Abstract: The internal combustion engine includes a working cylinder, a compressed air supply, an injector for supplying a gas and fuel mixture, an exhaust valve and an inlet valve. The inlet valve delivers both gas under pressure and gas and fuel mixture to the working cylinder, commencing delivery of the gas under pressure at an earlier stage in the engine cycle to the commencement of delivery of gas and fuel mixture. The inlet valve delivers gas under pressure to the cylinder at least for a part of the time that the exhaust port is open so as to drive combusted gases out of the working cylinder. The internal combustion engine also includes a rotary valve having a first member of a cylindrical external configuration, a sleeve surrounding the first member, and a mechanism for rotating the sleeve relative to the first member. The first member comprises a plurality of fluid passages and a plurality of apertures allowing communication of each of the passages with the exterior of the first member.