Air-fuel ratio feed back type fuel injection control system

.Iadd.RELATED CASE

This is a reissue of our U.S. Pat. 3,916,170 granted Oct. 28, 1975. .Iaddend.

1. Field of the Invention

The present invention relates to a fuel injection control system which provides an additional air-fuel ratio feedback control for an electronically controlled fuel injection system.

2. Description of the Prior Art

Fuel injection systems are known in the art in which the amount of air drawn into an engine is measured, and the amount of fuel predetermined in relation to the measured air quantity is programmed in terms of the injection time of electromagnetic valves, whereby the fuel injection quantity is regulated in accordance with the program.

A disadvantage of fuel injection systems of this type is that while various engine parameters such as the intake manifold vacuum and engine temperatures may be detected to control the fuel injection quantity, it is extremely difficult to correct for constantly changing operating conditions of the engine, the variations according to the type of engine, i.e., the variations with different engines, etc., and this particularly gives rise to serious difficulties when the vehicle is equipped with a catalytic exhaust gas cleaning system or the like for exhaust emission control purposes.

With a view to overcoming the foregoing difficulty, it is an object of the present invention to provide an air-fuel ratio feedback type fuel injection control system wherein the concentration of oxygen contained in the exhaust gases is detected by a sensor to correct the fuel injection quantity in accordance with the sensor output, and the feedback follow-up characteristic of the system during transient periods is improved, whereby permitting the engine to operate at a predetermined air-fuel mixture ratio.

In accordance with the present invention, there is thus provided an air-fuel ratio feedback type fuel injection control system comprising a comparator for comparing the detected output of an oxygen concentration sensor with a predetermined value, an adding circuit for adding a detected value of intake state and a detected value of engine rpms to produce a sum output, and correction means for determining the sense of correction in accordance with the output of the comparator and generating an error output having a correction factor varied in accordance with the sum output. A great advantage of this arrangement is that since the sense of correction is determined in accordance with the comparator output, the air-fuel mixture ratio indicated by the concentration of oxygen in the exhaust gases which is detected by the oxygen concentration sensor is controlled to a predetermined value, and moreover the correction factor of the error output is controlled in accordance with the sum output of the detected values of the intake state and engine rpms to thereby ensure improved feedback follow-up characteristic during transient perieds when the operating conditions of the engine vary over a wide range or rapidly.

When the air-fuel control cannot operate properly, for example because of a low exhaust temperature or under deceleration conditions, a holding circuit is connected to the integrater output to maintain the integrated output at a value intermediate the fluctuating range thereof.

In accordance with another form of the invention, the system further comprises stopping means for terminating the correction of the fuel injection quantity by the feedback control system when the supply of fuel is cut off during the deceleration periods. A great advantage of this arrangement is that by stopping the correction of the fuel injection quantity under the deceleration driving conditions, it is possible to eliminate the danger of supplying an abnormally rich mixture when the fuel injection is started again upon termination of the deceleration driving conditions.

FIG. 1 is a block diagram showing an embodiment of an air-fuel ratio feedback type fuel injection control system according to the present invention.

FIG. 2 is a wiring diagram showing a detailed circuit diagram of the principal part of the embodiment shown in FIG. 1.

FIG. 3 is an input-output characteristic diagram of the manifold vacuum sensor in the system of FIG. 2.

FIG. 4 is an input-output characteristic diagram of the engine rpm sensor in the system of FIG. 2.

FIG. 5 is an input-output characteristic diagram of the adding circuit in the system of FIG. 2.

FIG. 6 is a wiring diagram showing a detailed circuit diagram for the principal part of another embodiment of the invention.

The present invention will now be described in greater detail with reference to the illustrated embodiments.

Referring first to FIG. 1 showing a first embodiment of the invention, numeral 1 designates an integration constant generating adding circuit wherein the engine rpm, intake manifold vacuum, inlet air quantity, etc. may be detected, and the detected voltage is converted into an error from a reference voltage V B /2 having half the value of a supply voltage to produce a negative voltage integration constant output (the voltage smaller than the reference voltage V B /2 is hereinafter designated as a negative voltage and the greater one as a positive voltage). Numeral 1A designates a vacuum sensor for generating a vacuum voltage corresponding to the intake manifold vacuum representing the intake state, 1B an rpm sensor for generating an rpm voltage corresponding to the number of revolutions of an engine, 1C an adder for producing the sum of the vacuum voltage and the rpm voltage. Numeral 2 designates an inverting amplifier for inverting the integration constant with respect to the reference voltage V B /2 to generate a positive voltage inverted output. Numeral 3 designates an oxygen concentration sensor consisting of a solid electrolyte such as zirconium oxide which is mounted in an exhaust pipe 14 to detect the concentration of oxygen in the exhaust gases, whereby when the exhaust gas temperature exceeds an allowable temperature in the range between 450° to 600° C, the sensor comes into normal operation in response to the oxygen concentration and generates a concentration detecting signal. Numeral 4 designates a temperature sensor consisting of a thermistor for detecting the exhaust gas temperature 5, an allowable temperature detecting circuit responsive to the output signal of the temperature sensor 4 for generating an output when the exhaust gas temperature reaches the allowable temperature. Numeral 6 designates a gating circuit whereby in the absence of the output signal from the allowable temperature detecting circuit 5, that is, when the exhaust gas temperature is lower than the allowable temperature, the circuit is closed to prevent the passage of the concentration signal from the oxygen concentration sensor 3, whereas when the detecting circuit 5 generates an output, the circuit is opened to permit the concentration signal to pass therethrough. Numeral 7 designates a comparator for normally comparing the concentration signal from the oxygen concentration sensor 3 with a predetermined air-fuel ratio setting value V H to generate a 1 or 0 level output signal. Numeral 8 designates a switching circuit responsive to the output signal of the comparator 7 for determining the sense of correction to pass to the output thereof either the negative voltage integration constant from the integration constant generating adding circuit 1 or the positive voltage inverted output from the inverting amplifier 2. Numeral 9 designates an integrator for integrating the negative voltage integration constant or the positive voltage inverted output to vary the correction factor thereof. The fuel injection quantity is corrected in accordance with the output of the integrator 9. Numeral 10 designates an output holding feedback circuit which constitutes a correction stopping means and by which the output of the integrator 9 is received as an input and fed back through the gating circuit 6 when there is no output from the allowable temperature detecting circuit 5, whereby the output of the integrator 9 is held at the reference voltage V B /2 to stop the correction of the fuel injection quantity. Numeral 11 designates the computing section of a conventional electronically controlled fuel injection control system for generating injection pulses having a time duration corresponding to an engine parameter such as the intake manifold vacuum or engine temperature, whereby the time duration of the injection pulses is corrected in accordance with the difference between the integrated output of the integrator 9 and the reference voltage V B /2 . Numeral 12 designates electromagnetic valves which are opened by the injection pulses from the computing section 11 to inject the required fuel quantity, 13 an internal combustion engine, 14 the exhaust pipe of the engine 13.

With the construction described above, the operation of the first embodiment is as follows. The system is prearranged so that when the integrated output of the integrator 9 becomes equal to the value of the reference voltage V B /2, the amount of correction by the feedback control system which applies feedback in accordance with the oxygen concentration of the exhaust gases is reduced to zero and a predetermined basic fuel quantity required is injected. Assuming now that the exhaust gas temperature is below the allowable detecting temperature, the oxygen concentration sensor 3 does not respond and thus it produces no output. This situation corresponds to a weak mixture detecting condition so that the output signal of the allowable temperature detecting circuit 5 acts in such a manner that the gating circuit 6 prevents the passage of the output of the oxygen concentration sensor 3. In other words, the temperature sensor 4 and the allowable temperature detecting circuit 5 detects that the exhaust gas temperature is below the allowable temperature and thus an output for actuating the gating circuit 6 is generated. Consequently, the output holding feedback circuit 10 comes into operation to maintain the integrated output of the integrator 9 at the value of the reference voltage V B /2 and the computing section 11 generates the injection pulses corresponding to the basic fuel requirement to inject and supply the fuel into the engine 13 through the electromagnetic valves 12.

On the other hand, when the exhaust gas temperature exceeds the allowable temperature, the oxygen concentration sensor 3 operates in response to this temperature and the gating circuit 6 opens to pass the output signal of the oxygen concentration sensor 3. In this condition, if the oxygen concentration of the exhaust gases is high and the mixture is weak, the concentration detecting signal from the oxygen concentration sensor 3 is at the low level and this low level output signal is applied to the comparator 7 through the gating circuit 6. Consequently, the comparator 7 compares this signal with the air-fuel ratio setting value V H and generates a 0 level signal which is utilized to cause the switching circuit 8 to perform the selecting function and thereby to introduce the positive voltage inverted output of the inverting amplifier 2 into the integrator 9. As a result, the integrator 9 integrates this inverted output and generates an integrated output voltage which decreases gradually. In response to this integrated output voltage, the time duration of the injection pulses from the computing section 11 is increased and an increased amount of fuel is injected to supply an enriched mixture. Continuation of this process causes the oxygen concentration of the exhaust gases to decrease gradually. When the oxygen concentration eventually drops to a predetermined value, the output signal of the oxygen concentration sensor 3 goes to the high level and this high level output signal is applied to the comparator 7 by way of the gating circuit 6. When this high level signal becomes higher than the air-fuel ratio setting value V H , the 1 level signal from the comparator 7 causes the selecting operation of the switching circuit 8 to introduce the sum output or the negative voltage integration constant output of the adding circuit 1 into the integrator 9. The integrator 9 integrates this integration constant output and generates an integrated output voltage which rises gradually. This integrated output voltage decreases the time duration of the injection pulses from the computing section 11 and the fuel injection quantity is decreased to control the oxygen concentration to approach a predetermined value which corresponds to the optimum air-fuel mixture ratio. Further, in the above-described feedback control, the integration constant output by which the air-fuel mixture ratio or the oxygen concentration at the higher exhaust gas temperature than the allowable temperature is controlled to the predetermined value, is the sum output from the added 1C which produces the sum of the output voltages of the vacuum sensor 1A and the rpm sensor 1B. In this way, any time delay due to the influence of the velocity of exhaust gases or the response speed of the oxygen concentration sensor 3 during the time between the correction of the fuel injection quantity and the response of the sensor 3 is compensated for. In other words, when the amount of air drawn into the engine is increased greatly or in the high engine rpm ranges, the value of the integration constant is increased to increase the absolute value for the slope of the integrator output, whereas when the inlet air quantity is decreased greatly or in the low engine rpm ranges, the value of the integration constant is decreased to decrease the absolute value for the slope of the integrator output. In this way, it is possible to improve the control response characteristic of the feedback control system which maintains the air-fuel mixture ratio at a predetermined value during transient periods when the engine operating conditions change rapidly or over a wide range.

FIG. 2 shows the principal individual circuits of the embodiment which perform the abovedescribed operations. In FIG. 2 showing the individual circuits designated by numerals 1 through 10 in FIG. 1, numerals 1a 1 , 1a 2 , 1a 3 , 2a, 5a, 7a, 9a and 10a designate operational amplifiers, 1e 1 and 1e 2 diodes, 6b 1 , 6b 2 , 6b 3 , 8b 1 and 8b 2 transistors, 9c and integrating capacitor, 6d 1 and 6d 2 relays, V B a supply terminal to which a positive voltage is applied, G a grounding terminal, A a voltage divider for reducing the supply voltage to one half to supply the reference voltage V B /2 to the circuits 2, 5, 9 and 10.

FIG. 3 illustrates an intake manifold vacuum versus vacuum voltage characteristic diagram showing the input-output characteristic of the vacuum sensor 1A. In FIG. 3, designated as P D is a critical vacuum for preventing the integration constant from decreasing excessively when the intake manifold vacuum increases greatly, whereby when the manifold vacuum exceeds the critical vacuum, the vacuum voltage is clamped to an anode voltage V D of the diode 1e 2 . In FIG. 4, there is illustrated an engine rpm versus rpm voltage characteristic diagram showing the input-output characteristic of the rpm sensor 1B. In FIG. 5, there is illustrated an intake manifold vacuum versus integration constant characteristic diagram using the engine revolutions of the adding circuit 1 as an parameter. With the integration constants according to this characteristic diagram, the above-described correction of the fuel injection quantity with improved response characteristic during transient periods is ensured.

Referring now to FIG. 6, there is illustrated a wiring diagram showing the principal circuits of another embodiment of the system according to the invention. The circuits constitute second stopping means whereby the correction of the fuel injection quantity by the feedback control system is terminated when the supply of fuel is cutoff during deceleration periods. In FIG. 6, numeral 100 designates a cutoff condition sensor whereby when the sum of the outputs of a thermistor 100a for detecting the coolng water temperature and the engine rpm sensor 1B exceeds a predetermined value, a transistor 100b is turned off to generate a cutoff condition signal. Numeral 200 designates a cutoff switch circuit whereby when the accelerator pedal is released for deceleration driving, a cutoff switch 200a is closed and a transistor 200b is turned off to generate a cutoff switch signal. Numeral 300 designates an AND circuit whereby when both cutoff condition signal and the cutoff switch signal are on, a transistor 300a is turned off to generate a cutoff signal. Numeral 400 designates a cutoff signal conversion circuit whereby when the cutoff signal is generated, the output voltage of the temperature sensor 4 is raised to forcibly create a condition which indicates that the engine temperature is lower than the allowable temperature.

With the construction described above, when the supply of fuel is cutoff, the exhaust gas temperature is high, the oxygen concentration sensor 3 is in the normal operating condition and there is no injection of fuel. Consequently, the oxygen concentration sensor 3 continues to generate the low level output signal indicative of the fact that the concentration of oxygen contained in the exhaust gases is high and the mixture is weak. In this case, therefore, the feedback control system serves in such a manner that the amount of correction of the fuel injection quantity for enriching the mixture is prevented from increasing abnormally. In other words, during deceleration due to the releasing of the accelerator pedal, the cutoff condition sensor 100 generates a cutoff condition signal and the cutoff switch circuit 200 generates a cutoff switch signal with the result that the AND circuit 300 generates a cutoff signal. This cutoff signal brings the cutoff signal conversion circuit 400 into operation to forcibly raise the output voltage of the temperature sensor 4. Consequently, the output voltage of the sensor 4 indicates that the engine temperature is lower than the allowable temperature, and thus the output of the integrator 9 is held at the reference voltage V B /2 and the correction of the fuel injection quantity is stopped through the action of the previously mentioned allowable temperature detecting circuit 5, the gating circuit 6 and the output holding feedback circuit 10. The cutoff signal generating means shown in FIG. 6 and comprising the cutoff condition sensor 100, the cutoff switch circuit 200 and the AND circuit 300 is incorporated in the computing section 11 of the conventional electronically controlled fuel injection control system. In this case, therefore, it is necessary to add only the cutoff signal conversion circuit 400 to the circuitry of FIG. 2 and the circuit construction of this embodiment is thus greatly simplified.

Further, while, in the second embodiment described above, the second stopping means is designed to forcibly raise the output voltage of the temperature sensor 4, it is of course possible to use other means such as operating the gating circuit 6 with the cutoff signal.