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 and removal of heat, this process enabling a portion of the applied heat to be transformed into mechanical work. Just as the working substance which alternately takes in and gives out heat in the steam-engine is water (converted during a part of the action into steam), so in the air-engine it is air. The practical drawbacks to employing air as the working substance of a heat-engine are so great that its use has been very limited. Such attempts as have been made to design air-engines on a large scale have been practical failures, and are now interesting only as steps in the historical development of applied thermodynamics. In the form of motors for producing very small amounts of power air-engines have been found convenient, and within a restricted field they are still met with. But even in this field the competition of the oil-engine and the gas-engine is too formidable to leave to the air-engine more than a very narrow chance of employment.

One of the chief practical objections to air-engines is the great bulk of the working substance in relation to the amount of heat that is utilized in the working of the engine. To some extent this objection may be reduced by using the air in a state of compression, and therefore of greater density, throughout its operation. Even then, however, the amount of operative heat is very small in comparison with that which passes through the steam-engine, per cubic foot swept through by the piston, for the change of state which water undergoes in its transformation into steam involves the taking in of much more heat than can be communicated to air in changing its temperature within such a range as is practicable. Another and not less serious objection is the practical difficulty of getting heat into the working air through the walls of the containing vessel. The air receives heat from an external furnace just as water does in the boiler of a steam-engine, by contact with a heated metallic surface, but it takes up heat from such a surface with much less readiness than does water. The waste of heat in the chimney gases is accordingly greater; and further, the metallic shell is liable to be quickly burned away as a result of its contact at a high temperature with free oxygen. The temperature of the shell is much higher than that of a steam boiler, for in order to secure that the working air will take up a fair amount of heat, the upper limit to which its temperature is raised greatly exceeds that of even high-pressure steam. This objection to the air-engine arises from the fact that the heat comes to it from external combustion; it disappears when internal combustion is resorted to; that is to say, when the heat is generated within the envelope containing the working air, by the combustion there of gaseous or other fuel. Gas-engines and oil-engines and other types of engine employing internal combustion may be regarded as closely related to the air-engine. They differ from it, however, in the fact that their working substance is not air, but a mixture of gases—a necessary consequence of internal combustion. It is to internal combustion that they owe their success, for it enables them to get all the heat of combustion into the working substance, to use a relatively very high temperature at the top of the range, and at the same time to escape entirely the drawbacks that arise in the air-engine proper through the need of conveying heat to the air through a metallic shell.

A form of air-engine which was invented in 1816 by the Rev. R. Stirling is of special interest as embodying the earliest application of what is known as the “regenerative” principle, the principle namely that heat may be deposited by a substance at one stage of its action and taken up again at another stage with but little loss, and with a great resulting change in the substance’s temperature at each of the two stages in the operation. The principle has since found wide application in metallurgical and other operations. In any heat-engine it is essential that the working substance should be at a high temperature while it is taking in heat, and at a relatively low temperature when it is rejecting heat. The highest thermodynamic efficiency will be reached when the working substance is at the top of its temperature range while any heat is being received and at the bottom while any heat is being rejected—as is the case in the cycle of operations of the theoretically imagined engine of Carnot. (See and .) In Carnot’s cycle the substance takes in heat at its highest temperature, then passes by adiabatic expansion from the top to the bottom of its temperature range, then rejects heat at the bottom of the range, and is finally brought back by adiabatic compression to the highest temperature at which it again takes in heat, and so on. An air-engine working on this cycle would be intolerably bulky and mechanically inefficient. Stirling substituted for the two stages of adiabatic expansion and compression the passage of the air to and fro through a “regenerator,” in which the air was alternately cooled by storing its heat in the material of the regenerator and reheated by picking the stored heat up again on the return journey. The essential parts of one form of Stirling’s engine are shown in fig. 1. There A is the externally-fired heating vessel, the lower part of which contains hot air which is taking in heat from the furnace beneath. A pipe from the top of A leads to the working cylinder (B). At the top of A is a cooler (C) consisting of pipes through which cold water is made to circulate. In A there is a displacer (D) which is connected (by parts not shown) with the piston in such a manner that it moves down when the piston has moved up. The air-pressure is practically the same above and below D, for these spaces are in free communication with one another through the regenerator (E), which is an annular space stacked loosely with wire-gauze. When D moves down, the hot air is driven up through the regenerator to the upper part of the containing vessel. It deposits its heat in the wire-gauze, becoming lowered in temperature and consequently reduced in pressure. The piston (B) descends, and the air, now in contact with the cooling pipes (C), gives up heat to them. Then the displacer (D) is raised. The air passes down through its regenerator, picking up the heat deposited there, and thereby having its temperature restored and its pressure raised. It then takes in heat from the furnace, expanding in volume and forcing the piston (B) to rise, which completes the cycle. The engine was double-acting, another heating vessel like A being connected with the upper end of the working cylinder at F. The stages at which heat is taken from the furnace and rejected to the cooler (C) are approximately isothermal at the upper and lower limits of temperature respectively, and the cycle accordingly is approximately “perfect” in the thermodynamic sense. The theoretical indicator diagram is made up of two isothermal lines for the taking in and rejection of heat, and two lines of constant volume for the two passages through the regenerator. This engine was the subject of two patents (by R. and S. Stirling) in 1827 and 1840. A double-acting Stirling engine of 50 horse-power, using air which was maintained by a pump at a fairly high pressure throughout the operations, was used for some years in the Dundee Foundry, where it is credited with having consumed only 1·7 ℔ of coal per hour per indicated horse-power. The coal consumption per brake-horse-power was no doubt much greater. It was finally abandoned on account of the failure of the heating vessels.

The type survives in some small domestic motors, an example of which, manufactured under the patent of H. Robinson, is shown in fig. 2. In this there is no compressing pump, and the main pressure of the working air is simply that of the atmosphere. The whole range of pressure is so slight that no packing is required. Here A is the vessel in which the air is heated and within which the displacer works. It is heated by a small coke-fire or by a gas flame in C. It communicates through a passage