Free-piston engine

A free-piston engine is a linear, crankless internal combustion engine in which the piston's motion is not controlled by a crankshaft but instead results from the interplay of three forces: pressure from combustion in the chamber that pushes the piston, a rebound device such as a hydraulic cylinder or gas spring that returns it, and a load device that extracts the power the piston generates. Most free-piston engines run a two-stroke cycle, producing one power stroke per piston cycle. Power can be extracted in several ways: exhaust gases can drive a turbine that in turn powers pumps, propellers or generators; the piston can couple directly to an air compressor for a pneumatic output; or a linear alternator built into the piston assembly can generate electricity directly. Current research focuses on free-piston linear generators as range extenders for electric vehicles, with prototypes from manufacturers such as Toyota reporting thermal efficiencies of around 42 percent.