1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, and more particularly to a valve timing control system for an internal combustion engine having a pair of intake valves for each cylinder.
2. Description of the Prior Art
There has been proposed an internal combustion engine having a pair of intake ports and a pair of intake valves for each cylinder in which the intake valves are driven at predetermined different times to close and open the corresponding intake port. See Japanese Unexamined Patent Publication No. 56(1981)-44404, for example. By providing two intake ports for each cylinder, the effective area of the intake opening can be enlarged and the volumetric efficiency is improved, whereby the engine output can be improved.
In order to improve the engine output by making the volumetric efficiency high, generally it is preferred that the opening time of the intake valve, i.e., the time that the intake valve opens, be longer for a given area of the intake opening when the engine operates at a high rotational speed, and especially when the engine operates at a high speed under heavy load. When the engine operates at a high speed under heavy load, combustion in the engine is not adversely affected even if the opening time of the intake valve is extended and the valve overlap is extended, since the ratio of the residual exhaust gas to the intake gas is small and back flow of the intake gas does not occur because of the high inertia speed of the intake gas under such operating condition of the engine.
On the other hand, when the opening time of the intake valve is extended during operation of the engine at a low speed under heavy load, back flow of the intake gas occurs since the inertia speed of the intake gas is low, whereby the volumetric efficiency is lowered.