05/183907

05/07/1974

09/27/1971

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The Bendix Corporation (Southfield, MI)

123/493

F02D41/12; F02M51/00

123/32EA,119R

Goodridge, Laurence M.

Flint, Cort

Flagg, Gerald Benziger Robert K. A.

1. In an internal combustion engine having a signalling device for signalling an acceleration demand and a deceleration demand, and a fuel control system of the type having speed pulse generating means responsive to engine speed operative to generate a pulse train of fixed duration speed pulses having a pulse repetition rate varying directly with engine speed and an inter-pulse interval varying inversely with engine speed, computing means operative to generate an output signal indicative of engine fuel requirement, and injector valve means actuable in response to the computing means output signal to control the fuel delivered to the signal, an improved deceleration fuel control circuit comprising:

2. In an internal combustion engine having a first signalling device for signalling an acceleration demand and a deceleration demand, a second signalling device for signalling engine operation modes including a Park mode and a Neutral mode, and a fuel control system of the type having speed pulse generating means responsive to engine speed operative to generate a pulse train of fixed duration speed pulses having a pulse repetition rate varying directly with engine speed and an inter-pulse interval varying inversely with engine speed, computing means operative to generate an output signal indicative of engine fuel requirement, and injector valve means actuable in response to the computing means output signal to control the fuel delivered to the signal, an improved deceleration fuel control circuit comprising:

CROSS REFERENCE TO A RELATED APPLICATION

This is related to my co-pending commonly assigned patent application Ser. No. 193,824 "A Circuit for Controllably Driving a Schmitt Trigger in Response to Preselected Variations in an Analog Input Signal and in a Digitalized Input Signal" filed on Oct. 29, 1971 as a division hereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of electronic fuel control systems for internal combustion engines and more particularly to that portion of the above-noted field which relates to intermittent duty circuits for modifying or altering a fuel delivery pattern. In particular, the present invention relates to that portion of the above-noted field which is concerned with circuits for terminating delivery of fuel to an engine in the presence of selected deceleration conditions.

2. Description of the Prior Art

The prior art as represented by U.S. Pat. No. 3,570,460, issued Mar. 16, 1971, to Friedrich Rabus, illustrates a circuit for terminating fuel delivery to an engine upon the occurrence of two conditions; namely, closing of the air consumption controlling throttle and vehicle engine speed in excess of a first preselected value. This is achieved by strongly biasing a transistor into its nonconducting or "off" region so that a second transistor which is controlled thereby is strongly biased into the conducting or "on" condition to an extent that it may not be switched off. Switching off of this second transistor is necessary for the generation of an injection command. Once the first transistor is strongly biased off, that transistor is held off until the engine speed drops to a second predetermined value. Both speed signals are generated by integrating a pulse frequency through a resistive-capacitive network and thereafter applying the average voltage at one portion of that network to the control electrode of the first transistor. This bias must have obtained a first value in order to strongly bias the first transistor off and cross coupling with the collector of that transistor further accentuates the bias value. As the engine speed decreases, the integrated signal follows the speed decrease and the bias applied to the first transistor control electrode gradually changes to a point where the transistor is no longer strongly biased off. This then represents the second, lower, selected engine speed. The throttle position input is derived by establishing a strong "on" bias for the first transistor control electrode during open throttle position and by subsequently shorting out this bias to ground by closure of a grounded contact whenever the throttle is closed. Thus, both the high and low speed input signals and the throttle position signal are applied to the control electrode of the first transistor as noted above.

The approach of this patent contains three operational flaws which render it undesirable in use. Firstly, by generating both the high and low speed signals with the same collection of electrical elements, one cannot easily modify the high rpm point without concomitantly affecting the low rpm point. Thus, tailoring of the circuit to suit different engine applications becomes very complicated and expensive. Secondly, while the integrating technique for the high rpm point produces reasonably accurate results, use of the integrating technique to establish the low rpm point requires that the circuit be adjusted for a relatively high value of rpm, since the integrating circuit will have a slow response and the voltage signal will lag somewhat behind the actual engine speed. Since the low rpm point at which fuel delivery is restarted must be sufficiently high so that the engine will not stall out, the established low rpm speed must be somewhat higher than the desired speed. Thirdly, the technique which requires the turning off of electronic elements, while theoretically equivalent to the turning on of electronic elements, presents some problems in the practical aspects in that the interrelation of the various elements may not present a level of voltage which is sufficiently low to accomplish the desired turn-off effect, while a technique which contemplates the turning on of the electronic devices is, in a practical sense, easier to accomplish. In view of the foregoing problems discussed in relation to the above-noted United States Letters Patent, it is an object of the present invention to provide a circuit for generating a fuel cutoff signal during selected periods of engine deceleration which is readily tailorable for different engine installations. It is a further object of the present invention to provide such a circuit having a fast response time at low engine rpm operation. It is a still further object of the present invention to provide a circuit of the above-noted type in which the low speed sensing is accomplished substantially within one cycle of operation. It is a still further object of the present invention to provide a system of the above-noted type in which speed sensing is accomplished on a pulse-to-pulse basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an electronic fuel control system adapted to a reciprocating piston internal combustion engine.

FIG. 2 shows, in diagrammatic circuit form, an electronic fuel control system main computing means including a variable frequency, fixed duration pulse generator for use in the present invention.

FIG. 3 shows, in diagrammatic circuit form, the fuel cutoff circuit of the present invention responsive to engine deceleration commands over certain engine speed ranges.

FIG. 4 shows an alternative input portion for the circuit of FIG. 3 for use when the source of input signal is of comparatively low energy or voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, an electronic fuel control system is shown in schematic form. The system is comprised of an electronic control unit, or computing means 10, a manifold pressure sensor 12, a temperature sensor 14, an input timing means 16, and an additional sensor, such as an air temperature sensor, denoted as 18. Temperature sensor 14 may sense engine temperature directly or indirectly, or may also sense injection stream temperature directly or indirectly. The manifold pressure sensor 12 and additional sensor 18 are mounted on throttle body 20. The output of the computing means 10 is coupled to an electromagnetic injector valve member 22 mounted in intake manifold 24 and arranged to provide fuel from tank 26 via pumping means 28 and suitable fuel conduits 30 for delivery to a combustion cylinder 32 of an internal combustion engine otherwise not shown. While the injector valve member 22 is illustrated as delivering a spray of fuel toward an open intake valve 34, it will be understood that this representation is merely illustrative and that other delivery arrangements are known and utilized. Furthermore, it is well known in the art of electronic fuel control systems that computing means 10 may control an injector valve means comprised of one or more injector valve members 22 arranged to be actuated singly or in groups of varying numbers in a sequential fashion as well as simultaneously. The computing means is shown here as energized by battery 36 which could be a vehicle battery or a separate battery. Throttle switch 38 is illustrated as also coupled to throttle body 20 and by line 40 to the electronic control unit 10. The purpose of switch 38 will be discussed in further detail hereinbelow. Companion application docket number MOC 70/39 described a preferred form of switch 38.

Referring now to FIGS. 1 and 2 and particularly to FIG. 2, an electronic fuel control system main computation circuit 110 is shown. The circuit is shown as being energized by a voltage supply designated as B+ at the various locations noted. In the application of this system to an automative engine fuel control system, the voltage supply B+ could be the battery 36 and/or battery charging system conventionally used as the vehicle's electric power source. The man skilled in the art will recognize that the electrical polarity of the voltage supply could be readily reversed.

The circuit 110 receives, along with the voltage supply, various sensory inputs, in the form of voltage signals in this instance, indicative of various operating parameters of the associated engine. Intake manifold pressure sensor 12 supplies a voltage indicative of manifold pressure, temperature sensor 14 is operative to vary the voltage across the parallel resistance associated therewith to provide a voltage signal indicative of engine temperature and voltage signals indicative of engine speed are received from input timing means 16 at circuit input port 116. This signal may be derived from any source indicative of engine crank angle, but is preferably from the engine's ignition distributor (not shown).

The circuit 110 is operative to provide two consecutive pulses, of variable duration, through sequential networks to circuit location 118 to thereby control the "on" time of transistor 120. The first pulse is provided via resistor 122 from that portion of circuit 110 having inputs indicative of engine crank angle and intake manifold pressure. The termination of this pulse initiates a second pulse which is provided via resistor 124 from that portion of the circuit 110 having an input from the temperature sensor 14. These pulses, received sequentially at circuit location 118, serve to turn transistor 120 "on" (that is, transistor 120 is triggered into the conduction state) and a relatively low voltage signal is present at circuit output port 126. This port may be connected, through suitable inverters and/or amplifiers to the injector valve means (shown in FIG. 1) such that the selected injector valve means are energized whenever the transistor 120 is "on" and the low level signal appears at output port 126. It is the current practice to use switching means to control which of the injector valve means are coupled to circuit port 126 when the system is used for actuation of less than all injector valve means at any one time. Because the injector valve means are relatively slow acting, compared with the speed of electronic devices, the successive pulses at circuit point 118 will result in the injector valve means remaining open until after the termination of the second pulse.

The duration of the first pulse is controlled by the monostable multivibrator network associated with transistors 128 and 130. The presence of a pulse received via input port 116 will trigger the multivibrator into its unstable state with transistor 128 in the conducting state and transistor 130 blocked (or in the nonconducting state). The period of time during which transistor 128 is conducting will be controlled by the voltage signal from manifold pressure sensor 12. Conduction of transistor 128 will cause the collector 128c thereof to assume a relatively low voltage close to ground or common voltage. This low voltage will cause the base 134b of transistor 134 to assume a low voltage below that required for transistor 134 to be triggered into the conduction state, thus causing transistor 134 to be turned "off." The voltage at the collector 134c will, therefore, rise toward the B+ value and will be communicated via resistor 122 to circuit location 118 where it will trigger transistor 120 into the "on" or conduction state thus imposing a relatively low voltage signal at circuit port 126. As hereinbefore stated, the presence of a low voltage signal at circuit port 126 will cause the selected injector valve means to open. When the voltage signal from the manifold pressure sensor 12 has decayed to the value necessary for the multivibrator to relax or return to its stable condition, transistor 130 will be triggered "on" and transistor 128 will be turned "off." This will, in turn, cause transistor 134 to turn "on," transistor 120 to turn "off" and thereby remove the injector control signal from circuit port 126.

During the period of time that transistor 134 has been held in the nonconducting, or "off" state, the relatively high voltage at collector 134c has been applied to the base of transistor 136, triggering the transistor 136 "on." The resistor network 138, connected to the voltage supply, acts with transistor 136 as a current source and current flows through the conducting transistor 136 and begins to charge capacitor 140. Simultaneously, transistor 142 has been biased "on" and, with resistor network 144, constitutes a second current source. Currents from both sources flow into the base of transistor 146 thereby holding this transistor "on" which results in a low voltage at the collector 146c. This low voltage is communicated to the base of transistor 120 via resistor 124.

When transistor 128 turns "off" signalling termination of the first pulse transistor 134 turns "on" and the potential at the collector 134c falls to a low value. The current from the current source comprised of transistor 136 and resistor network 138 now flows through the base of transistor 136 and the capacitor 140 ceases to charge. The capacitor will then have been charged, with the polarity shown in FIG. 2, to a value representative of the duration of the first pulse. However, at the end of the first pulse when transistor 134 is turned "on" the collector-base junction of transistor 136 is forward biased, thus making the positive side of capacitor 140 only slightly positive with respect to ground as a result of being separated from ground by only a few pn junctions. This will impose a negative voltage on circuit location 148 which will reverse bias diode 150 and transistor 146 will be turned "off." This will initiate a high voltage signal from the collector of transistor 146 to circuit location 118 via resistor 124 which signal will re-trigger transistor 120 "on" and a second injector means control pulse will appear at circuit port 126. The time duration between the first and second pulses wll be sufficiently short so that the injector means will not respond to the brief lack of signal.

The duration of the second pulse will be a function of the time required for circuit location 148 to become sufficiently positive for diode 150 to be forward biased. This in turn is a function of the charge on capacitor 140 and the magnitude of the charging current supplied by the current source comprised of transistor 142 and resistor network 144. The charge on capacitor 140 is, of course, a function of the duration of the first pulse. However, the rate of charge (i.e., magnitude of the charging current) is a function of the base voltage at transistor 142. This value is controlled by the voltage divider networks 152 and 154 with the effect of network 154 being variably controlled by the engine temperature sensor 14.

It has been determined that the amount of fuel injected and hence the pulse width of the injection pulses must vary for varying engine rpm values under constant load conditions. With reference to FIG. 2 the rpm correction is achieved by the circuitry enclosed by dashed line 200. Circuit 200 is operative to control the voltage applied to the secondary coil 12s of pressure sensor 12. The voltage applied to the secondary coil 12s is comprised of two components. The first component is provided by the voltage divider network comprised of parallel resistances 201 and 202 coupled between B+ and the secondary coil 12s and diode 203 and resistance 204 going to ground. The second component, which is the variable component, is established by the additional resistances 205, 206 whose effect on the voltage at the secondary coil 12s is controlled by the conductivity of transistor 207. Transistor 207 is controlled in turn by circuitry which is responsive to the frequency of actuation of the monostable multivibrator (which is comprised of transistors 128 and 130) and thus engine rpm. This control is achieved as follows:

Transistor 208 is normally conducting due to the voltage applied to its base by way of resistance 209, diode 210, and resistance 211 interconnecting B+ to ground. The current flowing therethrough will also have the effect of charging capacitor 212 up to a voltage value intermediate B+ and ground by way of resistance 209 and resistance 213 which is normally receiving a ground signal at collector 130c of transistor 130. However, whenever transistor 130 ceases to conduct, as when a trigger pulse is received at input port 116, the voltage at collector 130c will immediately go toward B+ and the charge across capacitor 212 will adjust. When transistor 130 returns to the conductive state, the voltage at collector 130c will go to ground and the voltage applied to the anode of diode 210 will go immediately to a value more negative than the ground due to the capacitor action of capacitor 212 and diode 210 will be reverse biased. This will, in turn, cause transistor 208 to be triggered off and the voltage appearing at the collector of transistor 208 and terminal 214 will rise toward a B+ value. Alternatively, terminal 214 could be located at the collector of transistor 216 if the values of the elements associated therewith would permit the generation of fixed duration pulses. The voltage across capacitor 212 will immediately begin to readjust and the anode of diode 210 will eventually become forward biased. By properly selecting the resistance and capacitive values, the time period during which transistor 208 is off may be established at a fixed value due to the RC time constant of this network. For purposes of providing battery correction by way of lead 156, this time period is normally selected to be 1 millisecond. As the provision of battery correction, as well as the remaining portion of circuit 200, forms no part of the present invention, its mechanism will not be herein discussed. As soon as diode 210 becomes forward biased, transistor 208 will again turn "on" and the voltage at the collector of transistor 208 will go substantially to the ground potential. Thus, a pulse train of fixed width pulses will appear at circuit port 214 with a repetition rate directly indicative of engine speed. The interpulse interval will also be directly related to engine speed.

Referring now to FIG. 3 of the drawing, a circuit 300 according to the present invention and illustrative of a preferred embodiment thereof is shown. The circuit 300 is energized by B+ as noted and this may readily be the same source of energization as is illustrated and discussed with regard to FIG. 2. Circuit 300 has an output port 302 and three input ports denoted as 214, 304, and 306. Input port 214 corresponds to the similarly designated port illustrated in FIG. 2 and discussed in relation thereto. In this embodiment, circuit port 214 is communicated to base or first control electrode 308b of transistor 308 by resistive circuit means which include resistances 310, 312, and 314, diode 316, and integrating capacitor 318. The base 308b of transistor 308 is communicated to a source of voltage by resistance 320 and is also communicated to ground by resistance 322 and diode 324. In this embodiment, the resistance 320 is connected to a source of regulated voltage illustrated as zener diode 326 which is operative to establish a regulated voltage within common conductor 328 which communicates the cathode of the zener diode 326 to resistance 330 which in turn is connected to the supply B+ as shown. The collector of transistor 308 is coupled to the base 310b of transistor 310 through resistance 332 and the emit emitters of transistors 308 and 310 are coupled together and coupled to ground by resistance 334. The collector of transistor 308 is connected to the common connector 328 by resistance 336 and the collector of transistor 310 is connected to the common conductor 328 by resistance 338. The collector of transistor 310 is also coupled to the base 340b of transistor 340 through resistance 341. Transistors 308 and 310, together with their respective load and limit resistances form a Schmitt trigger and the resistive values are such that the Schmitt trigger will normally be biased with transistor 308 in the "off" or nonconducting mode and transistor 310 in the "on" or conducting mode. The emitter of transistor 340 is connected by resistance 342 to the common conductor 328 and by resistance 344 to the common or ground location. Resistances 342, 344 form a voltage divider network to establish a bias voltage at the emitter of transistor 340. The collector of transistor 340 is directly coupled to the base 346b of transistor 346 and to the source of energization, B+, by resistance 348. While all other transistors in circuit 300 are illustrated as npn transistors, transistor 346 is shown as a pnp transistor with its emitter electrode connected to the source of energization, B+, and its collector electrode connected to the output port 302 of the circuit 300. It should be noted, however, that transistor types herein are merely a matter of designer's choice.

Base or second control electrode 310b of transistor 310 is connected by way of resistance 350 and diode 352 to output port 306 which is also coupled to ground by diode 354. Diode 352 is connected so that its cathode is coupled to the output port 306 while its anoe, while coupled to resistance 350, is coupled to the B+ supply by way of resistance 356. Thus, in the absence of a low voltage signal at output port 306 which would forward bias diode 352, resistances 336, 332, 350, and 356 form a voltage divider network operative to establish the voltage at the base of transistor 310 at some value intermediate the B+ supply voltage and the regulated voltage existing in common conductor 328. By suitably arranging the resistive values, the Schmitt trigger can be so biased that transistor 310 is normally in conduction as hereinbefore stated. Circuit output port 306 may be coupled to throttle switch 38 (as shown in FIG. 1). The circuit as hereinabove described is adapted to operate when actuation of switch 38 applies a ground or common low voltage signal to a circuit port 306 to forward bias diode 352. Diode 354 as illustrated is operative to provide contact arcing protection.

Diode 358 interconnects circuit input port 304 with the base of transistor 340 and is arranged to have its cathode connected directly to input port 304. The cathode of diode 358 is also connected to the source of energy, B+, by resistance 360. Diode 362 interconnects input port 304 with ground and is connected relative to diode 358 in a cathode-to-cathode relationship. The presence of a very low voltage signal at circuit input port 304 is operative to provide a very low resistance path to ground from the base 340b of transistor 340 so as to prevent the turn-on of that transistor under any and all operating conditions. For instance, by coupling input port 304 to the vehicle transmission so that a ground signal appears whenever the transmission is in "park" or "neutral," transistor 340 may be forced off and the circuit may be inhibited. Resistance 360 provides noise protection for this portion of circuit 300.

Input port 214 is also coupled to the base 362b of transistor 362 by resistance 364. The collector of transistor 362 is coupled to the common conductor 328 by resistance 366 and interval determining means in the form of capacitor 368 are connected from the collector of transistor 362 to the common or ground point. The emitter of transistor 362 is also connected to ground so that interval determining means 368 are connected across the emitter and collector of transistor 362. The collector of transistor 362 is also connected to the anode of a bistable switch 370 having a control electrode 372. The cathode of the switch is connected to the base 310b of transistor 310. The control electrode 372 is coupled to the common or ground potential by a resistance 374 and by diode 376 to a voltage divider comprised of resistances 378 and 380. Resistances 378 and 380 interconnect the common conductor 328 with the common or ground point and operative to apply a predetermined level of voltage to the gate or control electrode 372 while resistance 374 is operative to provide a current flow path to ground for current flow from the control electrode 372. Bistable switch 370 is herein illustrated as a programmable unijunction transistor (PUT). Such a device is operative to switch from a nonconducting to a conducting state whenever the voltage applied to the anode thereof exceeds the voltage applied to the control electrode by the voltage drop across one pn junction (typically, 7/10 of a volt). Once conducting, such devices will remain conducting for any value of voltage applied to the gate, or anode, until the current flow through the device has dropped to a very low level. Bistable switching device 370 may also be a silicon controlled rectifier (SCR) and such devices are known to operate in substantially the same fashion as a PUT in this type of embodiment. Furthermore, other devices of this general character may be utilized for bistable switching device 370.

OPERATION

Upon the application of an energizing supply, B+, as for instance by way of an automotive ignition switch, regulating zener diode 326 will operate to establish a regulated voltage in common conductor 328. Since this voltage will be lower than the B+ voltage, a current will flow through resistor 356, resistance 350, resistance 332, and resistance 336 establishing a voltage at base 310b of transistor 310 which is intermediate the regulated voltage and the B+ supply voltage. By suitable sizing the resistors as hereinbefore stated, this voltage, relative to the voltage established at base 308b of transistor 308, can be sufficient to cause transistor 310 to go into conduction thereby holding transistor 308 off. By suitably arranging the resistances 338 and 334, the voltage at the emitters of transistors 308 and 310 can be established intermediate the regulated voltage and the ground level, and by suitably arranging the resistances 320 and 322 the voltage present at the base 308b will be less than the voltage at the emitters. This will then reverse bias the emitter base junction of transistor 308 and this transistor will be held off. Assuming normal vehicle operation over the entire speed range of which the engine is capable, and further assuming that input ports 304 and 306 do not receive a grounded signal, transistor 308 will be held off and transistor 310 will be held on so that no base current may flow into base 340b of transistor 340, thereby causing this transistor to be off which in turn will hold transistor 346 off. Upon closure of throttle switch 38, input terminal 306 will be grounded and will thus cause the current flow through resistances 336, 332, and 350 to reverse in direction. This will cause the voltage applied to the base of transistor 310 to drop to a value which is merely sufficient to maintain conduction thereof.

Normal operation of the FIG. 2 circuit will cause a sequence of pulses to appear at terminal 214 thereof, as well as terminal 214 of the FIG. 3 circuit, which pulses have a fixed duration and a repetition rate which is directly indicative of the engine speed. The pulses received at terminal 214 of circuit 300 will be applied to the integrating capacitor network comprised of resistance 312 and capacitor 318, and the capacitor discharge network which includes diode 316, resistance 314, resistance 322, and diode 324 going to ground. As the repetition rate of these pulses increases, the average voltage appearing at the anode of diode 316 will increase so that the voltage level is substantially directly indicative of the engine speed. By suitably sizing the capacitor this network may be arranged so that the voltage appearing at the base 308b of transistor 308 will be sufficiently high to drive that transistor into conduction when the engine speed is above a preselected rpm value and throttle switch 38 is closed. Resistance 332 will cause conduction of transistor 308 to provide a lower impedance path to ground for current flowing through resistance 336 thereby terminating base current flow into the base 310b of transistor 310. This will rapidly switch transistor 310 off thereby causing the voltage on the collector of transistor 310 to rise and thereby applying an increased voltage to base 340b of transistor 340. This will cause transistor 340 to go into conduction so that a current flows through resistance 348, thereby applying a voltage drop in the forward direction across the emitter-base junction of transistor 346. This will cause transistor 346 to go into conduction and thereby apply a high voltage signal at output terminal 302. Since the circuit 110 of FIG. 2 is arranged to provide a fuel injection command output pulse in the form of a low voltage signal at terminal 126, a high voltage signal at terminal 302 may be coupled through suitable blocking diodes for instance, directly to terminal 126 to thereby terminate the provision of fuel injection command pulses.

Closure at any time of the transmission "park" or "neutral" switch, operative to apply a ground signal at terminal 304, will short circuit base 340b to ground, thereby terminating or preventing flow of base current to transistor 340 causing that transistor to switch to, or maintain an off condition and therefore causing transistor 346 to switch or maintain an off condition. Furthermore, an opening of switch 38 operative to terminate the ground signal at port 306 during a fuel cutoff cycle will cause a base current to flow into base 310b by way of resistances 356 and 350. The provision of this base current will cause transistor 310 to switch back on again and by the intercoupling of emitters will concomitantly cause transistor 308 to rapidly switch off. This will also terminate the provision of a high voltage output signal at terminal 302. This, the fuel cutoff signal may be terminated by either an opening of throttle switch 38 or a closure of the transmission "park" or "neutral" switch. The present invention therefore initiates fuel cutoff only when the speed of the engine is above a preselected value and the throttle switch has been closed and, as an auxiliary condition, the transmission of the vehicle with which the engine is associated is not in a "park" or "neutral" setting.

The receipt of the fixed pulse width pulses at terminal 214 will be further operative to periodically cause transistor 362 to go into conduction due to the receipt of a high voltage signal through resistance 364 at base 362b. Transistor 362 will be in conduction for a time period which substantially corresponds, within the limits of solid state electronic device switching times, to the width of the fixed width pulse. When transistor 362 is nonconducting (i.e., during interpulse intervals), the regulated voltage from the common conductor 328 will be applied through resistance 366 to capacitor 368. The capacitor 368 will therefore charge to a value which is directly related to the interpulse interval and hence to engine rpm. The periodic switching on of transistor 362 will be operative to provide a voltage discharge path to ground for capacitor 368 and by suitably sizing this capacitor, it can be arranged to be fully discharged during the fixed pulse width duration. Furthermore, by suitably sizing the resistance 366 the RC time constant of this resistance and capacitor 368 can be arranged to be relatively long compared with the discharge time so that the charge-up time of the capacitor is of a comparatively long duration. Thus, when the interpulse interval is comparatively short, as, for instance, at relatively high speed operation, the capacitor will be periodically charged up to a low value. However, after a fuel cutoff signal has been generated at output port 302, the speed of the engine can be readily presumed to be decreasing. Therefore, the voltage across capacitor 368 and hence the voltage applied to the anode of bistable switching device 370 will be somewhat higher for each successive interpulse interval. Resistances 378, 374, 380, and diode 376 form a voltage divider network which establishes a fixed level of voltage at gate electrode 372. It can therefore readily be arranged that, when the engine speed reaches a predetermined value which may be, for instance, the curb idle speed, the voltage accumulated across capacitor 368 during the next succeeding interpulse interval will be sufficiently high (one diode drop above the voltage applied to gate 372) so that bistable switching device 370 goes into conduction. Once conducting, the current flowing therethrough will be applied to the base of 310b of transistor 310 to cause that transistor to go into conduction. Conduction of that transistor will cause the termination of conduction of transistor 340, the termination of conduction of transistor 346, and the termination of the fuel cutoff signal at output terminal 302. Thus, the present invention provides a circuit which can readily examine the operating speed of the engine by measuring a pulse width interval to determine when that interval has reached a predetermined width so as to terminate fuel cutoff and thereby prevent stalling of the engine. By separating the high speed and low speed signaling circuitry, tailoring of the overall circuit to suit particular applications and different systems and engines may be readily accomplished. The use of interval determining means 368 for low speed signaling permits accurate low speed calculation so that the low speed stall point may be closely approached without danger of stalling the engine. Furthermore, by making throttle closure and attainment of the preselected low rpm the positive switching criterion and the presence of a high rpm signal a condition for switching, the determination of circuit element values for a multi-conditional responsive Schmitt trigger is greatly simplified.

Referring now to FIG. 4, an alternate input stage is illustrated for those situations when the voltage or energy level of the fixed width pulse applied to terminal 214 is not sufficiently great to charge capacitor 318 and to drive transistor 362 into conduction. As can be seen, this alternate circuit includes a transistor 380 whose emitter is connected to ground through resistance 382 and also to the cathode of a second voltage level regulating device, zener device 384, through resistance 386. This input stage would otherwise be coupled to circuits 300 as indicated by the commonly-designated resistances 312 and 364.