. Applied thermodynamics for engineers. tter is small as compared with the transmittingpipe line, the specific volume and pres- psure of the fluid, and consequentlyits temperature as well, remain un- ^changed. BO represents expansionafter the supply from the mains iscut off. If the temperature at B is that fof the external atmosphere, and ex-pansion proceeds slowly, so that anyfall of temperature along BO \^ offset dby the transmission of heat from theoutside air through the cylinder walls, othis line is an isothermal. If, however,expansion is rapid, so that no transfer of heat occurs, ^(7 wil
. Applied thermodynamics for engineers. tter is small as compared with the transmittingpipe line, the specific volume and pres- psure of the fluid, and consequentlyits temperature as well, remain un- ^changed. BO represents expansionafter the supply from the mains iscut off. If the temperature at B is that fof the external atmosphere, and ex-pansion proceeds slowly, so that anyfall of temperature along BO \^ offset dby the transmission of heat from theoutside air through the cylinder walls, othis line is an isothermal. If, however,expansion is rapid, so that no transfer of heat occurs, ^(7 will be an adiahatlc. In practice, the expansionline is a polytropic, lying usually between the adiabatic and theisothermal. CD represents the expulsion of the air from -the cyl-inder at the completion of the working stroke. At i>, the inletvalve opens and the pressure rises to that at A. The volumesshown on this diagram are not specific volumes, but volumes of air inthe cylinder. Subtended areas, nevertheless, represent external Fig. 57. Arts. 180-183, 189, 222, 223,226, Prob. 6.—Air Engine Cycles. 181. Modified Cycle. The additional work area LMC obtained by ex-pansion beyond some limiting volume, say that along xy^ is small. Aslight gain in efficiency is thus made at the cost of a much larger cylin-der. In practice, the cycle is usually terminated prior to complete expan-sion, and has the form ABLMD, the line LM representing the fall ofpressure which occurs when the exhaust v? opens. 108 APPLIED THERMODYNAMICS 182. Work Done. Letting p denote the pressure along AB^ Pthe pressure at the end of the expansion, q the back pressurealong MD (slightly above that of the atmosphere), and letting vdenote the volume at B^ and J^that at the end of expansion, bothvolumes being measured from OA as a line of zero volumes, then,for isothermal expansion, the work done is and for expansion such that pi = PV^, it is ^ n —1 ^ (In these and other equations in the present chapter,
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