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The hot-air engine is an external combustion engine in that gas burners underneath the cylinder provide the necessary heat. Output: about 1/10 hp. It was invented by the Rev. Stirling in 1816, and the Heinrici version became very popular towards the end of the 19th century. They were cheaper and more convenient than steam engines.

The hot-air engine has now completely disappeared as an every-day engine apart from in highly specialized and advanced technology. Its operation is more complex than the steam engine. Invented in 1816 and successfully operated by a Scotsman, Robert Stirling, it is an external combustion engine. The fuel is burnt outside the cylinder and it is possible to run the engine on almost any material that can be burnt. Although the largest hot-air engines could generate only a few horse-power they were, nevertheless, developed and used widely in the nineteenth century. Their principal use was for pumping domestic water supplies, or for generating electricity in rural areas where they were applied for a variety of purposes — from ringing telephone bells, driving dentist's drills, to pumping air into church organ blowers. This small hot-air engine could have been used for any of these purposes. With the advent of the electric motor and the oil engine, the hot-air engine became obsolete. Today, however, it attracts a certain amount of attention from those concerned with the environment because it runs almost silently and is a pollution-free device. (Source: Welsh Industrial & Maritime Museum Guidebook, 1984).