Note: Descriptions are shown in the official language in which they were submitted.
CA 02298305 2000-02-04
SYSTEM FOR DETECTING FUEL INJECTION TIMING
Background of the Invention
The invention relates to a system and method for measuring the timing of a
fuel
injection pulse.
In a electronically controlled fuel injected Diesel engine, there is a need
for an
accurate measurement of the timing of a fuel injection pulse as feedback to
the fuel injection
control system so that the control system can adjust the fuel injection timing
to match the
desired timing. Desired timing of fuel injection can be determined as a
function, at least
partially, of the engine load (which can be represented by fuel quantity) or
fueling rate. The
quantity and timing of a fuel injection pulse can be determined from a fuel
pressure signal
supplied by a pressure sensor coupled to a fuel injection line. For example,
it is known to
determine the fuel quantity of a fuel injection pulse from the area, height
and width of a fuel
pressure signal. For example, US Patent 4,426,981 shows a system which
determines fuel
injection quantity via the integral of a pressure signal from a pressure
sensor coupled to the
fuel pump. It is also known to determine the timing from a fuel pressure
signal. For
example, the system of US Patent 4,426,981 also determines the onset of a fuel
injection
pulse by comparing a differentiated pressure signal to a threshold. US patent
No. 5,107,700
shows a system which detects the beginning of fuel injection by comparing a
fuel pressure
signal to a threshold which is derived from a peak value of the injection
pressure signal of
the preceding injection period.
Summary of the Invention
Accordingly, an object of this invention is to provide a system and method for
determining the timing of a fuel injection pulse for use in an electronic fuel
injection control
system.
This and other objects are achieved by the present invention, wherein an
engine has
a source of pressurized fuel, a fuel line for delivering fuel to a combustion
chamber of the
engine supply. A control system includes a fuel pressure sensor for sensing
fuel pressure
and determines the timing of a fuel quantity delivered to the combustion
chamber by a
method including the following steps. The engine speed is sensed. The fuel
injection
quantity is sensed and an engine load signal derived therefrom. A variable
threshold signal
is generated as a function of the engine speed signal and the engine load
signal. The fuel
pressure signal is compared to the variable threshold signal, and a timing
signal is generated
when the fuel pressure signal crosses the variable threshold signal while the
fuel pressure
signal is increasing in magnitude.
CA 02298305 2000-02-04
Brief Description of the Drawinas
Fig. 1 is a simplified schematic diagram of a Diesel engine fuel injection
control
system according to the present invention;
Fig. 2 is a circuit diagram of the timing circuit of FIG. 1;
Fig. 3 is a circuit diagram of the pressure signal integration circuit of FIG.
1; and
Fig. 4 is a logic flow diagram of an algorithm executed by the microprocessor
of the
control system of FIG. 1.
Description of the Preferred Embodiment
This application includes a microfiche appendix including one microfiche and
21
frames.
Referring to Fig. 1, a Diesel engine 10 includes fuel injectors 11 supplied
with fuel
from a fuel pump 12 via fuel lines 14. The fuel pump is preferably a rotary
type fuel injection
pump with a stepper motor timing actuator 16. A control circuit 20 includes a
microprocessor
22, an integration circuit 24 and a timing circuit 26, and provides a timing
control signal to the
actuator 16 as a function of sensed inputs, including a temperature signal
from temperature
sensor 28, an engine speed signal from engine speed sensor 30, a crank angle
signal from
crank angle sensor 32 and a pressure signal from fuel pressure sensor 34.
Fuel pressure sensor 34 is preferably a piezoelectric pressure sensor such as
is
available from Texas Instruments, (although many other sensor technologies
would also
work) and is mounted on a fuel line 14 between the pump 12 and the
corresponding injector
11 or in the timing advance piston (not shown) of the pump 12. The sensor 34
preferably
generates a voltage proportional to the strain of the sensing element. If the
sensor element
generates a charge proportional to the pressure in the injector line, the
charge can be
converted to a voltage.
As best seen in Fig. 2, the timing circuit 26 includes a comparator U3 (LM2901
) with
one input coupled to receive the pressure signal from pressure sensor 34 and a
second
input coupled to receive a trigger threshold signal from the microprocessor
22. Preferably,
the trigger threshold signal from the microprocessor 22 is a pulse width
modulated square
wave signal which has duty cycle proportional to the trigger threshold which
is converted to a
proportional d.c. voltage by a resister/capacitor filter consisting of
resistor R8 (40K Ohm) and
capacitor C3 (0.1 pF). The output of the comparator U3 is connected to an
integrator trigger
signal input of the microprocessor 22.
The integration circuit 24 includes the following components connected as
shown in
Fig. 3: resistor R1 (40K Ohm), resistors R2, R6 and R7 (100K Ohm), resistor R3
(10K Ohm),
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resistor R4 (50K Ohm), resistor R5 (1 K Ohm), capacitor C1 (0.1 p.F) capacitor
C2 (0.02 pF),
transistors Q1 and Q2 (2N7002), comparators U1 and U2 (LM2901). Thus,
integration
circuit 24 is a standard hardware based integrator with reset and windowing
logic. The
trigger thresholds to begin and end the integration are determined by the
measurement of
background signal levels and generated by the microprocessor 22. The windowing
of the
integration is controlled by the microprocessor 22 as a function of the engine
position signal
from the crank angle sensor 32, and allows better noise rejection in the
system. The triggers
at the beginning and end of integration are used by the microprocessor 22 to
correct the
integrated value for the background levels and to determine the beginning of
injection. With
respect to Figs. 2 and 3, the components values set forth herein are merely
exemplary and
other components could be utilized without departing from scope of the present
invention.
The micro 22 executes an algorithm represented by Fig. 4. For further details
regarding this algorithm, reference may be made to the computer program
listing included in
the microfiche appendix. The timing calculation algorithm is entered at step
200. Step 202
directs the algorithm to step 204 if the signal from crank angle sensor 32
indicates that the
engine 10 is at the start of a fuel injection window period, otherwise the
algorithm is directed
to step 214. Step 204 obtains an initial pressure threshold or trigger
threshold value from a
stored look-up table, based on the engine speed from sensor 30 and based on
the fuel
quantity value which results from the integration of the pressure signal from
sensor 34. This
fuel quantity value is representative of the load on the engine 10. Thus, if
the engine speed
varies, and/or if the engine load varies, the initial trigger threshold will
also vary.
If the pressure signal is noisy, then step 206 can be used to filter and
smooth the
pressure signal. Step 206 can be dispensed with if the pressure signal is
sufficiently free of
noise, or alternatively, this function can be performed by a hardware circuit.
Step 208 then calculates a new trigger threshold value by adding an offset to
the value from
step 204. The offset may be derived from the filtered background fuel
injection pressure
level from step 206. Preferably, the threshold is set relative to the
background level, but this
is done with a table rather than calculated directly by the microprocessor.
The background
pressure is determined for all engine speed and fuel flow rates. The
background pressure
level changes with changes in speed and fuel, but the level can be predicted
based on
engine speed and fuel flow. Therefore, the trigger level offset can be mapped
into a stored
data table of speed vs. fuel flow. The values in such a table represent the
sum of the
background level and a percentage of the peak pressure pulse. Using a single
table to
represent 2 physical values reduces the microprocessor requirements for
memory, speed
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CA 02298305 2000-02-04
and overall cost. Piezoelectric pressure sensors typically have an output
which responds to
changes in pressure. While such an output will increase or decrease when the
pressure
changes, the output level may not correspond to an absolute pressure, and the
output of
such a sensor can drift several volts even while the pressure remains
constant. When the
pressure changes, the sensor accurately measures the change relative to the
sensors output
voltage before the pressure change. The algorithm described here is designed
to function
with such a pressure sensor.
Step 210 outputs the calculated trigger threshold value to an input of the
timing circuit
26 so that the timing circuit 26 will generate a start of injection timing
signal when the signal
from pressure sensor 34 exceeds the trigger threshold value. Step 212 enables
an interrupt
so that the start of injection timing signal can be received by the
microprocessor 22, after
which step 230 returns control to a main fuel injection control algorithm (not
shown).
If, in step 202, if the signal from crank angle sensor 32 indicates that the
engine 10 is
not at the start of a fuel injection window period, step 202 directs the
algorithm to step 214.
Step 214 directs the algorithm to step 216 if the signal from crank angle
sensor 32 indicates
that the engine 10 is at the end of a fuel injection window period, otherwise
the algorithm
returns via step 230. Step 216 disables the threshold interrupt. Step 218
determines
whether or not a single interrupt occurred. If more than a single interrupt
occurred, it means
that a failure has occurred and step 218 directs the algorithm to step 220. If
the number of
errors detected by step 220 is high, step 220 directs the algorithm to step
222 which sets a
timing value to a default value, such as 10 degrees from top-dead-center, and
then step 222
directs the algorithm to a fault management step 224, and then returns via
step 230. If the
number of errors detected by step 220 is not high, step 220 directs the
algorithm to step 221
which sets the corrected angle value to the previous corrected angle.
If, in step 218 only a single interrupt occurred, then step 218 directs the
algorithm to
step 226 which calculates the crank angle value (relative to top-dead-center)
at which the
timing circuit 26 generated the timing signal. Then, step 228 generates a
corrected angle
value from a stored look-up table and as a function of engine speed from
sensor 30 and the
fuel quantity derived from the integration of the fuel pressure signal. This
corrected angle
value represents the crank angle at which a fuel injection pulse begins, and
this value can be
used to control fuel injection timing in a closed-loop control system.
A portion of the disclosure of this patent document contains material which is
subject
to a claim of copyright protection. The copyright owner has no objection to
the facsimile
reproduction by anyone of the patent document or the patent disclosure, as it
appears in the
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CA 02298305 2000-02-04
Patent and Trademark Office patent file or records, but otherwise reserves all
other rights
whatsoever.
While the present invention has been described in conjunction with a specific
embodiment, it is understood that many alternatives, modifications and
variations will be
apparent to those skilled in the art in light of the foregoing description.
Accordingly, this
invention is intended to embrace all such alternatives, modifications and
variations which fall
within the spirit and scope of the appended claims.
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