Variable compression ratio system for internal combustion engine and method for controlling the system

A variable compression ratio system for an internal combustion engine, including a variable compression ratio mechanism (10) for continuously varying a compression ratio of the engine, the variable compression ratio mechanism including a control shaft (18) rotatably moveable to a rotational position corresponding to the compression ratio, a hydraulic actuator (31) driving the control shaft to the rotational position depending on engine operating conditions, a hydraulic pressure source (60) mechanically driven by the engine to produce a hydraulic pressure supplied to the hydraulic actuator (31), and a hydraulic control (40) for variably controlling the hydraulic pressure supplied to the hydraulic actuator (31) on the basis of the engine operating conditions.