# Hot air engine

Illustration of a low temperature differential (LTD) hot air engine. 1. Power piston, 2. Cold end of cylinder, 3.Displacer piston 4. Hot end of cylinder Q1. Heat in, Q2. Heat out.

A hot air engine (historically called an air engine or caloric engine[1]) 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. [2]

## Definition

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.

## 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,[3] 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),[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]

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.[7] 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

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:

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

Cycle Process 1-2
(Compression)
Process 2-3
Process 3-4
(Expansion)
Process 4-1
(Heat Rejection)
Notes
Power cycles normally with external combustion - or heat pump cycles:
Bell Coleman adiabatic isobaric adiabatic isobaric A reversed Brayton cycle
Carnot isentropic isothermal isentropic isothermal
Ericsson isothermal isobaric isothermal isobaric the second Ericsson cycle from 1853
Rankine adiabatic isobaric adiabatic isobaric Steam engine
Hygroscopic adiabatic isobaric adiabatic isobaric Hygroscopic cycle
Scuderi adiabatic variable pressure
and volume
Stirling isothermal isochoric isothermal isochoric
Power cycles normally with internal combustion:
Brayton adiabatic isobaric adiabatic isobaric Jet engines
the external combustion version of this cycle is known as first Ericsson cycle from 1833
Lenoir isobaric isochoric adiabatic Pulse jets
(Note: Process 1-2 accomplishes both the heat rejection and the compression)
Otto adiabatic isochoric adiabatic isochoric Gasoline / petrol engines

Yet another example is the Vuilleumier cycle. [8]