Patent ID: 7165389
Filing Date: 2007-01-23
Classification: F01N,F02D,Y02T

Abstract:
1. 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 rich air-fuel ratio that is rich of a stoichiometric air-fuel ratio so that the catalyst completely releases oxygen stored in the catalyst, wherein the rich air-fuel ratio changes from an initial rich air-fuel ratio to a second rich air-fuel ratio that is less rich than the initial rich air-fuel ratio and is equal to or larger than a lower limit value of a window range for the air-fuel ratio; then controlling the upstream-of-catalyst air-fuel ratio to a lean air-fuel ratio that is lean 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 lean of the stoichiometric air-fuel ratio, wherein the lean air-fuel ratio changes from an initial lean air-fuel ratio to a second lean air-fuel ratio that is less lean than the initial lean air-fuel ratio and is equal to or smaller than an upper limit value of the window range; estimating a maximum oxygen storage amount of the catalyst by taking into account the value of the lean 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.