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ENGR5003 / Week 5
Lecture 14 example - turbocharged Otto engineTutorialCurrent spec2:118 min

ENGR5003 Lecture 14 example turbocharger question (slides 13-18)

A petrol engine is modelled as an Otto cycle. The air temperature and pressure at intake are 290 K and 101 kPa, and the cycle has a compression ratio of 4. The temperature after combustion is 1150 K. You may assume γ=1.4\gamma = 1.4 and cv=0.7175 kJ/(kg K)c_v = 0.7175\ \mathrm{kJ/(kg\,K)}.

Formulas you may need
  • Adiabatic process: T2/T1=rγ−1T_2/T_1 = r^{\gamma-1}; T2/T1=(p2/p1)(γ−1)/γT_2/T_1 = (p_2/p_1)^{(\gamma-1)/\gamma} (on the formula sheet)
  • Constant-volume heat: q=cvΔTq = c_v\Delta T (on the formula sheet)
  • Adiabatic work: w=p2v2−p1v1γ−1=cvΔTw = \dfrac{p_2v_2 - p_1v_1}{\gamma - 1} = c_v\Delta T (on the formula sheet)
  • Otto efficiency: η=1−r−(γ−1)\eta = 1 - r^{-(\gamma-1)} (on the formula sheet)
  • Ideal turbocharger: work on the intake air = heat taken from the exhaust (learn this, Lecture 14)
  1. (a)
    Determine the temperature at each stage of the cycle, the heat addition and rejection, and the thermal efficiency.
  2. (b)
    A turbocharger is used to increase the thermal efficiency; it provides a pressure ratio of 2 on the air before the compression stroke. Following the Lecture 14 method: (i) determine the temperature of the exhaust gases after the turbocharger and the heat exchanged; (ii) determine the new temperature T3′T_3', given that the specific heat addition is the same as before; (iii) determine the new thermal efficiency.