signal that a desired
recirculation rate is 5%, the desired reference voltage Vr is determined
to be 50 mV as seen from FIG. 5. This operation is done in Step S3. After
the determination of the desired reference voltage of 50 mV, the reference
voltage of 50 mV is set in the oxygen concentration detecting device 28 by
the electronic control unit 30 (Step S4). Therefore, correction of the
opening of the control valve 24 should be made so that the pump current Ip
becomes 50 mA.
Therefore, it is necessary to detect the output of the oxygen sensor 18
corresponding to the actual pump current Ip (Step S5). Then, comparison is
made between the detected pump current Ip with the supposed or set pump
current Ips in Step S6. If the detected pump current Ip actually flowing
through the resistance Ro is larger than the set value Ips of 50 mA, it is
seen from the characteristic curve in the case of 50 mA in Ip that the
present EGR rate is smaller than the desired value of 5%. Therefore, the
electronic control unit 30 causes the control valve 24 to open by a
certain degree (Step S7). This causes the amount of recirculated exhaust
gas to be increased to correspondingly increase the amount of the exhaust
gas in the inlet air so that the oxygen concentration in the inlet air is
reduced.
Thus, the oxygen concentration in the inlet air is reduced, thereby
reducing the pump current Ip flowing through the resistor Ro to 50 mA, and
thereupon it is found that the present EGR rate has now become equal to
the desired value of 5%.
On the other hand, if the detected pump current Ip is less than 50 mA, the
control valve 24 is closed by a certain degree (Step S8). As a result,
like the above case, the EGR rate is to be controlled to the desired value
of 5%.
In another case where a desired EGR rate is, for example, 40% depending
upon the operating condition determined by the speed signal and the
pressure signal, the reference voltage Vr may be set to 300 mA so that the
pump current Ip assumes the same supposed value of 50 mA whereby the
opening of the EGR control valve 24 is corrected.
While in the description of the above embodiment that the reference voltage
Vr is variable and the opening of the EGR control valve 24 is controlled
using feedback control so that the pump current Ip may assume a fixed
value, it is also possible that the opening of the control valve 24 be
controlled with the reference voltage Vr being fixed so that the EGR rate
will assume a desired value by the detection of the variation of the pump
current Ip, as seen also from the characteristic curves shown in FIG. 5.
This possible modification is made in a second embodiment of an EGR control
system for an engine according to this invention, as illustrated in FIG.
7.
FIG. 7 is a flow chart of the processes executed by the electronic control
unit 30 in a second embodiment of this invention. In the figure, after
having obtained a speed signal and a pressure signal from the respective
sensors 32 and 16 in Steps T1 and T2, Step T3 is performed in which a
desired pump current Ipd corresponding to an EGR rate determined by the
combination of speed data and pressure data is read out from the memory
which also stores therein data of FIG. 5. Then, in Step T4, the actual
pump current Ip of the oxygen sensor 18 is detected from the voltage drop
across the resistor Ro in the oxygen concentration detecting device 28.
This detected Ip signal is compared in Step T5 with the desired pump
current Ipd as determined above. If the pump current Ip is less than the
desired pump current Ipd, then in Step T6 the EGR control valve 24 is
closed by a certain degree. This operation continues until the pump
current Ip become equal to the desired pump current Ipd, at which point
the EGR rate becomes a desired value. On the other hand, if it is found in
Step T5 that the pump current Ip is larger than the desired pump current
Ipd, then in Step T7 the EGR control valve 24 is opened by a certain
degree. This operation also continues until the EGR rate becomes a desired
value corresponding to the operating condition of the engine 10 determined
by the engine speed and the engine pressure.
In FIG. 8 is shown another embodiment of an EGR control system for an
engine according to this invention. This embodiment is different from the
embodiment shown in FIG. 1 in that the embodiment of FIG. 8 has two inlet
paths 13 and 15 in which throttle valves 14 and 17 are respectively
disposed. Furthermore, the oxygen sensor 18 connected to the oxygen
concentration detecting device 28 projects into the inlet path 15. The
oxygen sensor 18 is preferably provided with a heater 19 so that the
sensor 18 is heated to a certain temperature (for example, 600 degrees or
more) in which the sensor 18 is normally operated. The fuel injection
valve 20 is disposed in the inlet path 13 and on the upstream side of the
throttle valve 14, in contrast to the first embodiment. The electronic
control unit 30 similarly receives as inputs the output signals of the
oxygen concentration detecting device 28, the pressure sensor 16, and the
rotational speed sensor 32, and provides an output to the control valve
24. It is to be noted that the heater 19 need not be used as in the
embodiment shown in FIG. 1.
In operation, when the engine 10 is started, the inlet air flows through
the inlet paths 13 and 15. The inlet air flowing through the inlet path 13
is mixed with fuel injected from the fuel injection valve 20 and the
resultant mixed gas is introduced through the throttle valve 14 and the
inlet pipe 12 into the engine 10. On the other hand, the inlet air flowing
through the inlet path 15 passes through the throttle valve 17, is mixed
with the exhaust gas recirculated from the exhaust pipe 22 through the
pipes 26a, 26b and the control valve 24, and is then introduced into the
engine 10.
In the case where the sectional areas of the inlet paths 13 and 15 are the
same, the relationship between the oxygen concentration in the inlet air
mixed with the exhaust gas in the inlet path 15 and the EGR rate is
illustrated in FIG. 9 by characteristic curve A. In the case where the
sectional area of the inlet path 15 is half of that of the inlet path 13,
the relationship between the oxygen concentration and the EGR rate is
illustrated in FIG. 9 by characteristic curve B.
Thus, by mixing a part of the exhaust gas with an inlet air in the inlet
path 15, and by detecting the oxygen concentration in the inlet path 15
with the oxygen sensor 18, the output characteristic of the oxygen sensor
18, i.e., the variation of the output voltage as a function of the EGR
rate, becomes steep depending on the ratio of the sectional areas of the
inlet paths 13 and 15, and therefore the EGR rate can be detected more
sensitively.
Since the pump current is proportional to the oxygen concentration in the
inlet pipe as described above, if the steep characteristic curve B in FIG.
9 is available, the characteristic curves shown in FIG. 5 also become
steep so that the EGR rate can be controlled accurately without using a
high sensitivity oxygen sensor.
Also in the embodiment shown in FIG. 8, the same control processes as those
shown in FIGS. 6 and 7 can be executed by the same electronic control unit
30. Therefore, this invention further provides a third and a fourth
embodiment illustrated by the flow charts of FIGS. 6 and 7, providing more
sensitive control.
As set forth in the above, by utilizing an oxygen sensor for sensing the
oxygen concentration in the inlet air mixed with a part of the exhaust gas
of the engine, this invention performs feedback control so that the output
of the oxygen sensor may assume a desired value to provide a desired EGR
rate. Therefore, even if the EGR control valve deteriorates, the EGR
control system of this invention can recirculate the exhaust gas with good
precision. In addition, since the inlet pipe of the engine which is
divided into two paths is used and an oxygen sensor is mounted on one of
the paths to detect the oxygen concentration in the inlet air mixed with a
part of the exhaust gas and to control the EGR rate according to the
output of the oxygen sensor, more precise EGR detection and control are
realized. Furthermore, if a fuel injection valve or a fuel supply device
is disposed in the inlet path 13 as shown in the embodiment of FIG. 8, the
fuel supplied by th | | |
