Page 67 - Piston Engine-Based Power Plants

CHAPTER 6 6

Stirling Engines and Free Piston Engines

In addition to conventional spark ignition and compression ignition
engines there are a range of other engines types that rely on reciprocat-
ing pistons to generate power or work. Two important groups are
Stirling engines and free piston engines. 1

The Stirling engine is an external combustion engine in which the
piston or pistons and the working fluid are sealed and isolated from
the atmosphere. Heat and cooling are applied to the exterior of the
engine to drive the engine cycle. This has the advantage that the engine
can operate with a variety of different fuels and heat sources, although
in practice engines are usually designed for specific fuels and applica-
tions. The Stirling engine concept is simple but realising it in a practi-
cal engine is complex. There are a wide range of Stirling engine
designs but most can be understood in terms of one or two generic
types. Most are crankshaft engines that use pistons mechanically linked
to a crankshaft to provide power and to control the motion of the
pistons. However some are a type of free piston engine. These do not
use a crankshaft to control the movement of the piston(s).

Free piston engines are reciprocating engines in which the piston is
not attached to any form of mechanical linkage such as a crankshaft
in order to extract power. Instead the movement of the piston is nor-
mally controlled by the air pressure on opposing sides of the piston.
As with Stirling engines, there are a variety of configurations for these
engines. Since there is no crankshaft, power cannot be extracted from
a free piston engine through a rotating shaft so alternatives are
required. Some use linear generators attached to the pistons to produce
electricity. Others act as compressors, with the high-pressure exhaust
gases used to drive a turbine.

Stirling engines have a long history and the engines have been
developed for a variety of power applications, from solar power to

1 These two groupings overlap because some Stirling engines are free piston engines.
Piston Engine-Based Power Plants. DOI: https://doi.org/10.1016/B978-0-12-812904-3.00006-9
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