History
The expansive property of heated air was known to the ancients. Hero of Alexandria's Pneumatica describes devices that might be used to automatically open temple doors when a fire was lit on a sacrificial altar. Devices called hot air engines, or simply air engines, have been recorded from as early as 1699, around the time when the laws of gasses were first set out, and early patents include those of Henry Wood, Vicar of High Ercall near Coalbrookdale Shropshire (English patent 739 of 1759) and Thomas Mead, an engineer from Sculcoats Yorkshire (English patent 979 of 1791), the latter in particular containing the essential elements of a displacer type engine (Mead termed it the transferrer). It is unlikely that either of these patents resulted in an actual engine and the earliest workable example was probably the open cycle furnace gas engine of the English inventor Sir George Cayley c.1807
It is likely that Robert Stirling's air engine of 1818, which incorporated his innovative Economiser (patented in 1816) was the first air engine put to practical work. The economiser, now known as the regenerator, stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency of Stirling's engine and should be present in any air engine that is properly called a Stirling engine.
Read more about this topic: Hot Air Engines
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