Patent ID: 7677027

Claim:
A deterioration detecting apparatus for a catalyst provided in an exhaust system of an internal combustion engine for purifying exhaust gases, said deterioration detecting apparatus detecting deterioration of said catalyst and comprising: intake air flow rate detecting means for detecting an intake air flow rate of said engine; a first oxygen concentration sensor disposed upstream of said catalyst; a second oxygen concentration sensor disposed downstream of said catalyst; inflowing oxygen amount calculating means for calculating an amount of oxygen flowing into said catalyst according to an oxygen concentration detected by said first oxygen concentration sensor and the intake air flow rate detected by said intake air flow rate detecting means; air-fuel ratio switching control means for alternately switching a control command value of an air-fuel ratio of an air-fuel mixture supplied to said engine between a first value corresponding to a lean air-fuel ratio which is leaner than the stoichiometric ratio and a second value corresponding to a rich air-fuel ratio which is richer than the stoichiometric ratio, according to a result of comparison between the inflowing oxygen amount calculated by said inflowing oxygen amount calculating means and target values of the inflowing oxygen amount; excessive inflowing oxygen amount calculating means for calculating excessive inflowing oxygen amounts which are amounts of oxygen flowing into said catalyst from the time when the control command value is switched from the first value to the second value or vice versa to the time when the oxygen concentration detected by said first oxygen concentration sensor reaches a value corresponding to the stoichiometric ratio; target value correcting means for correcting the target values of the inflowing oxygen amount with the excessive inflowing oxygen amount; and deterioration detecting means for detecting a deterioration degree of said catalyst based on an output of said second oxygen concentration sensor during an operation of said air-fuel ratio switching control means, wherein said air-fuel ratio switching control means performs the switching of the air-fuel ratio using the target values corrected by said target value correcting means.