38 AIR-ENGINE 



AIR, COMPRESSED. For its employment in some mining operations, see 

 MINING. 



AZR-EXCINE. The considerable expansibility of ai r by heat naturally suggested 

 its use as a motive power long before theoretical investigation demonstrated its actual 

 value. The great advance made during the last few years in our knowledge of the 

 mechanical action of heat has enabled us to determine with certainty the practical 

 result which may be obtained by the use of any contrivance for employing heat 

 as a prime mover of machinery. We are indebted to Sir Wm. Thomson for the 

 fundamental theorem which decides the economy of any thermo-dynamic engine. It 

 is that in any perfectly constructed engine the fraction of heat converted into work 

 is equal to the range of temperature from the highest to the lowest point, divided by 

 the highest temperature reckoned from the zero of absolute temperature. Thus, if 

 wo have a perfect engine in which the highest temperature is 280 and the lowest 80 



280 80 



F., the fraction of heat converted into force will be ' or rather more than 



AO(J + 460, 



one quarter. So that, if we use a coal of which one pound in combustion gives out 

 heat equivalent to 10,380,000 foot-pounds, such an engine as we have just described 

 would produce work equal to 2,805,405 foot-pounds for each pound of coal consumed 

 in the furnace. From the above formula of Sir Wm. Thomson, it will appear that 

 the economy of any perfect thermo-dynamic engine depends upon the range of tem- 

 perature we can obtain in it. And as the lowest temperature is generally nearly 

 constant, being ruled by the temperature of the surface of the earth, it follows that 

 the higher we can raise the highest temperature, the more economical will be the 

 engine. The question is thus reduced to this: In what class of engine can we 

 practically use the highest temperature ? In the steam-engine worked with satu- 

 rated vapour, the limit is obviously determined by the amount of pressure which can 

 be safely employed. In the steam-engine worked with super-heated vapour i.e. in 

 which the vapour, after passing from the boiler, receives an additional charge of heat 

 without being allowed to toko up more water and also in the air-engine, the limit 

 will depend upon the temperature at which steam or air acts chemically upon the 

 metals employed, as well as upon the power of the metals themselves to resist the 

 destructive action of heat. It thus appears that the steam-engine worked with super- 

 heated steam possesses most of the economical advantages of the air engine. But 

 when wo consider that an air-engine may be made available where a plentiful supply 

 of water cannot be readily obtained, the importance of this kind of thermo-dynamic 

 engine is incontestable. The merit of first constructing a practical air-engine 

 belongs to Mr. Stirling. Mr. Ericsson has subsequently introduced various refine- 

 ments, such as the respirator a reticulated mass of metal, which, by its extensive 

 conducting surface, is able, almost instantaneously, to give its own temperature to the 

 air which passes through it. But various practical difficulties attend these refine- 

 ments, which, at best, only apply to engines worked between particular temperatures. 

 The least complex engine, and that which would probably prove most effectual in 

 practice, is that described in the 'Philosophical Transactions," 1852, Part I. It con- 

 sists of a pump, which compresses air into a receiver, in which it receives an additional 

 charge of heat ; and a cylinder, the piston of which is worked by the heated air as it 

 escapes. The difference between the work produced by the cylinder and that absorbed 

 by the pump constitutes the force of the engine ; which, being compared with the heat 

 communicated to the receiver, gives results exactly conformable with the law of 

 Sir Wm. Thomson above described. J. P. J. 



Dr. Joule has proposed various engines to be worked at temperatures below redness, 

 which, if no loss occurred by friction or radiation, would realise about one-half the work 

 due to the heat of combustion, or about four times the economical duty which has, as 

 yet, been attained by the most perfect steam-engine. 



A detailed account of Ericsson's Calorific Engine may bo useful, especially 

 as a certain amount of success has attended his efforts in applying the expansive 

 power of heat to move machinery. Ericsson's engines have been for some years 

 at work in the foundry of Messrs. Hogg and Delamater, in Now York; one 

 engine being of five and another of sixty horse-power. The latter has four 

 cylinders. Two, of seventy-two inches diameter, stand side by side. Over each of 

 these is placed one much smaller. Within these are pistons exactly fitting their 

 respective cylinders, and so connected, that those within the lower and upper 

 cylinders move together. Under the bottom of each of the lower cylinders a fire 

 is applied, no other furnaces being employed. Neither boilers nor water are used. 

 The lower is called the working cylinder ; the upper, the supply cylinder. As the 

 piston in the supply cylinder moves down, valves placed in its top open, and it 

 becomes filled with cold air. As the piston rises within it, those valves close, and the 

 air within, unable to escape as it came, passes through another set of valves into a 



