BACKGROUND OF THE INVENTION

The present invention relates to an air-fuel ratio control system for an internal combustion engine for vehicles; which controls the air-fuel ratio of the air-fuel mixture to a value approximately equal to the stoichiometric air-fuel ratio atwhich a three-way catalyst acts most effectively, and more particularly to an air-fuel ratio control system which is capable of preventing an air-fuel ratio adjusting electromagnetic valve from resonating with the vibration of the engine and with thepulsations of induced air.

In a conventional air-fuel ratio control system, the air-fuel ratio of the air-fuel mixture burned in the cylinders of engine is detected as an oxygen concentration of the exhaust gases by means of an O.sub.2 -sensor provided in the exhaustpassage of the engine, and a comparator compares the output signal from the O.sub.2 -sensor with a reference value and produces an output representing whether the signal is greater or smaller than the reference value corresponding to the stoichiometricair-fuel ratio. An electromagnetic valve is operated in dependency on the output for regulating the air to be mixed with the mixture to provide the stoichiometric air-fuel ratio.

The output of the comparator is applied to an integrating circuit which produces an output increasing or decreasing in accordance with the integration of the input. The output of the integrating circuit is applied to a comparator where theoutput is compared with a triangular pulse train applied from a triangular pulse generator for producing square pulses. The duty ratio of the square pulses varies in accordance with the output of the integrating circuit. The square wave pulses areapplied to an on-off type electromagnetic valve. Thus, the electromagnetic valve is opened and closed at duty ratios dependent on the applied square pulses for controlling the air to be supplied to the carburetor of the engine to provide the air-fuelmixture at the stoichiometric air-fuel ratio.

The electromagnetic valve is driven at the frequency of the square pulses which corresponds to the frequency of the triangular pulses. It is advantageous for improving the resonance of the air-fuel ratio control system to drive theelectromagnetic valve at a high frequency. However, since the high frequency driving of the electromagnetic valve causes a short life time of the valve, the valve is driven at a relatively low constant frequency, in practice.

On the other hand, there is necessarily an operating condition in which vibration of the electromagnetic valve body caused by its own operation coincides with the vibration caused by the engine body and/or with the vibration caused by pulsationsof induced air, that is resonance of the electromagnetic valve occurs. When the electromagnetic valve resonates, the air-fuel ratio fluctuates greatly, which will impair the driveability of the vehicle.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an air-fuel ratio control system in which the electromagnetic valve is driven at a frequency which is changed depending on engine speed, whereby resonance of the electromagnetic valve may beprevented.

According to the present invention, there is provided an air-fuel ratio control system for an internal combustion engine having an ignition device, an induction passage, a carburetor, an on-off type electromagnetic valve for correcting theair-fuel ratio of the air-fuel mixture supplied to said carburetor, an O.sub.2 -sensor for detecting oxygen concentration of exhaust gases, and a feedback control circuit responsive to the output of said O.sub.2 -sensor for producing pulses for drivingsaid electromagnetic valve for correcting the air-fuel ratio; having an improvement comprising

an engine speed detecting circuit for producing an output signal which varies with the engine speed; and

a frequency changing circuit responsive to said output signal of said engine speed detecting for changing the frequency of said pulses for the drive of said electromagnetic valve so as to prevent resonance of vibration of the electromagneticvalve caused by operation of the electromagnetic valve with vibrations caused by other operations of the vehicle.

The other objects and features are explained more in detail with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an air-fuel ratio control system according to an embodiment of the present invention;

FIG. 2 is a block diagram of the control system of the same;

FIG. 3 is an electric circuit of an embodiment of the present invention; and

FIG. 4 is an electric circuit showing another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 showing schematically the air-fuel ratio control system according to an embodiment of the present invention, reference numeral 1 designates a carburetor provided upstream of an engine 2. The carburetor 1 comprises a floatchamber 3, a main nozzle 5 of a venturi 4, and a correcting air passage 8 communicating with an air-bleed 7 which is provided in a main fuel passage 6 between the float chamber 3 and the nozzle 5. Another correcting air passage 13 communicates withanother air-bleed 12 which is provided in a slow fuel passage 11 which diverges from the main fuel passage 6 and extends to a slow port 10 opening in the vicinity of a throttle valve 9 in the induction passage 1a. These correction air passages 8 and 13communicate with respective electromagnetic valves 14, 15, induction sides of which communicate with the atmosphere through an air cleaner 16. Further, a catalytic converter 18 with three-way catalyst is provided in an exhaust pipe 17 at the downstreamside of the engine, and an O.sub.2 -sensor 19 is provided between the engine 2 and the converter 18 to detect the oxygen concentration of the exhaust gases as the air-fuel ratio of the mixture burned in the cylinder of the engine.

A control circuit 21 is applied with the output from the O.sub.2 sensor 19, to actuate the electromagnetic valves 14, 15 to open and close at a certain duty ratio according to the output signal. The air-fuel ratio is made lean by supplyingcorrection air to the carburetor at a large feed rate and the air-fuel ratio is made rich by reducing the correction air supply.

An engine ignition pulse generating circuit 22 is connected to an engine speed detecting circuit 20 which operates to convert the engine speed to an electric signal. The output of the circuit 20 is also connected to the control circuit 21.

Referring to FIG. 2 which is a block diagram showing the control system of the present invention, the output of the O.sub.2 -sensor 19 is applied to an integrating circuit 24 through a comparator 23. The output of the integrating circuit 24 isapplied to a comparator 25; and a triangular wave signal from a triangular pulse generator 26 is applied to the comparator 25 to produce square wave pulses. A driving circuit 27 is applied with the square wave pulses from the comparator 25 to drive theelectromagnetic valves 14, 15. On the other hand, the output of the engine speed detecting circuit 20 is connected to a frequency changing circuit 28, which in turn is connected to the triangular pulse generator 26.

In operation, the output signal of the O.sub.2 -sensor 19 is applied to the comparator 23, where the output signal of the O.sub.2 -sensor 19 is compared with a standard signal which is set by a variable resistor R.sub.2 (FIG. 3) so as tocorrespond to the stoichiometric air-fuel ratio for comparing the air-fuel ratio of the mixture. The output of the comparator 23 is applied to the integrating circuit 24 which produces a proportional and integrated output. The output is compared withthe triangular pulse train from triangular pulse generator 26 in the comparator 25 to produce square wave pulses, the duty ratio of which varies with the proportional and integrated output of the integrating circuit 24. The frequency of the square wavepulses is determined by the frequency of the triangular pulse train. The square wave pulses are sent to the electromagnetic valves 14, 15 through a driving circuit 27. The electromagnetic valves 14, 15 are driven at the duty ratios of the squarepulses. Thus the air-fuel ratio of the mixture is controlled to the stoichiometric air-fuel ratio.

On the other hand, the ignition pulse generating circuit 22 produces pulses varying in proportion to the engine speed. The pulses are applied to the engine speed detecting circuit 20 comprising a monostable multivibrator which produces an outputproportional to the engine speed. The engine speed detecting circuit 20 comprises a NOR gate, an inverter INV and a capacitor C.sub.3, which form a monostable multivibrator, and further comprises an integrating circuit comprising a resistor R.sub.15 anda capacitor C.sub.4. Thus, the circuit 20 produces an output, the voltage of which varies in proportion to the engine speed. The output voltage is applied to a gate of a semiconductor device, namely a FET of the frequency changing circuit 28, so thatthe current passing between the drain and the source of the FET varies with the output voltage.

In the triangular pulse generating circuit 26, the potential of a capacitor C.sub.2 is zero at the start of the engine. Since the output of an operational amplifier (opamp) OP.sub.5 is at a high level, an analog switch SW.sub.1 is turned on anda high voltage set by resistors R.sub.10 and R.sub.13 is applied to the positive input of the opamp OP.sub.5. The output of the opamp OP.sub.5 is applied to the capacitor C.sub.2 through a resistor R.sub.9 and the FET. When the voltage at the capacitorC.sub.2 reaches a high level, the output of the opamp OP.sub.5 is inverted to a low level, turning off the analog switch SW.sub.1. As a result, a low voltage by resistors R.sub.10 to R.sub.13 is applied to the positive input of the opamp OP.sub.5 andthe voltage at the capacitor C.sub.2 rapidly decreases by discharging through a diode D.sub.1.

The above described operation is repeated so that the voltage at the capacitor C.sub.2 varies in triangular wave form. Thus, the circuit 26 produces a triangular pulse train at a frequency. The frequency is determined by the capacitance of thecapacitor C.sub.2 and the current passing through the FET when charging the capacitor C.sub.2. The current passing through the FET increases with the increase of the output voltage of the engine speed detecting circuit 20. Thus, the frequency of thetriangular pulse train increases with the increase of the engine speed.

The resonance occurs when the engine speed is equal to an integer multiple of the frequency. Accordingly, when the frequency of the triangular pulse train is 20 Hz, the resonance occurs at engine speeds of 1200, 2400 and 3600 rpm. Therefore, ifthe frequency of the triangular pulse train is varied so as not to coincide with the engine speed in an integer multiple, resonance of the electromagnetic valve may be prevented. In the system, the frequency of the triangular pulse train is varied byvarying the resistance of the FET so that resonance of the electromagnetic valve does not occur.

Referring to FIG. 4 showing another embodiment of the present invention, the circuit is provided with four wind comparators 29, 30, 31 and 32 between the engine speed detecting circuit 20 and the frequency changing circuit 28. The windcomparator comprises a pair of opamps OP.sub.6 and OP.sub.7, resistors R.sub.16 to R.sub.21 and a diode D.sub.6. The wind comparator is so arranged that its output changes to a high level when the input voltage thereto exceeds a predetermined voltage. The wind comparators 29 to 32 are different in set input voltage for the occurrence of an output inversion. Each set input voltage is selected to a value corresponding to an engine speed which would cause the resonance of the electromagnetic valve.

When the output of the engine speed detecting circuit 20 coincides with one of the set input voltages, the corresponding wind comparator produces a high level voltage which is applied to an analog switch SW.sub.2 to turn it on. Thus, theresistance serving to change the frequency changes from the resistance of the resistor R.sub.9 to a composite resistance of resistors R.sub.9 and R.sub.22. Accordingly, the frequency of the triangular pulse train increases to prevent resonance.

Although the above described systems are constructed by analogue devices, the system may also be composed by a digital circuit using microcomputers.

In accordance with the present invention, resonance of the electromagnetic valve is prevented and fluctuation of the air-fuel ratio of the mixture may be reduced to prevent the impairment of driveability of the vehicle.

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