Heat engine

A heat engine that that is capable of converting reciprocating motion of hydraulic fluid exiting a boiler, under the influence of working fluid vapour pressure, to useful work, the heat engine including:

• the boiler;
• a working fluid injecting means for injecting working fluid into the boiler;
• a hydraulic fluid retaining means capable of releasing the hydraulic fluid for movement away from a start position once working fluid vapour pressure in the boiler reaches a predetermined level;
• a hydraulic fluid return means to return the hydraulic fluid to its start position following expansion of working fluid vapour in the boiler, resulting in reciprocating hydraulic fluid motion;
• an exhaust valve to release expanded working fluid vapour from the boiler;
• wherein the vapour pressure increase in the boiler prior to the hydraulic fluid moving away from the start position is caused by a substantially constant volume heating of working fluid causing 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.