AIR-FUEL RATIO CONTROL DEVICE FOR INTERNAL-COMBUSTION ENGINE

A present air-fuel ratio control apparatus, applied to an internal combustion engine having a catalyst disposed in an exhaust passage of the engine, comprises a downstream air-fuel ratio sensor 56 (oxygen concentration cell type oxygen concentration sensor) disposed at a position downstream of the catalyst, and air-fuel ratio control means for controlling, based on an output value of the downstream air-fuel ratio sensor, an air-fuel ratio of a mixture supplied to the engine 10 so as to change an air-fuel ratio of a catalyst inflow gas. Further, the air-fuel ratio control means controls the air-fuel ratio of the mixture supplied to the engine 10 in such a manner that the air-fuel ratio of the catalyst inflow gas becomes an air-fuel ratio richer than a stoichiometric air-fuel ratio when the output value of the downstream air-fuel ratio sensor decreases, and that the air-fuel ratio of the catalyst inflow gas becomes an air-fuel ratio leaner than the stoichiometric air-fuel ratio when the output value of the downstream air-fuel ratio sensor increases.