Patent ID: 7596941

Claim:
An air-fuel ratio control device for an internal combustion engine, comprising: a catalyst, disposed in an exhaust system of the internal combustion engine, that purifies exhaust gas; an upstream and a downstream air-fuel ratio sensor respectively disposed at an upstream side and a downstream side of the catalyst, the upstream and the downstream air-fuel ratio sensors detecting the air-fuel ratio of the exhaust gas; a first air-fuel ratio feedback control unit that adjust the air-fuel ratio supplied to the internal combustion engine using an output value of the upstream air-fuel ratio sensor; a filter processing unit that performs filter processing such that variation in an output value of the downstream air-fuel ratio sensor is suppressed; a second air-fuel ratio feedback control unit that adjusts a control constant of the first air-fuel ratio feedback control unit using the filter processed value of the output value of the downstream air-fuel ratio sensor; and a filter constant change unit that changes a filter constant of the filter processing unit so as to reduce suppression of the filter processing and set a filter effect smaller, when the air-fuel ratio supplied to the internal combustion engine is being controlled to be rich or lean using rich or lean control, and not when the air-fuel ratio is being controlled such that the atmosphere of the catalyst is in the vicinity of the stoichiometric air-fuel ratio.