Abstract: An internal combustion engine is provided having a pair of cylinders interconnected by means of a narrow connecting channel. A working piston reciprocates in one cylinder and is conventionally attached to a crankshaft by means of a connecting rod and crankpin. An auxiliary piston reciprocates in the other cylinder and is driven in timed relation to the working piston by means of a cam affixed to the crankshaft and a cam follower attached to a connecting rod. Means are provided for adjusting the relative timing of the two pistons so as to vary the compression ratio of the engine. The auxiliary piston may function as a compression and ignition means by moving rapidly toward its top dead center position only when the working piston is close to its top dead center position. Other movements of the auxiliary piston provide the advantages of low pressure internal supercharging, and minimizing the retarding effect on the working piston by the compressive forces within the combustion chamber.