Abstract: An air-fuel ratio control system for an internal combustion engine, including an oxygen concentration sensor producing an output signal whose level is substantially proportional to an oxygen concentration of the exhaust gas, effects a feedback control of the air-fuel ratio of mixture to be supplied to the engine toward a target value determined on the basis of various parameters of the engine operation and corrected in response to a magnitude of atmospheric pressure, so that the target air-fuel ratio is made lean as the magnitude of the atmospheric pressure decreases. Thus, the reduction of the engine output power and the increase of the fuel comsumption which have been encountered in conventional systems are prevented.

Abstract: An air intake side secondary air supply system for an internal combustion engine having an air intake side secondary air supply passage and an O.sub.2 sensor controls a duty ratio of opening and closing of an open/close valve disposed in the air intake side secondary air supply passage in accordance with a detected air-fuel ratio of mixture by means of an output signal of the O.sub.2 sensor. The system sets a base valve open period of the open/close valve every predetermined period and decreases the base valve open period by a first predetermined correction amount when the detected air-fuel ratio is leaner than a target air-fuel ratio, and increases the base valve open period by a second predetermined correction amount when the detected air-fuel ratio is richer than the target air-fuel ratio.