Importance

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Abstract

The device according to the present invention controls air-fuel ratio of the exhaust gas flowing into the catalytic converter when performing the determination of deterioration of the catalytic converter in such a manner that the amount of oxygen released from a three-way catalytic converter is maintained at a suitable value for determining the deterioration of the converter. When the air-fuel ratio of the exhaust gas flowing into the catalytic converter fluctuates between a rich air-fuel ratio and a lean air-fuel ratio, the catalytic converter absorbs oxygen from the exhaust gas when the air-fuel ratio of the exhaust gas is rich, and releases the absorbed oxygen when the air-fuel ratio of the exhaust gas is lean. According to the present invention, the amount of oxygen released from the catalytic converter during the rich air-fuel ratio period of exhaust gas is controlled to a predetermined amount. This amount is set at the value larger than the maximum oxygen absorbing capacity of a deteriorated catalytic converter, and smaller than the maximum oxygen absorbing capacity of a normal catalyst. Therefore, the air-fuel ratio of the exhaust gas flowing out from a deteriorated catalytic converter fluctuates largely, while the air-fuel ratio of the exhaust gas flowing out from a normal catalytic converter does not fluctuate. Thus, deterioration of the catalytic converter can be determined accurately.

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First Claim