|Stirling Engine Gas Circuit:
Graphic courtesy of Dr. Israel Urieli of Ohio University.
In order to construct a Stirling Engine, we really need to understand the basic functioning of
the gas circuit (heater, cooler and regenerator). But the Gas Circuit is wrapped in a cloud of
virtually impenetrable fact and fiction as evidenced by the noise level of the Stirling engine discussion sites.
From the early Schmidt equation which somehow ultimately indicates that the optimum heat exchanger
volume is zero (no heat exchanger at all) to the concepts of Scaling and Similarity which shows that
the scaled configuration may contain hundreds and sometimes even thousands
of tubes, the optimum gas circuit is very difficult to pinpoint empirically or mathmatically.
As there are no valves directing the flow of the working fluid and the
mechanics of the piston motion are reasonably simple,it is amazing that the
actual computation of the thermodynamics of the working fluid in this process
is so very complicated. Many of the components are complex approximations made on a very
non intuitive basis. That is to say that unlike the internal combustion
engine, the Stirling engine can not be designed at one size by a formula and scaled
to the desired size.(At least not without adding the concepts of Similarity to the mix).
The design of a real world Stirling engine must begin with an estimate and end
with an approximation. Even though we can design precisely the exact engine we wish to
build only with great dificulty, we can make certain reasonable estimates of the gross performance
we wish to achieve and then through scaling and similarity principles turn this into a design
which has a good chance at succeeding. The success or failure
of the final hardware will depend upon the care and consideration taken
to avoid three classes of defects in the engine.
1.) Friction in all its forms.
2.) Thermal Shorts in all guises.
3.) Inadequate heat exchanger performance This is in addition to the care paid to the creation
of the gas circuit itself.
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