Science
Beautiful, unusual model engine runs, baffles experts

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Watch this video. If you’re confused about how it works, check the diagram above.

If you’re still confused, don’t worry–you’re in good company.

Nederlander Jan Ridders, who is something of a legend in the model engineering world, built what he calls his Thermo Pulse Mobile without really understanding how it works, basing his model solely on a YouTube video demonstrating another one in operation. At first it didn’t work. Then…

After a lot of random experiments, which I shall not enumerate here because of their irrelevance, I almost gave up until I took a closer look at the video. I noticed a kind of restriction at the place were the brass cylinder was fixed in the glass tube… Because I hadn’t the slightest idea how the restriction should look, I used numerous small brass cylindrical plugs from my scrap box, including those with and without all kinds of bores in them and other deformities. I started random experiments, putting all kinds of plugs in the glass tube at the place where it is fixed in the stand…

By far, most of the plugs didn’t bring any positive effect, but with some of them the engine started to show some sign of life. I was pleasantly surprised and at the same time extremely astonished when I found one plug that caused the engine to keep running, although yet not fully reliable. Using the typical characteristics of this “reference plug” I made a new plug, adding step by step changes to the geometries of the reference plug. This was still a random way of working, but at least somewhat more systematic. Finally I succeed in making a restriction that allowed the engine to run reliably.

Even though they’ve now got a working model, Ridders and his fellow model engineers have been unable to come to a consensus explaining the thermodynamics.

If you’re interested in this or any of Jan Ridders’ engines, he will e-mail you high-resolution technical drawings for free upon request. Apart from this remarkable curiosity, Ridders’ more traditional model engine work is not to be missed.

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26 thoughts on “Beautiful, unusual model engine runs, baffles experts

  1. The vortex tube has a restriction as well, wonder if there is a similar fluid dynamic effect that’s driving this little heat engine?

  2. I would guess that the momentum of the flywheel is pulling heated gas through the restriction, causing it to contract and cool on the unheated side, which pulls the piston back into the glass tube.

    The momentum of the flywheel then forces the gas back into the chamber where it is heated and expands, pushing the flywheel out.

    It’s a little bit like a vacuum engine and an Einstein refrigerator had babies. Then again, I’ve been wrong before. Many, many, times.

  3. Does this qualify as a Stirling Engine? I’m thinking the restriction device acts as a heat sink and cools the air going through it giving it the difference in pressure it needs to move the piston in an out.

      1. Just wasn’t sure if a sterling engine had only to do with an external heat source or if it also had something to do with the number of pistons etc. I just see one so that was my confusion. Thanks for clarifying!

        1. Sorry, I meant to confusion of the builder, not of you. I couldn’t say if this technically qualifies a Stirling Engine, but it’s clear to me that it’s a heat exchanger driving a piston.

  4. I think what may have thrown people off is the fact that the engine uses it’s output to rotate itself. This isn’t important to the function of the engine, but it does look kind of neat.

    1. Actually, upon further thought, perhaps the rotation does serve a purpose. Since this version of the stirling engine does not have cooling fins on the compression portion of the test tube, maybe it needs the movement to create air flow, thus removing heat.

      It’s a clever engine, but to say it baffles experts is hyperbole.

  5. I believe there’s a natural resonance of heating the air, forcing it through the restrictor, pushing the piston and then having the excess momentum of the flywheel turning then pushing the piston back in and compressing everything. That coupled with the cooling effects greater on the open end of the test tube causing a low pressure area.

  6. Could something in the air or something in the steel wool be undergoing a pyrolytic reaction? Maybe the steel wool is acting as a catalyst for oxydation or breakdown of voc’s, or maybe the steel is oxidizing. How long does this run for, is the steel wool eventually consumed?

  7. It looks like a take on the Stirling engine. The steel wool is a thermal mass to help even out the internal temperature of the expansion chamber. The air heats up in the thermal chamber, gets forced through the restriction plug and pushing the piston out of its way. There is now a partial vacuum in the thermal chamber. The next chamber allows the air to cool slightly, stopping the air expansion and allowing it to rush back into the other chamber, pulling the piston with it. The restriction plate looks like it helps regulate the speed at which the air moves in and out of the two chambers. Small hole to large hole is lower pressure, large hole to small hole, higher pressure. I’m just an electrical engineer, so I’ll use that itself as a disclaimer ;)

    1. I pretty much agree with the last poster. Here’s how I see the operation (starting cold with the piston drawn): The air around the steel wool heats up and expands, running through the restriction and pushing the piston out. As the air enters the piston (cool) side of the restriction, it begins cooling. The expanded and cooler air stops pushing on the piston, we reach full throw, and momentum drives the piston (and air) back toward the hot side to warm up and expand again. The aperture is sized so that the air can flow at a rate “resonant” with the speed of the piston (following Mike S.’s intuitive understanding of the system).

      So the restriction serves to (more or less) thermally isolate the two chambers. This way we have a hot end and a cold end. I guess that’s basically the definition of a Stirling engine. It looks similar to an “alpha” type but without the two pistons to fully empty the hot and cold chambers at each end of the stroke.

  8. Jan,
    I wonder if a small regenerator, in the form of some steel wool at the entrance to the orifice, would improve the performance a little, by storing some heat, which would preheat the air on its return back through the orifice. I think it would be easy to try and would be interesting to see the result (possibly a little more speed).
    Jim in the USA

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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