Abstract: In order to improve an exhaust gas purification performance immediately after a cold engine is started, firstly for HC purification, an air-fuel ratio is controlled to be in a lean range immediately after the engine start. Next for NOx purification, the air-fuel ratio is switched to be from lean to rich based upon a parameter (engine load, namely, fuel injection amount) related to NOx density at an engine exit or a parameter (catalyst temperature or water temperature) related to the catalyst temperature. The air-fuel ratio to be rich is set as a plurality of values based upon the parameter and as a richer range as the parameter shows high density or high temperature.

Abstract: An oxygen storage amount of a catalyst is estimated by a controller in accordance with a sensed air-fuel ratio of an inflowing exhaust gas mixture flowing into the catalyst, and a sensed intake air amount of the engine, to control the air-fuel ratio of the engine. The estimated oxygen storage amount is corrected to reduce an error in computing the estimated oxygen storage amount when a downstream exhaust condition sensed by an exhaust sensor disposed on the downstream of the catalyst becomes equal to a predetermined threshold, which is modified in accordance with the intake air amount for better emission control performance.

Abstract: In order to improve an exhaust gas purification performance immediately after a cold engine is started, firstly for HC purification, an air-fuel ratio is controlled to be in a lean range immediately after the engine start.

Abstract: A controller computes an oxygen storage amount of a catalyst based on the characteristics of an exhaust flowing into the catalyst, and controls the air-fuel ratio of an engine so that the oxygen storage amount of the catalyst is a target value. When it is determined that the engine starts from the warmed-up state when the engine starts, the air-fuel ratio of the engine is controlled to rich until the exhaust flowing out of the catalyst becomes rich. In this way, all the oxygen stored by the catalyst is first released, the NOx purification performance of the catalyst is maintained, and the NOx release amount immediately after engine startup is suppressed.

Abstract: A controller controls an air-fuel ratio of an engine to maintain the oxygen storage amount of a catalyst provided in an exhaust passage at a predetermined amount. At this time, the controller first estimates the initial value of the oxygen storage amount based on the catalyst temperature on engine startup, and then computes an oxygen storage characteristic using this estimated initial value. In this way, the oxygen storage amount can be precisely computed even immediately after engine startup, and the conversion efficiency of the catalyst is maintained at a high level.

Abstract: An oxygen storage amount of a catalyst is estimated by a controller in accordance with a sensed air-fuel ratio of an inflowing exhaust gas mixture flowing into the catalyst, and a sensed intake air amount of the engine, to control the air-fuel ratio of the engine. The estimated oxygen storage amount is corrected to reduce an error in computing the estimated oxygen storage amount when a downstream exhaust condition sensed by an exhaust sensor disposed on the downstream of the catalyst becomes equal to a predetermined threshold, which is modified in accordance with the intake air amount for better emission control performance.

Abstract: A controller computes an oxygen storage amount of a catalyst based on the characteristics of an exhaust flowing into the catalyst, and controls the air-fuel ratio of an engine so that the oxygen storage amount of the catalyst is a target value. When it is determined that the engine starts from the warmed-up state when the engine starts, the air-fuel ratio of the engine is controlled to rich until the exhaust flowing out of the catalyst becomes rich. In this way, all the oxygen stored by the catalyst is first released, the NOx purification performance of the catalyst is maintained, and the NOx release amount immediately after engine startup is suppressed.

Abstract: A controller controls an air-fuel ratio of an engine to maintain the oxygen storage amount of a catalyst provided in an exhaust passage at a predetermined amount. At this time, the controller first estimates the initial value of the oxygen storage amount based on the catalyst temperature on engine startup, and then computes an oxygen storage characteristic using this estimated initial value. In this way, the oxygen storage amount can be precisely computed even immediately after engine startup, and the conversion efficiency of the catalyst is maintained at a high level.