Valve timing control system for an internal combustion engine

A valve timing control system for an internal combustion engine, comprising:

• operating state detecting means for detecting an operating state of an internal combustion engine;
• actual valve timing detection means for detecting valve timing of at least one of an intake valve and an exhaust valve;
• target valve timing setting means for setting a target valve timing for said valve timing, based on a detection result from said operating state detecting means;
• an actuator for changing the valve timing of at least one of said intake valve and said exhaust valve;
• oil pressure adjusting means for supplying oil to said actuator to drive said actuator and performing adjustment of the oil pressure thereof;
• actual valve timing control means for controlling said actuator by controlling said oil pressure adjusting means, in order to make said valve timing follow up said target valve timing;
• lock means for engaging with said actuator at a given relative angle, and having the engagement be released due to oil being supplied to one of an lead angle side or a delay angle side of the actuator;
• lock release control means for controlling said oil pressure adjusting means so that, when the lock means is in a lock position and said valve timing is going to be changed, a release operation for the lock means is performed before the valve timing changes;
• and
• oil temperature estimation means for estimating the temperature of the oil that said oil pressure adjusting means supplies to the actuator, based on a state of said internal combustion engine during its previous operation time, and based on its current operation state,
• wherein the lock release control means switches a control amount for controlling said oil pressure adjusting means, based on the oil temperature estimated by said oil temperature estimation means.