Thermoacoustic Engine has Only One Moving Part

Modern internal combustion engines have around 500 parts, with many of them moving in concert with the piston. But have you seen an engine with only one moving part, out of four in total? In the thermoacoustic engine, the power piston is the only part in motion. [YTEngineer] has built a very simple prototype that works on power provided by a tealight.

His little engine, slightly larger than a cigarette lighter, is composed of a test tube that serves as the cylinder, a smaller tube, called the choke, that fits inside the test tube, the stack, which is nothing more than some steel wool, and the power piston. [YTEnginer] nicely explains how the engine works: basically a temperature difference is used to induce high-amplitude sound waves that create the piston’s back-and-forth movement. The engine can be easily converted to an electricity generator by adding a magnet to the piston and a coil surrounding it.

The thermoacoustic engine is a particular type of Stirling engine. They have been proposed as electricity generators for space travel using radioisotopes as the heat source, among other applications. You may be interested in the history of Stirling engines, or perhaps even build a simple one.

19 thoughts on “Thermoacoustic Engine has Only One Moving Part

    1. I did exactly that back about 13 or 14 years or so ago. Much larger than this one, of course. It was made mostly of standard plumbing pipe. It worked okay, but not real great. There was a lot of tuning that needed done, as far as diameter of the pipe versus the length of each section and all that. I quit working on it when the flashlight part stopped working, cheap ass thing that it was. But it did work for a while. I intended to get back to playing with some day, but I guess some day never got here. Well, at least not yet…

      1. A youtube video some time ago showed a thermoacoustic Stirling made with a 4″ steel pipe, a VW beetle cylinder and piston at one end and a propane tank in the other. Heating was on the 4″ pipe just before the tank. I suppose it was also filled with steel wool at that point serving as a regenerator. I was producing in the 100’s of Watts. That makes the thermoacoustic Stirling attractive as it scales better than other types and the gas seals are far away from heat!

  1. Unfortunately the explanations in the video completely leave out the function of the stack (and the empty space between it and the closed end of the tube. It must be some kind of heat exchanger and there must be some phase shift (perhaps by the choke) to achieve the sustained oscillation. Simlar to a normal stirling engine where the displacer moves 90° out of phase with the working piston.

  2. These engines are used at a large scale for liquefying natural gas, with the advantage that they are also powered by some of the gas, but they could use solar thermal to reduce their consumption on sunny days.

    1. 30% is the highest efficiency figure I have seen for a heat driven cooler, and they do get more efficient when they are larger as they can operate at lower frequencies. The more advanced systems are not a simple tube, and the numerical simulations required to design them are not trivial at all.

      BTW “Mark” is a troll, of the pathological type, rather than the mildly amusing type.

    2. The relatively low efficiency (although pretty good for it’s simplicity), is complicated by the naturally linear motion and resonant frequency, which complicates conversion to electrical power in an efficient way.

  3. I would think it would be hard to harvest any real power from it without stopping the oscillation.

    Sure you can tune it to oscillate without load, and probabaly retune it to oscillate with a moderate fixed load, but a variable load would probably be harder to handle.

    But I would love if someone proved me wrong on this

    1. At that small scale you’re probably correct – but that would be true with nearly any kind of heat engine; think of the pittance of work produced by a tiny mass of gas being moved a short distance and you’d be lucky to light an LED, but this would be true of an IC engine or other type.

      Here is an older (2002) white paper on thermoacoustic refrigeration used for liquifying gas as mentioned by @Dan above [PDF]: https://www.lanl.gov/projects/thermoacoustics/Pubs/GasTIPS.pdf

  4. As an electric generator, the piston carries a magnet that moves through a solenoid. The solenoid (an inductor) is made resonant with the acoustic standing wave-length using a tuned capacitor. This improves both efficiency and stability. Even-so, these devices are not very efficient and do not scale well. But they are simple.

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