Abstract: There is disclosed an internal combustion engine within which is defined at least one cylinder accommodating a reciprocable piston, and within which is disposed a valve rotor mounted at the compression end of the or each said cylinder for rotation about the longitudinal axis of the cylinder and having a number of passages therethrough each extending between the cylinder interior and a portion of the structure having inlet and exhaust ports, to allow for conventional fourstroke operation with said valve rotor rotating at an appropriate fraction of crank-shaft speed, the construction being such that at top dead center following a compression stroke each passage forms a substantially independently enclosed combustion space at the compression end of the cylinder adjacent the crown of the piston.