Importance

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Abstract

In the present invention, the No. 1 cylinder of an engine is connected to a first exhaust passage and the No. 2 to No. 4 cylinders are connected to a second exhaust passage. A three-way catalyst and a NO.sub.X absorbent are disposed in the first and second exhaust passage, respectively. A denitrating catalyst is disposed in a common exhaust passage to which the first and second exhaust passage merge. The NO.sub.X absorbent absorbs NO.sub.X when the No. 2 to No. 4 cylinders are operated at a lean air-fuel ratio, and is regenerated, i.e., releases and reduces the absorbed NO.sub.X when the No. 2 to No. 4 cylinders are operated at a rich air-fuel ratio. However, NO.sub.X, without being reduced, is released from the NO.sub.X absorbent during a short period at the beginning of the regenerating operation. In the present invention, the No. 1 cylinder is operated at a rich air-fuel ratio during the short period at the beginning of the regenerating operation in order to produce NH.sub.3 at the three-way catalyst. Therefore, when NO.sub.X without being reduced is released from the NO.sub.X absorbent and flows into the denitrating catalyst on the common exhaust passage, NH.sub.3 produced at the three-way catalyst is also supplied to the denitrating catalyst. Thus, NO.sub.X released from the NO.sub.X absorbent at the beginning of the regenerating operation is reduced at the denitrating catalyst by NH.sub.3 produced at the three-way catalyst.

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First Claim