Patent ID: 7698886

Claim:
A catalyst deterioration degree acquiring apparatus that is applied to an internal combustion engine including a catalyst that is disposed in an exhaust passage of the internal combustion engine and has an oxygen storing function, a downstream air-fuel-ratio sensor that is disposed in the exhaust passage at a downstream side of the catalyst for outputting a value according to an air-fuel ratio of a gas flown out of the catalyst, and an upstream air-fuel-ratio sensor that is disposed in the exhaust passage at an upstream side of the catalyst for outputting a value according to an air-fuel ratio of a gas flowing into the catalyst, comprising: maximum oxygen storing quantity estimating unit that changes the air-fuel ratio of the gas flowing into the catalyst to a predetermined lean air-fuel ratio, which is on a lean side with respect to a stoichiometric air-fuel ratio, from an air-fuel ratio on a rich side with respect to the stoichiometric air-fuel ratio at a point when the output from the downstream air-fuel-ratio sensor is changed to a state indicating the air-fuel ratio on the rich side with respect to the stoichiometric air-fuel ratio from a state indicating the air-fuel ratio on the lean side with respect to the stoichiometric air-fuel ratio; changes the air-fuel ratio of the gas flowing into the catalyst to a predetermined rich air-fuel ratio, which is on the rich side with respect to the stoichiometric air-fuel ratio, from an air-fuel ratio on the lean side with respect to the stoichiometric air-fuel ratio at a point when the output from the downstream air-fuel-ratio sensor is changed to the state indicating the air-fuel ratio on the lean side with respect to the stoichiometric air-fuel ratio from the state indicating the air-fuel ratio on the rich side with respect to the stoichiometric air-fuel ratio; and estimates a maximum oxygen storing quantity, which is a maximum quantity of oxygen storable by the catalyst, by utilizing at least the point when the output from the downstream air-fuel-ratio sensor is changed after the changeover of the air-fuel ratio of the gas flowing into the catalyst, and the output from the upstream air-fuel-ratio sensor; and catalyst deterioration degree acquiring unit that acquires, as a deterioration degree of the catalyst, a degree of decrease of the maximum oxygen storing quantity estimated with respect to an increase in the air flow rate on the basis of plural maximum oxygen storing quantities, which are respectively estimated in case where the flow rate of air sucked into the internal combustion engine assumes plural different values, and the plural values of the air flow rate.