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ENGR5003 / Week 4
ENGR217 2024 Q B1Past paperOld spec ENGR2172:125 marks30 min

ENGR217 Summer 2024 Q B1[VALID]

Answer ALL parts (a) - (d).

A heavy goods lorry uses a 13.4 litre, 4-stroke, 6 cylinder inline diesel engine that generates 412 kW of power at 1700 rpm, with a cut off ratio of 6.3. The overall engine efficiency is 32.5% and the breathing and mechanical efficiencies can be assumed to be 75% and 85% respectively. We will model this engine as the Diesel cycle, operating on an ideal gas and you may assume that Rg=0.287 kJ/kg/KR_g = 0.287\ \mathrm{kJ/kg/K} and γ=1.4\gamma = 1.4, where γ\gamma is the adiabatic index.

Figure A1-1 (as printed): power curve for a petrol or diesel engine.
Figure A1-1 (as printed): power curve for a petrol or diesel engine.
Formulas you may need
  • Diesel efficiency: ηth=1−1rγ−1[rcγ−1γ(rc−1)]\eta_{th} = 1 - \dfrac{1}{r^{\gamma-1}}\left[\dfrac{r_c^\gamma - 1}{\gamma(r_c - 1)}\right] (on the formula sheet)
  • Overall efficiency ηo=ηthηbηm\eta_o = \eta_{th}\eta_b\eta_m; power P=WnetNηbηmP = W_{net}N\eta_b\eta_m, N=rpm/120N = \mathrm{rpm}/120; Wb=ηmWnetW_b = \eta_mW_{net} (learn this)
  • ηth=Wnet/Qin\eta_{th} = W_{net}/Q_{in}; isobaric heat addition ΔQ=mcpΔT\Delta Q = mc_p\Delta T; cp=γRgγ−1c_p = \dfrac{\gamma R_g}{\gamma-1} (on the formula sheet)
  • Adiabatic process: TVγ−1=constantTV^{\gamma-1} = \text{constant}; ideal gas pV=mRgTpV = mR_gT (on the formula sheet)
  • Isentropic efficiencies: ηs,c=ΔTs/ΔTreal\eta_{s,c} = \Delta T_s/\Delta T_{real}, ηs,e=ΔTreal/ΔTs\eta_{s,e} = \Delta T_{real}/\Delta T_s (given in the question)
  1. (a)
    Determine the thermal efficiency, compression ratio, brake work and net work per cylinder, per cycle.
    [10]2:2
  2. (b)
    Given that the compression and expansion strokes have isentropic efficiencies of 90% and 85% respectively, determine the actual temperature of each stage of the process, given that at the intake, the air pressure and temperature are p1=101 kPap_1 = 101\ \mathrm{kPa} and T1=288 KT_1 = 288\ \mathrm{K}. You may assume that the heat addition per cycle is the same as for the isentropic process. The equations for the isentropic efficiencies are given as: ηs,compression=ΔhsΔhreal=ΔTsΔTreal\eta_{s,\mathrm{compression}} = \dfrac{\Delta h_s}{\Delta h_{real}} = \dfrac{\Delta T_s}{\Delta T_{real}} and ηs,expansion=ΔhrealΔhs=ΔTrealΔTs\eta_{s,\mathrm{expansion}} = \dfrac{\Delta h_{real}}{\Delta h_s} = \dfrac{\Delta T_{real}}{\Delta T_s}
    [8]First
  3. (c)
    Without detailed calculations, briefly describe why petrol and diesel engines have a power curve (such as that shown in Figure A1-1). Discuss how the thermal, breathing and mechanical efficiencies vary with RPM, as well as the fuel burn rate of the air/fuel mixture.
    [5]
  4. (d)
    Discuss why a diesel engine is preferable for a heavy goods vehicle over petrol engines.
    [2]Third