Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

530 | Thermodynamics

Therefore, the irreversibility of the cycle is

The exergy destruction of the cycle could also be determined from Eq. 9–34.

Notice that the largest exergy destruction in the cycle occurs during the

heat-rejection process. Therefore, any attempt to reduce the exergy destruc-

tion should 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:

where

Thus,

which is equivalent to the exergy destruction for process 4-1. (Why?)

Discussion Note that 163.2 kJ/kg of work could be obtained from the

exhaust gases if they were brought to the state of the surroundings in a

reversible manner.

f 4
381.83 kJ>kg 1 290 K 21 0.7540 kJ>kg#K 2  0
163.2 kJ/kg

v 4 v 0 
v 4 v 1 
 0

u 4 u 0 
u 4 u 1 
qout
381.83 kJ>kg

s 4 s 0
s 4 s 1
0.7540 kJ>kg#K

f 4 
 1 u 4 u 02 T 01 s 4 s 02 P 01 v 4 v 02

245.4 kJ/kg

 0 82.2 kJ>kg 0 163.2 kJ>kg

xdest,cycle
xdest,12xdest,23xdest,34xdest,41

163.2 kJ>kg

1 290 K2c0.7540 kJ>kg#K

381.83 kJ>kg

290 K

d

Two-thirds of the oil used in the United States is used for transportation. Half

of this oil is consumed by passenger cars and light trucks that are used to com-

mute to and from work (38 percent), run a family business (35 percent), and

for recreational, social, and religious activities (27 percent). The overall fuel

efficiency of the vehicles has increased considerably over the years due to

improvements primarily in aerodynamics, materials, and electronic controls.

However, the average fuel consumption of new vehicles has not changed much

from about 20 miles per gallon (mpg) because of the increasing consumer

trend toward purchasing larger and less fuel-efficient cars, trucks, and sport

utility vehicles. Motorists also continue to drive more each year: 11,725 miles

in 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 section

is based largely on the publications of the U.S. Department of Energy, Environmental

Protection Agency, and the American Automotive Association.

TOPIC OF SPECIAL INTEREST* Saving Fuel and Money by Driving Sensibly