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Home How a Stirling Engine Works

Heat in, motion out

How a Stirling engine works

A flame outside, two teaspoons of air sealed inside, and a brass wheel that spins twenty times a second.

Try the quarter turn
A brass desk Stirling engine on a walnut base at night: the spirit lamp's blue flame under the glass hot end, copper cooling fins, and the spoked brass flywheel.
1,200rpm
20 turns a second
300°C / 45°C
hot end / cold end
9cm³
of air, sealed in for good
90°
the displacer runs ahead

Video

How a Stirling Engine Works - Heat In, Motion Out

Watch on YouTube

3D model · cut open

Run it, slow it, take it apart

The engine with its top half cut away, so you can see the air move. Drag to turn, scroll to zoom, click any part.

Try itClick the flywheel to flick it, or the lamp to put the flame out.
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Meet the parts

Seventeen parts on the bench

The engine taken apart. Tap a part or pick a name.

The Stirling engine's parts laid out on a dark bench: the spoked brass flywheel, the bearing standard, the crankshaft with its two crank discs, the cylinder block, the power cylinder, the finned cold sleeve, the glass socket, the glass hot tube, the rods, the piston, the displacer, the walnut base, the generator, the spirit lamp and the snuffer cap.

Hover or tap the picture.

Parts diagram

Looking down into the cut

The top half sawn off, as in the video. The numbers follow the heat from the flame to the lamp.

The Stirling engine seen from straight above with its top half cut away: the glass hot end over the spirit lamp on the right, the displacer inside it, the finned cold sleeve, the brass block with its transfer port, the power cylinder and piston at the back, the rods, the crank discs, the bearing standard, the flywheel along the front, the pulley and generator at the back left.
    How it works, step by step

    The video in 31 steps

    Press Show me in 3D to see a step in the model.

      The night bench.
      Step 1 of 31

      A flame and sealed air

        Demo · the key idea

        Heat the air

        Free air grows when it warms. Sealed air can’t, so it pushes harder instead.

        –in kelvin
        –balloon size
        –sealed pressure
        –net push on a 16 mm piston

        Demo · why it keeps going

        Tune the quarter turn

        The displacer leads the piston by 90°. Change the lead or the direction and watch the loop.

        –work each turn
        –highest pressure
        –lowest pressure
        –on its own

        Demo · the speed control

        Flame, cold and load

        The speed follows the difference between hot and cold. Change the flame, cool the fins or switch on the lamp.

        –speed
        –hot minus cold
        –best possible efficiency
        –work each turn

        More to know

        Kinds, history and care

        Six kinds

        Most powerAlphaTwo pistons in two cylinders, one hot, one cold.
        Stirling’s ownBetaDisplacer and piston share one cylinder.
        This oneGammaDisplacer and piston each have their own.
        Coffee-cupLow temperatureWide plates; runs on a few degrees.
        No crankshaftFree-pistonPistons bounce on gas springs.
        Run backwardsStirling coolerDriven by a motor, it makes cold.

        From a quarry pump to space

        1. Robert Stirling patents his engine, regenerator included.
        2. One pumps water from a Scottish quarry.
        3. Philips gives it the name “Stirling engine”.
        4. Ivo Kolin’s engine runs on a 15 °C difference.
        5. Sweden’s first Gotland submarine enters service.
        6. A solar dish turns 31.25 % of sunlight into power.
        7. NASA’s KRUSTY test makes power from reactor heat.

        Running one safely

        • Never refill a lit lamp. Cap the flame first.
        • The glass stays hot for minutes after.
        • No oil on the piston. Its fit is lapped dry.
        • Flick it the right way and keep fingers clear.

        Things worth knowing

        The same air, foreverNo valves, no exhaust.
        The displacer does no workIt only moves the air.
        It runs on a differenceOn ice it runs backwards.
        The flywheel lends energyAbout 1.1 J at 1,200 rpm.
        45 % at best hereThis toy manages under 1 %.
        Run by a motor, it coolsDown to about −196 °C.
        Quick check

        Four questions