Page 388 - Advanced thermodynamics for engineers

16.8 PROBLEMS 377

P16.4 This question is based on Fig. 16.9. An engine operating on an Otto cycle has a maximum
peak pressure of 60 bar. If the pressure and temperature at the beginning of the cycle are
1.0 bar and 300 K respectively, evaluate the compression ratio that results in the maximum
work output from the cycle. What is the thermal efficiency of that cycle? What effect does the
peak pressure have on the result?
[2:1; 24.2%]
P16.5 Compare the air-standard cycle thermal efficiencies of an Otto cycle obtained by calculating the
state points around the cycle with the value obtained from Eqn (3.16) for an ‘engine’ operating
with a compression ratio, r, of 15:1, and an air–fuel ratio, ε ¼ 28:1 The trapped conditions are
1.0 bar and 27 C. Assume Q ¼ 44000 kJ=kg, the specific heat capacity at constant volume,

0
p
c v ,is 0.721 kJ/kg K, and k ¼ 1.4. The trapped conditions are 1 bar and 27 C.

[66.1%; 65.62%]
P16.6 Compare the air-standard cycle thermal efficiencies of a diesel cycle obtained by calculating the
state points around the cycle with the value obtained from Eqn (3.20) for an ‘engine’ operating
with a compression ratio, r, of 15:1, and an air–fuel ratio, ε ¼ 28:1. The trapped conditions are
1.0 bar and 27 C. Assume Q ¼ 44000 kJ=kg, the specific heat capacity at constant volume,

0
p
c v ,is 0.721 kJ/kg K, and k ¼ 1.4. The trapped conditions are 1 bar and 27 C.

[56.8%; 55.4%]
It has been suggested in Fig. 16.2 that it is possible to evaluate the thermal efficiency of a diesel
cycle by considering it to be an infinite number of incremental Otto cycles. Examine the truth
of this statement by dividing the diesel cycle into 10 incremental Otto cycles.
[56.7%]
P16.7 Recalculate the Otto cycle in P16.5 using EQUIL2 to evaluate the conditions around the
cycle. Do these calculations both with and without considering dissociation. How has the use
of more accurate data, and a better evaluation of the combustion process affected the results?
Is the effect of dissociation appreciable? Explain your reasoning.
[47.98%]
P16.8 Recalculate the diesel cycle in P16.6 using EQUIL2 to evaluate the conditions around the
cycle. Do these calculations both with and without considering dissociation. How has the use
of more accurate data, and a better evaluation of the combustion process affected the results?
Is the effect of dissociation appreciable? Explain your reasoning.
[46.8%]
P16.9 It was shown that the brake thermal efficiency of an engine with friction peaked at relatively
low compression ratios (see Fig. 16.10). Recalculate this diagram with fmep, p ¼ 2 bar.
f
Comment on the effect of higher friction values.
[5.5, 37.1%; 7, 43.4%; 13, 54.9%]
The following problems are based on using the simulation program SPIE. This program can be
accessed by visiting http://booksite.elsevier.com/9780444633736/. The basic engine data for
the program is available in a file (basic.data), and this is based on the data used to calculate
Figs 16.12 to 16.21 – however, certain minor changes internal to the data mean that the results
obtained by you will not exactly replicate those diagrams. The trends and general conclusions
will, obviously, be the same.