| 摘要 |
PROBLEM TO BE SOLVED: To always and surely purify NOx when an operating state is switched from a lean air fuel ratio operation to a rich air fuel ratio operation and NOx occluded by an occlusion type NOx catalyst is reduced and removed, in an exhaust emission control device provided with the occlusion type NOx catalyst. SOLUTION: By an air fuel ratio gradually-changing means, an air fuel ratio is gradually changed (tailing processing) between a lean air fuel ratio and a rich air fuel ratio and an operating state of an internal combustion engine is gradually switched between a lean air fuel ratio operating state and a rich air fuel ratio operating state. At a prescribed air fuel ratio range (a value 12 to a value 18) in the vicinity of a theoretical air fuel ratio, the rate of change of an air fuel ratio is constituted to be large and to be changed to infinity, for instance, by an air fuel ratio rate of change changing means. |
