Abstract: An internal combustion engine is provided including at least one cylinder having at least one inlet port and at least one exhaust port; a piston mounted in the at least on cylinder for reciprocated movement therein, the piston having a piston rod; a crankshaft coupled to the piston rod for converting the reciprocating movement of the piston to rotational movement; a cylindrical sleeve surrounding an outside wall of a portion of the cylinder; and mounting structure for mounting the cylindrical sleeve for rotation with respect to the cylinder. The cylindrical sleeve has at least one slot defined therethrough which aligns periodically with the ports of the cylinder upon rotation of the cylindrical sleeve. The mounting structure includes low-friction bearing structure to minimize friction between the cylinder and the sleeve. A gear train is provided for rotating the cylindrical sleeve relative to movement of the crankshaft.