Page 76 - Piston Engine-Based Power Plants

68 Piston Engine-Based Power Plants

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Figure 6.5 Dual-piston, free piston engine. Source: Newcastle University, UK
A second common type of free piston engine is the dual-piston
engine. This has two internal combustion engine cylinders each with its
own piston. However these pistons are connected, back to back so that
as one piston moves through its power stroke it compresses the contents
of the second piston (as if it were a bounce chamber), and vice versa
(Fig. 6.5). This type of design eliminates the need for an actual bouncer
chamber. However it requires extremely accurate control of the combus-
tion cycles in each cylinder since each drives the other. Both the stroke
length and the compression ratio in each of the cylinders is partly under
the control of the second and this can lead to imbalances if synchronisa-
tion is not perfect. The engines are highly efficient, in principle. As with
the single-piston version, the engine can be used either for hydraulic
drive or to generate electricity via a linear generator.
A third type is the opposed-piston, free piston engine. This also has
two pistons but they are inserted at either end of a single combustion
chamber, or cylinder. The outside ends of each piston are attached
through a rod to a piston inside a bounce chamber. In this case, igni-
tion and firing of an air fuel mixture in the cylinder causes the two
pistons to move outwards, symmetrically. In order to ensure that syn-
chronisation is perfectly maintained, there is usually a mechanical link-
age controlling their relative motions. This is shown in Fig. 6.6 which
presents a schematic of an opposed-piston, free piston engine. When
the pistons reach the end of their strokes they are returned by the
bounce chambers and the cycle is repeated. As a consequence of the
two opposed pistons, this type of engine is vibration free. However it is
more complex than other types of free piston engine.
An alternative approach to the use of free piston engines for power
generation is the free piston gas generator. This exploits the perfor-
mance of an opposed-piston, free piston engine to produce a flow of
compressed gas which is supplied to the combustion chamber of a gas

3 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.