| 摘要 |
PROBLEM TO BE SOLVED: To perform discharge reduction of NOx reliably occluded in an NOx catalyst without worsening fuel economy despite of a change of discharge characteristics of NOx of the NOx catalyst with regard to an internal combustion engine. SOLUTION: By bringing an exhaust air fuel ratio into oxygen concentration decreasing atmosphere, discharge reduction of NOx occluded in an NOx catalyst 6A arranged in the exhaust passage of an internal combustion engine. When NOx concentration detected by an NOx sensor 10, arranged in a position situated downstream from the NOx catalyst and discharged from the NOx catalyst 6A, is reduced to a value lower than a given value it is permitted that an exhaust air fuel ratio is varied from oxygen concentration decreasing atmosphere to oxygen excessive atmosphere by an atmosphere varying means 23.
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