Cayley's 1807 Engine

The first successfully working hot air engine was Cayley's, in which much ingenuity was displayed in overcoming practical difficulties arising from the high working temperature.

This engine type was of those in which the fire is enclosed, and fed by air pumped in beneath the grate in sufficient quantity to maintain combustion, while by far the largest portion of the air enters above the fire, to be heated and expanded; the whole, together with the products of combustion, then acts on the piston, and passes through the working cylinder; and the operation being one of simple mixture only, no heating surface of metal is required, the air to be heated being brought into immediate contact with the fire.

The furnace was arranged so that the air pumped in could be conveyed above or below the fire as required ; and before reaching the fire the air in its passage was conducted round an annular space between the firebrick lining and the outer casing of the furnace, so as to keep the exterior as cool as possible.

The cylinder was surrounded by a water belt to avoid an excess of heat that would injure the packing of the piston. The cold air for working ihe engine was pumped in by a separate air-pump.

One of these engines was kept at work for many months to test its capabilities : for economy of fuel compared with the work done it surpassed any form of steam engine known at that time ; but the joints caused great trouble, and the cylinder and the piston packing were rapidly destroyed by the dust and particles of grit from the fuel, which acted as a grinding material and rendered lubrication impossible.

An attempt was made to filter the air before entering the cylinder, by means of sheets of wire gauze ; but these either gave way or were soon choked up, and so became useless.

The plan of enclosing the fire in the mass of air to be heated involves the utmost degree of economy, as there is nothing whatever lost in the absolute heating, and the products of combustion, varying in quantity according to the fuel employed, also add to the bulk of the mixture.

In a letter to M. Nicholson, published in The Emporium for Arts and Sciences, Cayley gives himself a description of his engine, for affording mechanical power from air expanded by heat.