Thermodynamics

Chapter 9 | 503Process 2-3 is a constant-pressure heat-addition process, for which theboundary work and u terms can be combined into h. Thus,Process 4-1 is a constant-volume heat-rejection process (it involves no workinteractions), and the amount of heat rejected isThus,Then the thermal efficiency becomesThe thermal efficiency of this Diesel cycle under the cold-air-standardassumptions could also be determined from Eq. 9–12.(c) The mean effective pressure is determined from its definition, Eq. 9–4:MEP Q in m 1h 3 h 2 2 mc p 1T 3 T 2 2W netW netV max V min V 1 V 2 110 psia 10.00498 lbm210.240 Btu>lbm # R2313432 17162 R4 2.051 BtuQ out m 1u 4 u 1 2 mc v 1T 4 T 1 2 10.00498 lbm2 10.171 Btu>lbm # R2311425 5402 R4 0.754 BtuW net Q in Q out 2.051 0.754 1.297 Btuh th W net 1.297 Btu 0.632 or 63.2%Q in 2.051 Btu1.297 Btu1117 6.52 in a 778.17 lbf # ft3 1 BtuDiscussion Note that a constant pressure of 110 psia during the powerstroke would produce the same net work output as the entire Diesel cycle.ba12 in.b1 ft9–7 ■ STIRLING AND ERICSSON CYCLESThe ideal Otto and Diesel cycles discussed in the preceding sections arecomposed entirely of internally reversible processes and thus are internallyreversible cycles. These cycles are not totally reversible, however, since theyinvolve heat transfer through a finite temperature difference during the nonisothermalheat-addition and heat-rejection processes, which are irreversible.Therefore, the thermal efficiency of an Otto or Diesel engine will be lessthan that of a Carnot engine operating between the same temperature limits.Consider a heat engine operating between a heat source at T H and a heatsink at T L . For the heat-engine cycle to be totally reversible, the temperaturedifference between the working fluid and the heat source (or sink) shouldnever exceed a differential amount dT during any heat-transfer process. Thatis, both the heat-addition and heat-rejection processes during the cycle musttake place isothermally, one at a temperature of T H and the other at a temperatureof T L . This is precisely what happens in a Carnot cycle.