Page 98 - Advanced thermodynamics for engineers

4.7 AVAILABILITY BALANCE FOR A CLOSED SYSTEM 83

1.2
Availability received
during compression
1.0 Availability destroyed
during combustion
Non-dimensional availability, a/q* 0.8 Availability delivered as

work during expansion
0.6

0.4

0.2

0
0 2 4 6 8 10 12
Volume ratio, V/V e

FIGURE 4.9
Variation of nondimensional availability with volume for an Otto cycle.

be obtained if the working fluid is taken down to the dead state by reversible processes. Turbo-
chargers are used to convert part of this availability into work but cannot convert it all because it can
only take the state of the fluid down to the pressure of the dead state, p 0 . If the full availability is to be
converted, it is necessary to use a turbine (e.g. a turbocharger) and then a ‘bottoming’ cycle (e.g. a
Rankine cycle based on a suitable working fluid) to take the exhaust gas down to the dead-state
temperature, T 0 .
The nondimensional availability, a=q , is a measure of the work that can be obtained from the
cycle. Hence, the total ‘nondimensional work’ that can be achieved is
da 1 da 4 da 23
þ ¼ 1:029375 0:332836 0:17105 ¼ 0:5255
q q q
|fflfflfflfflfflffl{zfflfflfflfflfflffl} |ffl{zffl}
net work from availability
availability generated
This is the ratio of the net work output to the energy supplied (q*), and is equal to the thermal
efficiency of the engine cycle. The thermal efficiency of this Otto cycle, with a compression ratio
of 12:1, is
1
h Otto ¼ 1 ¼ 0:5255
r ðk 1Þ