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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a diesel engine fuel control system.
Specifically, this invention is directed toward a system for limiting the
fuel quantity supplied to a diesel engine.
It is well known to employ exhaust gas recirculation and maximum fuel
limiting to inhibit certain emissions from a diesel engine. For example,
it is known to limit the fuel injected into the engine in order to prevent
the air-to-fuel ratio of the cylinder charge from decreasing below a rich
limit where unacceptable smoke emissions result. In general, the fuel
limit must be altitude compensated since the cylinder gas charge density
varies with atmospheric pressure resulting in a decrease in the
air-to-fuel ratio with increasing altitude levels. This is typically
accomplished by monitoring the barometric pressure and adjusting the
maximum fuel limit in accord with the sensed pressure to provide for
altitude compensation.
The recirculation of exhaust gases is commonly employed to inhibit the
formation and emission of oxides of nitrogen. Generally, the greater the
amount of exhaust gases recirculated, the lower the emission levels of
oxides of nitrogen. However, since recirculated exhaust gases displace air
that would otherwise be drawn into the cylinders of the diesel engine, the
air-to-fuel ratio of the mixture in the cylinders is decreased with
increasing levels of exhaust gas recirculation. Therefore, in order to
prevent undesirable smoke emissions, the amount of exhaust gases
recirculated must be limited to levels that do not result in excessively
rich air-to-fuel ratios which produce smoke emissions.
In Applicants' copending application Ser. No. 342,729, filed on Jan. 26,
1982, now U.S. Pat. No. 4,399,799, an exhaust gas recirculation (EGR)
control system is described wherein the amount of recirculated exhaust
gases is controlled by generating a signal representing a commanded
absolute pressure that is proportional to the operator commanded fuel per
engine revolution. A pressure modulator generates an EGR control pressure
that is equal to the commanded absolute pressure by proportionately mixing
air from a subatmospheric source and atmospheric air. An EGR valve is
positioned based on the difference between the EGR control pressure and
atmospheric pressure. In this form of exhaust gas recirculation control
system, the amount of exhaust gases recirculated is automatically
compensated for atmospheric pressure changes.
SUMMARY OF THE INVENTION
In one aspect of this invention, the amount of fuel per revolution of a
diesel engine is limited to a value that is represented by the EGR control
pressure generated in the aforementioned EGR control system that is the
subject of Applicants' copending application Ser. No. 342,729 now U.S.
Pat. No. 4,399,799. Since the EGR control pressure is equal to the
commanded absolute pressure which is proportional to the operator
commanded fuel per engine revolution, the fuel limit is adjusted so as to
be equal to the commanded fuel per engine revolution. However, since the
pressure modulator in the aforementioned EGR system generates the EGR
control pressure by proportionately mixing air from a subatmospheric
source and atmospheric air, the EGR control pressure is limited to a
maximum value equal to the atmospheric pressure even though the commanded
absolute pressure in response to an operator commanded fuel per engine
revolution is greater than atmospheric pressure. By limiting the maximum
fuel in accord with the EGR control pressure, the upper maximum fuel limit
is the fuel per engine revolution represented by the atmospheric pressure
value. Therefore, the maximum fuel limit is automatically altitude
compensated to prevent the air-to-fuel ratio from decreasing below the
rich limit producing undesirable smoke emissions.
When the engine operator commands a sudden increase in the fuel per engine
cycle, the abovedescribed EGR control system operates to reduce the
amounts of recirculated exhaust gases by increasing the EGR control
pressure. Due to factors including the time required to purge the excess
recirculated exhaust gases from the intake manifold of the diesel engine,
a delay exists before the new lower EGR levels are established in the
intake manifold. In accord with another aspect of this invention, the rate
of change in the fuel limit established in response to a change in the EGR
control pressure is limited to a value that imposes a delay in the
increase in the fuel amount per engine revolution that substantially
matches the delay required to decrease the amount of recirculated exhaust
gases in the intake manifold of the engine. This delay prevents the
air-to-fuel ratio in the combustion space from decreasing to below a value
resulting in undesirable smoke emissions.
DESCRIPTION OF THE DRAWINGS
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 exhaust gas recirculation controller with which the
fuel limit controller incorporating the principles of this invention is
used;
FIGS. 2 and 3 illustrate a maximum fuel limiter in a rotary diesel fuel
pump;
FIG. 4 illustrates a digital computer for controlling the maximum fuel
limiter of FIGS. 2 and 3 in accord with this invention; and
FIGS. 5 and 6 are flow diagrams illustrating the operation of the digital
computer of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is illustrated an exhaust gas recirculation
(EGR) controller 10 for a diesel engine which provides a signal used in
accord with the principles of this invention for limiting the maximum fuel
that can be supplied to a diesel engine with each injection stroke of a
fuel injection pump and, therefore, the maximum fuel per engine
revolution. This EGR controller forms the basis of Applicants' copending
application Ser. No. 342,729, filed Jan. 26, 1982 and assigned to the
assignee of this invention, the disclosure of which is hereby incorporated
by reference. In general, the EGR controller 10 determines the fuel per
stroke of a conventional rotary fuel injection pump in response to engine
speed and the position of the metering valve of the injection pump. These
parameters are sensed by a fuel metering valve position sensor 12 such as
a potentiometer, and an engine speed sensor 14.
The metering valve position and engine speed signals are applied to a fuel
per stroke computer 16 which establishes a signal that is proportional to
and a measure of the fuel per injection stroke of the fuel injection pump.
The fuel per stroke computer 16 may take the form of a lookup table having
memory locations addressed by metering valve position and engine speed.
Each memory location has a commanded absolute EGR control pressure value
stored therein that is a direct measure of the fuel per injection stroke
corresponding to the particular values of fuel metering valve position and
engine speed addressed thereby. The commanded absolute EGR control
pressure value retrieved from the lookup table in the fuel per stroke
computer 16 is provided to the positive input of a summer 18 where it is
compared with the value of the absolute EGR control pressure in an EGR
control valve 19 as measured by the absolute pressure sensor 20. The
measured absolute EGR control pressure is utilized as a feedback signal in
a closed loop system for maintaining the absolute EGR control pressure in
the EGR control valve 19 equal to the commanded absolute EGR control
pressure output of the fuel per stroke computer 16.
The summer 18 provides an error signal to an EGR amplifier 22 that is the
difference between the commanded and actual absolute EGR control
pressures. The EGR amplifier 22 generates a control signal for adjusting
the value of the absolute EGR control pressure via an EGR solenoid 24 in
direction to cause correspondence between the commanded and measured
absolute EGR control pressures. In this embodiment, the amplifier 22 takes
the form of a duty cycle modulator having integral and proportional terms
providing a duty cycle modulated signal to the EGR solenoid 24. The
solenoid 24 controls the mixing of air from a subatmospheric pressure
source, such as a vacuum pump 26, and atmospheric air provided via an
atmospheric vent 28. The EGR solenoid 24 admits air at subatmospheric
pressure into the EGR control valve 19 or air at atmospheric pressure
through the atmospheric vent 28 into the EGR control valve 19 at relative
proportions determined by the duty cycle of the output of the amplifier 22
to provide a net absolute EGR control pressure in the EGR control valve.
As previously indicated, the absolute pressure sensor 20 provides a signal
representing the sensed absolute EGR control pressure to the negative
input of the comparator 18. Via the closed loop established by the
comparator 18, the EGR amplifier 22, the EGR solenoid 24 and the absolute
pressure sensor 20, the absolute EGR control pressure applied to the EGR
control valve 19 is controlled to the commanded absolute EGR control
pressure established by the fuel per stroke computer 16.
As described in the aforementioned copending application, the EGR control
valve 19 includes a diaphragm on one side of which atmospheric pressure is
applied and on the other side of which the absolute EGR control pressure
is applied. The amount of EGR is varied in an inverse proportional
relationship to the absolute EGR control pressure. Accordingly, the EGR
control valve is positioned in accord with the commanded absolute EGR
control pressure, which is a measure of fuel per stroke of the fuel
injection pump, and adjusted automatically in accord with variations in
altitude to prevent the air/fuel ratio from decreasing to a value
producing excessive smoke emissions from the diesel engine.
Characteristic of the output signal from the absolute pressure sensor 20 is
the fact that the value of the signal is, except for the condition
hereinafter described, equal to the commanded absolute EGR control
pressure from the fuel per stroke computer 16 and is therefore a measure
of the commanded fuel per stroke established by the operator of the diesel
engine. Another characteristic of the output signal of the absolute
pressure sensor 20 is that even though the fuel per stroke computer 16
provides a commanded absolute EGR control pressure greater than
atmospheric pressure, the output signal from the sensor 20 is limited to a
maximum value equal to atmospheric pressure since the absolute EGR control
pressure signal provided to the EGR control valve 19 is established by the
proportional mixing of air at a subatmospheric pressure and atmospheric
pressure admitted through the atmospheric vent 28. These characteristics
of the output signal from the absolute pressure sensor 20 are utilized to
establish maximum fuel limiting in accord with the principles of this
invention. That is, the signal provided by the absolute pressure sensor 20
is utilized by a fuel limit controller 32 as a fuel limit control signal.
This fuel limit control signal has a value equal to the output of the fuel
per stroke computer 16 and therefore is a measure of the operator
commanded fuel per injection stroke of the fuel pump up to a limit fuel
per stroke value represented by the output of the absolute pressure sensor
20 when the absolute EGR control pressure is at the limit imposed by
atmospheric pressure. The signal output of the absolute pressure sensor 20
hereinafter referred to as a fuel limit control signal is applied to the
fuel limit controller 32 which responds thereto to adjust a stepper motor
34 whose output shaft position establishes the maximum allowable fuel per
stroke of the fuel injection pump of the diesel engine.
Referring to FIGS. 2 and 3, there is illustrated a rotary fuel injection
pump including the maximum fuel per stroke limiter adjusted by the stepper
motor 34 of FIG. 1. The rotary fuel injection pump is a conventional pump
in which a metering valve opening controls the flow of pressurized fuel
from a transfer pump into a pumping chamber 36 through an inlet port which
moves a pair of plungers 38 outwardly for a distance which is proportional
to the amount of fuel for the next injection stroke. The maximum outward
movement of the plungers 38 and accordingly the maximum amount of fuel
that may be admitted into the pumping chamber 36 is limited by a pair of
scroll plates 40 (one of which is shown). As the rotor of the injection
pump revolves, the inlet port to the pumping chamber 36 closes and a
discharge port registers with the outlet to the next firing cylinder. The
rollers of the pump then contact opposing cam lobes on a cam ring 44
(shown only in FIG. 3) forcing the plungers 38 inwardly to begin high
pressure fuel delivery. The delivery continues until the rollers 42 travel
over the cam noses and begin to move outward. The scroll plates 40 are
contoured so that when they are rotated relative to the pump housing and
the cam ring 44, the maximum outward movement of the plungers 38 may be
adjusted.
The mechanism for adjusting the angular position of the scroll plates 40 to
limit the outward travel of the rollers 42 and accordingly the plungers 38
is illustrated in FIG. 2. The stepper motor 34 is mounted to the rotary
fuel pump and includes an output shaft 46 having a threaded portion 48
extending through a scroll plate adjusting nut 50. The nut 50 is moved
left or right depending on the direction of rotation of the output shaft
46 to angularly position the scroll plates 40 to increase or decrease the
maximum fuel per stroke of the injection pump.
The scroll plates 40 may be driven by the stepper motor 34 between two
mechanical limits. A maximum fuel upper limit is established by a stop
member 52 and a maximum fuel lower limit is established by a stop member
54. The maximum fuel lower limit established by the stop member 54
prevents the fuel limit from decreasing below road load fuel. In this
manner, the fuel limiter would be unable to limit below road load fuel so
that a "walk-home" capability exists in the event of a failure.
The scroll plates 40 are moved to fuel limiting positions between the
limits established by the stop members 52 and 54 by the fuel limit
controller 32 which is illustrated in FIG. 4. The fuel limit controller 32
takes the form of a digital computer that includes a microprocessor 56
which executes the sensing and control functions under the control of an
operating program permanently stored in an external read-only memory (ROM)
58. Internal to the microprocessor 56 are conventional counters,
registers, accumulators, flag flip flops, etc. The controller 32 also
includes a random access memory (RAM) 60 into which data may be
temporarily stored and from which data may be read at various address
locations determined in accord with the program stored in the ROM 58.
A clock oscillator 62, which establishes the timing of the digital
computer, supplies a clock signal to the microprocessor 56 and to a
divider 64 which issues a periodic interrupt pulse to a maskable interrupt
input A of the microprocessor 56. These interrupt pulses may be spaced at,
for example, 121/2 millisecond intervals.
A counter input/output (I/O) circuit 66 is provided having an output
counter section for providing output pulses to a stepper motor drive
circuit 68. A discrete input/output (I/O) circuit 70 is provided having an
output section for issuing a bilevel output signal representing a forward
or reverse command to the stepper motor driver circuit 68. For example, a
high level output of the discrete input/output circuit 70 may represent a
forward command and a low-level output may represent a reverse direction
command. The discrete I/O 70 may include a flip flop that is selectively
set to provide the forward command and reset to provide the reverse
command. The stepper motor drive circuit 68 is a conventional logic
circuit responsive to each pulse output from the counter I/O 66 for
causing the stepper motor 34 to rotate its output shaft 46 one incremental
step in a direction determined by the logic level output of the discrete
I/O 70.
An analog-to-digital converted ADC 72 provides for the measurement of the
analog absolute pressure signal provided by the absolute pressure sensor
20 of FIG. 1. The analog absolute pressure signal is sampled and converted
under control of the microprocessor 56. The conversion process is
initiated on command from the microprocessor 56 and at the end of the
conversion cycle, the ADC 72 generates an interrupt after which the
digital data representing the absolute pressure value from the sensor 20
is read over the data bus on command from the microprocessor 56 and stored
in a ROM designated memory location in the RAM 60.
The various elements of the digital computer are interconnected by an
address bus, a data bus and a control bus. The microprocessor 56 accesses
the various circuits and memory locations in the ROM 58 and the RAM 60 via
the address bus. Information is transmitted between the circuits via the
data bus and the control bus includes conventional lines such as
read-write lines, reset lines, clock lines, power supply lines, etc.
The operation of the digital computer 32 in adjusting the scroll plates 40
to provide controlled limiting of the maximum fuel per injection stroke of
the fuel injection pump is illustrated in the FIGS. 5 and 6. Referring
first to FIG. 5, when power is first applied to the system such as by
operation of the vehicle starter switch (not shown), the computer program
is initiated at point 74 and then proceeds to a step 76 where the computer
provides for system initialization. At this step, initial values stored in
the ROM 58 are entered into ROM designated locations in the RAM 60 and
counters, flags and timers are initialized. After the initialization step
76, the program proceeds to a step 78 where the program allows interrupts
to occur such as by resetting the interrupt mask bit in the microprocessor
condition code register. After the step 78, the program shifts to a
background loop 80 which is continuously repeated. This loop may include
execution of other routines such as certain diagnostic and warning
routines. Alternatively, the background loop 80 may simply be a "wait"
instruction wherein the computer is idle until an interrupt occurs to
execute the fuel limiting control routine of FIG. 6.
While the system may employ numerous program interrupts at various spaced
intervals, it will be assumed for purposes of illustrating this invention
that a single interrupt A is provided at 121/2 millisecond intervals by
means of a divider 64 of FIG. 4 during which the fuel limit routine of
FIG. 6 is executed.
Referring now to FIG. 6, the fuel limit routine is entered at point 82 and
proceeds to a decision point 86 where the fuel per stroke limit
represented by the fuel limit control signal provided by the absolute
pressure sensor 20 is compared with the maximum fuel lower limit value
which is the fuel per stroke of the fuel pump of FIG. 2 when the scroll
plates are moved by the stepper motor 34 to the limit established by the
stop member 54. If the fuel per stroke limit represented by the fuel limit
control signal is greater than the maximum fuel lower limit, the program
proceeds to a step 88 where a flag A is set. Thereafter, the program
proceeds to a decision point 92 where the actual fuel per stroke limit
represented by the count in a counter A is compared to the fuel per stroke
limit represented by the fuel limit control signal. In this respect, the
actual fuel per stroke limit represented by the count in the counter A is
defined by the expression X+YZ where X is the maximum fuel lower limit
fuel per stroke established by the stop member 54, Y is the total count in
the counter A, and Z is the change in the fuel per stroke when the scroll
plates 40 are rotated by one step from the stepper motor. If the actual
feed per stroke limit is less than the fuel per stroke limit represented
by the fuel limit control signal, the program proceeds to step 94 where
the counter A is incremented and the stepper motor 34 is incremented in a
direction to move the scroll plates 40 toward the maximum fuel upper limit
stop member 52. This is accomplished by setting the output of the discrete
I/O 70 to the proper level for establishing the rotational direction of
the stepper motor 34 in an increasing fuel limit direction and issuing a
pulse from the counter I/O 66. The stepper motor driver circuit 68 of FIG.
4 issues the signals for incrementing the stepper motor 34 as commanded.
Following step 94, the program exits the fuel limit control routine.
As long as the fuel per stroke limit represented by the fuel limit control
signal exceeds the actual fuel per stroke limit represented by the count
in the counter A, the program repeats the above-described series of steps
at each interrupt interval to cause the stepper motor 34 to rotate the
scroll plates 40 in direction to increase the limited fuel value. After
the number of interrupts required to increment the counter A to a value
representing an actual fuel per stroke limit equal to the fuel per stroke
limit represented by the fuel limit control signal, the program exits the
routine directly from decision point 92 and the actual fuel per stroke
limit is then established at the limit represented by the fuel limit
control signal from the sensor 20. Assuming the commanded absolute EGR
control pressure is less than atmospheric pressure, the actual fuel per
stroke limit is at this time equal to the operator commanded fuel per
stroke as represented by the output of the fuel per stroke computer 16. If
the commanded absolute EGR control pressure is greater than atmospheric
pressure, the absolute EGR control pressure remains at atmospheric
pressure and the actual fuel per stroke limit remains at a fuel per stroke
value represented by an output of the fuel per stroke computer 16 equal to
atmospheric pressure.
Through the foregoing series of steps, the scroll plates 40 are adjusted to
a position to limit the fuel per stroke of the fuel injection pump to the
operator commanded value represented by the output of the fuel per stroke
computer 16 unless that value exceeds the fuel per stroke represented by
the atmospheric pressure value. If this condition exists, the output of
the absolute pressure sensor 20 remains constant so that the fuel limit
established can not exceed the amount represented by the atmospheric
pressure. In this manner, the maximum fuel per stroke established by the
fuel limit controller is automatically adjusted with altitude variations.
Another fuel limiting aspect of the routine established by steps 86 through
94 is to provide for a limit in the rate of increase in the fuel limit in
response to sudden increases in the commanded fuel per stroke established
by sudden increases in the metering valve position. This rate of increase
limit is determined by the resolution in the counter A (fuel limit
increase per pulse to the stepper motor 34) and the interrupt interval
provided by the divider 64 of FIG. 4. The interrupt period and the
resolution of the counter A are determined so that the delay in the
increase in the fuel per stroke allowed substantially matches the delay
required for the excess exhaust gases to be purged from the intake
manifold in response to commanded increases in the fuel per stroke of the
injection pump so that the air/fuel ratio does not decrease below the rich
ratio producing undesirable smoke emissions from the diesel engine.
Assuming now that the commanded fuel per stroke represented by the
commanded absolute pressure signal output of the fuel per stroke computer
16 decreases to a level below the maximum fuel lower limit established by
the stop 54, the program proceeds from decision point 86 to a decision
point 96 where the count in the counter A is compared to zero. If the
count is greater than zero, the program proceeds to a step 98 where the
counter is decremented and the stepper motor 34 is rotated toward the
maximum fuel lower limit position. This is accomplished by setting the
output of the discrete I/O 70 to the logic level causing motor 34 rotation
in the decreasing fuel direction and issuing a pulse from the counter I/O
66. From step 98, the program exits the fuel limit routine.
The steps 96 and 98 are repeated with each interrupt interval as long as
the fuel per stroke limit signal represented by the output of the sensor
20 is less than the maximum fuel lower limit until the counter A is
decremented to zero.
When the counter A is decremented to zero, the program proceeds from the
decision point 96 to a decision point 100 where the state of the flag A is
sampled. If flag A is reset, it represents that the counter A was not
previously incremented in response to the fuel limit control signal being
greater than the maximum fuel lower limit. Whe this condition exists, the
flag A is in a reset condition having not been set at step 88, the scroll
plates 40 are positioned against the maximum fuel lower limit stop member
54 and the program exits the fuel limit routine. However, if the flag A is
set indicating that the scroll plates were previously moved away from the
maximum fuel lower limit stop member 54 to provide for fuel limiting and
the counter A has subsequently been decremented to zero via the steps 96
and 98, the program proceeds to execute a series of steps to ensure that
the stepper motor 34 has repositioned the scroll plates 40 to the maximum
fuel lower limit position established by the stop member 54. These steps
begin at decision point 102 where the condition of a flag B is sampled.
Assuming this flag is reset, the program proceeds to a step 104 where a
counter B, which may be a register in the RAM 60, is set to a
predetermined count value K. Thereafter, the flag B is set at step 106.
The program then proceeds to a decision point 108 where the content of the
counter B is compared to zero. Since the counter was preset to the value K
at step 104, the count in the counter B is greater than zero and the
program proceeds to step 110 where the counter B is decremented and the
stepper motor 34 is stepped toward the maximum fuel lower limit stop 54 by
issuing the proper logic level from the discrete I/O 70 and issuing a
pulse from the counter I/O 66. During the next interrupt interval, the
program proceeds directly from step 102 to decision point 108 and
thereafter to step 110 where the counter B is again decremented and the
stepper motor 34 is issued the command to rotate the scroll plates 40
toward the maximum fuel lower limit position. The steps 108 and 110 are
repeated K times (the count set into the counter B at step 104) to ensure
that the scroll plates have been positioned to the maximum fuel lower
limit position. When the count in the counter B has been decremented to
zero, the program proceeds from the decision point 108 to a step 112 where
the flag flip flops A and B are both reset. Thereafter, the program exits
the fuel limit routine.
While the foregoing routine does not issue commands to rotate the scroll
plates toward the maximum fuel lower limit position established by the
stop 54 until the fuel per stroke limit signal provided by the sensor 20
decreases below the fuel per stroke value established by the stop 54, it
would be apparent to one skilled in the art that the stepper motor 34 may
be controlled to cause rotation of the scroll plates 40 toward the maximum
fuel lower limit position at any time the fuel per stroke represented by
the fuel limit control signal decreases below the fuel per stroke
represented by the count in the counter A. The scroll plates would then be
continuously adjusted to provide a fuel limit that is equal to the fuel
per stroke commanded by the vehicle operator as limited by the maximum
fuel lower limit stop 54 and as limited by atmospheric pressure.
The foregoing description of a preferred embodiment of the invention for
the purpose of illustrating the invention is not to be considered as
limiting or restricting the invention as other modifications may be made
by the exercise of skill in the art without departing from the scope of
the invention.
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