Stirling Engines

Monday, August 13th, 2007

Cory Doctorow of Boing Boing is fascinated by this Papercraft stirling engine that runs on coffee and shares a translation of its German description:

The Stirling Engine
Runs on a cup of coffee or an ice pack

The revolutionary concept for this hot-air engine was discovered in 1816 by the Scottish minister Robert Stirling and has been updated for today. The principle is as ingenious as it is simple: In a sealed cylinder, heated from the underside, a piston pushes the enclosed air back and forth between the hot and the cold side. The air therefore expands out and compress together every cycle and that movement is converted via a moving piston and crankshaft into rotary motion.

As an energy source, any type of warmth or cooling that produces a temperature differential can be used, from an open fire to solar energy or any other unused source of heat or cold.

Set this fully functional Stirling engine on a cup with boiling hot coffee (Tea or water also works of course) – give the flywheel a small push to the left – and the apparatus begins simply to pump up and down – for up to an hour!

This isn’t everything it can do: Set it on an ice pack or ice cubes from the freezer and turn the flywheel to the right and it will also pump up and down for an even longer time.

(Caption:It is really a marvel)

Kit made from sturdy punched cardboard with gold stamping, complete with all accessories including laser cut aluminium plates, low-friction plastic axle bearings and spring steel bent wire.

Height 16.5 cm, width and depth 12.6 cm.

The folks at the American Stirling Company have a similar device, the MM-5 Coffee Cup Engine Kit, only it isn’t made from card stock.

Stirling engines are fascinating for a number of reasons. Unlike typical gasoline or diesel engines, they aren’t internal combustion engines; they’re external combustion engines.

One side-effect of this is that they can run on any source of heat, not just literal combustion — solar heat, waste heat given off by heated coffee or electrical equipment, etc.

Stirling engines are also disturbingly simple, consisting of little more than a sealed cylinder of air (or helium or hydrogen). One side is heated, the other cooled, and the piston moves as the gas expands and contracts.

This simple design also makes the Stirling engine remarkably efficient.

So why don’t we see more Stirling engines in use? Well, the first major drawback is that a Stirling engine, like a steam engine, needs time to get going. You can’t simply step on the gas and get more power. Also, Stirling engines tend to have low power density; they tend to be big for the amount of power they put out.

Today, Stirling engines are primarily used in reverse, as cryocoolers.

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