| 摘要 |
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 NOX 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 NOX absorbent absorbs NOX 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 NOX when the No. 2 to No. 4 cylinders are operated at a rich air-fuel ratio. However, NOX, without being reduced, is released from the NOX 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 NH3 at the three-way catalyst. Therefore, when NOX without being reduced is released from the NOX absorbent and flows into the denitrating catalyst on the common exhaust passage, NH3 produced at the three-way catalyst is also supplied to the denitrating catalyst. Thus, NOX released from the NOX absorbent at the beginning of the regenerating operation is reduced at the denitrating catalyst by NH3 produced at the three-way catalyst.
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