Thermodynamics

Chapter 9 | 539PROBLEMS*Actual and Ideal Cycles, Carnot Cycle, Air-StandardAssumptions, Reciprocating Engines9–1C Why is the Carnot cycle not suitable as an ideal cyclefor all power-producing cyclic devices?9–2C How does the thermal efficiency of an ideal cycle, ingeneral, compare to that of a Carnot cycle operating betweenthe same temperature limits?9–3C What does the area enclosed by the cycle representon a P-v diagram? How about on a T-s diagram?9–4C What is the difference between air-standard assumptionsand the cold-air-standard assumptions?9–5C How are the combustion and exhaust processes modeledunder the air-standard assumptions?9–6C What are the air-standard assumptions?9–7C What is the difference between the clearance volumeand the displacement volume of reciprocating engines?9–8C Define the compression ratio for reciprocating engines.9–9C How is the mean effective pressure for reciprocatingengines defined?9–10C Can the mean effective pressure of an automobileengine in operation be less than the atmospheric pressure?9–11C As a car gets older, will its compression ratio change?How about the mean effective pressure?9–12C What is the difference between spark-ignition andcompression-ignition engines?9–13C Define the following terms related to reciprocatingengines: stroke, bore, top dead center, and clearance volume.9–14 An air-standard cycle with variable specific heats isexecuted in a closed system and is composed of the followingfour processes:1-2 Isentropic compression from 100 kPa and 27°C to800 kPa2-3 v constant heat addition to 1800 K3-4 Isentropic expansion to 100 kPa4-1 P constant heat rejection to initial state(a) Show the cycle on P-v and T-s diagrams.(b) Calculate the net work output per unit mass.(c) Determine the thermal efficiency.* Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.9–15 Reconsider Problem 9–14. Using EES (or other)software, study the effect of varying the temperatureafter the constant-volume heat addition from 1500 K to2500 K. Plot the net work output and thermal efficiency as afunction of the maximum temperature of the cycle. Plot theT-s and P-v diagrams for the cycle when the maximum temperatureof the cycle is 1800 K.9–16 An air-standard cycle is executed in a closed systemand is composed of the following four processes:1-2 Isentropic compression from 100 kPa and 27°C to1 MPa2-3 P constant heat addition in amount of 2800kJ/kg3-4 v constant heat rejection to 100 kPa4-1 P constant heat rejection to initial state(a) Show the cycle on P-v and T-s diagrams.(b) Calculate the maximum temperature in the cycle.(c) Determine the thermal efficiency.Assume constant specific heats at room temperature.Answers: (b) 3360 K, (c) 21.0 percent9–17E An air-standard cycle with variable specific heats isexecuted in a closed system and is composed of the followingfour processes:1-2 v constant heat addition from 14.7 psia and80°F in the amount of 300 Btu/lbm2-3 P constant heat addition to 3200 R3-4 Isentropic expansion to 14.7 psia4-1 P constant heat rejection to initial state(a) Show the cycle on P-v and T-s diagrams.(b) Calculate the total heat input per unit mass.(c) Determine the thermal efficiency.Answers: (b) 612.4 Btu/lbm, (c) 24.2 percent9–18E Repeat Problem 9–17E using constant specific heatsat room temperature.9–19 An air-standard cycle is executed in a closed systemwith 0.004 kg of air and consists of the following threeprocesses:1-2 Isentropic compression from 100 kPa and 27°C to1 MPa2-3 P constant heat addition in the amount of 2.76 kJ3-1 P c 1 v + c 2 heat rejection to initial state (c 1 andc 2 are constants)(a) Show the cycle on P-v and T-s diagrams.(b) Calculate the heat rejected.(c) Determine the thermal efficiency.Assume constant specific heats at room temperature.Answers: (b) 1.679 kJ, (c) 39.2 percent