Easy Free Piston Stirling Engine

Stirling engines are really cool machines, invented by Reverend Dr. Robert Stirling in 1816 to rival the steam engine, they are one of the most efficient engines ever conceived.  Building one is a very rewarding experience, but it has a certain level of difficulty. However, [Attila Blade]’s version of a free-piston type Stirling engine is simple enough to be built in a matter of minutes.

To build the engine you only need a test tube, steel wool, a latex glove, an O ring and some wire. The construction is straightforward as you can see in the video. The whole engine rocks on the wire frame which also makes it different to most other Stirling engines that you can watch on the net. The free piston is just one type of several possible configurations for a Stirling. The most common one, is the beta type, usually made with soda cans, but it is much more difficult to build than [Attila Blade]’s engine.

This is definitely a fun project that you may want to try, and is also a great way to learn  thermodynamics concepts. Even if you don’t build this particular version, there are many other possibilities using mainly household items, or you can also check the very interesting history behind the Stirling engine.


22 thoughts on “Easy Free Piston Stirling Engine

  1. “Dr. Robert Stirling in 1816 to rival the steam engine, they are one of the most efficient engines ever conceived.”

    I’m sure there are reasons this “efficient” engine isn’t more widespread.

    1. they are having a rather big comeback, they are used in RTG’s and as general engines, you can buy industrial units in the megawatt range.

      dish-stirling solar power is also one of the most efficient solar technologies out there, with common efficiencies around 30%, compared to the 15% of high end photovoltaic cells.

      they are also used in RTG as far as i know, i dont know if any ever got sent to space, but it wouldnt surprise me.

    2. The Stirling is more efficient than a steam engine because it doesn’t pass through a phase change between liquid and gas, but because the phase of its medium is entirely gaseous heat transfer is relatively slow compared to a boiler and power density is relatively low.

      1. The shame was that size & weight are constraints for mobile applications (same is true for batteries), but not so much for stationary applications. Of course, turbines are also very efficient and small (for power produced), so that pretty much displaced everything else.

    3. Several. But your citation marks are implying you don’t believe the efficiency claim, if so you are wrong. It is very efficient compared to other engines.

      To make a good Stirling engine one have to have excellent precision in components, very good seals with little friction and a good regenerator (essentially a heat accumulator). One also want to have a good working gas like helium which brings a number of problems with it. The materials and design of a highly effective engine also require tolerating high temperatures without too much variance due to thermal cycling. Stirling engines also require more weight (and even so volume) for a certain power level compared to e.g. an Otto engine. Finally it is harder to throttle heat engines and they require warm up time before they can be used from a cold start.

        1. Yes. However they are in use in many places as they have many advantages, it can use many types of fuel (as it is an external combustion engine), it can use general heat-sources like the sun or radioactive decay etc. too. They are silent with little vibration making them useful for e.g. submarine propulsion.

          The two big problems with them are that they require more volume (that can be compensated in some cases with the increased efficiency) and that they are more complicated to get right.

    4. I believe it’s not gotten attention simply due to a lack of public confidence. The concept of free mechanical motion does not garner public faith. This is quite a pity as free temperature differential is quite easy to find.

      The example here is easily converted to portable and should suffice to power a simple generator to recharge a cell phone on the trail using reflector for daytime and IR from campfire for night. Or perhaps use a reflector to make a winter trickle charger for your motorcycle battery. Possibilities abound.

      Heh… think there’s enough power there that you can build an R/C glider with near unlimited daytime flight time. 3 degrees more temperature differential per 1000′.

      Some cases solar cells would be superior, some cases not.

    5. Mostly just that the materials science was there to make them good when they were invented and by the time it was much more development had been put into internal combustion engines. Nasa did research into Stirling engines for vehicle use in the 80s and 90s and was very positive about it. They replaced the engine in an AMC Spirit and it made better power and better fuel mileage while being able to run on a variety of fuels.


  2. This is NOT a Stirling engine, it has no displacer or two separate cylinders. Instead it is a so-called thermoacoustic (heat) engine.

    Such an engine has a slightly different working principle and involves standing waves (To be honest I am still in the process of understanding how it works exactly to present it in one of my classes).

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