Hot air engine

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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.

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.

Video Hot air engine

Definition

The term "hot air engine" specifically excludes any engine performing a thermodynamic cycle in which the working fluid undergoes a phase transition, such as the Rankine cycle. 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.

Maps Hot air engine

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.

Nano Disc Manson Hot Air Engine - by Kontax Engineering by Kontax ...

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Thermodynamic cycles

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 (not all hot air cycles, as defined above) are as follows:

Yet another example is the Vuilleumier cycle.