Cam phasers, as is known in the art, are used to control the angular relationship of a pulley or sprocket to a camshaft of a combustion engine. A variable cam phaser (VCP) allows changing the phase relationship while the engine is running. Typically, a VCP is used to shift an intake cam on a dual overhead cam engine in order to broaden the torque curve of the engine, to increase peak power at high revolution speeds, and to improve the idle quality. Also, an exhaust cam can be shifted by a cam phaser in order to provide internal charge diluent control, which can significantly reduce HC and NOx emissions, or to improve fuel economy. The above objectives are in the following briefly termed as combustion demands. With this definition a VCP is used to account for combustion demands.
Cam phasers are controlled by hydraulic systems, which use pressurised lubrication oil from the engine in order to change the relative position between camshaft and crankshaft, thus altering the valve timing. An “advance” or “retard” position of the camshaft is commanded via an oil flow control valve (in the following briefly termed “OCV”). The OCV controls the oil flow to different ports entering a VCP, thus controlling the angular position of the camshaft relative to pulley or sprocket. However, the efforts in the valve train pressurise the oil contained in the chambers of the VCP such that the oil pressure inside the VCP reaches peaks, which can be higher than the oil control supply pressure, i.e., the oil pressure supplied by the engine. This can lead to a certain amount of reverse oil flow across the OCV, diminishing the phase rate performance of the cam phasing system.
To avoid the reverse oil flow under the above mentioned circumstances, recent approaches have proposed to employ a check valve, integrated in the oil passage of either the cylinder head or the crankcase. Such a check valve also ensures that the cam phaser does not empty out in cases when the oil pressure is reduced, for example when the engine is stopped. However, this approach adds significant cost to the cylinder head or engine block. Also, the implementation of the check valve can be difficult because of oil routing. Furthermore, the check valve should not be placed too far away from the cam phaser in order to be still effective.
The present invention relates to an improved control scheme for an oil control valve. The object of the invention is to provide a remedy for the defects of the prior art. It is an object of the present invention to overcome problems and drawbacks described above. The present invention specifically aims at addressing the problem of the reverse oil flow.