FUEL CONTROL SYSTEM FOR AN INTERNAL COMBUSTION ENGINE

Abstract: An i.c. engine fuel system includes a main fuel control (19) which controls fuel flow to the engine in accordance with one or more engine parameters. A roughness sensor circuit (11) produces an output related to the roughness of running of the engine and the error between this roughness output and a reference signal is integrated with respect to time by an electronic integrator 15. The output of this integrator (15) is used to modify the fuel flow to reduce the error to zero, by, for example, varying the frequency of a clock (18) which influences the operation of the main fuel control (19).

Description: This invention relates to fuel control- system for an internal combustion engine. It- has been observed that when the fuel-áir mixture supplied to an internal combustion engine is. too lean- the running of the engine becomes rough, that is to say the instantaneous speed of the engine fluctuates rapidly within an engine cycle. Such rough running has been found to be associated with undesirable increases in noxious exhaust emissions. Measurement of engine roughness has already beenproposed and it is possible to derive an electrical signal representative of the magnitude of the speed fluctuations in relation to the average engine'speed,- The application of a roughness sensor to an engine fuel control system is, however, problematic because of, among other difficulties, the lag which occurs between a change in the fuel supply rate and a resultant change in the engine roughness It is thus an object of the invention to provide an internal combustion engine fuel control system including an engine roughness control0 An internal combustion engine. fuel. control system in accordance with the invention includes a roughness sensor circuit providing an electrical roughness signaldetermined by the magnitude of fluctuations in the speed of the engine, an electronic integrating circuit connected t.o-said sensor circuit and providing at its output an electrical output signal representative of the tbe intealof fflhe error between 'the roughness signal and- a reference signal, and fuel flow control means for controlling the fuel flow to the engine in accordance with said output signal such that said error is reduced to zero. The time constants andgain of the integrating circuit are arranged so that changes in said output signal occur relatively slowly. The invention also provides a method of eontrelling an internal combustion engine comprising measuring the roughness of the engine running so as to derive an electri- cal signal dependent on the magnitude of .fluctuations in the. engine speed, deriving a further electrical signal depend ent on the time integral of the error between said first mentioned signal and a reference signal, and using said further electrical signal to modify the rate of fuel flow to the engine, so as to tend to reduce said error to sero In the accompanying drawings: Figure 1 is a block diagram of an example of a fuel control circuit in accordance with the invention; Figure 2 is an electric circuit diagram of a rough ness sensor circuit forming a part of the circuit shown in Figure 1; Figures 3 and 4 are circuit diagrams of a further part of the circuit of Figure 1; and Figure 5 is a graph showing wave forms at. a series of positions in Figures 2,r3'and 4e Referring firstly to Figure 1, the circuit shown includes an'engine crankshaft position transducer circuit 10 which produces output pulses at fixed positions of the engine crankshafto By way of example a transducer which produces one pulse for every 180 of rotation of the engine shaft may be used. This transducer provides input pulses- to a roughness sensor circuit 11 which produces an output pulse whenever the length of time interval between two transducer pulses exceeds the time interval between the preceding transducer pulse and the first of two pulses. The duration of this- output pulse is slightly less than the¯difference between these two time intervals as will be explained hererns after. This duration is dependent both on the difference in shaft speed in the two 1800 arcs involved, but is also dependent on the average speed. A normalising circuit 12 is provided to process the pulse from the roughness sensor circuit and produce an output signal dependent on the ratio of the change in speed to the average speed and this normalising circuit has an input from a speed signal generating circuit 13 which produces an output related to engine speed by processing the pulses from the transducer circuit lOo The output from the normalising circuit is fed to a sample and hold -circuit 14 up dated- periodically by pulses from the transducer circuit 10. As will be explained in greater detail hereinafter, the transducer circuit 10 in fact has several difference outputs which are variously used by the roughness sensor circuit ll, the normalising circuit 12, the speed signal generating circuit 13 and the sample and hold circuit 14. The output of the sample and hold circuit 14 is connected to one input of an integrating. circuit 15 via an electronic switch 16. A speed-shaping circuit 17 which receives its input from circuit 13 provides an input to a reference terminal of the integrator¯so that the integrator normally produces an output signal dependent on the integral of the error between the roughness signal from the sample and hold circuit 14 and the speed dependent reference signal from the speed shaping circuit 17. The output of the integrator 15 is used to vary the frequency of a clock circuit 8 which provides a clock input to a main fuel control circuit 195 which controls the flow of fuel to the vehicle engine. The circuit 19 which is of known construction. operates by periodically generating a multi-bit digital signal as- a function of input signals it receives for example from a throttle pedal transducer 20 and the position transducer circuit 10,- which-multi-bit digital signal represents the scneduled quantity of fuel required by the engine for that throttle/speed co-mbination. This- multi-bit signal is used to determine the quantity of fuel supplied to the engine by energising a fuel injection valve four the period of time required for the clock circuit 18 to produce the number of pulses represented by the multibit digital signal. When the output of the integrator 15 is at an upper reference level the frequency of the clock is such that the quantity of fuel injected is appromimately sufficient to provide a stoichiometric air/fuel ratio. The circuit is such however, that the output of the integrator is normally lower than this upper reference level and this has the effect of increasing the clock frequency and thereby reducing the quantity of fuel injected. The output of the integrator 15 is, in fact, not per mitted to rise above the reference level referred to, an active clamp circuit including a schmitt trigger circuit 21 and a diode 22 being provided for this purpose. When the output of the integrator 15 is at a lower reference level the frequency of the clock 18 is such that the quantity of fuel injected corresponds to the leanest air/ fuel ratio which is acceptable taking into consideration fac- tors such as vehicle drivability fuel consumption and noxious exhaust emissions. The output of integrator 15 is prevented from falling below this lower reference level by a second active clamp circuit including a schmitt trigger circuit 31, a resistor 32 and a diode 33. In order to prevent the fuel flow to the engine being affected by the roughness outputs which is produced during normal acceleration and deceleration of the engine resulting from movement of the throttle pedal by the driver of a vehicle in which the circuit is installed, two differentiating circuits 24, 25, are connected to the throttle pedal transducer 20 and the output. of the speed signal generator 13 respectively. The outputs of these two'differentiating circuits 24, 25 are connected to a humming amplifier 26, the output of which is connected to the inputs of an acceleration sensing circuit 27 and a deceleration sensing circuit 28. The outputs of these two circuits are connected to an OR gate 29 which controls the electronic switch 16 and the output of the circuit 27 is also connected to control a further electronic switch 30 which is in parallel with the diode 22. The effect of these circuits is that during acceleration, the output of the integrator 15 is driven rapidly to the reference level irrespective of the output of the roughness sensing circuit and in deceleration, the output of the integrator is held constant at the level it was at when the deceleration commenced. Turning now to Figure 2, the speed transducer circuit 10 includes an actual transducer 101 which produces a positive going pulse (graph A in Figure 5) at 1800 degree intervals of crankshaft rotation. The transducer 101 is connected to a first monostable circuit 102 which is triggered by the rising edge of each output pulse from the transducer 101 and produces at its Q output a positive going pulse of fixed duration (graph B in Figure 5) and a corresponding negative going pulse at its Q output. A second monostable circuit 103 is connected to be triggered by the rising edge of this negative going pulse and produces at its Q output a fixed length pulse immediately following each pulse at the Q output of the first monostable circuit 102, (graph C in Figure 5). The roughness sensing circuit utilises the output (A) of the transducer 101 which is used to trigger an input flipflop circuit 110, the wave form at the Q output of which is shown in graph D of Figure 5. The Q and Q outputs of the flip-flop circuit 110 are connected to the UP/DOWN terminals of two 12-bit-counters (each consisting of three 4516 type CMOS integrated circuits in cascade) llla,lllb. Each counts lily, lllb has its PRESET ENABLE terminal connected by a capacitor 112a, 112b, to its UP/DOWM terminal and by a resis- tor l13a, 113b to a ground rail. The CLOCK terminals of both counters are connected to a 4.5 A2s oscillator 114. Graphs E and F show the respective count states of the counters llla lllbe The data input terminals of the counters are connected to provide a small initial count in each counter when it starts counting up, so that no carry out signal is produced by the counter if the counter counts up and then down for exactly equal periods. A carry out signal is only produced if the count down period exceeds the count up period which, as shown in Figure 4 occurs in the case of counter Illb at the two points marked "X". The CARRY OUT terminal of each counter is connected to a NMI) gate 115a, 115b connected to act as a logical inver ter, and the output of that NAND gate is connected to one input of a further NAND gate 116a 116b which has its other input connected to the oscillator 114. The output,terminal of the NAND gate 116a. 116b is connected to one input ter minal of a toggle circuit constituted by two cross connected NAND gates 117a, 117b and 118a, 118b the other input of which is connected to the one Q or Q outputs of flip-flop circuits 110, which is connected to the other counter llla or 111b. The outputs of NAND gates, 118a, 118b are connected to an AND gate 119 the output of which is shown in graph G of Figure Se As will be appreciated, each toggle circuit is only set when the associated counter produces a carry-out signal dur ing count down. This toggle circuit is subsequently reset by the ne t transducer pulse, so that the duration of the nega- tive going output of AND gate 119 is the difference between the time period between first and second transducer pulse and the time period between the preceding transducer pulse and the first transducer pulse, less whatever small error is intro ducad by the small preset count introduced into the counters 111a, 111b. The speed signal generating circuit 13 is basically a frequency to voltage converter operated by the Q output of monostable circuit 102. The circuit 13 includes an input pnp transistor 130 having its base connected by a resistor 131 to the Q output of monostable circuit 102 and its emitter connected to a +lOv rail. An npn transistor 132 has its base connected to the junction of two resistors 133y 134 which are in series between the +lOv rail and a ground rail, its collector connected to the +10v rail and its emitter connected via a resistor 135 to the ground rail0 The collec tor of transistor 130 is connected to the emitter of trans istor 132. An output pnp transistor 136 has its base connec- ted to the emitter of transistor 132, its emitter connected to the +10v rail by a resistor 137 and its collector connec- ted to the ground rail by a resistor 138 and a capacitor 139 in parallel. When the transistor 130 is off, which occurs for the duration of the Q output pulse of monostable circuit 102, the emitter of transistor 132 is held at a fixed voltage so that a fixed current flows into resistor 138 and capacitor 139. When the transistor 130 is on, which is for the remaining time period, transistor 136 is held off and capacitor 139 discharges through resistor 138 The mean voltage at the collector of transistor 136 is directly proportion to engine speed. The normalising circuit 12 includes an operation amplifier 120 having its non-inverting input terminal connected to the collector of the transistor 136. The output terminal of this operational amplifier 120 is connected by a resistor 121 to the base of an npn transistor 122 the emitter of which is connected by a resistor 123 to the ground rail and also connected to the inverting input terminal of the operational amplifier so that the operational amplifier and transistor act as a voltage to current converter in known manner. The collector of the transistor 122 is connected by a capacitor 124 to the +5v rail so that this capacitor charges up at a rate directly proportional to the voltage at the non-inverting input of amplifier 120. An npn transistor 125 has its emitter connected to the ground rail and its collector connected to the base of trmsistor 122. The base of transistor 125 is connected by a resistor 126 to the output terminal of AND gate 119 so that transistor 125 is on and thereby holds transistor 122 off except when the output of AIND gate 119 is low. For periodically discharging the capacitor 124, there is a pnp transistor 127 which has its emitter connected to the +5v rail and its collector connected by a resistor 128 b the collector of transistor 122. A resistor 128 connects the base of transistor 127 to the Q output of the ijionostable circuit 103. Transistor 127 is conductive only while the Q output of circuit 103 is high. Each Q output from the circuit 103 discharges capacitor 124 and transistor 122 remains off until a negative going pulse is produced by the AND gate 119. Transistor 122 then turns on and capacitor 124 charges to a voltage corresponding to the product of the output of the speed sensing circuit 13 and the duration of the low output of AND gate 11) This voltage signal is held on the capacitor 124 for the dur- ation of the high output at the Q output terminal of circuit 102, which commences as transistor 122 is switched off again. The capacitor 124 is then discharged again. This voltage signal is representative of the speed-nor malised roughness. The sample and hold circuit 14 includes an input amplifier 140 which has its non-inverting input terminal connected by a resistor 141 to the collector of transistor 122. The output terminal is connected by an electronic switch element 142 (controlled by the Q output of circuit 102) and two resistors 143 144 in series to the non-inverting input of an output buffer amplifier 145, the junction of resistors 143, 144 being connected by a capacitor 146 to a +5v rail and by a resistor 147 to the inverting input terminal of amplifier 140. A resistor 148 connects the output of amplifier 145 to its inverting input. The output (shown in graph H of Figure 5) of the smpli^ fier 145 is at +5v in any period between two successive cranlcshaft transducer pulses if no roughness pulse was produced by AND gate 119 immediately before the first of those pulses. If a roughness pulse is produced the output of the amplifier 145 falls linearly with increasing normalised roughness, i.e. a short roughness pulse at a given speed causes the voltage to take up a level slightly below +5v and a longer roughness pulse at the speed causes it to take up an even lower level. Turning now to Figures 3 and 4, the output of amplifier 145 is applied via the. electronic switch element 16 to the integrator 15 which includes an operational amplifier 150 having its inverting input connected by a resistor 151 to the switch 16 and its output connected to its inverting input by a capacitor 152. The output of amplifier 150 is connected to the variable frequency clock by a resistor 153. The non-inverting input of the amplifier 150 is connected to the output of the speed-shaping circuit 17 which as shown in Figure 4, includes four operational amplifiers 170, 171 172 and 173. The amplifier 170 has its non-inverting input connected to the collector of transistor 136 and its inverting input connected to the junction of two resistors 174, 175 in series between the output terminal of the amplifier 170 and the ground rail. The other three amplifiers 171, 172, 173 are connected with various resistors and diodes as shown to operate in known manner to provide an output which is between 0 and 5v when the signal at the collector of transistor 136 is at 0 volts and which rises linearly in three segments of decreasing slope as the signal at the collector of transistor 136 rises. The output of amplifier 173 is connected to the cathode of a diode 176 the anode of which is connected to the output of the circuit 17. The output of amplifier 150 is thus normally the integral of the error between the signal at point H and at the reference signal generated by the speed shaping circuit 17. This output is shown in graph J of Figure 5 assuming the speed to be constant throughout. The throttle signal differentiating circuit 24 comprises an operational amplifier 240 with its inverting input terminal connected by a capacitor 241 to the output of the pedal transducer 20 and has a feedback circuit consisting of two resistors 242 243 in series between the output terminal of amplifier 240 and its inverting input and a capacitor 244 across one of these resistors 243 to.limit the high frequency gain of the amplifier. The differentiator 25 is similar, consisting of an operational amplifier 250 resistors 252, 253 and capacitors 251, 254 the inverting input of amplifier 250 being connected to the output of amplifier 170 of the speed shaping circuit 17. The non-inverting inputs of the amplifiers 240, 250 are connected by respective resistors 245 255, to the junction of two resistors 260, 261 which are shown in Figure 3 as part of the summing amplifier 25 Summing amplifier 26 includes an operational amplifier 262 which has its non-inverting input connected to the junction of resistors 260, 261 which are in series between the +lOv rail and the ground rail. The outputs of the amplifies 240, 250 are connected by respective resistors 253, 254 to the inverting input of amplifier 262 which has a resistor 265 connected between its output and its inverting input. The acceleration and deceleration sensing circuits 27 and 28 are constituted by a pair of voltage comparators 270 and 280 which have reference voltages applied to their noninverting and inverting inputs respectively by different points on a resistor chain 271, 272, 273. The output of amplifier 262 is connected to the non-inverting input of comparator 280 and the inverting input of comparator 270. The OR gate 29 is constituted quite simply by a diode 290 which has its cathode connected to the output of comparator 280 and its anode connected by a resistor 291 to the +10v rail. The anode of diode 290 is connected to the control input of switch element 16 as is the output of comparator 280. This switch element 16 goes open circuit if the output of comparator 270 is low, or if the output of comparator 280 is low. Comparator 270 output goes low only when the accelerator pedal is actually being depressed or when the engine speed is actually increasing, and similarly the comparator 280 output goes low only when the accelerator pedal is being raised or actual deceleration of the engine is in progress. In cruising conditions both comparator out puts are high so that the switch element 16 is closed. The Schmitt trigger circuit 21 comprises a voltage comparator 211 having its inverting input connected to the +IOv rail by a resistor 212 and to the output of amplifier 150 by the resistor 153. The output of amplifier 150 is connected by a resistor 214 to the non-inverting input of comparator 211 and the doc positive feedback needed for comparator 211 to operate as a Schmitt trigger is provided by a resistor 215 connected between the output of comparator 211 and its non-inverting input. The diode 22 has its anode connected to the output of comparator 211 and its cathode connected by a resistor 221 to the inverting input of amply fier 150. A resistor 222 is connected between the anode of the diode 22 and the +10v rail0 The Schmitt trigger circuit 31 comprises a voltage comparator 311, the inverting input of which is connected to the junction of two resistors 312, 313, these resistors being connected in series between the junction of resistors 212 and 153 and the ground rail. The output of amplifier 150 is connected via a resistor 314 to the non-inverting input of comparator 311 and the d,c, positive feedback needed for comparator 311 to operate as a Schmitt trigger is provided by a resistor 315 connected between the output of comparator 311 and its non-inverting input. The output of amplifier 311 is connected to the +lOv rail through a resistor 316 and also to the cathode of diode 33, the anode of which is connected through a resistor 32 to the inverting input of amplifier 150. As explained above, the Schmitt trigger circuits 21 and 31 act to limit the range of output voltages of the amplifier 150, and consequently the range of output voltages provided to the clock 18, by each providing an active clamp. Provided the output of the integrator 15 remains below an upper reference level (set by-resistors 212, 153, 312 and 313), the output o comparator 211 remains low, diode 22 preventing it from having any effect on the integrator output. Should the output of integrator 15 happen to rise above the upper reference level the output of the comparator 211 will go high so that extra current flows into the inserting input of the amplifier 150 causing the output to ramp down until the Schmitt trigger reset threshold is reached. Likewise, provided the output of tke integrator 15 remains above a lower reference level (also set by resistors 212, 153, 312 and 313), the output of comparator 311 remai: :ns high, diode 33 preventing it from having any effect on the integrator output. -Should the output of integrator 15 happen to fall below the lower reference level the output of comparator 311 will go low causing the output of the amplifier 150 to ramp up until the Schmitt trigger threshold is reached. Resistors 221 and 32 are an order of magnitude smaller than resistor 151 se that such resetting occurs rapidly, The acceleration sensing circuit 27 also includes a pnp transistor 274 which has its emitter connected to the 910v and its base connected to the junction of two resistors 27fS 276 which are in series between the output of the amplifier 270 and the +lOv rail. The collector of the transistor 274 is connected by a resistor 277 to the ground rail and is also connected to the control terminal of the electronic switch 30. Switch 30 "closes" whenever accelera- tion is demanded or is actually taking place. The control circuit described above provides for closed loop fuel control based on roughness sensing. The counter system used for generating the "raw'1 roughness pulse ensures an accurate roughness output with a reasonably high response speed. The normalising circuit employed also provides a good degree of accuracy and the inclusion of the integrating circuit ensures that stable operation is obtain ed The Schmitt trigger circuit 21 ensures that the closed loop roughness control can only reduce fuel flow below the scheduled flow for the specific throttle/speed r@lationship, so that "d gging in" caused by enrichment when the engine is already <RTI ID=14.4> running rich cannot occur and the Schmitt trigger circuit 31 ensures that the closed loop roughness control cannot reduce the fuel flew below the least acceptable airi fuel ratio. The acceleration and deceleration loop inhibit- ing controls have no effect on the roughness sensing circuit itself which continues to provide an output during deceleration (but not during acceleration, because each 1800 time interval will be shorter than the last unless the engine is running exceptionally roughly). Closed loop control is restored as soon as acceleration or deceleration ceases and in the case of deceleration the output of the integrator 15 is the same as it was before the deceleration commenced. In the case of an acceleration "closing" of electronic switch causes the output of integrator 15 to ramp up (since the output of Schmitt trigger 21 is low at this stage), until the Schmitt trigger fires. The integrator output then oscillates between the upper and lower Schmitt trigger thresholds until acceleration ceases at which time closed loop operation is re-established.

Claims: CLAIMS la An internal combustion engine fuel control system com- prising a roughness sensor circuit providing an electrical roughness signal determined by the magnitude o fluctuations in the speed of the engine, an electronic integrating cir cuit connected to said sensor circuit and providing at its output an electrical output signal representative of the time integral of the error between the roug'Iness signal and a reference signal, and fuel flow control means for controlling the fuel flow to the engine in accordance with said output signal,such that said error is reduced to zero0 2. A system as claimed in claim 1 in which said roughness sensor circuit includes a counter, a control circuit for said counter arranged to cause the counter to count up and down alternately, said control circuit being controlled by a transducer sensitive to the engine crankshaft position such that reversal of counting takes place at equi-angularly spaced positions of the engine crankshaft, and a logic circuit for detecting when the counter produces a carry-out signal during down-counting and producing an output signal conencing when such detection occurs and terminating when the counting direction next reverses, the duration of such output signal representing the roughness of running of the engine 3. A method of controlling an internal combustion engine comprising measuring the roughness of the engine running so as to derive an electrical signal dependent on the magnitude of fluctuations in the engine speed, deriving a further electrical signal dependent on the time integral of the error between said first mentioned signal and a reference signal, and using said further electrical signal to modify the rate of fuel flow to the engine, so as to tend to reduce said error to zero.

Inventors: WILLIAMS, MALCOLM, WOODHOUSE, RICHARD GRAHAM
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1980
Main classification: F02D41
IPC codes: F02D41