Variable camshaft timing system utilizing square-edged spool valve

In an internal combustion engine having a rotatable crankshaft and a rotatable camshaft, said camshaft being position variable in a circumferential direction relative to said crankshaft, means for varying the position of said camshaft relative to said crankshaft, said means for varying comprising a source of hydraulic fluid under pressure, a first hydraulic operator, the operation of said first hydraulic operator being effective to vary the position of said camshaft relative to said crankshaft in a given circumferential direction, first conduit means for delivering hydraulic fluid from said source to said first hydraulic operator to operate said first hydraulic operator, a second hydraulic operator, the operation of said second hydraulic operator being effective to vary the position of said camshaft relative to said crankshaft in an opposed circumferential direction, second conduit means for exhausting hydraulic fluid from said first hydraulic operator, third conduit means for delivering hydraulic fluid from said source to said 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 said first hydraulic operator and said second hydraulic operator to selectively permit hydraulic fluid from said 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 having a first primary orifice and a first secondary orifice, both in communication with said second conduit means, said first primary orifice and said first secondary orifice defining a first primary flow area and a first secondary flow area, respectively, said housing also having a second primary orifice and a second secondary orifice, both in communication with said fourth conduit means, said second primary orifice and said second secondary orifice defining a second primary flow area and a second secondary flow area, respectively, 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 first primary orifice in first, third and fifth positions of said valve member and permitting flow through said first primary orifice in second and fourth positions of said valve member,
• said first land also being capable of blocking flow through said first secondary orifice in first, second, third and fifth positions of said valve member and permitting flow through said first secondary orifice in a fourth position of said valve member,
• said second land being capable of blocking flow through said second primary orifice in first, second and fourth positions of said valve member and permitting flow through said second primary orifice in third and fifth positions of said valve member,
• said second land also being capable of blocking flow through said second secondary orifice in first, second, third and fourth positions of said valve member and permitting flow through said second secondary orifice in a fifth position of said valve member;
• force imposing means imposing a load on a first surface of said valve member to urge said valve member in a given direction;
• fifth conduit means for transmitting hydraulic pressure from the source to act on a second surface of said valve member to urge said valve member in an opposed direction, said fifth conduit means comprising a control member therein to controllably increase or reduce the pressure of the source that acts on said second surface of said valve member;
• and
• centering means for centering said valve member in a fixed position relative to said housing when the opposed forces acting on said valve member are in balance.