Abstract: The engine for converting thermal energy to stored fluid energy includes expansion cylinders (61a-f) with expansion chambers (62a-f) and flexible membranes (63a-f). Heating and cooling of working fluid inside the cylinders (61a-f) is carried out by fluid supply lines (73, 71) communicating with external heat resources and sinks. Pressure accumulator (66a) is adapted to store a pressurised fluid (64a-f), such as hydraulic oil, from the individual cylinders (61a-f). In use, this pressurised fluid is delivered at an elevated and above a minimum threshold pressure level, irrespective of the irregularities of the movement of the expansion cylinders (61a-f).