I motori diesel per la terra e il lavoro marino . pressione costante, l'espansione e la compressione del gasbeing adiabatica come prima. Fig. 5 rappresenta il ciclo constantpressure sulla pressione-volume. A partire dalla b quando la pressione, volume e tempera-ture sono rispettivamente Pi, V2, e T2, il gas è compressedadiabatically a c dove P2 è la pressione, V3 il volumeand T3 la temperatura. Il calore viene preso in lungo la linecd a pressione costante P2, il volume a d essendo V4, ela temperatura T4. Espansione adiabatica occursalong successivo la linea da, a, dove la pressione, volume, andtemperature

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Diesel engines for land and marine work . constant pressure, expansion and compression of the gasbeing adiabatic as before. Fig. 5 represents the constantpressure cycle on the pressure-volume basis. Starting from b when the pressure, volume and tempera-ture are respectively Pi, V2, and T2, the gas is compressedadiabatically to c where P2 is the pressure, V3 the volumeand T3 the temperature. Heat is taken in along the linecd at constant pressure P2, the volume at d being V4, andthe temperature T4. Adiabatic expansion next occursalong the line da, to a, where the pressure, volume, andtemperature become Pi, Vi and Ti respectively. Heat isthen rejected at constant pressure Pi, along the line ab tothe starting point b where the original conditions prevail. GENERAL THEORY OF HEAT ENGINES 19 As before let Q2 = heat taken in by the fluid, and Qi = heat rejected by the fluid. The efficiency of the cycle is then n = — ~-i- and, considering 1 lb of gas Qo = kp (T4 — T3)and Qi = k^ {T, - T, ). The efficiency is therefore n = I T. - T, . ^3 y. vz V, Fig. 5.—Constant Pressure Cycle Diagram. Expansion along da and compression along be are adiabatic80 that, from formula (3) page 11. T, _Tt -T, T. T= From this T4 -T, Pi5^^* T /P The efl&ciency is thus n = I — -i = l —f—-M but with adiabatic expansion f—j r = f—j 20 DIESEL ENGINES FOP. LAND AND MARINE WORK and the efficiency may be expressed » = i-(vj =1-7- which is the same expression as was obtained for theefficiency of the constant vokime cycle, and in fact theefficiencies of the constant temperature, constant volume, and constant pressure cycles are identical. It is at once apparent from the above results that what-ever cycle of operations a heat engine works upon, the higherthe compression ratio can be made, the less becomes the fraction — , by wliich the possible efficiency is reduced below unity, and hence the greater becomes the efficiencyof the engine if mechanical and other losses are not increasedin the sa

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