Abstract: Disclosed is an improvement whereby performance of an internal combustion engine, an Otto cycle engine in particular, is increased by the provision of two distinct in-piston combustion chambers (2, 3) per cylinder these take the form of pockets (4, 5) dissimilar in volume, sunk into the crown (6) of each piston (1) in position directly beneath the inlet and exhaust valves (7, 8) and provided each with a spark plug (13). In a preferred embodiment, such pockets (4, 5) exhibit the shape of sector to an annulus, and their side walls (9, 10) are splayed in such a way as to enhance circulation and escape of combustion gases.