A reciprocating piston engine has a cylinder with a transfer port and an exhaust port, wherein the transfer port and exhaust port are at least partially coincident and are provided with a port valve. The engine includes a pump having a divided chamber therein, one side of the chamber being connected to a crankcase via a connecting port, and the other side of the chamber having an inlet port, an outlet port, and a valve to ensure unidirectional flow therethrough. The chamber is divided by a moving member responsive to variations in pressure in the crankcase to cause flow through the other side of the chamber.