Importance

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Abstract

In an exhaust gas purification device for an internal combustion engine, a NO.sub.X occluding and reducing catalyst is disposed in the exhaust gas passage of an engine. The NO.sub.X occluding and reducing catalyst absorbs NO.sub.X in the exhaust gas when the air-fuel ratio of the exhaust gas is at a lean air-fuel ratio and releases and reduces NO.sub.X when the air-fuel ratio of the exhaust gas is at a rich air-fuel ratio. The air-fuel ratio of the exhaust gas flowing out from the catalyst is detected by an air-fuel ratio sensor disposed in the exhaust gas passage downstream of the catalyst. When the air-fuel ratio of the exhaust gas flowing into the catalyst is changed from a rich air-fuel ratio to a lean air-fuel ratio, the air-fuel ratio of the exhaust gas flowing out from the catalyst stays at a stoichiometric air-fuel ratio before it changes to a lean air-fuel ratio. The length of the period in which the air-fuel ratio of the exhaust gas flowing out from the catalyst stays at a stoichiometric air-fuel ratio corresponds to the magnitude of the ability of the NO.sub.X occluding and reducing catalyst as a reducing catalyst. Thus, by measuring the length of the stoichiometric air-fuel ratio period of the exhaust gas flowing out from the NO.sub.X occluding and reducing catalyst, the ability of the NO.sub.X occluding and reducing catalyst as a reducing catalyst can be precisely evaluated.

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First Claim