Patent ID: 7484504

Claim:
An air-fuel ratio control system for an internal combustion engine, comprising: a catalyst that is provided in an exhaust passage of the internal combustion engine and stores oxygen; an oxygen concentration sensor that is provided downstream of the catalyst and outputs a value corresponding to an air-fuel ratio of exhaust gas flowing out from the catalyst; an integral value calculation portion that calculates an integral value of a deviation which is updated by integrating the deviation between the value output from the oxygen concentration sensor and a reference value corresponding to a target air-fuel ratio; an air-fuel ratio control portion that controls an air-fuel ratio of exhaust gas entering the catalyst to be equal to the target air-fuel ratio based on at least the integral value of the deviation; a target air-fuel ratio switching portion that switches the target air-fuel ratio such that a rich target air-fuel ratio which is richer than a stoichiometric air-fuel ratio is set when the value output from the oxygen concentration sensor has been inverted from a value indicating a rich air-fuel ratio to a value indicating a lean air-fuel ratio while a lean target air-fuel ratio which is leaner than the stoichiometric air-fuel ratio is set when the value output from the oxygen concentration sensor has been inverted from the value indicating the lean air-fuel ratio to the value indicating the rich air-fuel ratio; and an integral value correction portion that corrects the integral value of the deviation when the air-fuel ratio of exhaust gas entering the catalyst is being controlled to be equal to a target air-fuel ratio switched by the target air-fuel ratio switching portion, based on whether the next inversion of the value output from the oxygen concentration sensor takes place within a predetermined time period after the value output from the oxygen concentration sensor has been inverted.