Abstract:
A closed loop air-fuel ratio control system for an internal combustion engine is adapted to produce a control signal to be supplied to an air-fuel ratio regulator for regulating the air-fuel ratio of the air-fuel mixture to be supplied to the engine. The control signal is varied in amplitude in dependence on a concentration of a component in the exhaust gases upstream of a converter or reactor in the exhaust system. The control signal is composed of an integral component and a proportional component, the magnitude of the proportional component being dependent on that of the integral component.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a closed loop air-fuel ratio control system for controlling the ratio of air to fuel of the air-fuel mixture to be supplied to an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art that the types and amounts of substances present in exhaust gases from an internal combustion engine are greatly affected by the ratio of air to fuel in the mixture supplied to the engine. Rich mixtures, with excess fuel, tend to produce higher amounts of hydrocarbons and carbon monoxide, whereas lean mixtures, with excess air, tend to produce greater amounts of oxides of nitrogen. It is also well known in the art that exhaust gases can be treated so as to reduce the amounts of these undersirable components. An example of this treatment is catalytic treatment in which carbon monoxide and hydrocarbons are oxidized and nitrogen oxides are reduced. 
     Such treatment can be achieved by providing a catalytic converter and/or a reactor in the exhaust system of the engine. It has been suggested that oxidation and reduction for minimization of undesirable constituents is preferably performed when the air-fuel ratio of the exhaust gases upstream of the converter or reactor is maintained within a narrow range at, for example, stoichiometry. 
     In order to maintain the air-fuel ratio of the exhaust gases upstream of the converter or reactor, various air-fuel control systems have been developped. A typical approach utilizes a detector for detecting the air-fuel ratio of the exhaust gases upstream of the converter etc., a control signal generator for generating a control signal varying in amplitude in accordance with variations in the air-fuel ratio of the exhaust gases, and an air-fuel ratio regulator for regulating the air-fuel ratio of the air-fuel mixture to be supplied to the engine in response to the control signal. 
     Since variations in the air-fuel ratio of the exhaust gases appear after an appreciably long delay in the exhaust gases, the control signal undergoes undesirable hunting about a central level corresponding to a desired air-fuel ratio of the exhaust gases. In order to suppress the amplitude in this hunting of the control signal, various techniques have been utilized with the control system. However, difficulty is still encountered in conventional air-fuel ratio control systems. 
     OBJECTS OF THE INVENTION 
     It is therefore a principal object of the present invention to provide an improved closed loop air-fuel ratio control system which can appreciably suppress the magnitude of hunting in the control signal used for controlling the air-fuel ratio regulator. 
     SUMMARY OF THE INVENTION 
     Briefly described, an air-fuel ratio control system according to the present invention produces a control signal containing proportional and integrated components, the magnitude of the proportional component being varied according to the magnitude of the integrated component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing a prior art air-fuel ratio control system; 
     FIG. 2 is a graph showing a typical output characteristic of an O 2  sensor as a function of air-fuel ratio in the exhaust gases; 
     FIGS. 3A, 3B, and 3D show waveforms of signals occurring in the system of FIG. 1; 
     FIG. 4 shows a circuit arrangement of a air-fuel ratio control system according to the present invention; 
     FIGS. 5A, 5B and 5C show waveforms of signals occurring in the circuit of FIG. 4; and 
     FIGS. 6A, 6B, 6C and 7 show waveforms of signals occurring in the circuit of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the instant invention is better understood in light of the prior art as exemplified by FIGS. 1-3. 
     Referring now to FIG. 1, there is shown a conventional air-fuel ratio control system, which utilizes a concentration detector 10, such as an O 2  sensor, for detecting a concentration of a component in the exhaust gases of an internal combustion engine 11. The engine 11 exhausts into the exhaust passage 12 upstream of a catalytic converter or a reactor 13 for promoting oxidation and/or reduction of harmful components in the exhaust gases. The concentration detector 10 produces a higher signal when a concentration of a component in the exhaust gases is higher than a preselected level, and a lower signal when the concentration is lower than the preselected level. It is to be noted that the conventional concentration detector cannot adequately detect with precision the concentration of a specific component. The signals from the detector 10 are applied to a signal converter 14 which produces an air-fuel ratio signal in response to the signals from the detector 10. The signal converter 14 preferably includes a comparator, a differential amplifier, or like circuitry. The detector 10 and signal converter 14 constitute an air-fuel ratio detector. The output signal from the air-fuel ratio detector is applied to a control circuit 15 which includes a proportional signal generator 16, an integrator 17 and a summing circuit 18. A control signal of the control circuit 15 is applied to an air-fuel ratio regulator 19 which regulates the air-fuel ratio of the air-fuel mixture to be supplied to the engine 11 in accordance with the output signal from the control circuit 15. 
     FIG. 2 shows a typical output characteristic of the concentration detector 10. As seen from this figure, the concentration detector cannot detect an instantaneous true magnitude of deviation from a central level of a concentration of a component in the exhaust gases, but merely produces signals indicative of the concentration of the component that is higher than a preselected level or vice versa. 
     FIG. 3A shows a typical waveform of the air-fuel ratio signal from the signal converter 14 at a relatively low vehicle speed. When such signal as shown in FIG. 3A is applied to the control circuit 15, the proportional signal generator 16 and the integrator 17 cooperate to produce a so-called P-I control signal such as that shown by a solid line in FIG. 3B. The P-I control signal consists of proportional components P and integral components I. As seen from FIG. 3B, the output signal from the control circuit varies about a center level or reference value with a deviation of a magnitude d. The reference level corresponds to a desired air fuel ratio in the exhaust gases. FIGS. 3C and 3D, respectively, show waveforms of output signals from the signal conveter 14 and the control circuit 15 at a relatively high vehicle speed. As seen from these figures, hunting in the control signals is caused by delay times t occurring in the overall air-fuel ratio control system. 
     In the conventional air-fuel ratio control system, the proportional component P in the control signal has a constant amplitude notwithstanding variation in vehicle speed, as seen from FIGS. 3B and 3D. Thus, difficulty has been encountered in air-fuel ratio control when using conventional control systems. This difficulty is more pronounced at relatively low vehicle speeds. 
     In order to overcome the above-mentioned problems, the present invention provides an improved control circuit for the air-fuel ratio control circuit. 
     FIG. 4 shows a preferred control circuit 15&#39; according to the present invention, which comprises an integrator 17 including an operational amplifier OP 1  having its non-inverting input grounded and its inverting input connected through a resistor R 1  to the output of the signal converter 14. A capacitor C 1  is connected across the inverting input and the output of the operational amplifier OP 1 . An improved proportional component generator 16a is provided which functions to produce a proportional component with an amplitude varying in accordance with the magnitude of the integrated component. The proportional component generator 16a includes an inverter 21 having an operational amplifier OP 2  that has its non-inverting input grounded and its inverting input connected through a resistor R 2  to the output of the circuit 17. A resistor R 3  is connected across the inverting input and output of the operational amplifier OP 2 . 
     The circuit 16a further includes switching circuitry for selectively passing therethrough the output signals from the integrator 17 and the inverter 21 in accordance with the sign, i.e. positive or negative, of the output signal from the signal converter 14. The switching circuit includes a transistor Q 1  having its base connected through a resistor R 4  to the signal converter 14 and its collector connected through a resistor R 5  to the output of the integrator 17. The emitter of the transistor Q 1  is connected to a negative voltage source -V. A voltage appearing at the collector of the transistor Q 1  is transmitted through a diode D 2  to the resistor R 6  which is in turn connected to the negative voltage source -V. A voltage then appears across the resistor R 6 , which is transmitted to one input of a summing circuit 18. 
     The switching circuit further includes a transistor Q 2  having its base connected through a resistor R 7  to the output of the signal converter 14 and its emitter connected to the negative voltage source -V. The collector of the transistor Q 2  is connected through a resistor R 8  to a positive voltage source +Vcc and connected through a resistor R 9  to the base of a transistor Q 3 . The transistor Q 3  has its emitter connected to the negative voltage source -V and its collector connected through a resistor R 10  to the output of the operational amplifier OP 2  and connected through a diode D 1  to the resistor R 6 . 
     The summing circuit 18 includes an operational amplifier OP 3  having its inverting input connected through a resistor R 10  to the output of the integrator 17 and connected through a resistor R 11  to the output of the proportional component generator 16a. The noninverting input of the operational amplifier OP 3  is grounded. A resistor R 12  is connected across the inverting input and output of the operational amplifier OP 3 . The circuit 18 functions to sum the output signals from the integrator 17 and the proportional component generator 16a. 
     Referring to FIGS. 5A, 5B and 5C, the operation of the circuit 16a will be described hereinbelow. 
     The integrator 17 is adapted to produce an integral of an input signal applied thereto. When, therefore, the integrator 17 receives pulses from the signal converter 14, the integrator 17 produces a continuous signal decreasing in such a manner as shown in dotted line in FIG. 5A. In this instance, the transistor Q 1  repeats ON and OFF states so that a signal appearing at a junction J A  has a waveform such as that shown by the solid line in FIG. 5A. The transistors Q 2  and Q 3  and the diode D 1  cooperate to produce at a junction J B  a signal having a waveform such as that shown by the solid line in FIG. 5B. The signals at the junctions J A  and J B  are superposed on each other at a junction J C  therefore causing a signal having a waveform such as that shown by the solid line of FIG. 5C. 
     In FIGS. 6A, 6B and 6C, there are illustrated three different output signals from the signal converter 14, respectively illustrated by a phantom line a, a dotted line b and a solid line c. When these signals or signals such as these are supplied to the control circuit 15&#39; in different time periods, the circuit 15&#39; produces output signals corresponding to the input signals and having such waveforms such as those shown by lines a&#39;, b&#39; and c&#39; of FIG. 7. As seen from FIG. 7, the respective output signals have different proportional components P 1 , P 2 , and P 3  the magnitudes of which are proportional to the magnitudes of the corresponding integral components. 
     As is apparent from the above description, the control circuit according to the present invention produces control signals having a proportional component P with magnitudes varying directly with the magnitude of the integral component. Accordingly, the hunting amplitude in the control signal supplied to the air-fuel ratio regulator can be suppressed, and more accurate air-fuel ratio control can be performed. 
     It will be understood that the invention is not to be limited to the exact construction shown and described and that various changes and modifications may be made without departing from the spirit and scope of the invention, as defined in the appended claims.