Differential pressure control system for variable camshaft timing system

In an internal combustion engine having a rotatable crankshaft and a rotatable camshaft, the camshaft being position variable in a circumferential direction relative to the crankshaft, means for varying the position of the camshaft relative to the crankshaft, said means for varying comprising a source of hydraulic fluid under pressure, a first hydraulic operator connected to said crankshaft and to said camshaft, the operation of said first hydraulic operator being effective to vary the position of the camshaft relative to the crankshaft in a given circumferential direction, first conduit means for delivering hydraulic fluid from the source to the first hydraulic operator to operate the first hydraulic operator, a second hydraulic operator connected to said crankshaft and to said camshaft, the operation of said second hydraulic operator being effective to vary the position of the camshaft relative to the crankshaft in an opposed circumferential direction, second conduit means for exhausting hydraulic fluid from the first hydraulic operator, third conduit means for delivering hydraulic fluid from the source to the second hydraulic operator to operate the second hydraulic operator, fourth conduit means for exhausting hydraulic fluid from the second hydraulic operator and control means for controlling the exhausting of hydraulic fluid from the first hydraulic operator and the second hydraulic operator to selectively permit hydraulic fluid from the source to operate one or another of said first hydraulic operator and said second hydraulic operator, said control means comprising:

• a spool valve in communication with said second conduit means and said fourth conduit means, said spool valve comprising a housing and a valve member, said valve member being reciprocable within said housing and comprising first and second opposed ends and first and second spaced apart lands between said opposed ends, said first land being capable of blocking flow through said second conduit means in first and third positions of said valve member and permitting flow through said second conduit means in a second position of said valve member, said second land being capable of blocking flow through said fourth conduit means in said first and second positions of said valve member and permitting flow through said second conduit means in said third position of said valve member;
• fifth conduit means for transmitting hydraulic pressure from the source to act on a first surface of said valve member at substantially the pressure of the source to urge the valve member in a given direction;
• force imposing means for imposing a load on said valve member to urge said valve member in an opposed direction, said force imposing means having a second surface with an area that is substantially greater than the area of said first surface;
• sixth conduit means for transmitting hydraulic pressure from the source to the force imposing means to act on said second surface of said force imposing means, said sixth conduit means comprising a control member therein to controllably reduce the pressure of the source that acts on said second surface of said force imposing means;
• and
• centering means for centering said valve member in a substantially fixed position relative to said housing when the hydraulic forces acting on said valve member are in balance.