Abstract: An internal combustion engine comprises a cylinder and a piston axially movable within the cylinder and formed with cylindrical projections extending axially outwardly from the piston and being of a smaller diameter than that of the piston. The piston and its projections are formed with ducts which are connectable with inlet ports formed in the wall of the cylinder for admitting air and/or air/fuel mixture into combustion chambers of the engine and outlet ports for expelling exhaust gases from the combustion chambers, upon the movement of the piston. The ducts are formed one each within one of two longitudinal piston halves. The inlet ports, outlet ports and ducts are assigned to two longitudinal halves of the piston and its projections and arranged in the engine so that the admission of air and/or air/fuel mixture into the combustion chambers and the removal of exhaust gases therefrom via short, low-turbulence paths are controlled only by the position of the piston and its projections within the cylinder.