A multi-stage heat engine comprising: an evaporator and a condenser; an expander section including more than two expander stages; a compressor section comprising more than two vapour compression stages that co-operate with the expander section; x tanks wherein x is at least three (e.g. T1 to Tx) for holding gaseous phases (e.g. G1 to Gx) and liquid phases (e.g. L1 to Lx) of a fluid; the expander section having x-1 expansion valves (e.g. V1 to Vx-1), the compressor section being adapted to compress the gaseous phase from a first tank to a higher pressure and to move the compressed fluid to a second next adjacent tank at a higher pressure, the expander section being adapted to move a part of the compressed fluid (liquid) from the second next adjacent tank, through the expansion valve of that tank, to expand the fluid in the first tank at a lower pressure, the compressor and expander sections being adapted to output the gaseous phase at the highest pressure to the condenser and the liquid phase at the lowest pressure to the evaporator, the output of the condenser being fed back to tank (T 1) at the highest pressure and the output of the evaporator being fed back to the tank (Tx) at the lowest pressure, characterised in that the expansion of the fluid in the first tank at a lower pressure brings the vapour in this latter tank at a saturated or close to saturated state, in that the multi-stage heat engine is adapted so that at every compression stage where there is expansion of some liquid returned from a higher pressure tank, the cooling effect of this expansion keeps the vapour in that tank at or close to saturated, and in that each tank has pressure, and/or temperature and/or liquid level sensors and controllable expansion values; the heat engine further comprising a controller adapted to regulate the controllable valves in accordance with the outputs of at least one of the sensors to maintain a level of liquid in each tank and maintain the vapour of each tank in the saturated state or close thereto. an evaporator and a condenser; an expander section including more than two expander stages; a compressor section comprising more than two vapour compression stages that co-operate with the expander section; x tanks wherein x is at least three (e.g. T1 to Tx) for holding gaseous phases (e.g. G1 to Gx) and liquid phases (e.g. L1 to Lx) of a fluid; the expander section having x-1 expansion valves (e.g. V1 to Vx-1), the compressor section being adapted to compress the gaseous phase from a first tank to a higher pressure and to move the compressed fluid to a second next adjacent tank at a higher pressure, the expander section being adapted to move a part of the compressed fluid (liquid) from the second next adjacent tank, through the expansion valve of that tank, to expand the fluid in the first tank at a lower pressure, the compressor and expander sections being adapted to output the gaseous phase at the highest pressure to the condenser and the liquid phase at the lowest pressure to the evaporator, the output of the condenser being fed back to tank (T 1) at the highest pressure and the output of the evaporator being fed back to the tank (Tx) at the lowest pressure, characterised in that the expansion of the fluid in the first tank at a lower pressure brings the vapour in this latter tank at a saturated or close to saturated state, in that the multi-stage heat engine is adapted so that at every compression stage where there is expansion of some liquid returned from a higher pressure tank, the cooling effect of this expansion keeps the vapour in that tank at or close to saturated, and in that each tank has pressure, and/or temperature and/or liquid level sensors and controllable expansion values; the heat engine further comprising a controller adapted to regulate the controllable valves in accordance with the outputs of at least one of the sensors to maintain a level of liquid in each tank and maintain the vapour of each tank in the saturated state or close thereto.