Just how small can you make an engine? Two researchers from the University of Stuttgart and the Max Planck Institute for Intelligent Systems, Valentin Blickle and Clemens Bechinger, successfully shrank the Stirling heat engine down to a single, microscopic particle. The engine is so small, in fact, that the random fluctuations in position due to Brownian motion cause variations in its work output. This microscopic Stirling engine is controlled using a pair of highly focused lasers.

Stirling engines, named after the Scottish inventor who created them in 1816, offer the highest theoretical efficiency of any heat engine—the same as the Carnot efficiency. Due to pesky entropy and the second law of thermodynamics, you can’t get all the heat you put in back out as work. The efficiency of any heat engine, then, is just the ratio of output work to input heat. The Carnot efficiency, conceived by Nicolas Léonard Sadi Carnot (the father of thermodynamics), gives the maximum theoretical efficiency of the engine and depends only on the temperature range within which the engine operates.

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