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Stirling Engine

by Brian Dunning, Sep 13 2012

Just a quick fun thing today, a view of my new Stirling engine:

This is a just a simple little toy one that I found on Amazon for $40, but if you look at the related videos on YouTube, you’ll see that many people have built much more sophisticated engines, including many that are capable of useful work.

Stirling engines run on temperature differentials. If there is a source of heat (in this case my wireless router it’s resting atop), then you have warmer air on one side of the driving diaphragm and cooler air on the other. It works equally well whether its on top of a cup of coffee or a cup of ice water. Give it a spin to get the diaphragm and attached piston moving, and thermodynamics takes over from there. The expansion and contraction of the warmer and cooler air, coupled with the moving diaphragm, does all the work.

These engines come in a variety of different configurations and use various working fluids (this one uses air). They can run on pretty much any heat source. Many commercial products have used them, though their adoption has never been widespread. Issues like size, weight, cost, torque, and other factors have limited their use. Interestingly for space geeks, Stirling radioisotope generators using the same plutonium heat sources as the RTGs currently powering Mars Curiosity are several times more efficient. Why aren’t they on Mars? Basically, weight, complexity, and reliability. For such applications, moving parts are always less desirable than the solid state RTGs currently in use.

Before the Segway was made public, rampant speculation suggested it was to be powered by a Stirling engine. It wasn’t, so we don’t know how this would have worked. It would have needed to burn some kind of fuel and would have had major trouble generating enough torque. If the Stirling component was used to generate electricity to drive a high-torque electric motor, you would have gotten only a tiny amount of use for each unit of time the generator ran.

For now, I’m enjoying my little toy. Some graphite will quiet down the friendly little noise it makes, but I might let the noise stay. It’s kind of like a comfortingly tick-tocking grandfather clock in the office.

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18 Responses to “Stirling Engine”

  1. Steve B says:

    Is that a Richard Simmons box set in the background? :-)

  2. Clara Nendleshaw says:

    I’ve always liked Stirling engine’s, mostly because of my physics background: they’re a very nice thermodynamics demonstration and an excellent teaching example.

  3. Phil says:

    How much power can a stirling produce? If I put one on an AC unit would it make enough power to run a light bulb or charge a laptop?

    • Brian Dunning says:

      Browse through a few examples on YouTube. These things can get pretty hefty; I’m sure you could get one to power a lightbulb but it would be an inefficient way to do so.

      • Tom says:

        Brian,

        What do you mean when you say inefficient? It seems like the idea is to use waste heat. To use the heat would increase the efficiency of the whole system, wouldn’t it?

        Or, do you mean inefficient in terms of the energy required to build the engine in the first place?

        Just want to clarify, thanks.

      • Brian Dunning says:

        They are a relatively inefficient way to convert fuel into electricity, as most such engines require some sort of fuel-driven fire to generate the heat source. If you happen to have a waste heat source (of which my wifi router is a weak example) then you’re good to go.

  4. Aaron says:

    As my eyes drifted across this article, before I had a chance to read a single word, I happened to catch a glimpse of the first sentence of the second paragraph, which my brain spontaneously interpreted as, “Stirling engines run on telepathic energies.”

  5. Max says:

    “Before the Segway was made public, rampant speculation suggested it was to be powered by a Stirling engine.”

    At the time, it was just called “IT” and people didn’t know it was a scooter called the Segway.

  6. Arnim says:

    Hey, you’re forgetting the coolest thing – the stirling engine works the other way round, too! If you put in mechanical energy, it produces a temperature differential.
    So, first industrial use of stirling engines was in a fridge…

  7. d brown says:

    Back in the 60′s FORD spent a lot of money trying to make one to power a car. The most efficient of motors, they are a old idea but are big for their power. I think it was in the 70′s that a solar powered, one moving part Stirling electric generator was made and worked great. Then the party of big oil took the country. They can be of any size, all you must do is make a new industry and spend lots of money. I think I remember pics of desk fans running on a small oil flame before electric fans.

  8. Mjn says:

    The Swedish Gotland class submarines are powered by diesel-fuelled Stirling engines. http://en.wikipedia.org/wiki/Gotland_class_submarine
    TheStirling engines are extremely quiet and stable. I remmember during studies when we had a lab-exercise with a Stirling engine we were shown a photo of the submarine engine with a coin balanced on edge on it during operation.

  9. Phil says:

    Yes but it would be a good way to use energy that is being wasted. For instance I could charge my iPod if I was cooking dinner. Or attach one to my fridge and use the waste heat for something.

  10. igloo says:

    For a commercial application, check out http://www.whispergen.com/

  11. Tim Zebo says:

    This is one of the very best summaries of how a stirling engine works and a terrific application for it using solar energy (which sadly for some reason never led to a commercial product ):http://www.ted.com/talks/bill_gross_on_new_energy.html Does anyone know what happened to it?

  12. Mark Richardson says:

    I’m not someone with a scientific background so forgive me if this is a dumb question.

    What would be to stop someone using a Stirling engine as a household power generator? As I understand it the engine in it’s simplest form works best when producing a constant, non-fluctuating power supply. Well that’s how my back-up generator works.

    However my generator uses gas. If the power source for the Stirling engine were constant – for example geo-thermal heat – and essentially free then it would seem that as long as the engine produced enough power to cover peak usage periods then there would little cost outside of the initial capital cost and periodic maintenance.

    Anyone got any insight on other obstacles? Is it just capital cost?