An engine comprising: a crankshaft rotatable about a crankshaft axis; an expansion piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston is operable to reciprocate through an expansion stroke and an exhaust stroke of a four stroke cycle during a single rotation of the crankshaft; a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston is operable to reciprocate through an intake stroke and a compression stroke of a four stroke cycle during a single rotation of the crankshaft; and a passage interconnecting the compression and expansion cylinders and operable to define a pressure chamber and connected to the expansion cylinder by a valve; characterised in that: the ratio of the volume in the expansion cylinder when the expansion piston is at its bottom dead center position to the volume in the expansion cylinder when the expansion piston is at its top dead center position is 20 to 1 or greater; wherein the valve is an outwardly opening valve to avoid interference between the valve and the expansion piston; and wherein the engine is operable to initiate combustion in the expansion cylinder while the expansion piston is moving from its top dead center position towards its bottom dead center position. a crankshaft rotatable about a crankshaft axis; an expansion piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston is operable to reciprocate through an expansion stroke and an exhaust stroke of a four stroke cycle during a single rotation of the crankshaft; a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston is operable to reciprocate through an intake stroke and a compression stroke of a four stroke cycle during a single rotation of the crankshaft; and a passage interconnecting the compression and expansion cylinders and operable to define a pressure chamber and connected to the expansion cylinder by a valve; characterised in that: the ratio of the volume in the expansion cylinder when the expansion piston is at its bottom dead center position to the volume in the expansion cylinder when the expansion piston is at its top dead center position is 20 to 1 or greater; wherein the valve is an outwardly opening valve to avoid interference between the valve and the expansion piston; and wherein the engine is operable to initiate combustion in the expansion cylinder while the expansion piston is moving from its top dead center position towards its bottom dead center position. the ratio of the volume in the expansion cylinder when the expansion piston is at its bottom dead center position to the volume in the expansion cylinder when the expansion piston is at its top dead center position is 20 to 1 or greater; wherein the valve is an outwardly opening valve to avoid interference between the valve and the expansion piston; and wherein the engine is operable to initiate combustion in the expansion cylinder while the expansion piston is moving from its top dead center position towards its bottom dead center position.