The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more particularly to an air-fuel ratio control which is capable of improving an engine performance in a transient operation during EGR (exhaust gas recirculation) control.
JP-A-10-288043 discloses an air-fuel ratio control system for an internal combustion engine. This air-fuel ratio control system is arranged to determine a preferable relationship between an excess air ratio and an EGR ratio in correspondence to each engine operating condition and to control the engine on the basis of this determined relationship in order to mainly ensure exhaust gas purifying performance.
However, this air-fuel ratio control system has limitations in varying the excess air ratio and the EGR ratio since the air-fuel ratio control is executed on the basis of this linear relationship.
It is therefore an object of the present invention to provide an improved air-fuel ratio control system which is capable of maintaining an optimum combustion condition of an internal combustion engine even when an excess air ratio and an EGR ratio of the engine are separately varied, such as during a transient operation.
An aspect of the present invention resides in an air-fuel ratio control system which is for an internal combustion engine and which comprises an engine condition detecting unit and a control unit coupled to the engine condition detecting unit. The engine condition detecting unit detects an engine operating condition of the internal combustion engine. The control unit is arranged to calculate a target engine torque on the basis of the engine operating condition; to calculate a target EGR ratio, a target excess air ratio and a target intake air quantity on the basis of the engine operating condition and the target engine torque; to calculate a target equivalence ratio on the basis of the target EGR ratio and the target excess air ratio; to calculate a target injection quantity on the basis of the engine operating condition and the target equivalence ratio; and to control an air-fuel ratio at a desired value by bringing a real intake air quantity to the target intake air quantity and by bringing a real fuel injection quantity to the target fuel injection quantity.
Another aspect of the present invention resides in a method for controlling an air-fuel ratio of an internal combustion engine. The method comprises a step for detecting an engine operating condition of the internal combustion engine; a step for calculating a target engine torque on the basis of the engine operating condition; a step for calculating a target EGR ratio, a target excess air ratio and a target intake air quantity on the basis of the engine operating condition and the target engine torque; a step for calculating a target equivalence ratio on the basis of the target EGR ratio and the target excess air ratio; a step for calculating a target injection quantity on the basis of the engine operating condition and the target equivalence ratio; and a step for controlling an air-fuel ratio at a desired value by bringing a real intake air quantity to the target intake air quantity and by bringing a real fuel injection quantity to the target fuel injection quantity.