Importance

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Abstract

In an exhaust gas purification device for an internal combustion engine, a three-way catalyst and a NO.sub.X occluding and reducing catalyst (a NORC) are disposed in the exhaust gas passage of an engine in this order from the upstream side. A first air-fuel ratio sensor is disposed in the exhaust gas passage between the three-way catalyst and the NORC, and a second air-fuel ratio sensor is disposed in the exhaust gas passage downstream of the NORC. An engine electronic control unit (ECU) changes the operating air-fuel ratio of the engine from a lean air-fuel ratio to a rich air-fuel ratio and a rich air-fuel ratio to a lean air-fuel ratio in order to evaluate the abilities of the three-way catalyst and the NORC. The ECU evaluates the catalytic abilities based on the output of the first air-fuel ratio sensor when the engine air-fuel ratio is changed. Further, the ECU evaluates the catalytic ability and the NO.sub.X absorbing capacity of the NORC based on the outputs of the first and the second air-fuel ratio sensor when the engine operating air-fuel ratio is changed. The evaluation of the abilities of both the three-way catalyst and the NORC in one successive changing operation of the engine operating air-fuel ratio is based on only the outputs of the first and the second air-fuel ratio sensor.

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First Claim