Page 247 - Advanced thermodynamics for engineers

234 CHAPTER 10 THERMODYNAMICS OF COMBUSTION

P10.6 A gas injection system supplies a mixture of propane (C 3 H 8 ) and air to a spark-ignition
engine, in the ratio of volumes of 1:30. The mixture is trapped at 1 bar and 300 K, the
volumetric compression ratio is 12:1, and the index of compression, k ¼ 1.4. Calculate the
equivalence ratio, the maximum pressure and temperature achieved during the cycle, and
also the composition (by volume) of the dry exhaust gas.
[0.79334, 119.1 bar, 2883 K; 0.8463, 0.1071, 0.0465]
P10.7 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, and the indicated mean effective pressure (imep, p ) of the cycle in
i
bar. Assume that the index of compression, k c ¼ 1:4, while that of expansion, k e ¼ 1:35.
[2495 K; 99.4 bar; 20.57 bar]
P10.8 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 (C 8 H 18 ) 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 how much
lower the maximum temperature is under this condition than when the engine was operated
stoichiometrically. What are the major disadvantages of operating in this mode?
[208 C]

P10.9 A gas engine with a volumetric compression ratio of 10:1 is run on a weak mixture of methane
(CH 4 ) and air, with an equivalence ratio, 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 if compression is
isentropic, and 10% of the heat released during the combustion period is lost by heat transfer.
Assume the ratio of specific heats, k, during the compression stroke is 1.4, and the heat of
5
reaction at constant volume for methane at 25 Cis 8.023 10 kJ/kmol CH 4
[2817 K; 84.59 bar].
P10.10 A jet engine burns a weak mixture (f ¼ 0.32) of octane (C 8 H 18 ) and air. The air enters the
combustion chamber from the compressor at 10 bar and 500 K; assess if the temperature of
the exhaust gas entering the turbine is below the limit of 1300 K. Assume that the
combustion process is adiabatic and that dissociation can be neglected. The enthalpy of
reaction of octane at 25 Cis 44,880 kJ/kg, and the enthalpy of the fuel in the reactants

may be assumed to be negligible.
[Maximum temperature, T P ¼ 1298 K; value is very close to limit]
P10.11 A gas engine is run on a chemically correct mixture of methane (CH 4 ) and air. The
compression ratio of the engine is 10:1, and the trapped temperature and pressure at inlet
valve closure are 60 C and 1 bar, respectively. Calculate the maximum temperature and

pressure achieved during the cycle if:
(a)combustion occurs at constant volume;
(b)10% of the energy added by the fuel is lost through heat transfer;
(c)the compression process is isentropic.
It can be assumed that the ratio of specific heats, k ¼ 1.4, and that the internal energy of
methane in the reactants is negligible.
[2956 K; 88.77 bar]