How does a Stirling engine work?

How the parts work together

1. Move the air to the hot end

   The power piston is almost at the inner end of its stroke, so the volume barely changes. The displacer is rising fast and pushes air out of the cold space, down the duct, through the regenerator and into the hot space under it, where the dots are orange-red (press Play to watch more of them gather there). The air warms at nearly constant volume, so its pressure climbs, 1.43 bar here on its way to a peak of 1.68 bar.

2. Hot air pushes the piston out

   Now 43% of the air is in the hot space, about the most it ever is. Hot air is thinner, so even with the displacer near the top the hot space holds less than half of it. The pressure is at its peak, 1.68 bar, and the power piston moves out, so the air does work on it. In the ideal model the hot end keeps the expanding air at the hot-end temperature, so heat flows in there; the orange arrows show the total taken in each turn.

3. Through the regenerator to the cold end

   The piston is near the outer end of its stroke and the displacer is coming down, pushing air up the duct into the cold space. On the way the air gives heat to the wire screens, which are hottest at the bottom and coolest at the top, so it arrives already cool. Less of the air is in the hot space (24%), so the pressure falls, to 1.27 bar here.

4. Squeeze the cold air

   Only 5% of the air is in the hot space now, the pressure is low (1.08 bar) and the piston moves back in. Pushing in at low pressure takes less work than the hot air gave at high pressure. On the chart the dot runs clockwise round the loop, and the area inside is the net work: 1.54 J per turn at 400 °C. The fins carry away the heat of squeezing; the blue arrows show the net heat given out at 25 °C each turn. On the next trip down, the screens give their stored heat back to the air.

5. No temperature difference, no work

   Here the hot end is at 25 °C like the cold end, and the lamp is out. At the same crank angle as step 2, 59% of the air is in the hot space instead of 43%: cool air is denser. The displacer still moves the air, but now the pressure depends only on the volume, so it is the same going out as coming back. The loop closes into a line with no area and the net work is zero. The readouts still show 1.53 J of heat going in at one end and out at the other each turn; that comes from the ideal model holding every space at its wall temperature, which Urieli notes real engines do not do, and it adds nothing because as much leaves as enters. Raise the hot-end temperature and watch the loop open up.

The main parts

Hot end

A spirit-lamp flame heats the thick hot head at the bottom of the displacer cylinder and the lower part of its wall. The space under the displacer is the hot space. The head turns a darker red as the hot-end temperature rises, and the flame grows. The lamp is out when there is no temperature difference and when the cycle is driven backwards as a heat pump, and it burns low when the cranks are in step or opposite, because the cycle then takes in no heat.

Cooling fins

Fins around the upper part of the displacer cylinder give heat to the room air and keep the space above the displacer, the cold space, near 25 °C. The cold head on top carries the gland, packed so that gas cannot leak out along the displacer rod.

Displacer

A light plug 68 mm across in a 70 mm bore. The pressure is the same above and below it, so it never has to hold the gas in, only move it. There is gas on both sides, so moving it changes where the gas is, not how much room it has: going up it pushes gas from the cold space through the duct into the hot space, going down it does the reverse. A seal ring near its top stops gas slipping round its side, so the gas goes through the regenerator. Its rod, clevis and crank are driven by the crankshaft.

Regenerator

A duct beside the cylinder joins the hot space (bottom) to the cold space (top). Most of it is packed with fine wire screens, the regenerator, drawn see-through because a screen is mostly holes. Gas going up gives heat to the screens and arrives cooled; gas coming back takes the heat back and arrives warmed. The screens hold a temperature gradient, drawn from orange at the bottom to blue at the top.

Power piston

Sealed against its bore by two rings. It sits in a separate cylinder joined to the cold space by the transfer pipe, so the whole charge pushes on it. Moving up (out) it makes room for the gas; moving down (in) it squeezes the gas. Its connecting rod turns the crankshaft.

Crankshaft

Two cranks on one shaft, set at the chosen angle apart, drive the displacer and the power piston. The flywheel stores energy from the expansion and carries the shaft round while the gas is compressed. The bright spoke shows the shaft turning.

Frame

The base, the three posts and arms that carry the crankshaft bearings, the pedestal under the power cylinder, the lamp shroud under the hot head and the dark insulating band that joins the hot and cold sections of the displacer cylinder.

Gas dots

About a quarter of a gram of air is sealed inside. Each dot stands for 1% of it, so counting the dots in a space gives that space's share. Hot air is thinner, so a hot space holds fewer dots than a cold space of the same size. Colour shows the gas temperature, blue at 25 °C and orange at 525 °C. Dots are drawn in the cut-open half of each space and its passages; the few in the 1 mm gap round the displacer are counted but not drawn.

Heat arrows

Drawn arrows, not machine parts. Orange arrows at the hot head show the heat taken in there each turn. Blue arrows beside the fins show the net heat given out each turn by all the gas at 25 °C: the cold space gives out more than this, while the gas in the power cylinder takes some in as it expands. Their length is proportional to the amount and does not change during a turn. If the engine is run as a heat pump the arrows reverse, with the hot end's heat drawn leaving the side of the hot head. With the hot end at room temperature no arrows are drawn: heat still passes through (see the readouts), but none of it comes from the lamp.

Questions worth exploring