| abstract |
A three way catalyst and an NO x storage catalyst are arranged in an engine exhaust passage. In an engine low load operating region, a combustion is performed under a lean air-fuel ratio, and when NO x should be released from the NO x storage catalyst, the air-fuel ratio in a combustion chamber is made rich. In an engine medium load operating region, the base air-fuel ratio is made lower as compared with the engine low load operating region, and the air-fuel ratio in the combustion chamber is made rich at a shorter period as compared with the engine low load operating region. When the operating state of the engine is switched from the engine low load operating region to the engine medium load operating region, the air-fuel ratio in the combustion chamber is temporarily made rich, and the degree of richness of the air-fuel ratio at this time is stepwise lowered. |