Abstract: A solar powered engine 10, e.g. for pumping water, comprises a solar collector 116 in which a low boiling point liquid is vaporized, an engine comprising a piston 84 reciprocably movable in a chamber 82 where the resulting vaporized liquid does work, a condenser 126 where the vaporized liquid is condensed back into a liquid, means for returning the condensed liquid to the solar collector 116 and a transmission 88 which connects the piston 84 to a load. When the vaporized liquid is being introduced below the piston 84 to move the piston 84 to perform useful work, the condenser 126 is at the same pressure as a reservoir 142 so that condensed liquid may flow from the condenser 126 to the reservoir 142, and when the vaporized liquid is exhausting from below the piston 84 to permit the piston 84 to descend again, the reservoir 142 is at the same pressure as the solar collector 116 so that the condensed liquid may flow from the reservoir 142 to the solar collector 116 to complete the cycle.