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Homework Help: Thermal Efficiency Equation

  1. May 12, 2013 #1
    Can you please help me? I am trying to calculate this equation but can't get correct answer:

    Thermal Efficiency, Brake = 1/(bSPC x QHV)

    bSPC (brake Specific fuel consumption) = 0.396 kg/KWh

    QHV (Heating Value for fuel) = 44 MJ/Kg

    The correct answer is meant to be: Thermal Efficiency, Brake = 0.207 = 20.7%.

    I do not know the correct SI units for bSPC or QHV, hopefully I am not using the right ones because I can't get that answer.

    Look forward to any help or advise, Thanks
    Last edited: May 12, 2013
  2. jcsd
  3. May 12, 2013 #2


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    Homework Helper

    Your equation is correct, but I think what is confusing you are the prefixed units. What you can do is reduce everything to the same set of units.

    kWh is a unit of energy so 1kWh = 1 kW * 1 hr = 1kJ/s * 3600 s = 3600 kJ = 3.6 MJ

    So your bSPC = .396 kg/kWh = 0.396/3.6 kg/MJ
  4. May 12, 2013 #3
    Wow solved perfectly, and can get correct answer,, must have been <30mins. You don't know how stupid I felt not being able to do that equation. Thank you very much
  5. May 12, 2013 #4


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    Your units are at fault. The bSPC is given in units of kg/kWh while the HV is given in MJ/kg.

    You need to find the heat equivalent of kWh and convert the value of bSPC before plugging it nto the efficiency formula.
  6. May 12, 2013 #5


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    Another thing that I found helpful to do is to write out your units so that you know if you are missing out a factor.

    So for efficiency, it would be output power/input power which means that you units would cancel.

    Just multiplying the numbers would have left you with something like MJ/kWh and you would have neglected a factor.
  7. May 14, 2013 #6
    Thank you both soo much. I am going to learn today about manipulating units, it will help me soo much with Mechanical Engineering.
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