Hot air engine

In a typical implementation, air is repeatedly heated and cooled in a cylinder and the resulting expansion and contraction are used to move a piston and produce useful mechanical work.

Continuous combustion types, such as George Brayton's Ready Motor and the related gas turbine, could be seen as borderline cases.

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.

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 heated air expanded and, via tubes, forced water from one chamber to another, unbalancing the wheel and causing it to turn.

It is stated by Chambers to have been unsuccessful, owing to mechanical defects and to “the unforeseen accumulation of heat, not fully extracted by the sieves or small passages in the cool part of the regenerator, of which the external surface was not sufficiently large to throw off the unrecovered heat when the engine was working with highly compressed air.” Parkinson and Crossley, English patent, 1828 came up with their own hot air engine.

The specification describes it more particularly, as consisting of a “circular chamber, in which a cone is made to revolve on a shaft or axis by means of leaves or wings, alternately exposed to the pressure of steam; these wings or leaves being made to work through slits or openings of a circular plane, which revolves obliquely to, and is thereby kept in contact with the side of the cone.” Ericsson built his third hot air engine (the caloric engine) in 1833 "which excited so much interest a few years ago in England; and which, if it should be brought into practical operation, will prove the most important mechanical invention ever conceived by the human mind, and one that will confer greater benefits on civilized life than any that has ever preceded it.

A hot air engine thermodynamic cycle can (ideally) be made out of 3 or more processes (typically 4).