Page 74 - Advanced Thermodynamics for Engineers, Second Edition

3.6 PROBLEMS 59

P3.15 The previous examples, P3.9–P3.13, have all been based on a boiler pressure of 20 bar. What
is the effect of raising the boiler pressure to (a) 40 bar and (b) 80 bar on the steam plant
described in P3.10?
[(a) 34.16%; 1020 kW/(kg/s); (b) 37.0%; 1077 kW/(kg/s)]
P3.16 Recalculate P3.15(a) with the condenser pressure lowered to 0.07 bar.
[36.41%; 1110 kW/(kg/s)]
P3.17 Considering P3.15 and P3.16, which is the most effective method for increasing the thermal
efficiency of the plant – raising the boiler pressure or decreasing the condenser pressure?
Explain your answer.
P3.18 An air-standard Otto cycle operates with a compression ratio, r ¼ 10:1. If the initial
conditions at bdc are 1 bar and 27 C, and the energy addition is 2000 kJ/kg of air, calculate

the salient points around the cycle and the thermal efficiency. Show that the efficiency
calculated from the cycle calculation is equal to that from Eqn (3.16). Assume the
compression and expansion are isentropic and k ¼ 1.4. How does this compare with the
efficiency of a Carnot cycle between the two temperature limits?
[60.2%; 91.5%]
P3.19 Recalculate P3.18 by assuming that the energy addition results from the combustion of fuel in the
cylinder – this increases the mass of gas after ignition. The combustion occurs when an air-fuel
mixture with a strength, ε, of 20:1 is burned at tdc: the calorific value ðQ Þ of the fuel is
0
p
40,000 kJ/kg. How does this compare with the efficiency of a Carnot cycle between the two
temperature limits?
[61.0%; 91.2%]
P3.20 An air-standard Diesel cycle operates with a compression ratio, r ¼ 10:1. If the initial
conditions at bdc are 1 bar and 27 C, and the energy addition is 2000 kJ/kg of air, calculate

the salient points around the cycle, and the thermal efficiency. Show that the efficiency
calculated from the cycle calculation is equal to that from Eqn (3.20). Assume the
compression and expansion are isentropic and k ¼ 1.4. How does this compare with the
efficiency of a Carnot cycle between the two temperature limits, and what is the value of b?
[45.02%; 89.07%; 3.6453]
P3.21 The Otto cycle in P3.18 achieved a peak pressure of 118.1 bar, whilst the Diesel cycle in
P3.20 only reached 25.12 bar. If the compression ratio of the Diesel cycle was increased to
reach 118.1 bar at the end of compression, what would be the cycle efficiency? How does this
fit in with the analysis in this chapter?
[67.57%]
P3.22 The final air-standard cycle associated with reciprocating engines is the dual-combustion
cycle. Assume the Otto cycle in P3.18 is modified so that half the energy is added at constant
volume and the other half at constant pressure. What is the efficiency of this cycle based on
the ratio of work output to energy addition? Evaluate a and b, defined in the text, and
calculate the efficiency using Eqn (3.21)?
[58.49%; 2.850; 1.465]
There are more problems relating to reciprocating engines in Chapter 16, and gas turbines in
Chapter 17.