Closed Loop Engine Control System

Abstract

A closed loop engine control system for internal combustion engines is described. The control system is responsive to signals indicative of the presence or absence of oxygen in the exhaust gas of the engine and is operative to generate an output signal for receipt by a fuel delivery controller which will cause that fuel delivery controller to increase fuel delivery in the presence of oxygen molecules in the exhaust gas and to decrease fuel delivery in the absence of oxygen molecules in the exhaust gas in order to maintain the fuel delivery at the predetermined, and preferably the stoichiometric, air/fuel ratio mixture point.

Description

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention is related to the field of internal combustion engine control systems in general and in particular to that portion of the above noted field concerned with closed loop control systems. In greater detail, the present invention is concerned with a closed loop control system in which the exhaust gases of an internal combustion engine are analyzed to indicate the ratio of the air/fuel mixture being consumed by the engine and through which signals are generated in order to modulate the fuel delivery mechanism in order to provide a predetermined air/fuel ratio mixture for the engine.

2. Description Of The Prior Art

Closed loop fuel control systems are well known in the internal combustion engine art, in particular in that portion of the internal combustion engine art which is devoted to the hot gas turbine engine, to provide short term and long term input control corrections. However, as applied to variable volume combustion chamber type engines, such as the reciprocating piston or rotary (Wankel) engine, closed loop fuel control systems are not well known. One known system involves the use of an exhaust gas oxygen concentration sensor one example of which is described in co-pending commonly assigned patent application Ser. No. 284,386 filed Aug. 28, 1972 by Thomas J. Hemak and titled "Protective Shield for Oxygen Sensor." This system relies upon the virtual step function output signal of the oxygen concentration sensor (as illustrated in FIG. 4) to directly effect the fuel delivery control mechanism to increase or decrease the volume of fuel delivered as a function of air being consumed by the engine with the magnitude and speed of the correction being a function of the sensor output signal. In working with a closed loop fuel control system based on this sensor it has been observed that the sensor output signal characteristic varies greatly as a function of sensor temperature and also as a function of sensor age. While the sensor output characteristic normally evidences a high to low transition at the stoichiometric air/fuel mixture ratio point, the magnitude of this transition (and hence the magnitude of the output signal) decreases for an aging oxygen sensor and also is a direct function of sensor temperature as illustrated in FIG. 4. This results in a closed loop control system which is least effective during the engine warm-up cycle and until the sensor reaches its normal operating temperature. Since accurate fuel control during the warm-up cycle is known to be important in reducing automotive exhaust emissions it is therefore an object of the present invention to provide a closed loop control system capable of examining engine exhaust and controlling fuel admission to the engine which is not sensitive to temperature induced variations in the sensor output signal. More particularly it is an object of the present invention to provide such a closed loop control system which is fully responsive to sensor output signal variations having a minimal magnitude. In this context. "minimal magnitude" is to be understood to mean a signal magnitude of about 10 percent of the maximum signal magnitude. It is also an object of the present invention to provide a closed loop control system whose responsiveness is not altered by degradation of the sensor device.

Closed loop controls for variable volume combustion chamber internal combustion engines utilizing the known oxygen sensors provide a corrective signal which is directly related to the sensor output signal. However, as the sensor ages, the magnitude of its output signal decreases so that the closed loop control response time will be gradually slowing thereby resulting in a long term decrease in the responsiveness of the closed loop control system rendering the known closed loop controls to be of little value. It is therefore a further object of the present invention to provide a closed loop control system whose output performance is not altered or affected by age-induced changes in the sensor signal. It is a more particular object of the present invention to provide a closed loop control system which gives uniform response without regard to variations in the magnitude of the input signal. It is a still further object of the present invention to provide a closed loop control system which provides uniform response without regard to age or temperature of the sensor

SUMMARY OF THE PRESENT INVENTION

I have determined that the typical sensor output signal, regardless of the sensor's age or operating temperature, will make a high-to-low or a low-to-high signal excursion through at least one intermediate sensor output signal voltage value which may be termed the "transitional value." In some instances, a sensor may have a "transitional band," comprised of a plurality of output signal values each of which could be a transitional value. That is to say, that the maximum low signal voltage value is less than the minimum high signal voltage value and that between these two voltage values there exists a nominal sensor output voltage value excursions through which may advantageously be used to trigger a comparator circuit so that whenever the sensor output signal is greater than the transitional value, the comparator will generate an output signal having a first predetermined constant magnitude which magnitude will be uneffected by variations in sensor temperature or age. Furthermore, whenever the sensor output signal is below the transitional value, the comparator will generate an output signal having a second predetermined constant magnitude different from the first predetermined magnitude which again will not be influenced by sensor, temperature or age. Thus, the comparator output signal may be applied to the fuel delivery controller to increase or decrease the quantity of fuel in the air/fuel mixture being provided to the internal combustion engine in response to the sensor signals. This system will therefore be rendered insensitive to variations which may be due to variations in the temperature of the sensor or which may result from aging of the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the closed loop control system of the present invention in a block diagram.

FIG. 2 illustrates an electronic circuit which may comprise a portion of the block diagram of FIG. 1.

FIG. 3 illustrates an electronic circuit which may receive the output signal of the circuit of FIG. 2.

FIG. 4 illustrates various voltage signal waveforms which may be produced by the oxygen sensor of FIG. 1 in response to variations in sensor temperature and/or age.

FIG. 5 illustrates the voltage output signal generated by the comparator of FIG. 1.

FIG. 6 illustrates a full cycle of the voltage waveform as a function of time generated by the circuit of FIG. 3 to control fuel delivery.

FIG. 7 illustrates the output signal generated by the circuit of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a block diagram of a closed loop control system according to the present invention and intended for association with a variable volume combustion chamber internal combustion engine 10 is illustrated. The engine 10 produces an exhaust gas stream through conduit 12 which stream is examined by an exhaust sensor 20. The presently preferred embodiment contemplates an oxygen sensor operative to determine the percentage of oxygen concentration present within the exhaust gas stream. According to the prior art this oxygen sensor would provide a virtual step function output signal to be applied directly to a fuel delivery controller 50. However, according to the present invention, the output signal of the oxygen sensor is applied to a summing device 30 which also receives the fixed value signal, termed the set point value. The output of the summing device 30 is then applied to comparator means 40 which generates an output signal having a first relatively low fixed value when the output of the oxygen sensor 20 exceeds the set point value and a second relatively high fixed value when the output of the oxygen sensor 20 is less than the set point value. This output signal is applied to the fuel delivery controller 50 to influence or modulate the amount of fuel being provided to the engine 10. The engine 10 receives various control inputs as illustrated at 60 which may be for example an air consumption controlling input in the form of a throttle setting (which may be operator controlled) as well as other inputs which may or may not be controlled such as the load placed upon the engine, ignition advance or retard signals or modulation of exhaust gas recirculation (EGR). The fuel delivery controller 50 also receives intelligence via communication link 70 which is indicative of the moment-to-moment operation of the engine. For example, in the known fuel injection systems, this intelligence may comprise information as to the speed of the engine, the temperature of the engine coolant, the density of the air being consumed by the engine, and such other input information as may be of use to the fuel delivery controller 50 in providing a gross fuel delivery control. The fuel delivery controller 50 would then control the quantity of fuel to be delivered to the engine through conduit 80 in accordance with these various sensed parameters. The closed loop control would be operative to modulate the gross fuel delivery control signal in accordance with a correction factor determined by oxygen sensor 20. In this manner, the system will automatically compensate for aging of the engine and other components associated therewith such as the fuel delivery mechanism, the EGR components, the engine valves and seals and any other components which would directly or indirectly effect the quantities of air and/or of fuel being measured, computed or delivered.

Referring now to FIG. 2, the summing device 30 and the comparator 40 are illustrated in a representative, and preferred, electronic embodiment. The summing device 30 is comprised of a pair of interconnected resistors 32, 34 with the resistor 34 arranged to receive the output signal from the exhaust sensor 20 and resistor 32 arranged to receive a fixed value voltage signal from potentiometer 36. Resistors 32, 34 are interconnected at circuit location 38. Circuit location 38 communicates with one input to an operational amplifier 42. The other input to the operational amplifier 42 is communicated to a fixed voltage reference which represents the set point value. Oppositely directed diodes 44, 45 provide a feedback path around operational amplifier 42 to establish maximum and minimum output signal levels. In the illustrated embodiment the fixed voltage reference is established by communicating a nonregulated source of voltage B+ through a resistance 41 to the cathode of a zenner diode 43 whose anode is communicated to ground. This source of regulated voltage is also applied to the potentiometer 36 of the summing device 30 and to the oxygen sensor to establish a reference voltage or middle ground at the oxygen sensor. This will permit the oxygen sensor to generate an output signal which is referenced to the middle ground value so that, in the application of the present invention to an automotive vehicle which uses a d.c. supply with chassis ground (positive or negative) an intermediate voltage value will be used by the oxygen sensor as its "ground" in order to provide both positive and negative voltage values (relative to the middle ground) for the amplifier 42. The potentiometer 36 should be adjusted so that circuit location 38 will be at a voltage value equivalent to the reference voltage established by zenner diode 43, the set point value, when the output from the oxygen sensor 20 is at the transitional value. That is, the set point value should be selected to correspond to the selected transitional value of the exhaust sensor. Alternatively, a voltage divider may be used in place of potentiometer 36 where adjustability is not required.

With reference to FIG. 4, a graph is shown illustrating the output signal characteristic of the typical oxygen sensor with three output signal characteristic curves shown demonstrating a high to low excursion at the stoichiometric air/fuel mixture ratio. The curve identified as 1 corresponds to the maximum output signal excursion which would be produced by a new sensor operating at its maximum operating temperature. Curves 2 and 3 are illustrative of the output signal characteristic evidenced by an oxygen sensor operating at successively cooler temperatures or which is successively older. The signal curve 1 evidences a maximum excursion which, for way of example, would go from an output signal value of approximately 1.0 volts to an output signal value of 0.1 volts for increasing air/fuel ratio with the excursion occurring substantially at the stoichiometric mixture ratio. Extreme aging of the device or operation of the device at a temperature far below its normal operating temperature will result in a minimal signal excursion of about 0.2 volts. In the case of the sensor whose output signal characteristics are here illustrated, the signal characteristics of the curves 1, 2, and 3 overlap for a narrow region of output signal (of about 0.05 volts) centered at a value of about 0.5 volts as the various signals demonstrate their excursion characteristic. The "transitional band" for this sensor is thus about 0.05 volts wide and a nominal value of 0.5 volts may be selected as the representative "transitional value." The set point value would then be selected to correspond to the transitional value of 0.5 volts.

FIG. 5 illustrates the output of comparator device 40. The comparator device output signal demonstrates a minimum to maximum excursion of from -0.7 volts to +0.7 volts for increasing values of the sensor output signal with the excursion occurring when the sensor output signal equals the set point value, in this instance 0.5 volts. Thus, the output of the comparator device will be independent of time based variations in the sensor output signal, due for example to sensor aging, and will also be independent of temperature band variations in the sensor output signal so long as the minimum sensor signal excursion is occurring and is passing through the selected transitional value.

Referring now to FIG. 3, an electronic circuit is illustrated which accomplishes the general functions of the fuel delivery controller 50. The illustrated circuit includes a major portion of the electronic fuel injection computer according to copending commonly assigned patent application, Ser. No. 226,498 filed on Feb. 15, 1972 issued May 23, 1973 as U.S. Pat. No. 3,734,068 in the name of J. N. Reddy and titled "Electronic Fuel Control System Including Electronic Means For Providing A Continuous Variable Correction Factor" and is intended to be illustrative of one method for modulating fuel delivery in response to modulation commands of a closed loop control. The circuit of this figure is comprised of a pair of current sources 101, 102 which are alternately applied to a pair of timing capacitors 103, 104 by a switching network 105 receiving triggering signals at terminals 51, 52. Also receiving triggering signals at terminals 51, 52 (separately shown for convenience) network 106 controls the level of the voltage on the selected capacitor 103, 104 prior to generation of the injection command signal. Threshold establishing circuit means 107 samples the highest voltage appearing across capacitors 103, 104 and compares this value with the level established by the signal received at input port 53 to compute the fuel injection command signal. This signal may be derived by various known techniques such as illustrated in the co-pending Reddy application.

The current source 101 is comprised of transistor 108 whose base is connected to the junction of a pair of voltage dividing resistors 110, 111 and whose emitter is connected to resistor 112. The resistors 110 and 112 are connected to a source of potential identified as B+ and resistor 11 goes to ground. Current source 102 is similarly comprised of a transistor 109 whose base is coupled to the junction of voltage divider resistors 114, 115 and whose emitter is connected to resistor 116 which is also connected to the B+ source. The base of transistor 109 is also connected to a modulating network 118 to be described hereinbelow. This arrangement is operative to establish readily calculable levels of current flow in the collectors of transistors 108, 109, respectively. The collector of transistor 108 is then connected in a parallel fashion to the collectors of a pair of transistors 131, 132. Similarly, the collector of transistor 109 is connected in parallel to the collectors of a pair of transistors 133, 134. The bases of transistors 131 and 134 are connected together through resistances 141, 142 while the bases of transistors 132, 133 are connected by way of resistances 143, 144. The junction of resistances 141, 142 is connected to terminal 51 while the junction of resistances 143, 144 is connected to terminal 52. The emitters of transistors 131 and 133 are connected to capacitor 103 while the emitters of transistors 132 and 134 are connected to capacitor 104. This circuit is arranged to provide the current flow from current source 101 through transistor 131 to capacitor 103 and the current from source 102 through transistor 134 to capacitor 104 whenever a high voltage signal appears at terminal 51 and a low voltage signal appears at terminal 52. Whenever a low voltage signal is present at terminal 51 and a high voltage signal is present at terminal 52, the current from source 101 will flow through transistor 132 to capacitor 104, while the current from source 602 flows through transistor 133 to capacitor 103

The threshold establishing circuit receives a signal indicative of, for example, an engine operating parameter such as the manifold pressure at terminal 53 and this signal is applied to the base of transistor 172. The base of transistor 171 receives, by way of diodes 161, 162, the signal from the one of capacitors 103, 104 whose accumulated charge, or voltage, is highest. As the emitters of transistors 171, 172 are coupled together, one of these transistors will be in conduction depending upon which has a base residing at a higher voltage value. When the value appearing on the base of transistor 171 exceeds the value appearing on circuit input 170, transistor 171 will go into conduction and transistor 172 will drop out of conduction. Termination of conduction of transistor 172 will consequently terminate conduction of transistor 173. While transistor 172 was conducting, transistor 173 was also conducting and a relatively high voltage signal, as illustrated in FIG. 7, was present at terminal 54 due to the voltage divider action of resistors 182, 183. However, termination of conduction of transistor 173 will result in a substantially zero or ground level signal appearing at circuit location 174 due to the lack of current flow through the resistors 182, 183. This output signal may be applied to any of the known injector valve driver circuits one of which is illustrated in Ser. No. 130,349 -- Junuthula N. Reddy -- "Control Means For Controlling The Energy Provided To The Injector Valves Of An Electronically Controlled Fuel System" to constitute an injection command signal.

The timing capacitor discharging and initial charge controlling circuitry 106 is comprised of a plurality of reference level establishing means 210, 212, and 214, a pair of discharging means 216, 218, switching means 220 and a current source means 222. The reference level establishing means 210, 212, and 214 are connected to the source of energy indicated as B+ and are comprised of voltage divider means 224, 226, and 228, respectively, and voltage signal communicating transistor means 230, 232, and 234 respectively. The voltage communicating transistor means 230, 232 and 234 are arranged to have their bases communicated to a portion of the voltage divider means so that a known level of voltage may appear thereon and their emitters are connected to a common point. The collectors of the transistors 230 and 232 are coupled together and are communicated to ground through a diode means 236 while the collector of transistor 234 is communicated to ground through a separate diode means 238. The collector/diode junction of the transistors 230, 232 and diode means 236 is communicated to the discharging means 216 while the collector/diode junction of transistor 234 and diode means 238 is communicated to the discharging means 218.

With reference to FIG. 6, a complete cycle of a voltage waveform on the capacitors 103, 104 is illustrated. The portion of the wave from a to f represents the voltage attributable to the current I.sub.1 from source 101 while the portion identified as 4 represents the portion attributable to the current I.sub.2 from source 102. The various level changes and slopes present in the I.sub.1 initial portion of the waveform are attributable to the action of the reference level establishing means 210, 212, 214 and the charging and discharging characteristics of the capacitors under the influence of the current I.sub.1 and the discharging means 106, 216, 218. A similar wavefrom 180 degrees out of phase with this waveform is generated on the other of the capacitors 103, 104 so that the initial points a and f of the first and second portions of the waveforms on the capacitors 103, 104 coincide in time and also coincide with the receipt of mutually exclusive triggering signals received on terminals 51, 52. Receipt of a relatively high signal at terminal 51 will result in a rapid dumping of the energy stored in capacitor 103 and the resultant application of current I.sub.1 to the capacitor 103 to charge that capacitor. The voltage appearing on that capacitor as a result of the application of current I.sub.1 and as modulated by the action of the reference level establishing means 210, 212 will result in a voltage waveform appearing on capacitor 103 substantially as shown in FIG. 6 from points a through b, c, d, e and to point f on the curve in FIG. 6. At the point in time corresponding to point f, the triggering inputs received at input terminals 51, 52 will be reversed so that capacitor 103 will receive the current I.sub.2. The value of current I.sub.2 will be a function of the voltage appearing on the base terminal of transistor 109 and will charge capacitor 103 as shown on the portion of the curve identified as 4 of FIG. 6. A representative threshold value is illustrated in FIG. 6 as the dashed line 5 and the second portion of the curve, 4, crosses the threshold 5 at a point in time identified as T.sub.3. The circuit of FIG. 3 would therefore be operative to provide a flow of fuel to the engines in accordance with the teachings of the above-noted pending applications for the time period between T.sub.1 and T.sub.3.

With reference now to FIG. 3, modulating network or means 118 is illustrated as communicating with the base of transistor 109 through resistance 119. As illustrated, the modulating means 118 is comprised of an operational amplifier 120 having a capacitor 121 in its feedback loop communicating with the inverting input which also communicates through resistor 122 with a terminal 123. This terminal communicates directly with a similarly designated terminal of the comparator device 40 of FIGS. 1 and 2. Upon receipt of a comparator device output signal as illustrated in FIG. 5, the operational amplifier will be operative to generate at the base of transistor 109 an output voltage which will either be gradually increasing in the case of a negative input signal from comparator 40 or will be gradually decreasing in the case of a positive input from the comparator 40 so as to add or subtract incremental units of base drive for transistor 109. This will result in increasing or decreasing the magnitude of the current I.sub.2 and hence changing the slope of the ramp voltage generated at the capacitor 103, 104 receiving this current. Again with reference to FIG. 6, this will result in a curve identified as 4b for decreasing values of current I.sub.2 and the curve 4a for increasing values of I.sub.2. For convenience, the deviations between curves 4a, 4b, and 4 have been greatly exaggerated. With reference now to FIG. 7, the circuit of FIG. 3 and the curve 4a would generate a fuel injection command pulse having a duration from T.sub.1 to T.sub.2 while the curve 4 would generate a fuel injection command pulse having a duration from T.sub.1 to T.sub.3 and the curve 4b would generate the fuel injection command pulse subsisting from time T.sub.1 to T.sub.4. It can thus be seen, that for a given set of operating conditions for the engine, exemplified by the fact that threshold curve 5 in FIG. 4 is unchanging, the quantity of fuel delivered to the engine may be varied by the closed loop control system of the present invention to maintain the air/fuel ratio at the predetermined value.

It will be seen that the present invention accomplishes its stated objectives. By providing a comparator responsive to the sensor with fixed maximum and minimum values, and switchable therebetween in response to sensor signal excursions through the selected transitional value, the characteristic of the closed loop control signal applied to the fuel delivery controller is rendered independent of variation in the characteristic of the sensor signal so that the fuel delivery controller response is uniform and will only respond to variations in engine performance with respect to the set point.

Claims

I claim:

1. An internal combustion engine control system for modulating an engine operating parameter substantially independently of environmentally induced sensor output variations, said control system comprising:

sensor means for examining an engine operating variable operative to generate a sensor means signal having an environmentally variable characteristic selected to be indicative of a quality of the combustion process occurring within the engine said variable characteristic being subject to environmentally induced variations;

comparator means responsive to the sensor means signal operative to generate a comparator means output signal having a predetermined constant level signal established when said combustion process quality is one of a greater or lesser quality than a predetermined quality;

integrator means receiving said comparator output signal operative to integrate said predetermined constant signal level to generate a gradually changing output signal changing in one of two change directions at a predetermined integrating rate as long as said predetermined constant level signal is established; and

fuel delivery controller means receiving said gradually changing output signal and operative to modulate fuel delivery in response thereto;

whereby said predetermined constant level signal and said predetermined integrating rate cause said fuel delivery modulation to be substantially independent of said environmentally induced variations in said sensor characteristic.

2. The system as claimed in claim 1 wherein said sensor means signal variable characteristic varies with air/fuel ratio and comprises a transitional band portion intermediate a relatively high strength signal and a relatively low strength signal, said transitional band portion being constant with variations in said high and low strength signals induced by variations of at least one environmental condition; and occurring substantially at a selected air/fuel ratio;

wherein said system comprises reference level establishing means operative to provide said comparator means with a set point level within said sensor transitional band portion indicative of said predetermined quality;

wherein said comparator means output signal comprises a second constant signal level different from said predetermined constant level signal, said second constant level signal being established as long as said quality deviation is the other of said greater and lesser qualities;

wherein said integrator means integrates said second constant signal level to gradually change said gradually changing signal level in the other of said two changes directions at said predetermined integrating rates; and

wherein said fuel delivery controller means modulates fuel delivery so as to gradually increase the air/fuel ratio when said comparator output signal is established at one of said predetermined and second constant levels signals and to otherwise gradually decrease said air/fuel ratio when said comparator output signal is established at the other of said predetermined and second constant levels signals;

whereby said predetermined integrating rate and the difference between said predetermined and second constant signal levels causes said fuel delivery controller to vary the air/fuel ratio about said selected air/fuel ratio within a range of air/fuel ratios substantially independent of said variation of said at least one environmental condition.

3. The system as claimed in claim 1 wherein said fuel delivery controller means comprises at least one timing capacitor and current source means controllable to selectively charge said timing capacitor at a controllable rate from an initial value determined in accordance with a second engine operating variable to a threshold value determine in accordance with a third engine operating variable, said controllable rate being varied in accordance with said gradually changing output signal to controllably modulate the quantities of fuel delivered to the associated engine independently of variations in fuel delivery commanded to accommodate changes in said second and third operating variables.

4. The system as claimed in claim 1 wherein said integrator measn comprises an operational amplifier having an input terminal and an output terminal and an integrating capacitor coupling said input terminal and output terminal and determining asid predetermined integrating rate.

5. The system as claimed in claim 2 wherein said relatively high strength signal level comprises a minimum high strength signal level and said relatively low strength sensor signal level comprises a maximum low strength signal level, said minimum high strength signal level and said maximum low strength signal level each varying with the temperature and aging characteristics of said sensor means and the difference between minimum high strength and maximum lower strength signal levels defining said constant transitional band so as to be substantially independent of the aging and temperature characteristics of said sensor means so that the fuel delivery controller means modulates fuel delivery substantially independent of the aging and temperature characteristics of said sensor means.

6. The system as claimed in claim 4 wherein said comparator means comprises an operational amplifier having an input terminal and an output terminal coupled by parallelly connected oppositely polled semiconductor current conducting devices each having a predetermined voltage drop when conducting, said predetermined voltage drops establishing the magnitude of said constant signal levels and permitting said comparator means to switch substantially instantaneously from one of said constant signal levels to the other when said sensor means signal passes through said set point level so that the range in which said sensor means signal varies about said set point level is determined substantially by said magnitudes of said constant signal levels and said predetermined rate at which said integrator means integrates said comparator output signals.

7. An internal combustion engine controlling an engine parameter varying with operating conditions of an internal combustion engine, said control system comprising:

a. sensor means operatively connected with said engine to provide a sensor means signal having a range of values varying with said engine parameter;

b. reference level establishing means for providing a set point level within said range of said sensor means signal;

c. comparator means connected to receive said sensor means signal and said set point level operative to establish one of a constant positive and negative polarity comparator output signal when said sensor means signal is greater than said set point level and to establish the other of said constant positive and negative polarity comparator output signals when said sensor means signal is less than said set point level;

d. integrator means operatively connected with said comparator means to generate a control signal having a magnitude changing with the integral of said constant positive polarity comparator output signal when said constant positive polarity comparator output signal is established and with the integral of said constant negative polarity comparator output signal when said constant negative polarity comparator output signal is established; and

e. control means operatively connecting said integrator means and said engine for controlling said engine to vary said engine parameter in accordance with said control signal,

whereby the integration of said constant positive and negative polarity comparator output signals causes said control signal to control said engine so as to vary said engine parameter so that said sensor means signal is alternately increased and decreased through said set point level.

8. The engine control system of claim 7 wherein said comparator means comprises an operational amplifier having an input terminal and an output terminal coupled by parallelly connected oppositely polled semiconductor current conducting devices each having a predetermined voltage drop when conducting, said predetermined voltage drops establishing the magnitudes of said constant output signals and permitting said comparator means to switch substantially instantaneously from one of said constant positive and negative output signals to the other when said sensor means signal passes through said set point level so that said range in which said sensor means signal varies about said set point level is determined substantially by said magnitudes of said constant positive and negative polarity comparator output signals and the rate at which said integrator means integrates said comparator output signals.

9. The apparatus of claim 8 wherein said semiconductor devices comprise forward drops of said unidirectional current conducting devices having nominal forward voltage drops establishing said constant output signals.

10. The apparatus of claim 7 wherein said control means comprises an operator controllable air/fuel delivery means for delivering controllable air/fuel mixture to the engine, said control means operative to control one of said air and fuel so as to vary the air/fuel ratio in a predetermined range about a selected air/fuel ratio.

11. An internal combustion engine control system for modulating an engine operating parameter substantially independently of environmentally induced sensor output variations, said control system comprising:

sensor means for examining an engine operating variable operative to generate a sensor means signal having an environmentally variable characteristic selected to be indicative of a quality of the combustion process occurring within the engine said variable characteristic being subject to environmentally induced variations;

reference level establishing means for generating a set point level having a value in the range of values of said sensor means signal;

comparator means connected to said sensor means and said reference level establishing means operative to provide a comparator signal having first and second constant magnitudes when said sensor means signal is respectively above and below said set point level;

integrator means operatively connected with said comparator means operative to generate a control signal having a magnitude changing at a first predetermined integrating rate when the magnitude of said comparator signal is one of said first and second constant magnitudes and a second predetermined integrating rate when the magnitude of said comparison signal is the other of said first and second constant magnitudes;

fuel delivery controller means receiving said gradually changing output signal and operative to modulate fuel delivery in response thereto;

whereby said first and second constant levels and said predetermined integrating rate cause said fuel delivery modulation to be substantially independent of said environmentally induced variations in said sensor characteristic.