Control system and method for an internal combustion engine

A control system (19, 20, 30, 40) for an internal combustion engine is provided which comprises a compression ratio control mechanism (19) and a control unit (40) for controlling the compression ratio control mechanism. The control unit including an acceleration parameter obtaining section for obtaining an acceleration parameter corresponding to a change of an engine speed, an acceleration determining section for determining whether the engine is in a slow or fast acceleration state on the basis of the acceleration parameter, and a control section for controlling at least one of a compression ratio change speed at which the engine compression ratio is changed and a compression ratio change start time at which a change of the engine compression ratio is started, on the basis of whether the engine is in a slow or fast acceleration state. A control method is also provided.