An exercise in some high-shool physics classes is assembling and operating a Stirling engine. It’s a fairly simple device and can be surprising because it’s a “hot air engine.” All you need is a source of heat (a candle will do for a small engine) and the engine will operate. The Stirling engine was designed by Robert Stirling, who was born October 25, 1790, in Scotland.
Stirling was born on a farm, and his father and grandfather were also inventors — his grandfather, Michael Stirling, invented the threshing machine that separates grain from the stalks it grows on. His father, Patrick Stirling, invented and improved a number of industrial agricultural machines.
Robert Stirling, though, originally set out to be come a church minister. He attended Edinburgh University to study divinity, and continued his academic work at Glasgow University. He didn’t study engineering or science, but nevertheless apparently had both talent and interest in those areas, even after he became a clergyman in 1816.
The year 1816 was also when Stirling received his first patent for a hot air engine. The engines were put into use, including as a water pump in a quarry in 1818. That engine eventually overheated when an attendant neglected his duties — and Stirling studied the results, coming up with an improved design in the 1820s. He received a second patent in 1827. He remained a clergyman, but also kept working on his hot air engines with his brother James, and earned a third patent in 1840.
Stirling engines are generally low-power units, and depend on air pressure. The greater the pressure you can achieve, the more power the engine will produce. Failures in early Stirling engines were generally due to flaws in the metal used to construct them. In 1856, Henry Bessemer patented a new process for producing higher quality steel in quantity. In 1876, Stirling wrote about his optimism that better materials would result in higher-performance hot-air engines.
Stirling engines were relatively popular in the mid-1800s, largely because of safety. The alternative was a steam engine, and those were also built with the kind of metal available at the time — and as a result catastrophic explosions were pretty common. Stirling engines never explode; if something goes wrong they just stop. The hot air inside is never at high enough pressure to pose any danger.
Nowadays the engines aren’t often used for any practical purpose, but they do make excellent demonstrations because it’s not at all clear, unless you understand how they work, how nothing but heat — from any source at all — produces rotary motion. There have been experiments with Stirling-powered cars, boats, and even airplanes, even recently. There are a number of Stirling-powered submarines in service right now that can stay submerged for weeks at a time, something only nuclear-powered subs were able to do. There are also some experimental solar power facilities paired with Stirling engines — they work not by photovoltaics, but by focusing sunlight to produce high temperatures, which are used to power a large Stirling engine. There was a site in Arizona that generated 1.5 megawatts, although the test has been concluded and the system shut down.
Another application of a Stirling engine is to use it in reverse — if you use another motor to turn the driveshaft of the Stirling engine, the Stirling becomes a heat pump that can, for example, cool air to 73° Kelvin, which is cold enough to liquify it. Robert Stirling himself would probably have been eager to hear about new applications of his engine. He was inducted into the Scottish Engineering Hall of Fame in 2014.
