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Homework Help: Car Fuel Problem, calorific values.

  1. May 2, 2012 #1
    1) I need to find the output power in kW of an engine when running a mix of 90% Nitromethane
    (CH3NO2), and 10% Octane (C8H18), their calorific values are 11.3 MJ/kg and 43.7 MJ/kg respectively.
    what i know:
    An engine that runs pure 100% octane with 20% excess air produces 100kW, and the volumetric, thermal and mechanical efficiency's remain the same.
    I need to find out how much power the same engine using a 10/90 fuel mix would produce.

    2)Im using this equation:

    Power (kW)=Power (kJ/sec)= [calorific value fuel(kJ/kg)] X [molar mass of fuel(kg/mol)] X

    Im assuming since the volumetric efficeincy will be the same then the [mol/sec] i get for the first engine will be the same as the second fuel mix.

    100kW = [(43.7 x 10^3)kJ/kg] X [(114.224)kg/mol] X mol/sec
    mol/sec for both engines = 0.02.

    But im stuck from there and dont know how to continue, as when i try subbing this value into the same equation with the second fuels mix, i get a lower power which it shouldnt be.

    Any help would be greatly appreciated.

    Cheers Miller11
  2. jcsd
  3. May 3, 2012 #2
    Is this problem from a thermodynamics course?
  4. May 4, 2012 #3
    To work this problem you will have to write the chemical equations for each fuel, balance them both, and determine how much fuel is being burned given the 20% excess air. In doing so you will find that you can burn more nitromethane fuel and therefore produce more power because nitromethane contains oxygen.
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