How I picked these: how they work, specs and buyer reviews. I haven't owned every one of them.
What to expect
These are for watching and understanding, not for doing work. Go in knowing that and they're a joy.
- Almost no power. A desk model makes a tiny fraction of a watt. It can turn its own flywheel and not much else. It won't charge a phone or light a bulb.
- It runs while the difference lasts. As the water cools, the engine slows, then stops. Expect tens of minutes from a mug of just-boiled water, depending on the mug and the room.
- It needs a push to start. You flick the flywheel once. After that it keeps itself going.
- Cheap ones vary. A low-temperature engine only works if friction is tiny. Buyer reviews show some arrive needing a little adjustment before they run well.
- Heat isn't free power. The energy comes from the hot water. The engine just turns a small part of the heat flowing through it into motion.
Why these
There are lots of model Stirling engines. I've picked by one question: what's the easiest way to see the idea working, safely, at home?
A low-temperature-difference engine runs on hot water or ice. No flame, no fuel, nothing to burn. It's safe on a kitchen table and you can run it again whenever you boil a kettle.
A ready-made engine works out of the box, which is best if you just want to watch it. A kit takes an evening to assemble, and putting it together shows you exactly what each part does.
A hot-air engine heated by a small spirit lamp has a much bigger temperature difference, so it runs faster and more strongly. It burns fuel, so it's not fuel-free, but it still needs no battery. Try it once the low-temperature one makes sense.
All three let you see the parts moving. Watching the displacer rise and fall a quarter turn ahead of the piston is how the cycle finally clicks.
How it works
Air expands when you heat it and shrinks when you cool it. A Stirling engine keeps one sealed pocket of air and moves it back and forth between a hot side and a cold side. The pressure rises and falls, and that pushes a piston.
The low-temperature kind is a flat drum. The bottom plate sits on the hot mug. The top plate stays at room temperature. Inside is a light foam disc called the displacer. It doesn't push anything. It just shuffles the air: when it lifts, the air goes underneath it to the hot plate; when it drops, the air goes above it to the cold plate. A small power piston in a cylinder on top feels the pressure change and moves the crank. The flywheel keeps everything turning between pushes.
The trick is the timing. The displacer's crank pin sits a quarter turn ahead of the piston's. So the air is moved to the hot side just before the piston needs pushing up, and to the cold side just before it needs to come back down. Fig 2 shows the four stages.
Why so little power? Physics puts a ceiling on any heat engine, set by how hot and cold its two sides are. With water at about 90°C and a room at about 20°C, even a perfect engine could turn only about a fifth of the heat flowing through it into motion. A hobby model manages a small fraction of that, and spends most of it beating its own friction.
What to buy
Start with one of the two low-temperature engines. Add the spirit-lamp kit later if you want to see what a bigger temperature difference does. Listings change, so check the details before you order.
These are affiliate links. If you buy through them I make a small commission, at no extra cost to you. It helps cover the cost of running this site. This is a hobby, not a shop.
Runs on hot water
Runs on a flame
You'll also want a wide mug with a flat rim, a kettle, and for the flame kit, the fuel its instructions name (usually methylated spirit or bioethanol).
First run
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Check it turns freely
Before any heat, give the flywheel a gentle spin by hand. It should turn smoothly and coast for a few seconds. Watch the displacer move up and down without touching the sides. For the kit, build it first using its instructions, and leave screws snug rather than tight.
Why: on a low-temperature engine the push from the air is tiny, so friction decides whether it runs. If the flywheel stops dead when you spin it cold, it won't run hot either.
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Find the right mug
Use a wide mug with a flat rim that the bottom plate covers fully. Put it on a flat, steady surface away from draughts.
Why: the more of the plate that touches hot water vapour, the faster heat gets in. Rocking or a draught across the top plate both rob it of its small temperature difference.
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Pour just-boiled water and wait
Fill the mug near the top, set the engine on it and wait about a minute for the bottom plate to warm through.
Why: if you flick it straight away, both plates are still at room temperature and there's no difference to run on.
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Flick the flywheel
Give it a firm flick. If it slows and stops, flick it the other way. Once it's going in the right direction, it will pick up speed and settle.
Why: the displacer has to lead the piston by a quarter turn. Turn it backwards and the timing works against you. One direction is always right for hot-underneath.
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Flame kit: same idea, more care
Fill the spirit lamp outside the engine, cap it, wipe off any spills, then light it and set it under the hot end. Wait a minute or so, then flick the flywheel. To stop, put the cap back on the lamp to snuff it.
Why: filling away from the engine and wiping spills stops a stray flame running across wet fuel. The snuffer cap starves the flame of air, which is safer than blowing it out.
Experiments + care
Things to try
- Count the speed as it cools. Every five minutes, count flywheel turns for 15 seconds. Note the water temperature if you have a thermometer. You'll see speed fall as the difference shrinks.
- Run it on ice. Put it on a bowl of ice or a frozen ice pack. Now the bottom is cold and the room is the hot side. It runs the other way round, so flick it the opposite direction.
- Cool the top. With it running on hot water, rest a cold, damp cloth on the top plate and watch it speed up. A bigger difference means more push.
- Try your hand. The best low-temperature engines can run on the warmth of a palm. Don't count on it from a cheap one, but it's worth a go once it's well run in.
Looking after it
- Don't oil the power piston. Most use a graphite or glass piston that runs dry. Oil makes it sticky and it stops. If a bearing squeaks, one tiny drop of light clock or sewing-machine oil on that bearing only.
- Keep it covered. Dust in the piston or bearings adds friction, and friction is the enemy here.
- Let it cool before you pack it away, and don't leave it sitting on the mug in steam for hours.
If it won't run
| What you see | Likely cause | Fix |
|---|---|---|
| Spins a few turns then stops, every time | Flicking it the wrong way, or not warmed up yet | Wait another minute, then flick the other direction. |
| Won't coast when spun cold | Too much friction: tight screws, rubbing displacer or bent rod | Loosen screws a touch, check the displacer clears the walls, straighten rods. |
| Runs, then stops as water cools | Temperature difference too small | Refill with just-boiled water. Cool the top plate. |
| Piston feels sticky | Dust or oil on the piston | Clean piston and cylinder with a dry, lint-free cloth. Never oil them. |
| Runs weakly even on fresh hot water | Air leaking past the piston or a seal | Check the cylinder is seated and any seals are in place. Turn the flywheel slowly by hand: you should feel a slight springiness as the air squeezes. None at all suggests a leak. |
| Flame kit won't run | Flame not on the hot end, or not warm long enough | Centre the flame under the hot end, wait longer, and keep it out of draughts. |
Safety
- Boiled water scalds. Keep the mug well back from the table edge and away from small children. Lift the engine off before moving the mug.
- Never put a low-temperature engine on a flame or hob. They're built from acrylic and thin metal for gentle heat. A flame will warp or melt them.
- Spirit lamp: fill it only when cold and unlit, never while burning. Don't overfill. Use it on a stable, heat-proof surface away from anything that can catch. Methylated spirit and bioethanol flames can be almost invisible in daylight, so treat the lamp as lit until you've capped it. Keep the fuel bottle well away while it burns.
- The hot end stays hot for a while after the flame is out. Let it cool before touching.
Notes
Robert Stirling, a Scottish minister, patented his hot-air engine in 1816. The low-temperature-difference design, the flat drum that runs on a mug, came much later: engineers such as Ivo Kolin and James Senft worked on it in the 1980s and 1990s.
The "about a fifth" ceiling comes from the standard heat-engine limit: the best possible efficiency is 1 minus the cold temperature over the hot one, measured from absolute zero. For 20°C and 90°C that's 1 − 293/363, roughly 19%. Run times and temperatures elsewhere on this page are rough guides; your mug, room and engine will change them.
Related guides: earth batteries and lemon cells are another way to see energy come from something ordinary, and a mechanical watch is a machine you can watch tick. All guides are on the battery-free page.