Variable valve timing system and method for controlling the same

A variable valve timing system that changes opening/closing timing of at least one of an intake valve and an exhaust valve provided in an engine, comprising:

• a changing mechanism that changes the opening/closing timing of at least one of the intake valve and the exhaust valve by an amount of change corresponding to an operation amount of an actuator;
• and configured such that a reference timing at which combustion takes place stably in the engine is present in a middle of a control range in which the opening/closing timing is changed;
• a target phase setting unit that sets target opening/closing timing of at least one of the intake valve and the exhaust valve based on an operating state of the engine;
• a control target value setting unit that sets a control target value of the opening/closing timing based on the target opening/closing timing set by the target phase setting unit;
• an actuator operation amount setting unit that sets an operation amount of the actuator based on a deviation of a current value of the opening/closing timing from the control target value;
• a phase change direction determination unit that determines, based on the current value of the opening/closing timing and the target opening/closing timing, whether a direction of a change in the opening/closing timing is a first direction in which the opening/closing timing approaches the reference timing or a second direction in which the opening/closing timing moves away from the reference timing;
• and
• a change rate control unit that sets a rate of change in the opening/closing timing to a lower value when the opening/closing timing changes in the second direction and the current value of the opening/closing timing is more delayed than the reference timing than when the opening/closing timing changes in the first direction, and also sets the rate of change in the opening/closing timing to the lower value when the opening/closing timing changes in the second direction and the current value of the opening/closing timing is more advanced than the reference timing than when the opening/closing timing changes in the first direction.