Air-fuel ratio control system for an internal combustion engine

An air-fuel ratio control system for an internal combustion engine including a source of combustible air-fuel mixture, an intake passage means for supplying the combustible mixture from the mixture source to the engine, and an exhaust passage means including an exhaust gas purifying unit installed thereon between the engine and the atmosphere, said control system comprising a composition sensor provided on the exhaust passage means at a position between the purifying unit and the engine for detecting the concentration of a particular component contained in exhaust gases emitted from the engine, said concentration being a function of the air-fuel mixing ratio of the combustible mixture burned in the engine, said composition sensor generating an output signal capable of respectively assuming first and second states when the concentration of the particular exhaust gas component is high and low;

• a control means for determining whether the output signal is in the first state or in the second state and for generating a control signal of a value either increasing or decreasing depending on the output signal when and so long as no change has occurred in the state of the output signal, said control means being further operable, when change has occurred in the state of the output signal, to determine whether the mean value of the highest and lowest values of the control signal has attained a generally identical value at least two times consecutively and also to generate the control signal of the value either increasing or decreasing depending on the output signal if the mean value has not attained a generally identical value at least two times consecutively and to generate a control signal of a value fixed at the means value if the mean value has attained a generally identical value at least two times consecutively;
• and an actuator operable in response to the control signal to adjust the air-fuel mixing ratio of the combustible mixture supplied to the engine so as to be a predetermined optimum value.