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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 See also:

state of See also:

COMPRESSION

compression, and therefore of greater See also:

DENSITY (Lat. densus, thick)

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 See also:

FOOT

foot swept through by the See also:

PISTON (through Fr. from Ital. pistone or pestone, a great pestle, from Late Lat. pistare, to pound, a frequentative form of classical Lat. pinsere)

piston, for the See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change of state which water undergoes in its transformation into steam invblves 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 See also:

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel. The air receives heat from an See also:

EXTERNAL

external See also:

FURNACE

furnace just as water does in the See also:

BOILER

boiler of a steam-engine, by contact with a heated metallic See also:

SURFACE

surface, but it takes up heat from such a surface with much less readiness than does water. The See also:

WASTE (O. Fr. wast, guast, gast, gaste; Lat. vastus, vast, desolate)

waste of heat in the See also:

CHIMNEY (through the Fr. cheminee, from caminata, sc. camera, a Lat. derivative of caminus, an oven or furnace)

chimney gases is accordingly greater; and further, the metallic See also:

shell is liable to be quickly burned away as a result of its contact at a high temperature with See also:

FREE

free See also:

OXYGEN (symbol 0, atomic weight 16)

oxygen. The temperature of the shell is much higher than that of a steam boiler, for in See also:

order to secure that the working air will take up a See also:

FAIR

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. See also:

COMBUSTION (from the Lat. comburere, to burn up)

combustion; it disappears when See also:

INTERNAL

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 See also:

FUEL (O. Fr. feuaile, popular Lat. focalia, from focus, hearth, fire)

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 See also:

TOP (cf. Dan. top, Ger. Topf, also meaning pot)

top of the range, and at the same See also:

time to See also:

ESCAPE (in mid. Eng. eschape or escape, from the O. Fr. eschapper, modern echapper, and escaper, low Lat. escapium, from ex, out of, and cappa, cape, cloak; cf. for the sense development the Gr. iichueoOat, literally to put off one's clothes, hence to sli

escape entirely the draw-backs 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. See also:

Stirling is of See also:

SPECIAL

special See also:

INTEREST

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 See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

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 See also:

case in the See also:

cycle of operations of the theoretically imagined engine of See also:

Carnot.(See THERMODYNAMICS and STEAM-ENGINE.) 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 See also:

JOURNEY (through O. Fr. jornee or journee, mod. Fr. journee, from med. Lat. diurnata, Lat. diurnus, of or belonging to dies, day)

journey. The essential parts of one form of Stirling's engine are shown in fig. 1.

There A is the externally-fired See also:

HEATING

heating vessel, the See also:

LOWER

lower part of which contains hot air which is taking in heat from the furnace beneath. A See also:

PIPE

pipe from the top of A leads to the working See also:

CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

cylinder (B). At the top of A is a cooler (C) consisting of pipes through which See also:

COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

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 See also:

ANNULAR, ANNULATE

annular space stacked loosely with See also:

WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

wire-See also:

GAUZE

gauze. When D moves down, the hot air is driven up through the re-generator 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 See also:

VOLUME

volume and forcing the piston (B) to rise, which completes the cyde. The engine was See also:

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 See also:

INDICATOR (from Lat. indicare, to point out)

indicator See also:

DIAGRAM

diagram is made up of two isothermal lines for the taking in and rejection of heat, and two lines of See also:

CONSTANT, BENJAMIN (1845-1902)

constant volume for the two passages through the regenerator. This engine was the subject of two See also:

PATENTS

patents (by R. and S. Stirling) in 1827 and 184o. A double-acting Stirling engine of 50 See also:

HORSE (a word common to Teutonic languages in such forms as hors, hros, ros; cf. the Ger. ross)

horse-power, using air which was maintained by a See also:

PUMP

pump at a fairly high pressure throughout the operations, was used for some years in the See also:

Dundee Foundry, where it is credited with having consumed only 1.7 lb of See also:

COAL

coal per See also:

HOUR

hour per indicated horse-power. The coal See also:

CONSUMPTION (Lat. consumere)

consumption per brakehorse-power was no doubt much greater. It was finally abandoned on See also:

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. See also:

Robinson, is shown in fig. 2. In this there is no compressing pump, and the See also:

main pressure of the working air is simply that of the See also:

ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

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 See also:

coke-See also:

FIRE (in O. Eng. f j'r; the word is common to West German languages, cf. Dutch vuur, Ger. Feuer; the pre-Teutonic form is seen in Sanskrit pu, pavaka, and Gr. irup; the ultimate origin is usually taken to be a root meaning to purify, cf. Lat. purus)

fire or by a gas See also:

FLAME (Lat. flamma; the root flag- appears in flagrare, to burn, blaze, and Gr. d, XEryew)

flame in C. It communicates through a passage (D) with the working cylinder (B). The displacer (E),which takes its See also:

motion through a See also:

rod (I) from a rocking See also:

lever (F) connected by a See also:

SHORT, FRANCIS JOB (1857– )

short See also:

LINK

link to the See also:

CRANK

crank-See also:

PIN (a doublet with " pen " from Lat. pinna, feather, pinnacle, which is said to contain the same root as irirvs, pine tree, and properly to mean a sharp point or end)

pin, is itself the regenerator, its construction being such that the air passes up and down through it as in one of the See also:

ORIGINAL

original Stirling forms. The cooler is a water vessel (G) through which water circulates from a tank (H). Messrs. See also:

Hayward and See also:

Tyler's " Rider" engine may be mentioned as another small hot-air motor which follows nearly the Stirling cycle of operations. An See also:

ATTEMPT (Lat. adtemptare, attentare, to try)

attempt to develop a powerful air-engine was made in See also:

AMERICA

America about 1833 by See also:

John Ericsson, who applied it to marine propulsion in the See also:

SHIP

ship " Caloric," but without permanent success. Like Stirling, Ericsson used a regenerator, but with this difference that the pressure instead of the volume of ,the air remained constant while it passed in each direction through the regenerator. Cold air was compressed by a pump into a See also:

RECEIVER

receiver, where it was kept cool during compression and from which it passed through a regenerator into the work- See also:

ing cylinder. In so passing it took up heat and ex- panded. It was then allowed to expand further, taking in heat from a furnace, under the cylinder and, falling in pressure.

This expansion was continued till the pressure of the Engine. that of the atmosphere. It was then discharged through the regenerator, depositing heat for the next See also:

charge of air in turn to take up. The indicator diagram approximated to a form made up of two isothermal lines and two lines of constant pressure. In the transmission of power by compressed air (see POWER TRANSMISSION) the air-driven motors are for the most part See also:

MACHINES

machines resembling steam-engines in the See also:

general features of their pistons, cylinders, valves and so forth. Such machines are not properly described as air-engines since their See also:

FUNCTION

function is not the See also:

CONVERSION (Lat. conversio, from convertere, to turn or ,change)

conversion of heat into work. Incidentally, however, they do in some cases partially See also:

DISCHARGE (adapted from the O. Fr. discharge, modern decharge, from a med. Lat. discargare, to unload, dis- and earn care, to load, cf. " charge ")

discharge that function, namely, when what is called a "preheater " is used to warm up the compressed air before it enters in the motor cylinder. The See also:

OBJECT

object of this See also:

DEVICE

device is not, primarily, to.produce work from heat, but to escape the inconveniences that would otherwise arise through extreme cooling of the air during its expansion. Without preheating the expanding air becomes so cold as to be liable to See also:

DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

deposit See also:

SNOW (in O. Eng. sndw; a common Indo-European word; cf. in Teutonic languages, Ger. Schnee, Du. sneeuw; in Slavonic snieg', Lith. snegas; Gr. vi4a, Lat. nix, nivis, whence the Romanic forms, Ital. neve, Fr. neige, &c.; Ir. and Gael. sneachd; the original

snow from the moisture held in suspension, and thereby to clog the valves. With preheating this is avoided, and the amount of work done by a given gtlantity of air is increased by the conversion into work of a part of the supplementary See also:

ENERGY (from the Gr. ivipyela; iv, in, ipyov, work)

energy which the preheater supplies in the form of heat. (J. A.

End of Article: AIRE

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AIREY, RICHARD AIREY, BARON (1803-1881)