Abstract: A hydraulic system for actuating gas-change valves in internal combustion engines or compressors wherein the output piston associated with each gas-change valve is capable of being moved against the force of a closing spring through a control circuit filled with control fluid by an input piston operable by a cam, with means for venting the control fluid at the highest point of the hydraulic system, and wherein the input cylinder communicates with a system for refilling with control fluid. The means for venting the control fluid are formed as permanent venting means, a variable area restrictor determining the amount of control fluid in the hydraulic system during operation is provided which is located in a discharge circuit, and the cam actuating the output piston is formed with a pre-ramp and a post-ramp.

Abstract: A controllable hydraulic valve mechanism for reciprocating engines or pumps including a drive piston, operable by a cam is provided with control edges and projects into a drive cylinder; a working piston which operates the valve directly and is guided in a working cylinder. The valve mechanism provides a continuous communication between the drive cylinder interior and a second reservoir which absorbs a substantial part of the energy produced by the lift of a cam. Control sleeves surround the drive piston and are operable by actuating means operable from outside the drive cylinder. Each of the control sleeves has a control ring groove which cooperates with control edges provided in the drive piston. A further sleeve in the working cylinder surrounds the working piston, is rotatable by actuating means operable from outside the working cylinder, and is provided with a control ring groove for varying the valve lift.