Abstract: A variable valve mechanism performs a valve lift switching operation for an intake valve. ECU defines valve lift switching characteristics corresponding to the engine operating region and executes the valve lift switching operation based on the switching characteristics. ECU calculates a parameter fluctuation amount of an engine occurring in response to the valve lift switching operation, and corrects the valve lift switching characteristics when the calculated parameter fluctuation amount exceeds a predetermined judgment value.

Abstract: A variable valve mechanism performs a valve lift switching operation for an intake valve. ECU defines valve lift switching characteristics corresponding to the engine operating region and executes the valve lift switching operation based on the switching characteristics. ECU calculates a parameter fluctuation amount of an engine occurring in response to the valve lift switching operation, and corrects the valve lift switching characteristics when the calculated parameter fluctuation amount exceeds a predetermined judgment value.

Abstract: A variable valve mechanism performs a valve lift switching operation for an intake valve. ECU defines valve lift switching characteristics corresponding to the engine operating region and executes the valve lift switching operation based on the switching characteristics. ECU calculates a parameter fluctuation amount of an engine occurring in response to the valve lift switching operation, and corrects the valve lift switching characteristics when the calculated parameter fluctuation amount exceeds a predetermined judgment value.

Abstract: A variable valve mechanism performs a valve lift switching operation for an intake valve. ECU defines valve lift switching characteristics corresponding to the engine operating region and executes the valve lift switching operation based on the switching characteristics. ECU calculates a parameter fluctuation amount of an engine occurring in response to the valve lift switching operation, and corrects the valve lift switching characteristics when the calculated parameter fluctuation amount exceeds a predetermined judgment value.

Abstract: Evaporated fuel gas from a canister is introduced into an intake path of an internal combustion engine. A target air-fuel ratio is changed to a value on the fuel rich side during the introduction of purge gas in accordance with the purge gas concentration. This change will restrict the shift of the air-fuel ratio toward the lean side, allowing a catalyst to operate at high purification efficiency even during the introduction of purge gas. The change may be varied in accordance with the volume of the evaporated fuel gas or a ratio of the evaporated fuel gas to the fuel supplied to the engine.

Abstract: An air-fuel ratio feedback control has an air-fuel ratio sensor and an oxygen sensor at an upstream side and a downstream side of a catalytic converter. A CPU feedback controls an air-fuel ratio detected by the air-fuel ratio sensor to a target air-fuel ratio, with a feedback gain being varied with an output of the oxygen sensor. The CPU further estimates an air-fuel ratio response speed based on a ratio between an exhaust gas flow amount and a capacity of the catalyst and determines a rate of deterioration of the catalyst. The CPU varies further the feedback gain based on the estimated response speed and the determined rate of deterioration.

Abstract: In an engine fuel supply system, the difference between the actual and target air-fuel ratios, an air-fuel ratio feedback correction coefficient and a learned correction amount are added as a parameter for diagnosing the fuel supply system. The diagnostic parameter is smoothed and the smoothed value is compared with a diagnosis reference value, thereby detecting a malfunction in the fuel supply system. Even if the learned correction amount is not updated, the malfunction in the fuel supply system can be promptly detected from the difference between the actual and target air-fuel ratios and the air-fuel ratio correction coefficient. The diagnosis reference value is determined variably in accordance with engine operating parameters such as the amount of intake air.