The Fluid Engine is a positive displacement prime mover to be utilised in the Organic Rankine Cycle process. The Organic Rankine Cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The working fluid allows Rankine cycle heat recovery from lower temperature sources such as solar thermal, geothermal, biomass and waste heat. The low-temperature heat is converted into useful work that can itself be converted into electricity.

For the Fluid Engine, the work is converted into electricity via expansion of this working fluid in its pistons. The technology is far more forgiving to formation of liquid droplets in the vapour, which works in favour of the designs efficiency as lower discharge temperatures can be utilised, something turbine technology cannot perform.