Simple Fluidyne Engine

[Mirslav] built this fuidyne engine himself. This is a single piston model but you won’t find any precision milled cylinders here. That’s because fluidyne engines use columns of water as the pistons. In the rig shown above you can see one metal pipe which serves as the cold side of the loop. There’s another hot pipe underneath the insulation that completes the circuit. When that pipe is heated it causes the air inside the loop to expand, forcing the liquid on the open side of the plastic tubing (to the left) to rise. Once that air escapes to the other side of the circuit the water piston in the open tube falls back again. This results in continuous oscillation that can be used to drive a pump using a pair of check valves.

We’ve embedded a couple of videos after the break. You’ll see the system tested by heating one pipe with a hot air gun. But the example seen above uses an induction coil to bring the heat.

20 thoughts on “Simple Fluidyne Engine

  1. Wow, I don’t know if I should call it an atmospheric steam engine or a water pulse jet but it’s a cool concept. No moving parts(except check valves) is hard to argue with. I wonder what the efficiency is with good aerogel insulation.

    OTEC applications?

  2. Yes, that is indeed a 386 motherboard :) I had no idea it will attract more interest than the engine ;) Seriously though, I hope to achieve much better results with my next Fluidyne, which will have much larger diam. pipes and be much more heat resistant. If anyone has any (inexpensive) ideas regarding tubing and ways to join them, I’m listening.

  3. I doubt other liquids would make a substantial difference. “Fluidyne” is just a fancy name for a stirling engine where the displacer is liquid. Like other stirling engines, the performance should improve if you can pressurize the system. Substituting hydrogen or helium would slightly improve performance.

  4. The goal is to heat the air on the hot-side, it looks like the metal hot-side pipe contacts the water – this will suck the heat out of the air. Heat lost to evaporation is heat the engine doesn’t use for pumping – look into adding an insulating float on the hot side. Also length of the tuning arm is very important – it is analogous to the mass in a flywheel.

  5. @ryan: Yes, water level was approx. at mid-hot metal tube level. So it should be lower, according to your explanation? I have tried adding insulating floats, but they would get stuck in a narrow tube so I gave up on that for now. Can you point me to some source of info regarding the optimal calculation of Fluidyne dimensions?

  6. Optimal calculations can be obtained from West’s book “Liquid Piston Engines.” To a first approximation, the natural frequency of the displacer and output water columns are matched.

    I’m trying to make a similar fluidyne; my copper tubes are much longer and it is not working. My diameter is also very small…about 2mm. Do you think these might be the problem?

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