Abstract: An internal combustion engine includes at least one piston (1) which is reciprocally received in a cylinder (2) which is coupled to a rotary output shaft (17) by a coupling (5, 22) which converts reciprocal movement of the piston into rotary movement of the output shaft. The engine is so arranged that, in use, the fuel/air mixture in the or each cylinder ignites at a predetermined ignition time in the operation cycle of the engine. The coupling is so arranged or programmed that on its compression stroke the speed of the piston deceases abruptly substantially at the ignition time and that the speed of the piston subsequently increases prior to reaching the top dead center position.