Air-to-fuel ratio feedback control system for internal combustion engines
BACKGROUND OF THE INVENTION 
     The present invention relates to an air-to-fuel ratio feedback control system for internal combustion engines, wherein feedback control is held and stopped during respective preselected operating conditions of the engine. 
     It is a well-known art in the field related to internal combustion engines that the air-to-fuel ratio of air-fuel mixture is controlled in accordance with the oxygen content in the exhaust gas of the engine for the purpose of reducing noxious components therein. These techniques are disclosed widely, in U.S. Pat. No. 3,815,561 and U.S. Pat. No. 3,782,347 for instance. It must be noted, however, that the engine is constantly supplied with the air-fuel mixture of the stoichiometric air-to-fuel ratio according to these techniques, whereas it requires the mixture richer or leaner than the stoichiometric one during some operating conditions of the engine. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of this invention to provide a system, wherein an engine is supplied with air-fuel mixture of various air-to-fuel ratio in accordance with operating conditions. 
     It is another object of this invention to provide a system, wherein air-to-fuel ratio feedback control is stopped and held during respective preselected operating conditions. 
     It is a further object of this invention to provide a system, wherein air-to-fuel feedback control is stopped during each one of engine starting, engine warm-up and inoperativeness of a ratio detector. 
     It is a still further object of this invention to provide a system, wherein air-to-fuel feedback control is held constant during full opening and full closing of a throttle valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram schematically illustrating one preferred embodiment according to this invention; 
     FIG. 2 is an electric wiring diagram showing detailed circuit construction of this invention shown in FIG. 1; 
     FIG. 3 is an electric wiring diagram schematically illustrating some parts of circuits shown in FIG. 2; and 
     FIG. 4 is a chart showing signal waveforms (A) and (B) for use in explaining the operation of the circuits shown in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reffering first to FIG. 1, an internal combustion engine 6 is provided with a mixture supply controller 5 at the intake side thereof. The mixture supply controller 5, such as an injection device or a carburetor, is adapted to supply the engine 6 with air-fuel mixture air-to-fuel ratio characteristics of which is preset in accordance with operating conditions of the engine 6. The engine 6 is further provided with an air-to-fuel ratio detector 1 at the exhaust side thereof. The ratio detector 1 is adapted to be responsive to the oxygen content in the exhaust gas which is indicative of the air-to-fuel ratio of the mixture supplied to the engine 6. In order to correct the air-to-fuel ratio of the mixture in accordance with the oxygen content, a discrimination circuit 2 and an integration circuit 4 which constitute a well-known feedback controller are connected between the ratio detector 1 and the mixture supply controller 5 and the mixture supply controller 5 is adapted to be responsive to this feedback controller. 
     Connected between the discrimination circuit 2 and the integration circuit 4 is a feedback hold circuit 3 which is controllable by a throttle detector 9. The hold circuit 3 disconnects electric connection between the circuits 2 and 4 in accordance with the movement of a throttle valve (not shown) with which the throttle detector 9 is associated. 
     Also connected between the discrimination circuit 2 and the integration circuit 4 are a monitor circuit 7 and a feedback halt circuit 8 for stopping the operation of the integration circuit 4. The monitor circuit 7 monitors the operation of the ratio detector 1 through the discrimination circuit 2 to detect the inoperativeness of the detector 1. A timer circuit 10 is connected between the throttle detector 9 and the halt circuit 8 to detect the deceleration duration of the engine 6. A start detector 11 for detecting the starting operation of the engine 6 and a warm-up detector 12 for detecting warm-up operation of the engine 6 are connected to the halt circuit 8. The halt circuit 8, as a result, is controllable by the monitor circuit 7, the timer circuit 10, the start detector 11 and the warm-up detector 12. 
     Referring next to FIG. 2, detailed construction and operation of each circuits shown in FIG. 1 are described hereinbelow. 
     The discrimination circuit 2, connected to the ratio detector 1 such as an oxygen-responsive sensor, is comprised of a zener diode 2a, resistors 2b, 2c, 2e, 2f, 2g, 2i, 2j, 2k, 2l, 2n, 2o, 2p, 2q, 2s and 2t, transistors 2d, 2h, 2r and 2u and a comparator 2m. The comparator 2m is applied with the constant voltage  V  B/ 2  ( V  B: regulated voltage) appearing at the junction of resistors 2j and 21 at the non-inverting input (+) and the variable voltage V O  appearing at the junction of the resistors 2f  and 2g. The voltage V O  is dependent on the output voltage of the detector 1, since the resistor 2g is connected to the detector 1 through the base-emitter path of the transistor 2h. The comparator 2m, as a result, produces a low voltage when the detector 1 produces a high voltage indicative of the absence of the oxygen in the exhaust gas, whereas it produces a high voltage when the detector 1 produces a low voltage indicative of the presence of the oxygen. The transistor 2r is rendered conductive in response to the low voltage applied from the comparator 2m, whereas the transistor 2u is rendered conductive in response to the high voltage applied from the comparator 2m. It must be understood herein that the absence and the presence of the oxygen are representative of the air-fuel mixture richer and leaner than the stoichiometric, respectively. 
     The hold circuit 3 is comprised of resistors 3a, 3b, 3d, 3e, 3i and 3k, transistors 3c and 3g, diodes 3f and 3h and a conventional photo-coupler 3j. Photo-coupler 3j is suitably of the type comprising a photo-diode which emits light in response to electric current flowing therethrough cooperating with a photo-transistor which is rendered conductive by the light emitted by the photo-diode, such as the OPTO-COUPLER 3N220 produced by Texas Instruments. As shown in FIG. 2, the photo-diodes of photo-coupler 3j are coupled to transistor 3g and resistor 3k. Current flows through the photo-diodes in response to conduction through transistor 3g, thus rendering the photo-transistors of the photo-coupler 3j conductive. Conversely, when transistor 3g is nonconductive, no current flows through the photo-diodes and the photo-transistors are also rendered nonconductive. Photo-coupler 3j thus functions, in effect, as a switch to selectively transmit the output voltage of comparator 2 to integrator 4. Integration circuit 4 is comprised of resistors 4a,  4b and 4e, a capacitor 4c, an operational amplifier 4d and a diode 4f. The amplifier 4d is applied with the voltage appearing at the junction of the collectors of the transistors 2r and 2u through the photocoupler 3j and the constant voltage  V  B/ 2  appearing at the junction of the resistors 2o and 2p. The capacitor 4c connected across the amplifier 4d charges and discharges in response to input voltage changes. The integration circuit 4 integrates the output voltage of the discrimination circuit 2 with respect to time during the conduction of the photo-coupler 3j and produces output voltage which changes in opposite changing directions. That is, the integration output voltage keeps decreasing during the conduction of the transistor 2r, whereas it keeps increasing during the conduction of the transistor 2u. It must be noted, however, that the output voltage of the discrimination circuit 2 is cut off by the hold circuit 3 during the nonconduction of the photo-coupler 3j and the capacitor 4c does not charge nor discharge. The integration circuit 4 holds or maintains the integration output voltage, while the hold circuit 3 disconnects the electrical connection between the circuits 2 and 4. The throttle detector 9 for controlling the operation of the hold circuit 3, on-off operation of the photo-coupler 3j in particular, is comprised of a first and a second switches 9a and 9b, coupled to the throttle valve and diodes 9c and 9d. The first switch 9a is adapted to close while the throttle valve is fully opened for engine acceleration, whereas the second switch 9b is adapted to close while the throttle valve is fully closed for engine idling and engine deceleration. As a result, the throttle detector 9 generates a high voltage which renders the transistors 3c and 3g of the hold circuit 3 conductive and nonconductive, respectively. When the photo-coupler 3j is rendered nonconductive during the nonconduction of the transistor 3g, the output voltage of the integration circuit 4 is maintained while the throttle valve is fully closed or opened. The integration output voltage does not change during this period, even though the output voltage of the ratio detector changes. 
     The monitor circuit 7 is comprised of diodes 7a, 7c and 7l, resistors 7b, 7d, 7e, 7g, 7h, 7i and 7j, a capacitor 7f and a comparator 7k which receives a constant voltage across the resistor 7h and a variable voltage across the capacitor 7f. The capacitor 7f is connected to the discrimination circuit 2 and the throttle detector 9 to be charged by the output voltages thereof. Owing to the fact that the ratio detector 1 (oxygenresponsive sensor) is inoperative during low temperature (below some 300° C) because of high impedance thereof, the comparator 2m of the discrimination circuit 2 keeps generating a low voltage by which the capacitor 7f cannot be charged. Therefore the voltage across the capacitor 7f is low enough to cause the comparator 7k to produce a high voltage during the inoperative condition of the detector 1. This high voltage is indicative of the inoperativeness of the detector 1. It is a well-known fact on the other hand that the detector 1 is operative during high temperature and produces the output voltage which alternatively changes high and low. One cycle period of this output voltage is much shorter in general than the period in which the detector 1 becomes responsive to the oxygen content in the exhaust gas. For this reason the comparator 7k produces a low voltage during the normal operation of the detector 1, provided that the discharging time constant of the capacitor 7f is set rather long and the constant voltage across the resistor 7h is set rather low. This low voltage is indicative of the operativeness of the detector 1. 
     The halt circuit 8 adapted to control the integration circuit 4 is comprised of transistors 8a and 8d, resistors 8b, 8f, 8g and 8h and diodes 8c and 8e. The transistors 8a and 8d are so inter-connected as that both are rendered conductive to short-circuit the capacitor 4c of the integration circuit 4 in response to the application of high voltage thereto. Receiving the high voltage from the monitor circuit 7, the transistors 8a and 8d become conductive to cause the capacitor 4c to discharge therethrough. Therefore the integration circuit 4 produces the constant voltage  V  B/ 2  even while the output voltage of the discrimination circuit 2 is applied through the hold circuit 3. Integrating operation of the circuit 4 is thus stopped by the halt circuit 8 while the ratio detector 1 is inoperative. This halt circuit 8 is further adapted to be controllable by other circuits 10, 11 and 12. The timer circuit 10 is comprised of resistors 10a, 10b, 10 d, 10h, 10i, 10k, 10n, 10p, 10q, 10r and 10u, transistors 10c, 10j and 10l, diodes 10e, and 10f and 10t, a capacitor 10m and a comparator 10s. The transistor 10c is connected to the second switch 9b of the throttle detector 9 to be rendered conductive while the switch 9b is closed. The transistor 101 is connected to the transistor 10c to be rendered conductive in response to the conduction of the transistor 10c, whereas the transistor 10j is connected to the same to be rendered nonconductive contrary to the transistor 101. The capacitor 10m is charged through the transistor 101 during the closure of the second switch 9b and discharged through the transistor 10j during the opening of the same switch 9b. As the closing period of the switch 9b becomes long, the voltage across the capacitor 10m becomes high to exceed the constant voltage across the resistor 10r. That is, the comparator 10s produces a high voltage at the time when the closing period of the throttle valve has exceeded the predetermined time period. This high voltage is applied to the halt circuit 8 to stop the integrating operation of the integration circuit 4. 
     The start detector 11 is comprised of a switch which is adapted to close when a starter motor (not shown) is driven and the warm-up detector 12 is comprised of resistors 12a, 12b, 12d, 12f, 12g, 12h, 12i and 12l, a transistor 12c, a thermally sensitive resistor 12e adapted to detect engine coolant temperature, a comparator 12j and a diode 12k. During engine starting the switch 11a closes to provide a high voltage which renders the transistor 12c conductive. The voltage across the resistor 12g becomes lower than the voltage across the resistor 12e during the conduction of the transistor 12c and the comparator 12j produces a high voltage. Since the resistance of the resistor 12e becomes low as the engine coolant temperature becomes high, the comparator 12j produces the high voltage until when the voltage across the resistor 12e becomes lower than that of the resistor 12g. This high voltage produced by the comparator 12j is indicative of either engine starting or engine warm-up and is applied to the halt circuit 8 to stop the integrating operation of the circuit 4. 
     It can be easily understood from the foregoing description that the halt circuit 8 stops integrating operations of the circuit 4 to thereby control the integration output voltage to the present constant value  V  B/ 2  during preselected first operating conditions of the engine and that the hold circuit 3 prevents the air-to-fuel ratio detection signal from being applied to the circuit 4 during preselected second operating conditions of the engine to thereby maintain the integration output voltage which is equal to the one of the time when one of the second operating conditions is detected. For this reason the hold circuit 3 and the halt circuit 8 can be illustrated as respective switches 3&#39; and 8&#39; for simplification only as in FIG. 3, wherein same numerals designate the same component parts shown in FIG. 2. 
     Overall feedback control operation of the system is described hereinunder with reference to FIG. 3 and FIG. 4 in which signal waveform (A) of the discrimination circuit 2 and signal waveform (B) of the integration circuit 4 are shown with respect to the time. 
     It is first assumed that the engine 6 is started at the time t 0  and warmed up until the time t 1 . During this time period t 0  -t 1  the switch 8&#39; which is normally-open type closes to stop the integrating operation and to apply the constant voltage  V  B/ 2  to the input terminal B of the mixture suply controller 5. Adjusting the controller 5 to be responsive to the voltage difference between the input voltage of the terminal B and the constant voltage  V  B/ 2 , it is made possible that the controller 5 doesn&#39;t correct the air-to-fuel ratio of the mixture during the period t 0  -t 1 . The engine 6 is therefore supplied with the mixture of the ratio desired and preset. The controller 5 is preset to supply the richer mixture during the engine starting for example. 
     Assuming next that the ratio detector 1 is operative and the engine 6 is operating normally during the time period t 1  -t 2 , the hold circuit 3 receives the discrimination output voltage shown in (A) of FIG. 4 at a terminal A and applies to the integration circuit 4 through the normally-closed switch 3&#39;. The discrimination output voltage in (A) of FIG. 4 changes high and low in accordance with the output voltage of the detector 1 and the preset voltage corresponding to the stoichiometric air-to-fuel ratio. It must be recalled that the high level voltage and the low level voltage in (A) of FIG. 4 are indicative of the leaner mixture and the richer mixture, respectively. The integration circuit 4, integrating this output voltage with respect to time, producesthe output voltage as shown in (B) of FIG. 4. The integration output voltage increases from the preset constant voltage  V  B/ 2  after the time t 1  in response to high input voltage and thereafter changes changing direction in accordance with input voltage level. The mixture supply control 5 adapted to supply the mixture leaner than the stoichiometry and to correct fuel supply amount in response to the integration output voltage, for example, gradually increases and decreases the fuel supply amount while the comparison voltage remains high and low, respectively. As a result the air-to-fuel ratio of the mixture is controlled toward the preset value, the stoichiometric ratio (λ = 1). 
     Provided that the throttle valve of the engine 6 is fully closed or opened at the time t 2  upon respective deceleration or acceleration, the switch 3&#39; of normally-closed type opens and cuts off the discrimination output voltage applied to the terminal A. The integration circuit 4, therefore, maintains the integration voltage as shown in (B) of FIG. 4 until the time t 3  when the switch 3&#39; is released to close again. The mixture supply controller 5 corrects the air-to-fuel ratio of the mixture in response to the maintained integration voltage which is not dependent on the change of the discrimination output voltage. Since the mixture supply controller 5 is so designed as to supply the richer mixture upon engine acceleration, it rarely occurs that the mixture becomes too lean due to the integration voltage indicative of the decrease of the fuel. Holding the integration voltage during the time period t 2  -t 3  becomes advantageous when the air-to-fuel ratio feedback control is established again at the time t 3 . That is, the air-to-fuel ratio of the mixture is returned to the stoichiometric ratio in a very short time period after the time t 3 , because the integration voltage has been maintained near the value indicative of the stoichiometry. 
     As in the case during the period t 1  -t 3 , the engine 6 is supplied with the stoichimetric mixture during the time period t 3  -t 4  in which the engine 6 operates normally and the integration voltage is held again during the time period t 4  -t 5  in which the switch 3&#39; opens. It is assumed again that the throttle valve of the engine 6 is fully closed at the time t 4  and that the closing duration t 4  -t 5  exceeds the time period which is preset by the timer circuit 10. The integration voltage is maintained during the period t 4  -t 5  and thereafter forcibly controlled to the constant value  V  B/ 2  due to the closure of the switch 8&#39; which is controllable by the timer circuit 10. Since the integration voltage is maintained by the capacitor 4c, the voltage is apt to change due to the leakage thereof as the time passes. Therefore holding the integration voltage too long is not desirable to prevent the feedback control from being resumed from the erroneous integration voltage. Due to the operation of the timer circuit 10, holding the integration voltage is limited to the preset time period and thereafter the integration voltage is kept to the constant voltage  V  B/ 2  upon which the mixture supply controller 5 does not depend for correcting the ratio of the mixture. As a result the engine 6 is supplied with the mixture the ratio of which is preset by the controller 5 during the period t 5  -t 6  and the feedback control after the time t 6  is resumed in the same manner as has been described hereinabove with respect to the period t 1  -t 4 . 
     It would be easily understood from the foregoing description that halting the feedback control is accomplished during the preselected first operating conditions which generally last rather long and holding the constant feedback control is accomplished during the preselected second operating conditions which generally last rather short, resulting in the feedback control which is well-matched to various operating conditions of the engine. It must be also understood that other operating conditions of the engine, moving speed of the throttle valve for instance, may be added for halting and holding the feedback control without departing the scope of this invention.