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Combustion of Lean C8H18?

  1. Jan 21, 2010 #1
    I am doing a thesis work and I am trying to figure out the amount of C02 and the products in combustion of Air and C8H18. Its known that the engine is operating on lean fuel, but I am having trouble with the reaction formula. This is not my subject. The engine is operating at lambda 1.2 to 1.6. (Phi = 0.83 - 0.62).

    Here is my try:

    0.83 C8H18 + 12.5O2 + 94N2 <--> 6.64C02 + 7.47 H20 + 94 N2 + 2.12 O2 ?

    0.62 C8H18 + 12.5O2 + 94N2 <--> 4.96C02 + 5.58 H20 + 94 N2 + 4.7 O2 ?

    Is there something wrong here? Does this really happen as the fuel + air ignite?
  2. jcsd
  3. Jan 21, 2010 #2


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    Unfortunately it's not that easy. There are many complex reactions taking place during the combustion process, and even with a lean burn engine, you'll still have a fair amount of various hydrocarbons in the exhaust. Depending on the conditions, you'll also produce a significant quantity of NO and NO2, and CO too.

    Is this a degree thesis? What are you trying to achieve with this?
  4. Jan 22, 2010 #3
    Yes, its a little complicated to describe here. But in short I am trying to 'estimate' the C02 molefraction for an estimation of the width of the line profile in simulated spectra and a very approximate 'estimation' of the amount of C02 from total in a simulation for optical thickness of investigated bands. I know that this does not meet the reality and that you probably have unburned fuel and NOx, CO, internal ERG and so on. But I do not see how I can determine those fractions of NOx, C0 etc. The ultimate goal is to determine the C02 concentration in a measurement.
  5. Jan 22, 2010 #4


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    Why not assume that something like 99% of the carbon in the fuel ends up as CO2?

    Why are you considering the fuel to be Octane?
  6. Jan 22, 2010 #5
    Depending on what kind of accuracy is needed, a program like STANJAN (there may be newer/better programs; it's been a while since I did this sort of calc) could be used to determine exhaust gas composition.
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