high pressure accumulator 56 flows via a first magnetic valve 58 into chamber 60 opens intake valve 18 or exhaust valve 36 and displaces the hydraulic fluid in chamber 62 when the desired valve lift has been achieved first magnetic valve 58 is closed by valve control unit 34 in order to close intake valve 18 or exhaust valve 36 second magnetic valve 64 is opened in this context first magnetic valve 58 remains closed the pressure of the hydraulic fluid that is constantly present at chamber 62 closes intake valve 18 or exhaust valve 36 hydraulic fluid flowing out of chamber 62 is collected in a reservoir 66 with the aid of the valve actuator shown individual control may be provided for each valve in this context the charge of the combustion chamber with fresh air may be set by the duration of being open and or the lift of intake valve 18 at least at low rotary speeds an intake valve may be opened several times per operating period fig3 shows a flow diagram as an exemplary method of the present invention the way it may proceed in the composite of control units 32 34 and 42 that communicate via bus system 46 in this context block 3 1 represents a main program for controlling the piston engine as it runs with respect to control injection and ignition in engine control unit 32 and as it runs with respect to control of intake valve 18 and exhaust valve 36 in valve control unit 34 in a step 3.