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Stirling Engine

  1. Sep 20, 2010 #1

    Drakkith

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    Hello all. I've been wanting to build a Stirling engine at home out of parts that i could easily afford/scrounge up. Not one of those small ones that you hold in your hand, but one that could generate some real work. Enough to run a small generator maybe. The problem is that i have absolutely no idea where to start. I was thinking about making one out of some small RC car/plane engines(combustion engines), or even a lawn mower engine, but i dont not know if i could convert one. Any help would be appreciated.

    Stirling engine: http://en.wikipedia.org/wiki/Sterling_engine (Funny, the link is spelled Sterling, but the article has it as Stirling...)
     
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  3. Sep 20, 2010 #2

    Danger

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    I'm certainly no expert in this area, but I'm wondering if perhaps a twin-cylinder air compressor might be a better starting point than an IC engine. Just a thought.
     
  4. Sep 21, 2010 #3

    Drakkith

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    Perhaps. I have about zero experience at any of this, so I'd never even thought of that.
     
  5. Sep 21, 2010 #4
    That's an interesting thought. The Stirling design needs two pistons, one for power and one to push the air back & forth.

    I built a toy one many years ago, a learning exercise combining thermo with practical machine shop skills. I still have it, and it still runs (it turns a fan). Building one to run a generator would be a leap from that. What are you planning on using for fuel/heat source? Considering the Stirling is (generally?) an external combustion device, which can be a plus or a minus for a given fuel type or machine location. I mean, maybe it's good that you can burn wood chips or dung; but maybe you don't want to do that in your basement...
     
  6. Sep 21, 2010 #5

    Danger

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    Depending upon the climate, Sterlings can also be pretty effective when heated with focused solar energy. (They don't work so great at night, though.)
     
  7. Sep 21, 2010 #6

    Drakkith

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    The heat source would be heat from the sun or from an internal combustion engine.
     
  8. Sep 21, 2010 #7
    In order to achieve moderate efficiencies, you need a two cylinder engine with a heat capacitor (usually something like copper wool) in between them. However, this adds complexity and cost. If you want to start simple, a little RC engine might be a good place to start if you can machine a new sleeve but I would recommend starting with an oil-less air compressor. Try buying a toy stirling engine to start off with and study and go from there.

    http://www.thinkgeek.com/geektoys/science/9e84/?cpg=froogle
    http://scientificsonline.com/produc...-DB81-DE11-8C0A-000423C27502&mr:referralID=NA
     
  9. Sep 22, 2010 #8

    Ranger Mike

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    My thinking may be wrong regarding this matter but I do not think it is a good idea to use an air compressor as a starting point to make a two cylinder engine. the A/C is DRIVEN by an external electric motor that turns a crankshaft that is sized to deal with force required to squeeze the two cylinders. If you reverse the process and use the cylinders to build more pressure than the typical 200 psi requirement you will be introducing a lot more stress and i fear the housing ( usually cheap cast variety) will crack or even shatter..no provisions for crank lubrication, con rod lub , heat dissipation etc..
     
  10. Sep 22, 2010 #9

    Danger

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    Good points, Mike. I hadn't thought it through too well.
     
  11. Sep 22, 2010 #10
    Well, they sound like good points, but on the other hand, I would be surprised if anyone (especially the OP, who says he has 'absolutely no idea where to start'), could get power out of the compressor end (running a a Stirling cycle) approaching the motor power the device is built to use in its compressor configuration. So, I would think the crankshaft will be OK. Overheating the cylinder used as the hot end might cause problems. But, you might learn alot by trying.
     
  12. Sep 22, 2010 #11
    I'm thinking that using 2 separate lawnmower (or single cylinder engines) would work best. You would have to hook the cylinders up to each other, (just like any other stirling engine). And you would have to synchronize them (kinda tough to explain but I'm sure you know what I mean). It is something that would be just a bit complicated for me to do but this may be a start. The reason I say 2 separate engines is because if you have twin cylinders, they would be right next to each other and would conduct heat and you would have to modify the crankshaft so that they are timed accordingly for a stirling engine.
     
  13. Sep 23, 2010 #12

    Drakkith

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    Hrmm, i wonder if it would be possible to make one using a turbine instead? (Just thinking aloud...)

    Could you have your heat source heat the air and have a valve open at a certain pressure, releasing the hot air into the turbine to turn it. As the pressure falls, the valve would close and another valve would open, releasing cool air into the heating area again. I'm not sure how you would get the air from the turbine cooled down and transferred back to the hot side though. I'm assuming you would have to use work from the turbine?
     
  14. Sep 23, 2010 #13

    Danger

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    That doesn't sound too practical to me, but I stress again that my knowledge is severely limited. The only thing that I can say for sure is that an engine operating along those guidelines would not be a Sterling. Those are pretty strictly defined.
     
  15. Sep 23, 2010 #14

    Drakkith

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    True, it kinda popped into my head as i was thinking. I would still work on the same principles as far as i can tell. You should just need something to pump the cool air back into the heating element. I think it could be practical. The only moving parts would be the turbine, whatever pumps the cold air, and the valves. How it compares to a normal heat engine i couldnt say.
     
  16. Sep 24, 2010 #15
    You might look into using "liquid pistons" in your sterling engine, where columns of water or oil do the compression and expansion of the working gas in the cylinders. Using liquid pistons avoids the need for precision machining. A sort of hybrid approach between a pure mechanical piston and liquid piston is a "Oil immersed piston". These work well (I've build working stirling engines with them), and allow large ,sloppy tolerances between piston and cylinder wall. I made the piston by directly casting acrylic resin into a pvc pipe, which served both as a mold and the final cylinder wall. Due to sloshing of the liquids, such pistons can't run real fast. If you search the archives of "Home Power" magazine, you can find a design for a liquid piston sterling engine.
     
    Last edited: Sep 24, 2010
  17. Sep 24, 2010 #16

    jambaugh

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    I would suggest starting with the gamma configuration, displacement cylinder and displacer piston separate from the power cylinder/piston which you might get from a compressor or small ICE. The displacer piston can simply be packed bronze wool in a pipe to act as the regenerator and the displacer cylinder just a length of slightly larger pipe. You can connect the displacer to a small engine by threading a connecting pipe to the sparkplug hole.

    It is then a matter of mechanically linking up the power piston, displacer piston, and a flywheel.

    The tricky part, I think, is getting a good seal on the displacement cylinder where the rod pushing the displacer enters.
     
  18. Sep 24, 2010 #17
    I should note that the power produced per cubic inch of displacement of a simple, atmospheric pressure Sterling engine is very low. The friction in a lawnmower engine is so high that a sterling engine made out of one is likely to not run at all--I know, I tried it once. If you use pressurized air in the engine (which increases specific power) you might be able to get it to run, but now you would have to deal with the air leakage problems.
     
  19. Sep 24, 2010 #18

    Drakkith

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    Thanks, I was actually wondering about that.
     
  20. Sep 24, 2010 #19
    Oh and another tip: I managed to make a good running small Stirling Engine using a bicycle pump ( a portable mountain bike model designed to clip onto the bike's frame) as a piston. Bicycle pump pistons have low friction and a fair amount of displacement. They also pressurize the engine slightly due to the design of the piston, that lets air flow past it one way. Increases the power of the engine somewhat.

    Obviously you want to keep heat away from the delicate piston in a bicycle pump. In a properly designed engine, the only really hot area will be on the heated end of the displacer piston--the power piston will stay cool.
     
  21. Sep 25, 2010 #20

    Drakkith

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    Thats pretty cool, i had never even thought about something like that before lol.
     
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