An engine comprises a compressor and a closed fluid circuit connected to the input and output of the compressor such that compressed gas can be driven through the circuit by the compressor. The output of the compressor is connected, through the fluid circuit, to a turbine component comprising at least one set of turbine blades connected to a rotating shaft that acts as the output of the engine in use. A condenser receives fluid in the circuit that has passed through the turbine component and is arranged to reduce the temperature and pressure of the compressed gas, the outlet of the condenser being connected to the inlet of the compressor.