Abstract: An engine with one or more ring cylinders of round cross-section is disclosed. In each ring cylinder are provided at least two rotary pistons whose cross-sections match that of the ring cylinder, the pistons being mounted on the circumference of a rotary disc which is mounted on a shaft and incapable of rotating. Each cylinder is provided with disc cams in the form of rotary valves which protrude at their peripheral areas through slots which extend transversely in relation to the course of the ring cylinders. Rotary valves engage in overflow channels and in the ring cylinder between an intake compression chamber and an expansion chamber. The rotary valves engage in the ring cylinder between an expansion chamber and an intake compression chamber. The rotary valve grooves are so arranged as to clear the ring cylinder for the rotary pistons.