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Combustion when there's left over fuel, what happens to it?

  1. May 16, 2009 #1
    hi, I'm wondering what happens when you burn a liquid fuel in a combustion chamber, eg. suppose you've got octane (C8H18) and you're using air as the comburent, but the equivalence ratio is less than one, so that not all the octane can get used up... Does the 'left over' octane come out of the chamber as liquid..? - suppose the temperature in the chamber is around 2000K . . . well no, I guess it wouldn't be liquid at that temperature assuming the pressure is the atmospheric pressure :P . . . so would it just evaporate into gaseous octane and form part of the flue gases? That is, assuming no dissociation occurs . . . um, could the gaseous octane dissociate too?

    Thank you :p
  2. jcsd
  3. May 16, 2009 #2


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    Science Advisor

    Welcome to the world of emissions. Incomplete combustion is always a balancing act with combustion temps and other forms that exist in the exhaust stream. I'm not a chemist so I can't talk to the dissociation, but I would imagine that it is going to dissociate like any other hydrocarbon into water, CO2 and oxides of nitrogen.
  4. May 16, 2009 #3
    Haha lol well I'm really just passing by.. :P But I'm happy with your answer, it seems logical :)
    Fanku so xD
  5. May 16, 2009 #4


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    Put another way: there is generally no such thing as "left over fuel" unless you are waaaay off the equivalence ratio. The fuel is disasociated and combined with whatever is available to make new compounds.

    You did miss the primary product of burning a hydrocarbon without enough oxygen, though: CO instead of CO2. What happens when you don't even have enough oxygen to make CO? Well.....I'm not a chemist either...

    My last thermo book came with a dos-based combustion products calculator we played with a little in class. I let someone at work borrow it and I'm not sure if the cd is still in it (I'll check next week). Perhaps there are combustion product calculators available online, but a quick google only turns up one that doesn't work.
  6. May 16, 2009 #5
    Oooooooooh Russ, I'd not even been thinking of that! :P

    -Nods- so let's take for example, the stoichiometric combustion of C8H18, and for simplicity I'm considering the air to be composed of 21%O2, 78%N2 and 1%Ar :

    C8H18 (l) + 12.5 O2 + 12.5 (78/21) N2 + 12.5 (1/21) Ar ----> 8CO2 + 9H2O + 12.5 (78/21) N2 + 12.5 (1/21) Ar

    Now let's say, instead of that we have a 20% excess of fuel, so that firstly I'd write:
    1.2 C8H18 (l) + 12.5 O2 + 12.5 (78/21) N2 + 12.5 (1/21) Ar ----> 8CO2 + 9H2O + 12.5 (78/21) N2 + 12.5 (1/21) Ar + 0.2 C8H18(g)

    And then I was thinking, but wait, the fuel is not going to just sit there without recombining itself with someone . . . so I was thinking on the lines of, it dissociates and we'd have something like:

    1.2 C8H18 (l) + 12.5 O2 + 12.5 (78/21) N2 + 12.5 (1/21) Ar ----> a CO2 + b H2O + 12.5 (78/21) N2 + c H2 + d CO + e OH + 12.5 (1/21) Ar

    but of course, the formation of CO, OH and H2O will not only come from the 'leftover' fuel (-bites tongue and promises not to say 'leftover fuel' again- :P) but from the incomplete oxidation of the carbon, and possibly even the disassociation of the other compounds, so that we'd really have a kind of soup of compounds in the exhaust stream:

    1.2 C8H18 (l) + 12.5 O2 + 12.5 (78/21) N2 + 12.5 (1/21) Ar ----> a CO2 + b H2O + cN2 + d H2 + e CO + f OH + g H + h O + i NO + j N + 12.5 (1/21) Ar

    I'd even not dare to remove the atomic and molecular oxygen from the soup even though there's a shortage of oxygen . . .

    To determine the molar relations, I'd apply the four equations stating the conservation of the elements (I've already used the one that applies to Argon), and then derive the remainder of equations needed by considering the values of dissociation constants of some of the compounds involved at the temperature of the furnace (assuming a state of equilibrium exists) . . . would that be the correctest way of determining what species really exist in the exhaust gases? Not that I'm needing to do this, I was just wondering what happens to the . . . um, superfluous (:P) fuel injected into the combustion chamber - usually one is used to hearing how much excess air is used, not fuel, so I'd never thought about this before :P

    Oh and about:
    you are very kind :)
  7. May 16, 2009 #6
    You are too young to remember carbuerators with the manual choke knob on the dashboard, and starting the car with an over-rich fuel mixture, and getting black smoke out the tailpipe.
  8. May 16, 2009 #7
    -rolling on the floor laughing- I'm not that young!:P At least I've seen in movies:P
    So that's just suspended droplets of liquid fuel, or vapourized fuel, actually coming out of the exhaust? hmm.
    I was thinking more along the lines of power plants, but yeah, I see.

    Oh and -quotes himself- "but of course, the formation of CO, OH and H2O will not only come from the 'leftover' fuel" . . um, that was meant to read CO, OH and H2, btw :P I never manage to fully avoid typos :(
  9. May 16, 2009 #8
    Black smoke contains Buckyballs and all kinds of strange stuff doesn't it? So the hydrogen would be more likely to oxidize, leaving an excess of carbon..
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