Motor Stirling, Silnik Stirlinga, Dokumenty
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Stirling Engine Reference
Guide and Catalog
The MM-6 is Powered by the Heat of Your Hand
Extremely Interesting
Model Stirling Engines
Figure 1.
Key to Stirling Engine. The air flows through and around the porous displacer. The displacer looks like a piston but is not.
Stirling Engines
another gas such as helium) inside a leak tight
container and using the pressure changes to drive a
piston. The heating and cooling process works like
this: One part of the engine is kept hot while
another part is kept cold. A mechanism then moves
the air back and forth between the hot side and the
cold side. When the air is moved to the hot side, it
expands and pushes up on the piston, and when the
air is moved back to the cold side, it contracts and
pulls down on the piston.
by Brent H. Van Arsdell
Reprinted from MacMillan Encyclopedia of Energy
Used by Permission
The principle that makes Stirling engines possible is
quite simple. When air is heated it expands, and
when it is cooled it contracts. Stirling engines work
by cyclically heating and cooling air (or perhaps
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While Stirling engines are conceptually quite
simple, understanding how any particular engine
design works is often quite difficult because there
are hundreds of different mechanical configurations
that can achieve the Stirling cycle. Figure 1 shows a
schematic of a transparent educational
demonstration engine that runs on the top of a cup
of hot coffee. This engine uses a piece of foam
similar to what would be used as a filter for a
window air conditioning unit to "displace" the air
between the hot side and the cold side. This foam
displacer is carefully mounted so it does not touch
the walls of the cylinder. Figure 2 shows how this
particular engine achieves the Stirling cycle. In this
engine, the air flows through and around the
displacer from the hot side then back to the cold
side, producing a power pulse during both the hot
and cold portion of the cycle. Stirling engines can
be mechanically quite simple since they have no
valves, and no sparkplugs. This can result in
extremely high reliability as there are fewer parts to
fail.
combustion each minute, burners in Stirling engines
burn fuel continuously. It's much easier to make a
continuous combustion engine burn very cleanly
than one that has to start and stop. An excellent
demonstration of this principle is to strike a match,
let it burn for a few seconds, then blow it out. Most
of the smoke is produced during the starting and
stopping phases of combustion.
A Brief History
In the early days of the industrial revolution, steam
engine explosions were a real problem. Metal
fatigue was not well understood, and the steam
engines of the day would often explode, killing and
injuring people nearby. In 1816 the Reverend
Robert Stirling, a minister of the Church of
Scotland, invented what he called "A New Type of
Hot Air Engine with Economiser" as a safe and
economical alternative to steam. His engines
couldn't explode, used less fuel, and put out more
power than the steam engines of the day.
It is worthwhile to compare Stirling engines to other
more familiar engines and note their similarities as
well as their differences. Stirling engines are a type
of heat engine. They turn heat into mechanical
work and in this sense they perform the same
function as other well known heat engines such as
gasoline, diesel, and steam engines. Like steam
engines, Stirling engines are external combustion
engines, since the heat is supplied to the engine
from a source outside the cylinder instead of being
supplied by a fuel burning inside the cylinder.
Because the heat in a Stirling engine comes from
outside of the engine, Stirling engines can be
designed that will run on any heat source from fossil
fuel heat, to geo-thermal heat, to sunshine. Unlike
steam engines, Stirling engines do not use a boiler
that might explode if not carefully monitored.
The engines designed by Robert Stirling and those
who followed him were very innovative engines,
but there was a problem with the material that was
used to build them. In a Stirling engine, the hot
side of the engine heats up to the average
temperature of the flame used to heat it and remains
at that temperature. There is no time for the
cylinder head to cool off briefly between power
pulses. When Robert Stirling built his first engines,
cast iron was the only readily available material,
and when the hot side of a cast iron Stirling engine
was heated to almost red hot, it would oxidize
fairly quickly. The result was that quite often a hole
would burn through the hot side causing the engine
to quit. In spite of the difficulties with materials,
tens of thousands of Stirling engines were used to
power water pumps, run small machines, and turn
fans, from the time of their invention up until about
1915.
When operating on sunshine, or geo-thermal heat,
Stirling engines obviously produce no pollution at
all, but they can be exceedingly low emissions
engines even when burning gasoline, diesel, or
home heating oil. Unlike gasoline or diesel engines
that have many thousands of start stop cycles of
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As electricity became more widely available in the
early 1900s, and as gasoline became readily
available as a fuel for automobiles, electric motors
Insert figure 2 here
Figure 2.
Four phases of the Stirling engine power cycle.
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and gasoline engines began to replace Stirling
engines.
to run on any heat source. Every imaginable heat
source from fossil fuel heat to solar energy heat can
and has been used to power a Stirling engine.
Regeneration
Stirling engines also have the maximum theoretical
possible efficiency since their power cycle (their
theoretical pressure volume diagram) matches the
Carnot cycle. The Carnot cycle, first described by
the French physicist Sadi Carnot, determines the
maximum theoretical efficiency of any heat engine
operating between a hot and a cold reservoir. The
Carnot efficiency formula is: (T(hot)-
T(cold))/T(hot). T(hot) is the temperature on the
hot side of the engine. T(cold) is the temperature on
the cold side of the engine. These temperatures
must be measured in absolute degrees (Kelvin or
Rankine).
Robert Stirling's most important invention was
probably a feature of his engines that he called an
"economiser." Stirling realized that heat engines
usually get their power from the force of an
expanding gas which pushes up on a piston. The
steam engines that he observed dumped all of their
waste heat into the environment through their
exhaust and the heat was lost forever. Stirling
engines changed all that. Robert Stirling invented
what he called an "economiser" that saved some heat
from one cycle and used it again to pre-heat the air
for the next cycle.
It worked like this: After the hot air had expanded
and pushed the piston as far as the connecting rod
would allow, the air still had quite a bit of heat
energy left in it. Stirling's engines stored some of
this waste heat by making the air flow through
economiser tubes that absorbed some of the heat
from the air. This pre-cooled air was then moved to
the cold part of the engine where it cooled very
quickly and as it cooled it contracted, pulling down
on the piston. Next the air was mechanically moved
back through the pre-heating economiser tubes to the
hot side of the engine where it was heated even
further, expanding and pushing up on the piston.
This type of heat storage is used in many industrial
processes and today is called "regeneration."
Stirling engines do not have to have regenerators to
work, but well designed engines will run faster and
put out more power if they have a regenerator.
Stirling Applications
Stirling engines make sense in applications that take
advantage of their best features while avoiding their
drawbacks. Unfortunately, there have been some
extremely dedicated research efforts that apparently
overlooked the critical importance of matching the
right technology to the right application.
In the 1970s and 1980s a huge amount of research
was done on Stirling engines for automobiles by
companies such as General Motors, Ford, and
Philips Electronics. The difficulty was that Stirling
engines have several intrinsic characteristics that
make building a good automobile Stirling engine
quite difficult. Stirling engines like to run at a
constant power setting, which is perfect for pumping
water, but is a real challenge for the stop and go
driving of an automobile.
Continued Interest
Automobile engines need to be able to change
power levels very quickly as a driver accelerates
from a stop to highway speed. It is easy to design a
Stirling engine power control mechanism that will
change power levels efficiently, by simply turning
up or down the burner. But this is a relatively slow
method of changing power levels and probably is
not a good way to add the power necessary to
In spite of the fact that the world offers many
competing sources of power there are some very
good reasons why interest in Stirling engines has
remained strong among scientists, engineers, and
public policy makers. Stirling engines can be made
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