Abstract: This disclosure relates to an external combustion engine including an engine housing forming at least one pair of opposed cylinders, a piston mounted for reciprocating motion in each cylinder, a rotatable power output shaft mounted between the pistons, and a roller assembly connecting the pistons with the shaft. A boiler and a condenser are connected to supply a pressurized vapor to the cylinders in order to reciprocate the pistons and rotate the shaft. The moving pistons operate as an air pump for moving air through the condenser and the boiler.