Catalyst degradation detection apparatus and catalyst degradation detection method

A catalyst degradation detection apparatus that includes:

• an oxygen storage amount calculating portion that calculates, every preset short time, an amount of oxygen stored in a catalyst provided in an exhaust passage of an internal combustion engine, or an amount of oxygen released from the catalyst, during the short time, and sets a value obtained by accumulating the amount of oxygen at each calculation as an oxygen storage amount of the catalyst, and
• a catalyst downstream sensor that is provided downstream of the catalyst in the exhaust passage and that outputs a signal based on an oxygen concentration in exhaust gas, wherein a determination of whether the catalyst is degraded is made based on the oxygen storage amount calculated by the oxygen storage amount calculating portion, when there is a reversal, between a lean side and a rich side with respect to a value corresponding to the oxygen concentration in the exhaust gas when fuel is combusted at a stoichiometric air-fuel ratio, of the signal of the catalyst downstream sensor,
• the catalyst degradation detection apparatus comprising:
• a correcting portion that, when determining whether the catalyst is degraded based on the oxygen storage amount calculated by the oxygen storage amount calculating portion, applies a correction of a correction amount that corresponds to a response delay time with respect to a change in the oxygen concentration of the signal of the catalyst downstream sensor to the oxygen storage amount;
• a first storage portion that stores the oxygen storage amount calculated by the oxygen storage amount calculating portion in memory in place of a last value as a storage value in each storage cycle that is a time interval that is longer than the short time;
• a slope calculating portion that calculates a slope of a change in the oxygen storage amount calculated by the oxygen storage amount calculating portion with respect to the storage value every short time;
• and
• a second storage portion that stores the slope calculated by the slope calculating portion in the memory in each storage cycle,
• wherein the second storage portion stores, separately in the memory, only the slopes for the number of the storage cycles corresponding to the response delay time of the signal of the catalyst downstream sensor, from among the slopes calculated by the slope calculating portion,
• and wherein the correction amount is calculated based on the slopes stored in the memory, the storage cycle, the response delay time, and a period of time that has passed after the end of the last storage cycle.