High temperature liquids held in a subcooled state are capable of storing large amounts of energy and then explosively releasing this energy when depressurized in a phase change process known as “Flashing”. The rapid volume expansion that results from this flashing has been harnessed to drive an expansion engine working on a cycle called “The Superheated Liquid Flash, Boiling” (S.L.F.B) cycle. The first stage showed that multiphase convective boiling of the unflashed water off the heated walls of the expansion unit supplemented the Flash work output. Furthermore, Flashing was seen to improve the effectiveness of convective boiling off the walls. The results were shown to be repeatable in a modified piston engine. Convective boiling was again shown to be able to supplement the power output under specific conditions. Engine power was seen to be directly related to the peak In-Cylinder pressure, which in turn was directly related to the mass and temperature of the injected subcooled water. The efficiency of the engine was directly related to the mass and level of subcooling of injected working fluid up to a boiling plate temperature of 215°C but inverted in relationship at boiling plate temperatures between 289°C and 297°C. This is hypothesised to be due to the effects of residual liquid mass caused by the Leidenfrost effect. The effective removal of the residual liquid mass during the exhaust stroke is expected to improve the overall efficiency of the engine.