Patent ID: 6857403

Claim:
A hydraulically controlled actuator for activating a gas-exchange valve in a combustion cylinder of an internal combustion engine, comprising: two fluid-filled pressure chambers whose chamber volume is controllable; and an operating piston acting upon the valve and being movable out of and into a valve-closure position and a valve-opening position, the operating piston delimiting the pressure chambers by piston sides facing away from one another, and having an effective closing area, acted upon by the fluid pressure in the pressure chambers to close the valve, and an effective opening area acted upon by a fluid pressure in the pressure chambers to open the valve, wherein the operating piston operates so that a real surface of at least one of the effective closing and opening areas changes along the sliding path of the operating piston wherein: the operating piston operates so that, when the operating piston moves out of its valve-closure position, the effective opening area is reduced by a predefined value following at least one predefined sliding path; and the operating piston has multiple parts and is made up of concentric partial pistons, which have differing axial lengths and are moveable relative to each other, and which are insertable into each other so that a second one of the pressure chambers is delimited by all, and a first one of the pressure chambers only by a portion of partial pistons, and sliding paths of the partial pistons not delimiting the first one of the pressure chambers are reduced in a step-wise manner relative to an overall sliding path of the operating piston.