The Stirling Engine (or hot air engine) Explained

1820, when steam engines operated everywhere, Robert Stirling, a Scottish vicar, and his brother James built a new engine. They used an outside burn, additionally hot air was sufficient for the operation of the engine. This principle has theoretically already been developed by the English flight pioneer Sir George Cayley twenty years earlier.
Stirling proceeds from the fundamental characteristic of air that it needs more space with rising temperature. If the volume remains constant, the pressure rises. All thermal engines work because of this characteristic. The basic principle of the stirling engine is described in the following.

Animation: Stirling engine.
enlarge (51 K, 800 * 600), slow animation (51 K, 330 * 320), described (51 K, 400 * 340)

Mode of operation of the stirling engineA machine, whose cylinder is alternating cooled and heated up again, would work, too. But for the change of temperature of the cylinder large energy quantities would be wasted. Stirling developed therefore a new principle: He uses two cylinders and two pistons: a working and a displace piston. The cylinder left is constantly heated up on the top while it is cooled at the lower part. The displace piston left seals not, it is of a porous structure, which lets air pass through. If the displace piston is now in the lower dead-center, air is strongly heated up and the pressure pushs on the working piston on the right, which slides to the right now. The left piston gets now pulled upward by the coupling of the two pistons. Air is strongly cooled, and together with compression work from the flywheel the working piston is brought again to the left, the displace piston slides down and the air is heated up again.

The regenerator

Robert Stirling called the displace piston also regenerator. In the upper animation only a spongelike piston is used, but in other constructions e.g. a close wire mesh is used as piston. This regenerator can, like the name already says, “regenerate” the air. If the piston goes upward, it is flowed through by warm air and takes up a part of the energy of the air then – it stores it. If the piston goes downward, it is flowed through by cold air now and delivers its stored energy to it. At the same time the wire mesh cools down. The piston helps so to avoid losses of energy. Also because of the regenerator, stirling engines reach excellent efficiencies.

Grafic: Regenerator. T1 is a higher temperature than T2. The green arrow displays in each case the change of temperature of the regenerator, the red-blue arrows the change of temperature of the gas.
animated (54 K, 244 * 292), enlarge (76 K, 1020 * 292)

Modern stirling enginesThe stirling engine appeared at the beginning as a good alternative to the steam engines, but the success was not so big because of high production costs (two cylinders) and the small performances compared with its size. In this century, when people began to think about the environment, the stirling engine could play out its strengths partially.

Grafik: α-type-stirling engine. Source source:
You’ll find further views of the α-type stirling engine on that site, too
described (17 K, 500 * 300)

It convinces by very quiet running, an excellent efficiency and good exhaust quality, since the burn takes place outside of the cylinder. The company Phillips for example designed 1938 a stirling engine, which operated with helium as working gas (conductivity better than in air). The Phillips engine was quite well sold and could be operated with solar energy. So it was used as mobile refrigerator, for the power supply of smaller devices as well as on satellites.

The so-called α-type stirling engine uses two pistons, which are around 90° out of phase in V-arrangement. The displace piston is missing, replaced by a fix installed regenerator. The α-type stirling engine functions because of the different lever lengths, with which the piston rods attack at the main shaft. For a closer explanation of this engine you can read the excellent description on the homepage of Peter Fette.

Grafic: Model stirling engine. Source: Ki¶nig & Kraft GbR,
There are other model stirling engines on this site, too.


Although the stirling engine would have a large development potential, there won’t be a larger production soon. The relatively small performance compared with the size and the weight of the engine hardly permit a use in cars or airplanes. But as small mobile generators, stirling engines are a good choice. It enjoys a big popularity in the model construction, where small steam engines and stirling engines experience a real revival.



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