Page 75 - Piston Engine-Based Power Plants

Stirling Engines and Free Piston Engines 67

was launched in 1933. He later produced a generator based on the free
piston principle. The development of this type of engine continued
with their use as gas generators in which the high-pressure exhaust
from the engine is used to drive a gas turbine. Free piston engines have
also been coupled with some form of linear generator that can exploit
the back-and-fro movement of the piston, rather than the more normal
rotary motion, to provide an electrical output.

TYPES OF FREE PISTON ENGINE

The simplest type of free piston engine has a single cylinder that oper-
ates in a manner identical to the cylinder of a two-stroke spark ignition
engine. Air and fuel are mixed and fed into the cylinder, the mixture is
compressed and then ignited, providing a power stroke and at the
same time exhausting the combustion gases and the cycle is repeated.
In the free piston version, the piston is connected not to a crankshaft
but to a bounce chamber, a second piston that is sealed inside a cylin-
der containing air or another gas. This is shown schematically in
Fig. 6.4. During the power stroke the main engine piston forces the
bounce chamber piston to compress the gas inside the chamber. At the
end of the power stroke the gas inside the bounce chamber is under
high pressure and can force the engine piston to return to the top of its
chamber, starting the compression and ignition stroke again.

This type of free piston engine is extremely simple. The pressurisa-
tion of the gas inside the bounce chamber can be exploited to provide
a form of hydraulic drive. Alternatively it will act simply to return
the piston while a linear generator exploits the back-and-fro motion of
the piston to generate electricity.

Figure 6.4 Single-piston, free piston engine. Source: Newcastle University, UK . 2

2 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.