The Stirling Engine: a steam engine, without the steam

By Ben Everard. Posted over 6 years ago.

In 1816, Robert Stirling created a closed-cycle air engine which worked by heating one side of the engine while the other remained cool. Air in the hot side heats up and expands, and is shuffled through to the cold side where it contracts. This expanding and contracting drives pistons, which in turn drive a fly-wheel which itself keeps the pistons in both sides (the hot and cold parts) moving in synchronisation. It’s an external combustion engine in that the fuel and heat source are entirely outside the main engine, in a similar way to a steam engine.

However, unlike a steam engine, it’s a closed-loop system so you don’t need to top up the water (or any other fluid) – just apply heat and you’re ready to go.
Typically the hot side is heated by a flame, while the cold side is left to cool to room temperature, though this doesn’t have to be the case. Theoretically, the hot side can be left at room temperature and the cold side cooled – provided you can get it cold enough, the engine will still run – or some combination of both heating and cooling.

There are a few different configurations available – the most mechanically significant difference is whether there’s one cylinder that holds both the cold and hot sides (beta type), or two cylinders, with one hot and one cold (alpha type). Beyond this, there are dozens of different ways of mounting them, though most are purely aesthetic, including propeller mounts and setups designed to look like an internal combustion engine. There are also high and low temperature differential options, with the low temperature differential options needing a much larger surface area on the hot and cold sides (leading to a short, wide cylinder).

We got an alpha-type engine driven by a small spirit burner mounted on a small buggy from the Cassiel Alice store on Ali Express for £30.81, (a wide variety of similar models is available on many direct-from-China sites). The unit came fully assembled, with the engine mounted via a threaded bolt on a short section of U-profile aluminium. A couple of axles support plastic wheels that allow the buggy to move.

Stirling2

A STIRLING EFFORT

We applied a flame and waited. It took around 30 seconds for the engine to heat up enough that a flick of the fly-wheel spurred it into life. The speed of the engine depends on the size of the flame, but we’d put it at between 100 and 500 RPM.
The drive system comprises of a couple of gears joined by elastic bands to the front axle. Frankly, it does not inspire confidence. However, it worked and drove our little buggy along at a fairly sedate pace. We have no doubt that this drive system could be improved to power the buggy at a higher speed, but if you’re just after movement, this elastic band system does the job.

Unlike internal combustion engines with their repeated ignitions, there’s not an inherent source of noise in a Stirling engine, and they can operate very quietly. However, this buggy is quite noisy in operation. This appears to be from intolerances in the fly-wheel and attachments. There’s quite a lot of slack and wobble in the system, which causes it to judder about more than this type of engine really should. This doesn’t really affect the buggy too much (though it may shortened its life), but it does limit what other projects you could build off this engine.

We were slightly disappointed that the spirit burner is difficult to remove – this makes it quite difficult to hack the system to work with a different heat source. Although it’s bolted on, the nuts appear to be glued in place, so removing this to allow space for a different burner is difficult. An alternative hack would be to direct the power from the engine into some other mechanism. This should be perfectly possible, and the aluminium base of the buggy could be used as the base of other makes, or simply kept as a mounting for the engine.

Despite these limitations, this Stirling engine is still easy to work with, and in general, Stirling engines are probably the simplest of the mechanical engines to work into a build. They can run at much lower powers than internal combustion engines, and don’t have any high-pressure parts liable to explode if pushed to their limits, (as with steam engines).

Electric motors might be an easier option, but there’s something just undefinably nice about building something with a mechanical engine driven with real fuel. Whether you want it for pure interest, or you’re looking to add a steampunk aesthetic to the power source in your next make, Stirling engines are a fascinating option.