| 摘要 |
PROBLEM TO BE SOLVED: To overcome such a problem that when changing over from the com bustion in a lean region to the combustion in a rich region under a condition where catalyst disposed before NOx catalyst has an oxygen storage capacity, the catalyst has a storage effect to cause the oxidation-reduction reaction of HC and CO with oxygen so that the air-fuel ratio is not quickly changed over to the rich region and the time required for passing through the area of air-fuel ratio (16-18) having a lot of NOx exhaust becomes long to generate a lot of NOx. SOLUTION: In a rich purge control performed to reduce and emit NOx occluded in NOx occlusion catalyst, the air-fuel ratio is so controlled as to be temporarily richer than the air-fuel ratio under the rich purge in changing over from the lean region to the rich region so that oxygen stored in a three way catalyst having the oxygen storage capacity and disposed on the upstream side of an exhaust pipe passage can be quickly consumed and quickly pass over the air-fuel ratio region generating a lot of NOx in changing over from the lean region to the rich region.
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