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ENGR5003 / Week 4
Week 4 Q3Exercise sheetCurrent spec2:125 min

ENGR5003 Week 4 workshop sheet Q3 (worked in Lecture 12) official answers

A petrol engine has a volumetric airflow of 70 litres/s, a swept volume of 1.3 litres at an engine speed of 4000 rpm. This engine has a brake work of 1.3 kJ per cycle and a net work output of 1.5 kJ per cycle. The compression ratio of the cycle is 8 and we will assume γ=1.4\gamma = 1.4.

Formulas you may need
  • Otto efficiency: ηth=1−1rvγ−1\eta_{th} = 1 - \dfrac{1}{r_v^{\gamma-1}} (on the formula sheet)
  • Volumetric (breathing) efficiency: ηv=V˙aVsN\eta_v = \dfrac{\dot V_a}{V_sN} (learn this, Lecture 11)
  • Mechanical efficiency: ηm=WbWnet\eta_m = \dfrac{W_b}{W_{net}} (learn this, Lecture 11)
  • Overall efficiency: ηo=ηthηvηm\eta_o = \eta_{th}\eta_v\eta_m (learn this, Lecture 11)
  • Fuel energy: Qin=mf×CVQ_{in} = m_f \times CV (learn this)
  1. (a)
    Determine the thermal, volumetric and mechanical efficiencies of the engine.
  2. (b)
    Determine the power output of the engine at 4000 rpm.
  3. (c)
    Given that the fuel has a calorific value of 43,710 kJ/kg, and a density, ρ=800 kg/m3\rho = 800\ \mathrm{kg/m^3}, determine the mass and volume of fuel consumed per cycle and per second.
  4. (d)
    If one revolution of the engine corresponds to 0.48 m distance travelled by a car, determine the speed of the car and the fuel economy in km/litre. How far can the car travel on a 40 litre tank of fuel?