Device for controlling compression

A device for controlling the compression ratio of a multi-cylinder reciprocating piston internal combustion engine is described for vehicles, in particular passenger cars. This device has an actuator piston (5) which is assigned to each cylinder (2) and is kept adjustable by an actuator drive (14) in a secondary combustion space (4), connected to the cylinder combustion space (3), as a function of the loading of the internal combustion engine. In addition, a spark plug is provided which can be adjusted with the actuator piston and has an insulator (8) which is kept gas pressure-tight with a spark plug housing and has a central electrode (11) which can be fed with ignition current. In order to achieve the best possible conduction away of heat from the base of the spark plug which is particularly highly thermally stressed, the spark plug housing of the spark plug is to be directly designed as an actuator piston with a piston collar (6) which slides in the cylindrical secondary combustion space (4) and with a piston neck (7) which engages with the actuator drive (14).