Importance

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Abstract

An NOx occluding and reducing catalyst is disposed in an exhaust gas passage of an internal combustion engine to absorb NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, and to release and reduce the absorbed NOx when the air-fuel ratio of the exhaust gas is rich. A recovery operation is executed only under particular operating conditions of the engine to heat the NOx occluding and reducing catalyst so as to release NOx as well as SOx absorbed thereby. The engine operating conditions for executing the recovery operation are expanded with an increase in the amount of SOx held by the NOx occluding and reducing catalyst while preventing a drop in the engine fuel efficiency. The recovery operation is easily executed with an increase in the holding amount of SOx, and a state in which SOx are held in increased amounts by the NOx occluding and reducing catalyst is prevented from lasting long.

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First Claim