| 摘要 |
PROBLEM TO BE SOLVED: To prevent unpurified NOx from being emitted out of NOx storage reduction catalyst due to an engine operational condition. SOLUTION: NOx storage reduction catalyst 7 is disposed to the exhaust passage 2 of an engine 1, so that NOx is thereby absorbed by the catalyst 7 while the engine is in operation at a leans air-fuel ratio combustion. An ECU 30, depending on the requisition of acceleration caused by the change of an acceleration pedal operation, permits the specified quantity of fuel to be injected asynchronously to the respective cylinders of the engine during the period of time before a requisition for acceleration is detected to start the opening control of a throttle valve 15 by the ECU 30, so that an air-fuel ratio for exhaust gas flowing through catalyst 7 is temporarily made rich. By this constitution, NOx is emitted out of catalyst 7 so as to be purified simultaneously. Therefore, when an engine operational condition is changed into a condition where NOx is apt to be naturally emitted out of catalyst 7 with the opening controlled of the throttle valve, the amount of NOx storaged by catalyst 7 is lowered, and no natural emission of NOx will take place.
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