Friday, April 18, 2008

Syringe Stirling

I just realized I've never done a post on Stirling engines. How can that be?? Stirling engines are so, so awesome.

Stirlings are a member of the class called "hot air engines". As a group, hot air engines work by exploiting the expansion and compression of a gas when it is alternately heated and cooled. There's a good explanation/animation here.

(That particular engine is a "true" Stirling because of those green cross-hatches. That's a "regenerator" that vastly improves efficiency. However, most people call all generic hot air engines, regenerator or not, "Stirlings".)

Stirling engines are awesome for a lot of reasons, but one of the best reasons is that you can use any source of heat. This isn't an internal combustion engine that can use only one type of fuel that's later impossible to wean away from. This is an external combustion engine. You could use natural gas or wood or ethanol or solar energy or geothermal energy. In fact, some satellites/probes use Stirlings to convert nuclear energy to electricity. That's right, Stirling engines in spaaaaace.

Anyway, back to Earth and me. Because Stirlings are so simple and efficient, you can make even a really crappy one run fairly well. They are very common first projects in machine tool classes, for instance, whereas building an IC engine from scratch would be...challenging for a beginner. However, I'm not even at the level of machine tool student, so even that option isn't open to me.

Fortunately, lots of people have been coming up with plans for engines that don't require machining. A few years ago, I tried one that used water-based pistons but I couldn't get it to run. I really have no idea what the problem was, probably multiple things. More recently, I found some "plans" for a test tube Stirling. It took me a while to actually get it going, because it turns out that glass syringes are incredibly finely made and also very rare nowadays. I actually had to buy one on ebay as an antique!

I think the functioning is clear enough to explain itself. (Note: This is technically not a Stirling since I have no regenerator, although I could stuff a bit of steel wool in there to fix that.)

After viewing the video again, actually that might not be so clear. Here's what is happening:

  1. Candle heats air inside left end of test tube.
  2. Air expands, pushing "piston" up.
  3. Marbles roll to the left, displacing the air to the right.
  4. Air cools down, contracting.
  5. Pulling piston down and causing marbles to roll right.
  6. Air displaces leftward.
  7. GOTO 1


  1. Very cool. But what's the white marble for, and why doesn't it roll? Is that a bumper to keep the steelies from bumping the cork off?

  2. The white marble is just another marble. I didn't think 3 would be enough so I added another.

    The reason it doesn't roll is that it's a little too big. It just barely fits in there and I think there isn't quite enough room for the air to slip past.

    Your idea of a bumper is a good one, though. Putting steel wool in at each end as a regenerator could also act as a bumper.

    Oh and speaking of things: Where the HECK can I get lamp alcohol? Candles are VERY sooty.

  3. Are the glass syringes so much less sticky and have so little friction that they only work? How about some sort of low friction plastic or rubber-less plastic syringe?

    Does a glass syringe move so freely that you do not even have to touch it and it moves when holding in the air? i.e. absolutely no sticky spots? or does it still have some notable friction?

    I ask because I do not have a glass syringe and need to know what properties they have.. because there ARE some plastic syringes out there that may work which have no rubber in them.. but I have never tried one nor know if they offer low friction!

  4. Before I had a glass syringe I wondered exactly the same thing. I tried 2 or 3 different plastic ones, plus various lubricants. The main problem was the static friction. Once you get the plunger moving the expanding air can keep it moving, but getting it moving was really, really hard.

    Then I got the glass one. They are really incredible. It's a perfectly smooth movement with a perfect seal.

    You can find them on ebay. Mine is only 2cc IIRC.