. Applied thermodynamics for engineers. ply and giving the instan-taneous rise of pressure indicated by de. 424. Expansion. This has been shown to be an inefficient cycle(Art. 417), and it would be impossible, for mechanical reasons, tomore than approximate it in practice. The inlet port is nearlyalways closed prior to the end of the stroke, producing such a diagram as debgq, Fig. 184, inwhich the supply ofsteam to the cylin-der is less than thewhole volume of thepiston displacement,and the work areaunder hg is obtainedwithout the supply ofheat, but solely inconsequence of theexpansive action
. Applied thermodynamics for engineers. ply and giving the instan-taneous rise of pressure indicated by de. 424. Expansion. This has been shown to be an inefficient cycle(Art. 417), and it would be impossible, for mechanical reasons, tomore than approximate it in practice. The inlet port is nearlyalways closed prior to the end of the stroke, producing such a diagram as debgq, Fig. 184, inwhich the supply ofsteam to the cylin-der is less than thewhole volume of thepiston displacement,and the work areaunder hg is obtainedwithout the supply ofheat, but solely inconsequence of theexpansive action ofthe steam. Appar-ently, then, the actual steam engine cycle is that of Rankine * (). But if we apply an indicator (Art. 484) to the cyhnder,—an instru- * It need scarcely be said that the association of the steam engine indicator dia-gram and its varying quantity of steam with the ideal Rankine cycle is open toobjection (Art. 454). Yet there are advantages on the ground of simplicity in thismethod of approaching the Fig. 187. Arts. 424, 425, 427, 430, 431, 436, 441, 445, 446,448, 449, 450, 451,452, 454.—Indicator Diagram andRankine Cycle. WIREDRAWING 301 ment for graphically recording the changes of pressure and volumeduring the stroke of the piston, — we obtain some such diagram asabodes^ Fig. 187, which may be instructively compared with the cor-responding Rankine cycle, ABODE. The remaining study of thesteam engine deals principally with the reasons for the differencesbetween these two cycles. 425. Wiredrawing. The first difference to be considered is that along thelines ah, AB. An important reason for the difference in volumes at b and B willbe discussed (Art. 430) ; we may at present note that the pressures at a and b areless than those at A and B, and that the pressure at b is less than that at a. Thisis due to the frictional resistance of steam pipes, valves, and ports, which causesthe steam to enter the cylinder at a pressure somewhat less than
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