Abstract: An internal combustion engine comprises a cylinder of stepped configuration having a working portion and a pumping portion and a piston of stepped configuration having a working portion and a pumping portion, the working and pumping portions respectively co-operating to define a combustion chamber and a pumping chamber each of which varies in volume upon reciprocation of the piston in the cylinder. Air is admitted to the pumping chamber through a duct from where, on volume reduction thereof, if passes via a plurality of passages and a one-way valve into a transfer chamber. On volume expansion of the combustion chamber, air in chamber is discharged therefrom via a plurality of passages, common discharge port and valve to scavenge the combustion chamber and provide combustion air. The valve may be self-acting or operated by a crankshaft controlled push-rod.