As water, wind, and animal power gave way to the Industrial Revolution, steam engines provided the cheap power necessary for mass production. They were notoriously dangerous and unreliable, often shoddily built of unsuitable materials, and they exploded with tragic frequency. Moved by such calamities, Scottish clergyman Robert Stirling (1790-1878) decided to find a safer alternative, one without a pressurized boiler. The result was an external-combustion engine that alternately heats, compresses, and cools a gas (Stirling used air) to produce energy. Although it never replaced the steam engine as Stirling hoped, it proved to be a quiet and efficient machine that still intrigues scientists today.

The Stirling engine has five major components: a heater, which burns fuel in an external-combustion chamber, is connected to a gas-filled cylinder. Inside, the power piston compresses the gas as it is heated. The hot gas expands, and the force generated is transmitted to a drive mechanism. At this point the displacer piston moves down, pushing the gas through the regenerator and the cooler. Then the cycle starts again as the gas enters the heater. The regenerator, or heat exchanger, stores the thermal energy produced as the gas is compressed and returns it to the system. This process, known as a thermodynamic cycle, was discovered by Nicolas Carnot (1796-1832) in his search for an efficient engine. Stirling was the first to put the concept to practical use.

Stirling's engines were patented in 1816 and manufactured through the 1920s. Small and reliable, they performed limited tasks like pumping water. A factory model was built in 1843, but it proved too expensive for commercial use. Stirling engines powered portable generators during World War II in the Netherlands. In the 1970s, Ford and Philips tried without success to develop a Stirling engine for automobiles.

Today the Stirling engine uses hydrogen or helium instead of air, but has found application only as a cryogenic refrigerator. There have been sporadic efforts to adapt it for wider industry, but results have been disappointing, although its theoretical efficiency continues to tantalize researchers.