Abstract: The engine comprises an elongated, vertical casing within which a buoyancy piston is vertically reciprocal between upper and lower positions. The buoyancy piston bottomly carries a bottom piston plate which is a sliding seal within the interior of the casing. A flapper check valve is positioned on the piston plate to alternately open or close the valve opening to thereby allow the liquid to pass through the plate to impinge buoyancy forces upon the underside of the piston. A circulating pump draws its suction below the piston plate and discharges to the top of the casing at a height elevated above the piston plate. Cyclical opening and closing of the flapper check valve causes corresponding reciprocal movement of the power piston within the casing to power an output shaft for reciprocal motion.