A hot air engine (historically called an air engine or caloric engine) is any heat engine that uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work. These engines may be based on a number of thermodynamic cycles encompassing both open cycle devices such as those of Sir George Cayley and John Ericsson and the closed cycle engine of Robert Stirling. Hot air engines are distinct from the better known internal combustion based engine and steam engine.
The term "hot air engine" specifically excludes any engine performing a thermodynamic cycle such as the Rankine cycle, in which the working fluid undergoes a phase transition. Also excluded are conventional internal combustion engines in which heat is added to the working fluid by combustion of fuel within the working cylinder. Continuous combustion types such as George Brayton's Ready Motor and the related gas turbine could be seen as borderline cases.
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.
Thermodynamic cycles 
- isothermal process (at constant temperature, maintained with heat added or removed from a heat source or sink)
- isobaric process (at constant pressure)
- isometric / isochoric process (at constant volume)
- adiabatic process (no heat is added or removed from the working fluid)
- isenthalpic process (the enthalpy is constant)
Some examples (not all hot air cycles, as defined above) are as follows:
|Power cycles normally with external combustion - or heat pump cycles:|
|Bell Coleman||adiabatic||isobaric||adiabatic||isobaric||A reversed Brayton cycle|
|Ericsson||isothermal||isobaric||isothermal||isobaric||the second Ericsson cycle from 1853|
|Power cycles normally with internal combustion:|
the external combustion version of this cycle is known as first Ericsson cycle from 1833
(Note: Process 1-2 accomplishes both the heat rejection and the compression)
|Otto||adiabatic||isochoric||adiabatic||isochoric||Gasoline / petrol engines|
See also 
- Robert Sier (1999). Hot air caloric and stirling engines. Vol.1, A history (1st Edition (Revised) ed.). L.A. Mair. ISBN 0-9526417-0-4.
- Stirling (or Hot air) engine
- In 1699, Guillaume Amontons (1663-1705) presented, to the Royal Academy of Sciences in Paris, a report on his invention: a wheel that was made to turn by heat. The wheel was mounted vertically. Around the wheel's hub were water-filled chambers. Air-filled chambers on the wheel's rim were heated by a fire under one side of the wheel. The heated air expanded and, via tubes, forced water from one chamber to another, unbalancing the wheel and causing it to turn. See:
- Amontons (20 June 1699) "Moyen de substituer commodement l'action du feu, à la force des hommes et des chevaux pour mouvoir les machines" (Means of conveniently substituting the action of fire for the force of men and horses in order to move [i.e., power] machines), Mémoires de l'Académie Royale des Sciences, pages 112-126. The Mémoires appear in the Histoire de l'Académie Royale des Sciences, année 1699, which was published in 1732. The operation of Amontons' moulin à feu (fire mill) is explained on pages 123-126; his machine is illustrated on the plate following page 126.
- For an account of Amontons' fire-powered wheel in English, see: Robert Stuart, Historical and Descriptive Anecdotes of Steam-engines and of Their Inventors and Improvers (London, England: Wightman and Cramp, 1829), vol. 1, pages 130-132 ; an illustration of the machine appears on page 351.
- Robert Sier (1999). Hot air caloric and stirling engines. Vol.1, A history, page 56 (1st Edition (Revised) ed.). L.A. Mair. ISBN 0-9526417-0-4.
- Stirling engine history
- Detailed contents of the book Hot air caloric and stirling engines. Vol.1, A history
- Finkelstein, T and Organ, A.J (2001). Chapter 2.2 Air Engines. Professional Engineering Publishing. ISBN 1-86058-338-5.
- Wurm, Jaroslav (1991). Stirling and Vuilleumier heat pumps: design and applications. McGraw-Hill. ISBN 0-07-053567-1.
|Wikimedia Commons has media related to: Hot air engines|
- Introduction to Stirling-Cycle Machines
- Pioneers in Air Engine Designs (Select the desired biography)
- Stirling Engine's Function Principle
- Project for making one at home
- Apparatus for the Method of Heat Differentiation Vuilleumier patent