Page 77 - Piston Engine-Based Power Plants

Stirling Engines and Free Piston Engines 69

Bounce chamber Combustion chamber

Piston synchronisation
4
Figure 6.6 Opposed-piston, free piston engine. Source: Newcastle University, UK

turbine. In this application the free piston gas generator replaces the
compressor of the gas turbine. The free piston device can provide a
high compression ratio and the devices were of considerable interest in
the middle of the 20th century but as gas turbines developed through
the latter part of the century, interest waned.

LINEAR GENERATORS
In order to generate electricity from a free piston engine, some form of
linear generator is required. There has been a considerable amount of
interest in free piston linear generators during the 21st century because
they could be used as range extenders for electric vehicles. Most of
these devices integrate the free piston engine and the generator in a sin-
gle device. For vehicle applications, the idea is to use the engine to
provide an efficient source of electricity from combustion of a standby
supply of combustion fuel in case the vehicle battery runs low.

A number of configurations have been explored. For example, one
design developed at the Sandia National Laboratory in the United
States utilises magnetised pistons, with coils around the cylinder to
5
produce an electrical output. Most use opposed-piston engines with a
variety of ignition options including spark ignition, compression igni-
tion and homogeneous charge compression ignition in which an
air fuel mixture is compressed to the point of spontaneous ignition, as
in a diesel engine.

4 A review of free-piston engine history and applications. R. Mikalsen, A.P. Roskilly Sir Joseph
Swan Institute for Energy Research, Newcastle University, Newcastle upon Tyne, NE1 7RU,
United Kingdom.
5 http://crf.sandia.gov/free-piston-engines-a-possible-route-to-hybrid-electric-vehicles/