Page 68 - Piston Engine-Based Power Plants

60 Piston Engine-Based Power Plants

domestic heat and power. Free piston engines have had limited appli-
cation since their invention but they are attracting more interest in the
21st century as potential power sources in hybrid vehicles.

THE STIRLING ENGINE
The Stirling engine is a heat engine that converts heat into mechanical
energy. It does this by exploiting a heat source and a heat sink to alter-
nately cause a gas, the working fluid, to expand and contract. The
actual physical process that the engine exploits is the transfer of heat
from the heat source to the heat sink and the working fluid, the gas, is
the medium through which this transfer occurs. It is probably the clos-
est approach in a practical engine to the idealised Carnot heat engine
proposed by Sadi Carnot in the early 19th century.

The engine was designed by a Scottish Presbyterian minister, Robert
Stirling, who received his first patent in 1816. The original Stirling
engines used air within the cylinders and were called air engines but
modern Stirling engines usually employ helium or hydrogen since these
gases can absorb and release heat rapidly. The operation of early
Stirling engines, which require high temperatures to operate efficiently,
was limited by the materials available during the 19th century.

There are many configurations for Stirling engines, including Alpha,
Beta, Gamma and free piston variants. All of these configurations
require two pistons to function although this is not always immedi-
ately apparent from their designs. The engine is a sealed unit so that the
working fluid and pistons are isolated from the atmosphere. Movement
of linked pistons cycle this working fluid from the hot part of the engine,
where an external heat source provides heat to the fluid at constant tem-
perature, and an external cold sink where heat is extracted from the
engine. The engine cycle is reversible and if a Stirling engine is driven
mechanically it can take heat energy from the cold end and move it to
the hot end, essentially acting as a refrigerator.

STIRLING ENGINE CYCLE

The Stirling engine working fluid passes through four stages during its
cycle: cooling, compression, heating and expansion. These four are
similar to the same four stages that characterise all heat engine cycles