Apparatus for changing operation timing of valves for internal combustion engine

In a valve operation arrangement of an internal combustion engine including intake or exhaust valves normally urged by springs to close intake or exhaust ports of a combustion chamber;

• cam means rotated in synchronism with the engine crankshaft for operating said valves to open said ports;
• and a hydraulically operated valve operation timing changing device for varying the operation of said valves between a first condition and a second condition, said device comprising:
• spring means for normally biasing said valve operation timing device in a mode for operating said valves in said first condition;
• a fluid circuit operative to conduct operating fluid to said valve operation timing device in opposition to the bias of said spring means, said hydraulic circuit including a hydraulic pressure generating means for producing a fluid pressure in said hydraulic circuit that varies directly with the rotational speed of said engine;
• and
• said spring means having a spring force sufficient to place said valve operation timing device in a mode for operating said valves in said first condition while said hydraulic pressure in said fluid circuit is in a range of fluid pressures below a predetermined level and in a mode for operating said valves in said second condition when the hydraulic pressure in said fluid circuit exceeds said predetermined level.