A Brief History of the Stirling Engine
The Stirling-Cycle Engine was originally patented by the Reverend
Robert Stirling, a 26 year old Scottish Minister in 1816. He first
proposed the idea of the regenerator or economiser as a way of
using less coal in industrial heating processes and went on to
develop the Stirling engine as a safe and efficient alternative
to the steam engine. Once popular as a small source of mechanical
power, with no potentially explosive boiler like a steam engine,
the Stirling was used to pump water and run machinery, often in
remote rural areas. Superseded by the more powerful internal combustion
(IC) engine at the turn of the century, it fell into obscurity,
and existed only as an interesting model. The Philips Company
of Holland revived it in 1937, as a potential source of generating
electrical power for their valve radio sets, which was clean,
quiet and produced no electrical interference.
In the late 1960's and early 1970's, large companies including
Ford and GM and United Stirling of Sweden took out licenses on
Philip's work, with the aim of developing the Stirling engine
for vehicle use. This work was funded to a large extent by the
American government as part of the Heat Engine Bill, and tens
of millions of dollars went into the various projects. Prototype
vehicles including cars, trucks and buses were fitted with Stirling
engines. However, the engine that they had developed for this
purpose was a far cry from the simple Stirling known to the Victorians,
and had become a high-tech, space-age energy conversion machine.
It was considered too highly priced to suit the automotive market,
as a result of manufacturing difficulties of the heat exchanger
components and the high cost of reliable gas sealing systems.
The automotive Stirling research projects were abandoned in the
early 1980's, as the US car companies switched their research
efforts to other projects, during major restructuring programmes.
How Stirlings Work
The Stirling Engine relies on the principle that when a quantity
of gas is heated (usually air, but sometimes helium or hydrogen),
it will expand, and its volume will increase. If the gas is sealed
in a container, then the pressure inside the container will rise.
When cooled, the gas contracts, the volume decreases and thus
the pressure will fall.
In practical Stirling engines, rather than alternately heating
and cooling the cylinder containing the gas, it is easier and
more efficient to move the gas, from one end of the cylinder which
is kept hot to the other end which is kept cool. A loose fitting
piston, known as the displacer, is made to move back and forth
in the cylinder, thus shuttling the gas from one end to the other.
As the displacer moves, the gas leaks around the peripheral gap
between the displacer and the cylinder wall. The displacer produces
no power itself, but uses a little power from the working piston
in order to move the gas through the engine and produce the changes
in gas pressure. The displacer is moved by means of a linkage
connected to the crankshaft, and is set so that it reaches top
dead center, one quarter revolution, or 90 degrees, ahead of the
power piston.
