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

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Discussion Overview

The discussion revolves around the behavior of unburned liquid fuel, specifically octane, in a combustion chamber when the equivalence ratio is less than one. Participants explore the implications of incomplete combustion, the state of leftover fuel, and the resulting emissions, including potential dissociation products.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether leftover octane would exit the combustion chamber as liquid or evaporate into gaseous octane at high temperatures.
  • Another participant suggests that incomplete combustion leads to dissociation of hydrocarbons into water, CO2, and oxides of nitrogen, but expresses uncertainty about the specifics of dissociation.
  • A participant emphasizes that there is generally no "leftover fuel" unless the equivalence ratio is significantly off, indicating that fuel typically dissociates and combines with available oxygen.
  • Discussion includes the formation of carbon monoxide (CO) instead of carbon dioxide (CO2) when there is insufficient oxygen, with one participant acknowledging this point and expanding on the stoichiometric combustion equations.
  • Participants propose that the leftover fuel would not remain static but would likely dissociate and contribute to a complex mixture of compounds in the exhaust, including CO, H2, and OH, among others.
  • One participant reflects on historical experiences with carburetors and over-rich fuel mixtures, relating it to the discussion of emissions and black smoke from combustion.
  • Another participant speculates about the nature of black smoke, suggesting it contains various compounds, including carbon structures like Buckyballs, and discusses the implications for hydrogen oxidation.

Areas of Agreement / Disagreement

Participants express a range of views on the behavior of leftover fuel and the products of incomplete combustion. There is no consensus on the specifics of dissociation or the exact nature of emissions, indicating that multiple competing views remain.

Contextual Notes

Participants acknowledge the complexity of combustion chemistry, including the influence of temperature, pressure, and the presence of various compounds, but do not resolve the uncertainties surrounding these factors.

BobbyBear
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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
 
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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.
 
Welcome to the world of emissions.
Haha lol well I'm really just passing by.. :P But I'm happy with your answer, it seems logical :)
Fanku so xD
 
FredGarvin said:
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.
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.
 
You did miss the primary product of burning a hydrocarbon without enough oxygen, though: CO instead of CO2.
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) ArNow 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) Arbut 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 :POh and about:
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).
you are very kind :)
 
BobbyBear said:
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:
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.
 
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.
-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 :(
 
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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