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Description  |
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This invention relates to a method and apparatus for controlling the value
of an operating parameter of an internal combustion engine as a function
of at least one engine operating condition in which an actuator is
effective to establish the operating parameter at a desired value a
predetermined time lag after receiving a command to do so.
Generally, engine operating parameters are controlled either by open loop
or feedback control systems. Open loop engine control systems respond to
sensed engine operating conditions to provide for control of engine
operating parameters. The control law in this type of controller is based
upon the calibration of the control loop. However, open loop control
systems are incapable of responding to calibration changes over the useful
life of the vehicle.
Feedback control systems control the operating parameter based on the
difference between actual and desired values of the parameter, the desired
value being determined from the sensed engine operating conditions. These
systems are more sophisticated and usually more accurate than the
calibrated open loop control systems. Feedback control systems also
overcome the problems caused by undesirable calibration changes over the
useful life of the vehicle. While having advantages over open loop
control, feedback control is not usually fast responding and, in some
cases, actually can have much slower response than a typical calibrated
open loop control system. In fact, with feedback control, a compromise
between control accuracy and response time usually must be made.
Because both calibrated open loop and feedback control systems set engine
parameters in response to the existing engine operating condition, when
there is a time lag in the control system, the value of the controlled
parameter will always lag that which is actually required for the existing
engine operating condition during transients in the engine operating
condition. For example, a typical EGR control system for an internal
combustion engine may include a pneumatic controller responsive to an
electronic control signal for adjusting the EGR control valve. Due to the
delay in the pneumatic system, a time lapse occurs between the application
of a control signal and the actual time that the pneumatic system responds
to position the EGR valve to the desired position. Because of this time
lapse, if the engine conditions dictating the desired amount of EGR are
changing, the EGR control valve position at any instant in time lags the
position required for the existing engine operating conditions.
It would be desirable for an engine control system to provide for the
control of an engine operating parameter having a delay in its response to
an input control signal in a manner such that even during engine transient
conditions, the engine operating parameter does not lag the corresponding
engine operating condition.
In general, this invention provides for the control of an engine operating
parameter having an PG,4 inherent delay in its response to an input
control signal in a manner such that the value of the engine operating
parameter is set substantially to the value corresponding to the existing
engine operating condition and without a delay even during engine
transient conditions. By monitoring the value of the engine operating
condition and the variation of the engine operating condition, the
controller of this invention predicts the engine operating condition at a
predetermined future time. Based on the predicted value of the engine
operating condition, the controller determines the corresponding value for
the engine operating parameter (such as exhaust gas recirculation valve
position). The control signal for adjusting the enigne operating parameter
is then applied to the actuator for adjusting the parameter at a period
preceding the future time by the known delay time in the control system.
In this manner, at the future time, the engine operating parameter has
been adjusted to the value corresponding to the predicted engine operating
condition. By this adjustment based upon the predicted value of the engine
operating condition, the operating parameter more closely corresponds to
that required by the actual engine operating condition at the future time.
The invention may be best understood by reference to the following
description of a preferred embodiment and the drawings in which:
FIG. 1 illustrates an internal combustion engine and a controller for
controlling the exhaust gas recirculation of the engine in accord with the
principles of this invention;
FIG. 2 is a diagram of the digital engine control module of FIG. 1
responsive to engine operating conditions for adjusting the position of
the EGR valve;
FIGS. 3 thru 5 are diagrams illustrating the operation of the digital
engine controller in controlling exhaust gas recirculation; and
FIG. 6 is a timing diagram illustrating the operation of the digital
controller of FIG. 2 in controlling exhaust gas recirculation.
Referring the FIG. 1, the invention is illustrated with respect to the
control of exhaust gas recirculation of an internal combustion engine 10.
Fuel is provided to the engine 10 by means of a single fuel injector 12
controlled by an engine control module 14 to inject fuel into the throttle
bore of the engine 10 at a location above a conventional throttle valve
16. The fuel injected by the fuel injector 12 is mixed with the air drawn
into the engine 10 through the throttle bore to provide a combustible
mixture. The exhaust gases are directed to the atmosphere through the
exhaust manifolds of the engine and the exhaust conduit 18. The engine is
controlled by the operator by adjusting the position of the throttle valve
16 to vary the air and fuel mixture drawn into the cylinders of the engine
10. The fuel injector 12 is controlled by the engine control module 14 to
maintain a scheduled air-fuel ratio in response to engine operating
parameters including manifold absolute pressure (MAP) as measured by a
conventional pressure sensor and engine speed as measured from pulses
provided by a conventional ignition distributor. While not illustrated,
the fuel quantity injected by the injector 12 could also be made a
function of additional parameters including engine temperature.
An exhaust gas recirculation control system is also provided for the engine
10 for control of exhaust gas emissions. The exhaust gas recirculation is
provided via an EGR actuator and valve assembly 20 receiving exhaust gases
from the exhaust manifold of the engine 10 via a conduit 22 and which
provides a controlled amount of recirculated exhaust gases to the intake
of the engine 10 via a conduit 24. The EGR actuator and valve assembly 20
in this embodiment includes a pneumatic motor such as a vacuum operated
diaphragm for positioning a valve element in the exhaust gas passage in
response to the value of a control signal provided by the engine control
module 14. The engine control module 14 provides the control signal for
adjusting the position of the EGR valve in response to predetermined
engine operating parameters such as engine speed as monitored by the pulse
input from the distributor and the position and velocity of the throttle
valve 16 as monitored by a throttle position and velocity sensor 26. The
throttle position and velocity sensor 26 may include a potentiometer
positioned by the throttle valve 16 to indicate position and a permanent
magnet rotated by the throttle valve 16 relative to a coil to provide an
induced voltage in the coil representing the instantaneous throttle
velocity. The control signal from the engine control module 14 modulates a
vacuum input from the manifold of the engine 10 to the EGR actuator and
valve assembly 20. The EGR valve in the assembly is positioned in accord
with the modulated vacuum to control the exhaust gases recirculated via
conduit 24.
An EGR valve position sensor (not shown) provides a position signal
feedback to the engine control module 14 so as to provide for closed loop
positioning of the EGR valve.
A characteristic of the EGR control above described is the delay that is
experienced between the application of a control signal by the engine
control module 14 to the EGR actuator and valve assembly 20 and the
achievement of the corresponding desired EGR valve position. This delay is
substantially dominated by the pneumatic delay in the adjustment of the
vacuum level in the vacuum motor positioning the EGR valve. Because of
this delay, if the position of the throttle valve 16 is changing, the
position of the EGR valve and accordingly the amount of exhaust gases
recirculated via the conduit 24 lags the desired amount dictated by the
current position of the throttle valve 16 and the engine speed. Only after
the transient condition of the throttle valve and engine speed stops can
the EGR valve position be adjusted by the engine control module 14 to the
position corresponding to the actual throttle valve position and engine
speed.
In accord with this invention, the engine operating parameters including
the position of the throttle valve 16 and engine speed are monitored as
well as the variation in the position of the throttle valve. Based on
those values, the engine control module 14 predicts the value of throttle
position at a predetermined future time. The engine control module 14 then
applies a control signal to the EGR valve 20 commanding the EGR valve 20
to a position corresponding to the predicted future position of the
throttle valve 16 at a time preceding the future time by the known delay
time of the EGR control loop. At the future time, the EGR valve will have
been positioned to the position corresponding to the predicted position of
the throttle valve 16 so as to more closely provide the desired exhaust
gas recirculation to the engine 10 during engine transient conditions.
Referring to FIG. 2, the engine control module 14 takes the form of a
digital computer. The digital computer is standard in form and includes a
central processing unit (CPU) which executes an operating program
permanently stored in a read-only memory (ROM) which also stores tables
and constants utilized in determining the fuel and exhaust gas
recirculation requirements of the engine. Contained within the CPU are
conventional counters, registers, accumulators, flag flip-flops, etc.,
along with a clock which provides a high frequency clock signal.
The digital computer also includes a random access memory (RAM) into which
data may be temporarily stored and from which data may be read at various
address locations determined in accord with the programs stored in the
ROM. A power control unit (PCU) receives the voltage from the vehicle
battery through the vehicle ignition switch and provides regulated power
to the various operating circuits in the engine control module 14.
An input/output circuit (I/O) includes a pair of output counter sections
each independently controlled by the CPU to provide EGR and injector
control pulses to respective driver circuits 28 and 30 for controlling the
EGR valve position and the injector opening time. The I/O also includes an
input counter section which receives a pulse output from the vehicle
distributor which generates a pulse for each cylinder during each engine
cycle. The pulses from the distributor are used for determining the engine
speed and also may be used for initiating the energization of the fuel
injector 12. In this respect, the engine speed may be determined by
counting clock pulses from the internal clock between pulses from the
vehicle distributor.
The engine control unit 14 further includes an analog-to-digital unit (ADU)
which provides for the measurement of analog signals. In the present
embodiment, the analog signals include a manifold absolute pressure signal
(MAP) provided by a conventional pressure sensor, the throttle position
and velocity signals provided by the sensor 26, and the EGR valve position
signal provided by the EGR actuator and valve assembly 20.
The analog signals are each sampled and converted under control of the CPU.
The conversion process is initiated from command of the CPU which selects
the particular analog input channel to be converted. At the end of the
conversion cycle, the digital data is read over the data bus on command
from the CPU and stored in ROM designated RAM memory locations.
The clock signal from the CPU internal clock is provided to an external
divider 32 which issues a periodic interrupt pulse to an interrupt A of
the CPU. These interrupt pulses may be spaced at, for example, 10
millisecond intervals. A second interrupt is provided to an interrupt B
input of the CPU by a programmable timer 34. In general, the programmable
timer 34 is a conventional timer including counter registers that are
preset by the CPU to a digital number representing a desired timed
interval. Clock pulses from the CPU are also supplied to the timer 34 to
count down the number preset into the counter registers. When the counter
registers are clocked to zero, the interrupt signal is generated and
provided to the interrupt B input of the CPU. Each of the interrupt
signals to the A and B inputs of the CPU functions to initiate a
particular program routine in the program stored in the ROM.
The operation of the engine control module 14 in controlling the exhaust
gas recirculation of the engine 10 in accord with the principles of this
invention is illustrated in FIGS. 3 thru 6. Referring first to FIG. 3,
when power is first applied to the engine control module such as by the
operation of the vehicle ignition switch, the computer program is
initiated at point 36 and then proceeds to a step 38 where the computer
provides for system initialization. For example, at this step, initial
values stored in the ROM are entered into ROM designated locations in the
RAM and counters, flags, and timers are initialized. After the
initialization step 38, the program proceeds to a step 40 where the
program is then allowed to respond to the inputs to the interrupt A and B
inputs of the CPU. After the step 40, the program proceeds to a background
loop 42 which is continuously repeated. This loop may include execution of
routines such as diagnostic and warning routines.
As previously indicated, in the present embodiment, the input to the
interrupt A input of the CPU from the divider 32 is provided at 10
millisecond intervals. When the CPU senses the interrupt input from the
divider 32, the background loop 42 is interrupted and an interrupt A
routine as illustrated in FIG. 4 is executed.
Referring to FIG. 4, the background loop 42 is interrupted at step 44 to
begin execution of the interrupt A routine and the program proceeds to a
step 46 where the throttle velocity read and stored during execution of
the prior interrupt A routine is saved in a ROM designated RAM location.
Thereafter, the various engine operating conditions are read and stored.
At this step, the program executes the analog-to-digital conversions of
the throttle position and velocity signals, the manifold absolute pressure
signal and the EGR valve position signal and stores the resulting digital
numbers at ROM designated RAM locations. The engine speed is also sampled
from the input counter section of the I/O and stored in a ROM designated
RAM location.
Following the read routine of step 46, the program proceeds to a step 48
where the acceleration of the throttle is calculated and saved based on
the previously stored value of throttle velocity saved at step 46 and the
new value of throttle velocity read and stored at step 46.
Thereafter, the program proceeds to a decision point 50 where the count i
in an index counter is compared to a calibration constant n. The
calibration constant n in conjunction with the period of the interrupt
signal generated by the divider 32 defines the time interval between
updates of the control signal supplied to the EGR actuator and valve
assembly 20 to command a desired position of the EGR valve. If i is less
than n, the program proceeds to a step 52 where the index counter is
incremented. Thereafter, the program proceeds to a step 54 where a fuel
control routine is executed to determine the time duration of energization
of the fuel injector 12 to achieve a desired air-fuel ratio.
If at decision point 50, it is determined that the index counter is equal
to the value n, the program proceeds to a step 56 where the index counter
is reset. Thereafter, a step 58 is executed which determines the desired
value of the EGR valve position and the control signal to produce that
position based on the current EGR dependent engine operating conditions of
engine speed and throttle position. This value may be determined via a
three-dimensional lookup table stored in the ROM addressed by the values
of engine speed and throttle position. The EGR signal producing the
desired EGR valve position retrieved from the lookup table is provided to
the input/output circuit of the engine control module 14 to immediately
readjust the value of the signal applied to the driver 28 for positioning
the valve in the EGR actuator and valve assembly. The signal input to the
driver 28 may be a pulse width modulated signal having a duty cycle
determined by the EGR control signal value retrieved from the lookup table
at step 58.
From step 58, the program proceeds to a step 60 where the program predicts
the position of the throttle 16 at a future time t+.DELTA.t, where t is
the present time. While other predictor equations may be used, in the
preferred embodiment the prediction of the future throttle position at
t+.DELTA.t is based on the predictor equation
TP.sub.t+.DELTA.t =TP.sub.t +(.DELTA.t)(TP')+(.DELTA.t/4)(TP")
where TP' is the measured throttle position velocity and TP" is the
calculated throttle position acceleration. This predicted future throttle
position at t+.DELTA.t over short intervals of time provides an accurate
prediction of the future throttle position.
From step 60, the program proceeds to a step 62 where the EGR control
signal producing the EGR valve position corresponding to the predicted
engine operating condition at time t+.DELTA.t is determined. As before,
this value may be determined by the ROM stored lookup table. This step
assumes that the engine speed at the future time t+.DELTA.t is equal to
the present engine speed. This assumption does not introduce any
substantial error in the EGR valve position determined for the future time
since engine speed does not vary at a rapid rate and over the short time
interval defined by the calibration constant n of decision point 50 can be
assumed a constant.
At step 64, a number is set into the programmable timer 34 of FIG. 2 that
represents the time remaining until the EGR signal established at step 62
is to be issued to the driver circuit 28 to readjust the EGR valve
position in the EGR actuator and valve assembly 20. This time can be best
illustrated by reference to the time diagram of FIG. 6. At time t, the
interrupt A occurred initiating the interrupt A routine at step 44. The
time period until the next interrupt A is defined by the calibration
constant n which establishes the period .DELTA.t. Accordingly, at time
t+.DELTA.t, the next interrupt A is provided to the CPU wherein the
routine is again repeated. In order that the EGR valve is positioned to
the desired position at the future time t+.DELTA.t and corresponding to
the predicted engine operating condition at that future time, the EGR
valve control signal corresponding to the desired EGR valve position at
time t+.DELTA.t is provided in accord with this invention at a time
preceding the future time by a period equal to the time delay in the EGR
actuator and valve assembly 20. This time delay T.sub.D is primarily due
to the delay in the pneumatic actuator of the assembly 20. In order to
accomplish this, the value set into the programmable timer 34 at step 64
is equal to the time period .DELTA.t minus the time delay period T.sub.D.
Since the interrupt A routine is substantially instantaneous in its
execution, it is assumed that the value .DELTA.t-T.sub.D is inserted into
the programmable timer subtantially at the present time t. In another
embodiment, compensation for the execution time of the interrupt A routine
44 may be accounted for and the numerical value inserted into the
programmable timer 34 adjusted accordingly. From this point in time, the
programmable timer 34 is decremented by the clock pulses to time the
interval .DELTA.t-T.sub.D. From step 64, the program proceeds to the fuel
control routine 54. Thereafter at step 66, the program returns to the
background loop 42 of FIG. 3.
When the programmable timer 34 has timed the programmed period established
at step 64, an interrupt signal is provided to the interrupt B input of
the CPU. Upon receipt of this interrupt signal, the background loop 42 is
interrupted and the program proceeds to execute an interrupt B routine
initiated at step 68. From step 68, the program proceeds to a step 70
where the value of the EGR signal determined to establish the desired EGR
valve position at the future time t+.DELTA.t is provided to the
input/output circuit and outputted to the driver circuit 28. The output of
the driver 28 is applied to the actuator in the EGR actuator and valve
assembly 20 to effect adjustment of the vacuum level for controlling the
EGR valve position. Following step 70, the program returns to the
background loop 42 at step 72. The signal output of the I/O circuit of the
computer may take the form of a duty cycle modulated signal having a duty
cycle value determined by the interrupt routine of FIG. 4. The duty cycle
value is set at time t to the value established at step 58 and at time
t+(.DELTA.t- T.sub.D) to the value established at step 62.
In summary and with reference to FIG. 6, at time t, the interrupt A routine
is executed and the control signal to establish the desired EGR valve
position corresponding to the current values of the engine operating
conditions is provided to the EGR actuator and valve assembly 20. At
substantially the same time, (A) the values of the engine operating
conditions at a future time t+.DELTA.t are predicted and the control
signal required to establish the corresponding desired EGR valve position
is determined and (B) a time value equal to the time to the future time
(.DELTA.t) minus the known time delay (T.sub.D) in the EGR control loop is
set into the timer 34. At a time preceding the future time by the EGR
control loop time delay T.sub.D, the timer 34 initiates the interrupt B
routine which causes the control signal value provided to the EGR actuator
and valve assembly 20 to be adjusted to the previously determined value
establishing the desired EGR valve position at the future time t+.DELTA.t.
Thereafter upon expiration of the time period T.sub.D, the EGR valve is
positioned to the desired position corresponding to the engine operating
conditions predicted to exist at that time.
The interrupt A and interrupt B routines are fully executed at each
.DELTA.t interval as defined by the calibration constant n so that the
amount of exhaust gases recirculated is continuously established at the
value determined by the current engine operating condition even during
engine transient conditions.
The foregoing description of the preferred embodiment of the invention for
purposes of illustrating the invention is not to be considered as limiting
or restricting the invention since many modifications may be made by the
exercise of skill in the art without departing from the scope of the
invention.
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