| 摘要 |
PROBLEM TO BE SOLVED: To compensate for lowering of emission control performance due to the increase of the temperature of an NOx catalyst during lean operation after rich operation of an engine when a lean NOx catalyst to purify NOx in a lean operation state in an air-fuel ratio higher than a theoretical air-fuel ratio and a catalytic converter rhodium catalyst situated downstream from the lean NOx catalyst are arranged. SOLUTION: A lean NOx catalyst 51 on the upper stream side, a catalytic converter rhodium catalyst 52, and a lean NOx catalyst 53 on the downstream side are arranged in the exhaust passage 31 of an engine 1, in the order, toward a spot situated downstream such that a distance L2 between the catalytic converter rhodium 52 and the downstream lean NOx catalyst 53 is longer than a distance L1 between the upstream lean NOx catalyst 51 and the catalytic converter rhodium catalyst 52. Even when in the rich operation state of the engine 1 and in a state that an exhaust gas temperature on the inlet side of the upstream lean NOx catalyst 51 is increased, the coefficient of NOx purification of the lean NOx catalyst 51 on the upper stream side is decreased owing to switching to a lean operation state, NOx not purified thereby is purified by the downstream lean NOx catalyst 53 wherein temperature on the inlet side is low and NOx emission control performance is high. |
