Free-piston engine

The free-piston engine is a linear, 'crankless' combustion engine, in which the piston motion is not controlled by a crankshaft but is a result of the interaction of forces from the combustion cylinder gases, a rebound device and a load device. Free-piston engines are commonly configured as single piston, dual piston or opposed pistons, referring to the number- and arrangement of combustion cylinders. As the engine does not have an energy storage to drive a gas exchange stroke, the free-piston engine is in practice restricted to the two-stroke operating principle.

Free-piston engines have the potential advantages of compactness, reduced manufacturing and maintenance costs, and low frictional losses due to the reduced number of moving parts. High operational flexibility and operation on multiple fuels are possible due to the variable compression ratio. The main challenge with the concept is the piston motion control: as the engine does not have a crankshaft to limit the dead centres of the piston motion, other means of control must be introduced in order to avoid excessive in-cylinder gas pressures and, in the worst case, the piston hitting the cylinder head, while at the same time ensure a sufficiently high compression ratio for fuel autoignition and efficient combustion.

The free-piston engine concept was first described by Pescara (1928), and the first generation of free-piston engines were in use in the period 1930-1960 as air compressors and gas generators. In recent years, the free-piston engine has attracted attention for use in the generation of hydraulic and electric power, with the use of modern control technology to address the control challenges associated with the concept. A comprehensive review of free-piston engine history and applications was presented by Mikalsen and Roskilly (2007).