A four-stoke reciprocating internal combustion engine includes the capability to change the distance between the crankshaft axis and the big end of the connecting rod and hence the maximum cylinder displacement within a combustion cycle. This provides for a given inducted volume of gases to be expanded over a greater volume. An exhaust gas aperture is uncovered by the piston towards the end of the expansion stroke. The piston and crankcase space serves as a pre-compression space for supercharging air to be inducted into the cylinder via ducts. A unitary valve device sliding axially co-operates with intake and exhaust apertures of the combustion cylinder. The unitary device extends at times into a recess in the head of the piston. A diffuse ignition source is provided. A fuel pump is operated by action of the piston.
