Page 305 - Advanced Thermodynamics for Engineers, Second Edition

294 CHAPTER 12 CHEMICAL EQUILIBRIUM AND DISSOCIATION

also the composition (by volume) of the dry exhaust gas. What has been the effect of
dissociation? (See Chapter 10, P10.6.)
[0.79334, 119.1 bar, 2883 K; 0.8463, 0.1071, 0.0465]
P12.18. A 10% rich mixture of heptane (C 7 H 16 ) and air is trapped in the cylinder of an engine at a
pressure of 1 bar and temperature of 400 K. This is compressed and ignited, and at a
particular instant during the expansion stroke, when the volume is 20% of the trapped
volume, the pressure is 27.06 bar. Assuming that the mixture is in chemical equilibrium and
contains only CO 2 ,CO, H 2 O, H 2 and N 2 , find the temperature and molar fractions of the
constituents. This solution presupposes that there is no O 2 in the products; use your results to
confirm this.
[2000 K; 0.1037; 0.0293; 0.1434; 0.00868; 0.7148]
P12.19. A turbocharged, intercooled compression ignition engine is operated on octane (C 8 H 18 )and
achieves constant pressure combustion. The volumetric compression ratio of the engine
is 20:1 and the pressure and temperature at the start of compression are 1.5 bar and 350 K
respectively. If the air–fuel ratio is 24:1, calculate maximum temperature and pressure
achieved in the cycle, taking into account the dissociation of the carbon dioxide and water
vapour. Assuming that the combustion gases do not change composition during the
expansion stroke, calculate the indicated mean effective pressure (imep, p )of the cycle in
i
bar. What has been the effect of dissociation on the power output of the engine? Assume that
the index of compression, k c ¼ 1:4, while that of expansion, k e ¼ 1:35 (see Chapter 10,
P10.7).
[2487 K; 99.4 bar; 15.10 bar]
P12.20. One method of reducing the maximum temperature in an engine is to run with a rich
mixture. A spark-ignition engine with a compression ratio of 10:1, operating on the Otto
cycle, runs on a rich mixture of octane and air, with an equivalence ratio of 1.2. The trapped
conditions are 1 bar and 300 K and the index of compression is 1.4. Calculate, taking into
account dissociation of the carbon dioxide and water vapour, how much lower the
maximum temperature is under this condition than when the engine was operated
stoichiometrically. How has dissociation affected the products of combustion? What are the
major disadvantages of operating in this mode? (See Chapter 10, P10.8.)
[3020 K; 3029 K]
P12.21. A gas enginewith a volumetric compression ratio of 10:1 is run on a weak mixture of methane
(CH 4 ) and air, with f ¼ 0.9. If the initial temperature and pressure at the commencement of
compression are 60 C and 1 bar respectively, calculate the maximum temperature and

pressure reached during combustion at constant volume, taking into account dissociation of
the carbon dioxide and water vapour, under the following assumptions
(a) that 10% of the heat released is lost during the combustion period and
(b) that compression is isentropic.
Assume the ratio of specific heat, k, when the compression stroke is 1.4, and the heat of
5
reaction at constant volume for methane is 8.023 10 kJ/kmol CH 4 (see Chapter 10,
P10.9).
[2737 K; 71.57 bar].