An internal reciprocating engine (10) effective to operate at one engine cycle per revolution is provided with a differential stroke piston (14) having an inner piston part (14a), for sealing the cylinder (12), operating at a cycle different from its corresponding outer piston part (14b), and a differential stroke actuating means (62) for operating the inner and outer piston parts (14a, 14b) in the same and the opposite directions within the cylinder (12) and to provide differential stroke periods and/or stroke lengths for the inner piston part (14a) cycle.