Hot air engine

hot air engine (historically simply 'air engine' or 'caloric engine') is a catch-all term for any heat engine which uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work. It encompasses closed cycle devices such as the Stirling engine (which should include a regenerator), thermoacoustic engine and open cycle devices such as those devised by Sir George Cayley and John Ericsson. Its use is sometimes extended to describe engines employing other permanent gasses as their working fluid, but specifically excludes any engine performing a thermodynamic cycle such as the Rankine cycle, in which the working fluid undergoes a phase change. 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.

In a typical implementation, air is repeatedly heated and cooled in a cylinder and the resulting expansion and contraction is used to move a piston and produce useful mechanical work. ^{[1] [2]}

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History

The expansive property of heated air was known to the ancients and Hero of Alexandria's Pneumatica contains descriptions of devices which 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 gases were first set out, and early patents include those of Henry Wood, Vicar of High Ercall near Coalbrookdale Shropshire (English patent 739 of 1759) ^[3] and Thomas Mead, an engineer from Sculcoats Yorkshire (English patent 979 of 1791) ^[4], 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 ^{[5] [6] [7]}

It is likely that Robert Stirling's air engine of 1818 which incorporated his innovative Economiser patented in 1816 was the first to be 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 a component of every air engine worthy of the title Stirling engine. A full description is available at that article.

Thermodynamic cycle

Main article: Thermodynamic cycle

A hot air engine thermodynamic cycle can (ideally) be made out of 3 or more processes (typical 4). The processes can be any of these:

• 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)
  • isentropic process, reversible adiabatic process (no heat is added or removed from the working fluid - and the entropy is constant)
• isenthalpic process (the enthalpy is constant)

Some examples are as follows:

Yet another example is Vuilleumier refrigeration. ^[8]

See also

• Thermoacoustic hot air engine
• Carnot heat engine
• Timeline of heat engine technology

References

More references