1. Field of the Invention
The present invention relates generally to a valve timing control system for an internal combustion engine, for adjusting open and close timing of intake and/or exhaust valves depending upon engine driving condition. More specifically, the invention relates to a valve timing control device for an internal combustion engine which has a pair of clutch for selectively restricting phase shift of a camshaft relative to engine revolution in phase advancing and retarding direction.
2. Description of the Background Art
In the modern automotive technologies, it is important to achieve both of high driving performance and fuel economy. High engine performance is especially important at high engine load range. On the other hand, at low engine load range, fuel economy will be regarded as more important factor than the engine performance. Furthermore, in the recent days, anti-polution is becoming more important factor for avoiding contamination of atmosphere. Fuel consumption and purity of exhaust gas can be adjusted by adjusting induction efficiency of an air/fuel mixture into engine combustion chambers. For adjusting mixture gas induction efficiency, some of advanced automotive internal combustion engines employ variable cam timing technologies for advancing and retarding valve open timing with respect to top-dead-center (TDC) in the engine revolution cycle.
For example, the U.S. Pat. No. 4,535,731, issued on to, and assigned to the common owner to the present invention, proposes a valve timing control device for an internal combustion engine. The device adjusts open timing of an intake valve and/or exhaust valve of an internal combustion engine. The device includes a helical intermediate gear formed with an external helical gear teeth meshing with an internal gear teeth of an engine revolution synchronous component, such as a timing sprocket or timing pulley, and an internal gear teeth meshing with an external gear teeth of an internal gear which is rigidly connected to a camshaft. The intermediate gear is axially movable for varying rotational phase relationship between the engine revolution synchronous component and the internal gear. The axial position of the intermediate gear is adjusted hydraulically depending upon an engine driving condition so that open timing of the intake valve and/or the exhaust valve is advanced or retarded with respect to engine revolution cycle.
Such prior proposed valve timing control device is successfully in effectively adjusting valve timing. However, on the other hand, such prior proposed valve timing control device employs the helical gear teeth for adjusting the phase relationship between the rotational input torque and rotational cam drive torque. For holding accuracy in engagement between gears, it is required substantially high accuracy in production. Therefore, machining of the gears becomes difficult and costful. Furthermore, during long use, wearing and secular variation may loose tight engagement between the gears for causing variation of the valve timing out of an optimal range.
Furthermore, since the prior proposed device is required to drive the helical intermediate gear meshing with the timing sprocket or timing pulley, and the inner gear, relatively great hydraulic and/or mechanical force is required. For reliably accepting relatively large operational force, the whole assembly of the valve timing control device necessarily becomes bulky to increase engine weight.