| 摘要 |
PROBLEM TO BE SOLVED: To efficiently execute rich spike control for controlling reducingly NOx occluded in a NOx catalyst to enhance a NOx control rate, drivability and the like. SOLUTION: The rich spike control is executed to make rich an air-fuel ratio transitorily in every establishment of rich-spike control executing conditions during a lean operation. NOx is occluded thereby in the NOx catalyst 24 during the lean operation, and the occluded NOx is reducedly controlled by the rich spike control to be discharged. The rich spike control is prohibited when an air intake (exhaust gas flow rate) is low as in an idling operation, a deceleration operation, a low speed.low load operation or the like, because the occluded NOx is not reduced sufficiently by the NOx catalyst 24. The rich spike control is executed in the every establishment of the rich spike control executing conditions when brought into an operation condition capable of reducingly controlling the occluded NOx efficiently. The rich spike control is also prohibited during fuel cut, for a prescribed time after finish of the fuel cut, and during a cylinder cut-off operation.
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