Fuel control system for an internal combustion engine

A fuel control system for an internal combustion engine, including a fuel flow control sensitive to one or more engine operating parameters and controlling the rate at which fuel is introduced into the engine, an exhaust gas sensing device for producing an output signal corresponding to the exhaust gas composition, feed back means for feeding back to the fuel flow control, a signal derived from said output signal to correct the fuel flow, said feed back means including a signal storage device, the signal stored in which is altered in accordance with variations in the output signal of the exhaust gas sensing device, and overrun detection means connected to said feed back means and arranged to prevent alteration of said stored signal by the exhaust gas sensing device during overrun.

This invention relates to a fuel control system for an internal combustion 
engine. 
The invention overcomes the inability of closed-loop fuel control systems 
to cope with the conditions of overrun, also known as engine-braking. 
Overrun occurs when a vehicle moves forward under its momentum, but the 
throttle or accelerator is not depressed. The vehicle is essentially 
coasting in gear. 
It has already been proposed to include in a fuel control system an exhaust 
gas sensor which provides closed-loop control of the air/fuel ratio to the 
stoichiometric condition. With such a system when an overrun condition 
occurs it is desirable to override the closed-loop control because the 
signal produced by the sensor due to poor combustion is not truly 
representative of the ratios of air and fuel supplied to the engine. 
Consequently the control system will attempt to modify the quantity of 
fuel supplied until the sensor detects what it takes to be a 
stoichiometric condition. During overrun, therefore, the exhaust gas 
sensing circuit will detect this anomolous condition and provide a 
feedback signal tending to alter the fuel input. At the end of the overrun 
period, however, when normal closed-loop control is restored, the 
erroneous feedback signal now formed will tend to cause the mixture fed to 
the engine to become incorrect until the system lag is overcome. This 
could result, for a short time after overrunning, in a highly polluted 
exhaust emission. 
It is an object of the invention to provide a closed-loop fuel control in 
which this disadvantage is overcome. 
Broadly, the invention resides in a fuel control system for an internal 
combustion engine, including a fuel flow control sensitive to at least one 
engine operating parameter and controlling the rate at which fuel is 
introduced into the engine, an exhaust gas sensing device for producing an 
output signal corresponding to the exhaust gas composition, feedback means 
for feeding back to the fuel flow control, a signal derived from said 
output signal to correct the fuel flow, said feedback means including a 
signal storage device, the signal stored in which is altered in accordance 
with variations in the output signal of the exhaust gas sensing device, 
and overrun detection means connected to said feedback means and arranged 
to prevent alteration of said stored signal by the exhaust gas sensing 
device during overrun. 
Preferably, said overrun detection means includes delay means connected to 
extend the period during which alteration of said stored signal is 
prevented for a predetermined length of time after the overrun condition 
has ceased. 
The invention may be applied both to fuel injection systems (with either 
analog or digital electronic controls) and to carburettor systems. 
In a digital electronic control system for fuel injection, the feedback 
means may include means for varying the frequency of a clock which clocks 
a counter periodically programmed with a count corresponding to the 
required amount of fuel per stroke. In this case the clock may be a 
voltage controlled oscillator the control voltage of which is supplied by 
an electronic analog integrator (the feedback capacitor of which 
constitutes said signal storage device) receiving an input from the 
exhaust gas sensor, the overrun detection means including switch means for 
disconnecting the exhaust gas sensor from the integrator. 
In the case of an analog electronic control system for fuel injection the 
feedback control may likewise include an electronic analog integrator 
which receives its input from the exhaust gas sensor device, and the 
overrun detection means may include a switch means for disconnecting the 
integrator from this sensor device. In this case, however, the integrator 
would be arranged to control a controlled current source which discharges 
a capacitor periodically charged to a voltage corresponding to the fuel 
demand. Such a system (without the integrator in the feedback means) is 
described in our co-pending application no. 717,058, now abandoned. 
In the carburettor system feedback is obtained in varying the air pressure 
in the float chamber of the carburetor. In one possible arrangement the 
exhaust gas sensor device causes a valve to connect a plenum chamber via 
an orifice to a vacuum source when the mixture is too rich and to 
atmosphere when the mixture is weak, the plenum chamber being connected to 
the carburetor float chamber. The plenum chamber acts in this case as the 
signal storage device and, in accordance with an aspect of the present 
invention the overrun detector means is arranged to close the connection 
of the sensor controlled valve to the plenum chamber during overrun. This 
may be achieved either by adding a further shut off valve controlled by 
the sensor or by utilizing a single valve with two solenoids for moving 
its control element to extreme positions connecting the plenum chamber to 
the vacuum source and to atmosphere respectively, and an off position 
which it occupies when both solenoids are de-energized, the overrun 
detection means effecting overriding de-energization of the solenoids.

Referring firstly to FIG. 1 the system includes a known air mass flow 
measuring device 10 mounted in the air intake 11 of the engine 12. The 
device 10 includes an electrode 13 which is connected to a controlled high 
voltage source 14 and two collector electrodes 16, 17 the flow of current 
to which from the electrode 13 depends upon the air mass flow through the 
intake 11. A current differencing circuit 18 is connected to the two 
electrodes 16 and 17 and produces a voltage output dependent upon the 
difference between the current and hence upon the air mass flow. The 
voltage output from the circuit 18 is applied to a voltage controlled 
oscillator 19 the output of which is applied to the clock input of a 
count-up counter 20. The voltage controlled oscillator also applies pulses 
to a control logic circuit 21 which controls the inhibition and clearing 
of the count-up counter 20. The control logic circuit 21 also has an input 
connection from a distribution/timing device 22 on the engine 12, The 
circuit 21 utilizes the first three pulses from the oscillator 19 
following each pulse from the device 22 to produce output pulses at 
terminals A, B and C respectively. Terminal A is connected to the INHIBIT 
terminal of the counter 20 and terminal C is connected to the RESET 
terminal of counter 20. A second counter 23 is connected as a presettable 
count-down counter so that when a pulse is received at the LOAD terminal 
of the counter 23 the count currently in the counter 20 will be 
transferred to the counter 23 in well known manner. A clock 24 is 
connected to the clock input terminal of the counter 23 so that, in each 
cycle of operation, the time taken to count out the count transferred to 
the counter 23 will depend both upon the value of the count transferred 
and upon the frequency of the clock. For the duration of this count-out 
period, in known manner, the counter 23 supplies a signal to an injector 
control circuit 25 which controls the injection of fuel into the engine, 
the amount of fuel injected in each engine cycle depending upon this 
count-out period. 
The system includes an exhaust feedback arrangement making use of an 
exhaust gas sensor 26 in the exhaust pipe 27 of the engine. This, in known 
manner, has a heater 28 and the resistance of the sensor (which is 
electrically isolated from the heater) varies in accordance with the 
concentration of the oxygen or carbon-monoxide in the exhaust gas. The 
sensor 26 is connected to a clock frequency control 27 so that if, for 
example, there is an excess of oxygen in the exhaust (indicating that the 
mixture supplied to the engine is too lean) the clock frequency will be 
decreased to increase the amount of fuel injected per cycle. Conversely if 
the oxygen content is too low the fuel supplied will be increased. 
The system further includes an overrun detector circuit 29 which has 
connections from terminals B and C the control logic circuit 21 and also 
from the outputs of the count-up counter 20. The overrun detector 29 is 
connected to the frequency control 27 as will be described in more detail 
hereinafter and also supplies the LOAD terminal of the counter 23. 
Turning now to FIG. 2 there is shown therein, in some detail, the clock and 
its frequency control circuit. The clock itself is a type 8038 integrated 
circuit having its terminal 1 connected by a capacitor C1 to an earth rail 
30 and its terminal 11 connected directly to the earth rail 30. Terminals 
7 and 8 are interconnected and terminals 4 and 5 are connected via a 
variable resistor RV1 and a resistor R1 in series to a positive supply 
line 31. Terminal 6 of the clock is connected directly to the supply line 
31. Frequency control is effected by varying the voltage at terminal 4 of 
the clock as will be hereinafter described. 
For such frequency variation there are several variables including an 
engine temperature measuring thermistor 32 and an engine start-up circuit 
33 neither of which are directly pertinent to the present invention and 
which will not, therefore, be described in detail. As regards the present 
invention which is primarily concerned with the question of exhaust gas 
sensor feedback to the clock 24 thereis an n-p-n transistor T1 having its 
collector connected to terminal 4 of the clock 24 and its emitter 
connected via a resistor R2 to the rail 30. The base of the transistor T1 
is connected via a resistor R3 to the output terminal of an operational 
amplifier A1 connected as an integrator having a feedback capacitor C2. 
The non-inverting input terminal of the amplifier A1 is connected to the 
common point of two resistors R4, R5 connected between the rails 30, 31 
and the inverting input terminal of the amplifier A1 is connected via a 
relay contact RL1a to a resistor R6 the other end of which is connected to 
the common point of two bias resistors R7, R8 connected in series between 
the rails 30, 31 and also via a resistor R9 to a further relay contct RL2a 
which connects the resistor R9 to the rail 30 when closed. 
The relay contact RL1a is operated by a relay coil RL1 shown in FIG. 3, the 
relay RL1 being operated by an amplifier 33. FIG. 3 in fact, shows the 
overrun detector circuit 29 in detail and this detector circuit simply 
consists of an AND gate 34, a first flip-flop 35, a NAND gate 36 an 
inverter 37, a second flip-flop 38 a further inverter 39, a retriggerable 
monostable circuit 40 and a further NAND gate 41. Both flip-flops are of 
the type known as D-type flip-flops and the flip-flop 35 has its CLEAR 
terminal connected to an output terminal of the logic circuit 21 which is 
also connected to the RESET terminal of the counter 20. The D input 
terminal of the flip-flop 35 is permanently connected to a logical 1 and 
the output terminal of the AND gate 34 is connected to the clock terminal 
of the flip-flop 35. The AND gate 34 has three input terminals connected 
to three of the output terminals of the counter 20. 
The Q output terminal of the flip-flop 35 is connected to one input 
terminal of the NAND gate 36 the other input terminals of which are 
connected via the inverter 37 to an output terminal B of the logic circuit 
21. This B terminal is also connected to the clock terminal of the 
flip-flop 38 and the Q terminal of the flip-flop 35 is connected to the D 
input terminal of the flip-flop 38. The NAND gate 36 has its output 
terminal connected to the LOAD terminal of the counter 23. The Q terminal 
of the flip-flop 38 is connected via the inverter 39 to the `B` input 
terminal of the circuit 40 which has external timing components 42, 43 
setting its output pulse length to about 2 seconds. The gate 41 has one 
input connected to the Q output of the circuit 40 and its other input 
terminals connected to the output terminal of the inverter 39. The output 
of gate 41 is connected to the relay amplifier 33 so as to open contact 
RL1a during overrun and for 2 seconds thereafter. 
The circuit shown in FIG. 3 detects overrun by determining whether the 
count reached by the count-up counter 20 has attained a certain minimum 
value. In non-overrun conditions this overrun count is always exceeded and 
the output from the AND gate 34 goes positive which clocks the flip-flop 
35 and provides a logic 1 on its output Q. This enables the load pulse B 
from the control logic 21 to be passed forward to the count-down counter 
23 and injection pulses are obtained normally. The flip-flop 35 is cleared 
before each count-up by pulse C. The clear input puts a logic 0 on the 
output Q of flip-flop 35. Flip-flop 38 transfers to the output Q the 
complement of output Q of flip-flop 35 when clocked by load pulse B. Hence 
flip-flop 38 provides a constant output level on output Q of logic 1 
during overrun conditions and logic 0 during non-overrun conditions. The 
output Q of the flip-flop 38 being at a logic 0 at this stage, keeps the 
relay contact RL1a closed in the inverter 39 and gate 41 during 
non-overrun conditions, When overrun occurs the count required to clock 
the flip-flop 35 does not occur and the output Q remains at logic 0. This 
inhibits the load pulse B and no injection pulses are obtained. The fuel 
is cut off during overrun. The output Q of the flip-flop 38 now goes to 
logic 1 causing the output of the gate 41 to go to logic 1 and thereby 
opening the contact RL1a. At the end of the overrun condition the Q output 
of the flip-flop 38 goes to logic 0 again, but this transition sets the 
circuit 40 so that its Q output goes to 0 for the 2 second interval 
mentioned. This maintains the exhaust loop inhibition for an extra two 
seconds after overrun, ensuring that transient conditions set up during 
overrun have disappeared before exhaust feedback is reestablishes. It will 
be understood that the delay will ensure that the exhaust gas composition 
has had time to reach a steady value during this delay, allowing for the 
time taken for the exhaust gases generated during overrun to be swept away 
from the sensor. 
Turning now to FIG. 4 the exhaust gas sensor circuit will be seen to 
include three biasing resistors R10, R11 and R12 connected in series 
between the rails 30 and 31 and the sensor itself is connected in series 
with another resistor R13 across the resistor R11. The common point of the 
resistor R13 and the sensor 26 is connected to the invert input terminal 
of an operational amplifier A2 connected as a comparator with a feedback 
resistor R14 from its output terminal to its non-inverting input terminal. 
The non-inverting input terminal is also connected to the common point of 
two resistors R15 and R16 connected in series between the rails 30 and 31. 
The output of the amplifier A2 is connected via resistor R18 to the relay 
RL2 which controls the contact RL2a so as to close the contact RL2a 
whenever the mixture is lean. 
Turning now to FIG. 5 there is shown a system in which the engine uses a 
conventional carburetor 100 through which air enters the air intake 101 of 
the engine 102. Closed-loop control is obtained by utilising a sensor 103 
in the exhaust pipe of the engine and this sensor is, as in FIG. 1, a 
known element incorporating a heater 104. A circuit identical to that 
shown in FIG. 4, with the sensor 103 substituted for the sensor 26 therein 
constitutes an air fuel ratio control 105 and the relay RL2 is used to 
control the solenoid 106 of a valve 107. In one position of the valve 107 
a plenum chamber 108 is connected via a restrictor 109 to atmosphere. In 
the other position of the valve 107 the plenum chamber is connected via 
the restrictor 109 to a source of constant vacuum provided by a regulator 
110 connected to the engine air intake manifold 101. The plenum chamber 
108 is connected to the float chamber of the carburettor so that the fuel 
flow from the carburetor is modified in accordance with the pressure in 
the plenum chamber, which itself varies in accordance with the output of 
the sensor 103. In fact the valve 107, the restrictor 109 and the plenum 
chamber 108, effectively form in combination an integrator with the plenum 
chamber 108 forming the equivalent of a capacitor in an electronic analog 
integrator. 
The feedback loop established via the valve 107 is interrupted during 
overrun conditions by means of a pressure switch 112 which senses the air 
pressure in the manifold 111. In overrun conditions this pressure becomes 
very low and the pressure switch 112 closes and, via a monostable circuit 
115 energises a solenoid 113 operating a shut off valve 114 between the 
restrictor 109 and the plenum chamber 108. Thus, in overrun conditions, 
the pressure in the plenum chamber 108 remains substantially constant, 
irrespective of the output of the sensor 103, during overrun and for a 
fixed delay (set by the monostable circuit 115) after the overrun 
condition has ceased. 
The system shown in FIG. 6 is similar in undelying principle to that shown 
in FIG. 1 except that it makes use of electronic analogue techniques 
instead of digital techniques. A similar system, but lacking the overrun 
exhaust feedback interruption and signal storage concept employed herein 
is described in copending application Serial No. 717,058. 
The engine 202 incorporates a mass flow sensor 200 in its air intake 201 
exactly the same as that employed in FIG. 1. The output voltage signal 
therefrom is, however, fed to an analogue integrator 220 with a capacitor 
220 a and a switch 220b for periodically resetting the capacitor. A 
further switch 221 connects the output of the integrator to a signal 
storage capacitor 222. The switch 221 is operated periodically 
(immediately before resetting of the capacitor 220a) to permit up-dating 
of the signal stored on capacitor 222. The signal on capacitor 222 is 
applied to a bank of voltage comparators 223 which produce output signals 
at terminal a during high engine load conditions, at terminal b in idling 
conditions and at terminal c in overrun conditions. 
Two further switches 224 and 225 which are operated alternately in 
synchronism with the operation of the switch 221, serve to transfer the 
integrator output signal to two capacitors 226 and 227 respectively. Two 
comparators 228 and 229 serve the voltages on the respective capacitor 226 
and 227 and their outputs control two sets of injectors via two power 
amplifiers 230 and 231. 
Discharge of the capacitors 226 and 227 is controlled by a controlled 
current source 234 operation of which is fully explained in application 
Serial No. 717,058. Normally, provided there is no output at any of the 
terminals a, b and c and the engine is warm and running normally, the 
source 234 is controlled by an exhaust feedback control 233. In high 
engine load, idling, start or warmup conditions, however, exhaust feedback 
control is inhibited and the signals from terminals a or b, or from a cold 
start circuit 232 are used to control the source 234. 
The exhaust feedback control 233 consists of the sensor circuit of FIG. 4 
together with the components R.sub.3 to R.sub.9, C.sub.2 and A.sub.1 of 
FIG. 2, the output voltage of the amplifier A.sub.1 providing the input to 
the source 234. The relay which controls the contacts RL2a of FIG. 2 is a 
relay 235 connected via a monostable circuit 236 to the c output terminal 
of the comparator bank 223. As in the previous examples the circuit 236 
acts to provide a delay in re-establishing the exhaust feedback loop after 
overrun has ceased. The output c is also connected to disable the 
comparators 228 and 229. 
It will be appreciated that the combination of the capacitor 226, current 
cource 234 and comparator 228 is functionally equivalent to the 
combination of the counter 23, and the clock 24 of FIG. 1. 
It will be seen that in all the examples described above the overrun 
detector effectively interrupts the feedback loop and causes a signal 
storage device in the feedback loop to hold a feedback signal 
corresponding to that which existed at the instant when overrun commenced. 
In this way sudden over-fuelling commencing when the overrun condition is 
terminated is avoided.