PASCO SE-8575 VISIBLE STIRLING ENGINE - Manual (Page 6)

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The Visible Stirling Engine 012-06055A
®
History and Theory of Operation
Why STIRLING Engines are important
Every practical engine ever built compresses a gas then
expands it and moves it through a cycle.
The Reverend Robert Stirling, a minister of the Church of
Scotland, was troubled by some of the dangerous engines
that were used at the beginning of the industrial revolu-
tion. Steam engines would often explode with tragic
effects to anyone unfortunate enough to be standing
nearby. So in 1816 he invented and patented “A New
Type of Air Engine With Economizer.”
Hot air engines, as they were initially called, couldn’t
explode and often put out more power than the steam
engines of their day. The only trouble was that the readily
available metal of the early 1800’s was cast iron, and cast
iron oxidizes rapidly when you leave it in a very hot flame.
In spite of these difficulties, Stirling engines were widely
used as water pumping engines at the turn of the century.
They required little service, never exploded, were fairly
quiet and the water provided a good cooling source for
the cold side of the engine. Thousands of these engines
were sold .
In the mid 1800’s a bright Frenchman named Sadi Carnot
figured out the maximum limits on efficiency. His
formula is an accepted standard for determining the
maximum possible efficiency of an engine. No engine
can exceed the Carnot efficiency. [The first law of
Thermodynamics says you can’t get out more than you
put in. You can only break even.] In fact no real engine
can achieve carnot efficiency. [The second law of thermo-
dynamics says you can’t break even.]
It takes good engineering and complex machines to
achieve a significant fraction of the Carnot efficiency.
On the simplest level, a Stirling engine operates as
follows. When a gas in a closed cylinder is heated it
expands and pushes up on a piston. When the same gas is
cooled, it contracts and pulls down on the piston.
The next level of understanding is to realize how regen-
eration works. Robert Stirling realized that the engines he
built would be much more efficient if some of the heat
that was used to heat the air for one cycle was saved and
used again in the next cycle. Robert Stirling called the
device that saved heat in his engines an economizer.
Today, these are usually called regenerators and probably
are Robert Stirling’s most important invention.
In the model SE-8575, regeneration works as follows.
When the yellow foam inside the engine is near the top of
the cylinder (and the engine is running on a cup of hot
water) most of the air is on the bottom side (the hot side)
where it is heated. When the air gets hot it expands and
pushes up on the piston. When the foam moves to the
bottom of the engine it moves most of the air (it displaces
the air) to the top of the engine. The top of the engine is
cool, allowing the air inside the engine to cool off (reject
heat to the environment) and the piston receives a down-
ward push. This engine would run even if the “displacer”
(the yellow foam) was made of solid Styrofoam. It runs
much better because it is made of a special air filter foam.
When the air is flowing from the hot side of the engine to
the cold side it flows through and around the yellow foam
(the displacer). Since the air is hotter than the foam, some
of the heat from the air will flow into the foam. The air
cools off and the foam heats up. This is called regenera-
tion and is very important in many industrial processes.
When the air makes the return trip to the hot side of the
engine it once again flows through and around the foam.
This time the air heats up and the foam cools off. The
heat that would have been wasted in an engine without
regeneration is saved and a much more efficient engine is
the result.
A Stirling engine with a regenerator has a cycle that
matches the Carnot cycle. It has the same theoretical
maximums and the same theoretical efficiencies.