photo George cayley

Sir George Cayley (1773-1857)

Sir George Cayley, of Bromton Hall in Yorkshire, first published details of his invention in the form of a letter to the editor of Nicholsons Journal in 1807.

drawing of Cayley engine To Mr. Nicholson.
Brompton, Sept. 25, 1807.

I observed in your last vol. p.368 that some experiments have been lately made in France upon air, expanded by heat, applied as a first mover for mechanical purposes. This idea, as you justly remark, is by no means new in this country; yet I have not yet heard that any successful experiments have been made, exclusively upon this principle, in England, though you hint that something promising has been accomplished relative to it.
The subject is of much importance as the steam engine has hitherto proved too weighty and cumbrous for most purposes of locomotion; whereas the expansion of air seems calculated to supply a mover free from these defects. Under this impression I send to you a sketch of an engine I projected upon this principle several years ago; it was made on a considerable scale at Newcastle, though I must confess without success in the result, which I attributed to the imperfect manner in which it was executed, the cylinders being made of sheet copper and so irregular, as not to be rendered tolerably air-tight by any packing of the piston. I think that there can be no doubt that the scheme is practicable in some way or other; and I conceive that the form of the engine here sketched will be the basis of whatever experience may prove to perfection in the apparatus of the air engine.
A and B, are two cylinders, placed one above the other; C and D, their respective pistons connected by one rod. F is a cylinder, containing a fire in a vessel within a vessel within it in such a manner, that any air passing between the upper and lower portions of it must go through the fire. This vessel also contains a long cylinder, open at the bottom, and directly over the centre of the fire, for the purpose of holding coke or other fuel. This cylinder is covered at the top, and packed air-tight when it has served the purpose of permitting the fire to be kindled through it: and has been filled with fuel.
The cylinder B is fitted up to answer the purpose of a double stroke forcing pump, or bellows, to drive the air into the upper portion of vessel F, from whence it passes downward through the fire for the purpose of consuming the smoke (the fresh fuel being supplied from the reservoir above) in its passage through the more completely ignited cinders below. In this act the air is expanded; and by means of pipes from the lower portion of F, it is conveyed alternately above and below the piston of the cylinder A. In each pipe is fixed a stop cock or valve, so constructed as to open a passage to the external air, when it shuts the connection with the fire vessel. These cocks are worked by a plug frame.
From this construction it will appear evident that whatever expansion the air receives its pressure will operate alike upon the piston of the bellows and of the receiver; and that always in opposition to each other; hence the power of the stroke will be in proportion to the excess of the area of the receiving piston, over that of the feeding one, multiplied by the expansive force of the contained air, and by the length of the movement.
If when the engine is well constructed, the expansion of the air in keeping up the fire be not found sufficiently sensible, still the form of the engine is such as to admit of either inflammable gas, oil of tar, or some other inflammable matters, being injected each stroke upon the fire; so that all the heat generated by the united combustion may operate without waste; perhaps even a light sprinkling of water, either upon, or round the sides of the fire, might answer the purpose. It scarcely need be observed, that a tube connected with a small forcing pump are the only things required for producing these effects.

I remain Sir,
Your obedient Servant,

Although not the first to propose using an engine with pistons of unequal diameters Cayley does seem to have been the first to developed a form of internal combustion air engine now called the gradual combustion or furnace gas engine. Sir George did not patent his ideas until 1837 when the invention was taken up on both sides of the Atlantic in the form of the heavy and cumbersome furnace gas engine. Cayley was interested in aviation and was it seems trying to find a suitable power plant to fit in his aircraft designs, also the 1837 patent details the application of his air engine to a road carriage.


  1. Sir George Cayleys Aeronautics
    C H Gibbs-Smith
    London. 1962.

  2. Sir George Cayley
    J Laurence Pritchard
    London. 1961.

  3. The life of a Genius
    Gerard Fairlie and Elizabeth Cayley

  4. A History of Hot Air Caloric and Stirling Engines. Vol one, A History.
    Robert Sier.
    Chelmsford. 1999.