Device for compression control

A device for controlling the compression ratio of a reciprocating piston internal combustion engine has an auxiliary piston (7) assigned to each cylinder (1) and varying the volume of an auxiliary combustion chamber (5) connected to the cylinder working chamber (4), which piston is connected to an adjusting piston (10) sliding in a hydraulic cylinder (11) and acted upon by a pressure medium fed under essentially constant pressure. The position of this adjusting piston (10) is controlled by a control element adjustable as a function of a variable characterising the operating state of the internal combustion engine, which control element has a control edge interacting with the pressure-actuated end of the adjusting piston (10) in order to control the flow of pressure medium. In order to achieve a relatively small axial design space above the adjusting piston, it is proposed that the control element be designed as a cylindrical rotary slide valve (12), which is provided with a connecting groove (13) arranged on its periphery and running in a spiral shape and is arranged parallel to the longitudinal axis of the hydraulic cylinder (11) so that it engages in part in a partially cylindrical longitudinal recess (24) of the hydraulic cylinder (11) and in part engages positively with a partially cylindrical longitudinal recess (25) of the adjusting piston (10), fixing the adjusting piston in the peripheral direction.