Abstract: The compression ratio is variable in that the piston hub may be adjusted, since the connecting rod is mounted at the crankshaft side on an eccentric pin. The eccentric crank pin can be adjusted around its axis of rotation by control means while the engine is running. The control means include a toothed wheel that turns concentrically to the axis of rotation of the eccentric crank pin and is fixed thereto. This toothed wheel acts as an external gear inside a larger diameter internal gear inside which it rolls. The internal gear is concentrically mounted around the axis of the crankshaft and its rotating position may be adjusted. The external gear turns exactly once upon itself every time it rolls round the internal gear.