So You Want to Build a Stirling Engine ?

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So You Want to Build a Stirling Engine ?

Postby stan.hornbaker » Tue Apr 14, 2009 6:20 pm

The following arrived in my email this AM.

"Subject: Seeking your advice

Hi there. I understand that you are a Stirling engine expert.

I'm trying to create a Stirling engine powered generator to be
manufactured using recycled material in developing nations as part of
a microenterprise development program I am working on. If you don't
mind, I have a few questions I need your help with:

1. How do you size the engine? 2. How do you determine its power
output? 3. How do you determine the number of RPMs it makes from its
design parameters? 4. How do you determine its power input requirements?

I hope I'm not imposing on you. If I am, I want to apologize for it in
advance and hope you can help me with my problem.

Thanks & best regards."

My Reply follows:

"I offer you a one word piece of advice. D O N ' T . Not flippant but
serious advice.

Designing a power producing Stirling engine from the starting line is
a long and arduous task frought with a steep learning curve and much
time devoted to preparation and knowledge acquisition.

Even the experts (very few and far between) have to build a prototype
and then modify it to reach a really good engine. This requires more
R&D financing than is likely to be available.

To find more information go the < > and look at
some of bottom of list topics on the Power Producing, Waste Heat, &
Model Engine Forums.

A CD with items of interest is available for the cost of mailing & a
blank CD.

Also many links are to be found at < >

Engineering Thermodynamics < > Is
required understanding of the subject of Stirling engines. Dr. Izzy
published this jr-sr level web site for his engineering students to
make it available to anyone interested. It is a quite complete
treatment of the subject as well as providing the necessary background
for any attempt at construction of a real Stirling engine. Note that
is is quite long and has many more pages linked to one another.

There are may sources on the web with plans and instructions to build
Model Stirling engines.

This has been presented to provide the best advice and information
that I can think of at present. While not a designer but having
followed the progress or lack thereof of the topic of Stirling engines
the last 15 or so years and being a retired mechanical engineer I have
a well rounded knowledge of what is involved in the design and
production of Stirling engines. Anyone with even modest skills can
build a low temperature (LTD) engine from plans and/or design one from
observing what others have done. When you want real power output you
have a different "Kettle of Fish."

Here is an additional email message from another person's email to me.

"...have to build a prototype and then modify it to reach a really
good engine. This requires more R&D financing than is likely to be

There's the answer you might reconsider giving this individual. Ask
him/her if they have the money required to complete the above process.
If so, recommend that they spend it on an already experienced "expert"
who's in the R&D stages already. If, by some strange chance, the reply
comes back that the money can be spent in this manner, then inform
them that they must offer the money in the same manner as if they were
designing it themselves. In other words, high risk, no guarantees, no
set goal timeframes or deadlines, no outside (or entrenched) 'expert'
advice and no bean-counters.
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First Name: William S.
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Response to So You Want to Build a Stirling Engine ?

Postby fvnorman » Mon Apr 20, 2009 6:49 pm

I just signed up yesterday so this will be my first post.

The complete answer to this person's questions is not possible in a post. However, I'll do my best to answer what I can:

"1. How do you size the engine?"

One of the best ways is to first use the Beale formula. It's available online via Wikipedia: Adjust the mean pressure and swept volume until you get the power you want. This will be a decent first estimate of the necessary "size" of your engine to give you the power you desire. And from there you can further refine the engine, but that will take some thermodynamic analysis and additional testing during the building stage. I'll get more into that below.

"2. How do you determine its power output?"

See above.

"3. How do you determine the number of RPMs it makes from its design parameters?"

Again, this takes some analysis. It's almost impossible to know beforehand unless you do some analysis, but even that will at best give you a range of possible engine speeds. The only way to know the exact speed is to build an actual engine and run it.

"4. How do you determine its power input requirements?"

This depends on the thermal efficiency of the engine. If the thermal efficiency of the engine is (say) 40%, and you have a power output of 1000 Watts, then you need 1000/0.4 = 2500 W of heat input into the heater.

I recently created a software program that will help one design a high-power Stirling engine. This program gives you the design data needed to build an optimal engine that produces "X" amount of power. The program also tells you the approximate range of speed of the engine.

I created this program because I was inspired and challenged by forums like this one to come up with a way to help design an engine using some predictive method, which removes most of the trial and error from the design. I have a background in mechanical engineering but it was still a challenging task. Stirling engine theory is not at all easy to understand right off the bat. There's thermodynamics, fluid mechanics, and some basic machine design concepts. My software program incorporates the basic physics of Stirling engines, and tells you how to optimize the engine by varying the number of heater/cooler tubes and regenerator volume, until maximum power is reached.

Once the analysis is done you are halfway there. But you still have to build the thing, and that's not trivial, as William said. There's material selection requirements, such that the high pressures and temperatures can be withstood. There's the size requirements of your drive mechanism such that the piston forces can be withstood. There's the careful heater and cooler fabrication such that no gas leaks. There's the sealing requirements for the pistons. There's buffer space considerations, etc.

Ideally, you want to make the engine as modular as possible so that it's easy to change parts of it as you tweak the design. And once you are satisfied that you have built the best engine you can, you can build the final version.

Although the necessary determination and effort can be mustered, one may find the budget requirements of building such an engine to be excessive. So to help reduce that financial burden you want to make sure that you have theoretically come up with the best design you can before you start building, and any further refinement from there can be done in the fabrication stage.

In my searches I have found a fair bit of secrecy surrounding the specifics of Stirling engine design, especially in modern engines where company secrets are not so easily divulged. So much of the information found tends to be recycled from previous works. But there are gems out there. IMO the best sources of information are papers written in the 1980s (which are available online, such as from the NASA technical reports server and Oak Ridge National Laboratory). The 1980s seems to be the peak period of attempts to commercialize Stirling engines for use in automobiles, so it spawned a lot of quality information. Nowadays the interest is renewed but more for purposes of alternative energy - for example, Stirling Energy Systems.

If anyone wants to know more about my Stirling engine program I have a website which explains it in more detail. But just know that it's not free, as I put a lot of time and effort into it. But it is much cheaper than some of the other Stirling engine software available. Here's the website:

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Response to So You Want to Build a Stirling Engine ?

Postby cchagnot » Tue Apr 21, 2009 1:34 am

Here's a link to various photos of the ST-5 engine that I've posted on the Yahoo HAES site.
These were taken around the world in various places and at various times.

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