External combustion engine

An external combustion engine (1) comprising pressure vessel means defining a tubular working chamber (3) having spaced apart first and second ends and including first wall means (11) adjacent said first end of the chamber and second wall means (6) adjacent said second end of the chamber, heating means (10, 12) for heating said first wall means (11), cooling means for cooling said second wall means (6), piston means (7) having heat exchanging means (9) and drive means for reciprocating the piston means (7) within the tubular working chamber (3) between said first and second ends of the chamber so that the working fluid passes through said heat exchanging means (9), characterised in that said first wall means (11) has first heat exchange surface means (11d) and in that said piston means (7) has valving means including first valve means (20) positionable for directing the working fluid, after passage through said heat exchanging means (9), to flow over said first heat exchange surface means (11d) when the piston means (7) is moving towards said second end of the chamber (3) in order to move the working fluid from the second end to the first end of said chamber and to by-pass said first heat exchange surface means (11d) when the piston means (7) is moving towards said first end of the chamber (3) in order to move the working fluid from the first end to the second end of said chamber (3).