Abstract: A combustion engine comprises a plurality of cylinders. Each cylinder contains aligned pistons displaceable toward and away from one another. Rotary cam disks are connected to a drive shaft and are arranged to be contacted by respective ones of the pistons for transferring motion therebetween. Each cam disc includes a cam curve arranged to be engaged by respective pistons of each pair of pistons. Each cam curve has circumferentially spaced peaks and valleys interconnected by an interconnecting surface with which the cams make contact during a power stroke. The interconnecting surface is of concave configuration and presents a slope which progressively decreases from adjacent the peak to adjacent the valley.