Abstract: A heat engine includes a boiler, an injector for injecting working fluid into the boiler, a retainer for releasing the hydraulic fluid for movement away from a start position once working fluid vapour pressure in the boiler reaches a predetermined level, a returner for returning the hydraulic fluid to its start position following expansion of working fluid vapour in the boiler, resulting in reciprocating hydraulic fluid motion, and an exhaust valve to release expanded working fluid vapour from the boiler. The vapour pressure increase in the boiler causes a change of state from liquid to gas of a least a portion of the injected working fluid, followed by a substantially constant pressure heating of the working fluid, giving rise to a change of state from liquid to gas of substantially all of the remaining injected working fluid.