Patent ID: 8234853

Claim:
A catalyst degradation determining method for use with an emission control apparatus of an internal combustion engine that includes: a catalyst disposed in an exhaust passage of the internal combustion engine, and a downstream-of- catalyst air-fuel ratio sensor disposed in the exhaust passage downstream of the catalyst, the method comprising the steps of: acquiring an oxidizing-reducing capability index value that changes in accordance with a degree of an oxidizing-reducing capability of the catalyst; controlling an upstream-of-catalyst air-fuel ratio occurring upstream of the catalyst to a lean air-fuel ratio that is lean of a stoichiometric air-fuel ratio so that the catalyst stores oxygen in the catalyst up to a maximum storage amount of oxygen; then controlling the upstream-of-catalyst air-fuel ratio to a rich air-fuel ratio that is rich of the stoichiometric air-fuel ratio and that has a value that is determined in accordance with the oxidizing-reducing capability index value, until a time point when an output of the downstream-of-catalyst air-fuel ratio sensor indicates an air-fuel ratio that is rich of the stoichiometric air-fuel ratio, the value of the rich air-fuel ratio remaining constant from a time point when the upstream-of-catalyst air-fuel ratio is switched to the rich air-fuel ratio to the time point when the output of the downstream-of-catalyst air-fuel ratio sensor indicates the air-fuel ratio that is rich of the stoichiometric air-fuel ratio; estimating a maximum oxygen storage amount of the catalyst by taking into account the value of the rich air-fuel ratio to which the upstream-of-catalyst air-fuel ratio was controlled; and determining whether the catalyst has degraded based on the estimated maximum oxygen storage amount of the catalyst, wherein the rich air-fuel ratio is changed through a series of values between upper and lower limits in accordance with a degree of degradation of the catalyst such that the value of the rich air-fuel ratio changes for different iterations of the catalyst degradation determining method as the degree of degradation of the catalyst changes.