Thermodynamics

530 | ThermodynamicsTherefore, the irreversibility of the cycle isThe exergy destruction of the cycle could also be determined from Eq. 9–34.Notice that the largest exergy destruction in the cycle occurs during theheat-rejection process. Therefore, any attempt to reduce the exergy destructionshould start with this process.Disregarding any kinetic and potential energies, the exergy (work potential)of the working fluid before it is purged (state 4) is determined from Eq. 9–35:whereThus,381.83 kJ>kg 1290 K2 c0.7540 kJ>kg # K d290 K 163.2 kJ>kgx dest,cycle x dest,12 x dest,23 x dest,34 x dest,41 0 82.2 kJ>kg 0 163.2 kJ>kg 245.4 kJ/kgf 4 1u 4 u 0 2 T 0 1s 4 s 0 2 P 0 1v 4 v 0 2s 4 s 0 s 4 s 1 0.7540 kJ>kg # Ku 4 u 0 u 4 u 1 q out 381.83 kJ>kgv 4 v 0 v 4 v 1 0f 4 381.83 kJ>kg 1290 K210.7540 kJ>kg # K2 0 163.2 kJ /kgwhich is equivalent to the exergy destruction for process 4-1. (Why?)Discussion Note that 163.2 kJ/kg of work could be obtained from theexhaust gases if they were brought to the state of the surroundings in areversible manner.TOPIC OF SPECIAL INTEREST*Saving Fuel and Money by Driving SensiblyTwo-thirds of the oil used in the United States is used for transportation. Halfof this oil is consumed by passenger cars and light trucks that are used to commuteto and from work (38 percent), run a family business (35 percent), andfor recreational, social, and religious activities (27 percent). The overall fuelefficiency of the vehicles has increased considerably over the years due toimprovements primarily in aerodynamics, materials, and electronic controls.However, the average fuel consumption of new vehicles has not changed muchfrom about 20 miles per gallon (mpg) because of the increasing consumertrend toward purchasing larger and less fuel-efficient cars, trucks, and sportutility vehicles. Motorists also continue to drive more each year: 11,725 milesin 1999 compared to 10,277 miles in 1990. Consequently, the annual gasoline*This section can be skipped without a loss in continuity. Information in this sectionis based largely on the publications of the U.S. Department of Energy, EnvironmentalProtection Agency, and the American Automotive Association.