Importance

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Abstract

Start catalysts (SCs) having an O.sub.2 storage function are disposed in an exhaust gas passage of an internal combustion engine, and an NOx occluding and reducing catalyst is disposed in the exhaust gas passages downstream of the SCs. NOx in the exhaust gas is absorbed by the NOx occluding and reducing catalyst when the engine is in operation at a lean air-fuel ratio. When NOx is to be released, the engine is operated at a rich air-fuel ratio so that exhaust gas having a rich air-fuel ratio flows into the SCs and into the NOx occluding and reducing catalyst. An electronic control unit (ECU) executes the secondary fuel injection that does not contribute to the combustion in the expansion stroke or in the exhaust stroke of each cylinder by using a direct cylinder fuel injection valve at the time when the engine operating air-fuel ratio is to be changed from a lean air-fuel ratio operation to a rich air-fuel ratio, so that exhaust gas having a rich air-fuel ratio flows into the SC and that oxygen stored in the SC is released. At the time of changing over the air-fuel ratio, therefore, the air-fuel ratio is changed from the lean side to the rich side without delay, eliminating such an occurrence that NOx is released from the NOx occluding and reducing catalyst without being purified at the time of changing the air-fuel ratio.

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First Claim