AIR-FUEL RATIO CONTROL MEANS FOR INTERNAL COMBUSTION ENGINES

An air-fuel ratio control system for an internal combustion engine wherein a control unit is provided for producing a basic fuel quantity signal based on an engine operating condition. The system includes a feedback system including an O.sub.2 sensor provided in the engine exhaust passage for producing an ON-OFF type signal based on the oxygen content in the engine exhaust gas. The control unit produces a feedback signal which is based on the signal from the O.sub.2 sensor. The control unit further performs a learning control by providing supplementary signals periodically based on the average value of the feedback control signal and the gain of the feedback control is decreased as the learning progresses.