A free-piston engine is a type of diesel engine in which the power output is extracted from a turbine driven by the exhaust gases and not via any direct rod and crank drive. The combustion chamber is formed between two opposed pistons in a closed metal tube, the piston oscillating in and out in phase with each other, regularly exposing the exhaust and air-inlet ports. Fuel is injected at the midpoint. The engine, which has a good power-to-weight ratio and is use din some ships, has the advantage that the gasifier and turbine units need not be sited adjacently.