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Stoichiometric combustion. Products and dissociation

  1. Jan 24, 2012 #1
    Hey guys,
    Combustion in an SI engine.

    So an AF:R of 14.7:1 is stoichiometric for petrol (gas), but an AF:R of 12.5:1 makes more power.

    One common thought is that not all the fuel and oxygen react in a stoichiometric mix, so by upping the fuel there is more chance of all the oxygen reacting (excess fuel).

    For some reason I have got the notion that when the co2 and h2o in the cylinder dissociate, the liberated oxygen is free to react with the excess fuel.
    But I can't find any sources that state this, they all talk about equilibrium reactions and NOx production.

    So question is, what is happening to make the extra power when using a rich mixture.
  2. jcsd
  3. Jan 24, 2012 #2
    Hey guys, anyone wanna jump in here?

    The information in this link gives some insight:
    http://www1.gantep.edu.tr/~ozcan/kitaplar/makina/term_%20heat/Advanced%20Thermodynamics%20for%20Engineers,%20First%20Edition/7699X_13.pdf [Broken]

    The author [above] states:
    The author also states:
    The author doesn't state what he means by "substance in the products".
    Anybody have any idea what he might be talking about?

    Gordon Blair (design and simulation of four stroke engines) states on the subject of CO2 dissociation that:
    With 2.1% free oxygen in the cylinder, surely some would react with the excess fuel present in a rich mixture.

    I'm not making 2 + 2 = 5 am I?
    Last edited by a moderator: May 5, 2017
  4. Jan 25, 2012 #3
    Since there is a limit to how much oxygen gets inducted in each cycle, using up all the oxygen is important for full power. The rich mixture attempts to do that, as you noted.

    The better fuel distribution the engine has, the closer to stoichiometric it will want the mixture to be to get full power. 12.5:1 is a reasonable place to start for carbs and older engines; the last dyno test I did was an LS engine with aftermarket EFI and intake which made best power at 13.6:1.
    Last edited: Jan 25, 2012
  5. Jan 25, 2012 #4
    Thank you for the reply mender.
    But I can't accept that engines will make more power towards stoichiometric than at a rich condition with the only factor being fuel mixing.
    The theory says that as you never reach the ideal otto cycle model due to dissociation of CO2 reducing the temperature in the cylinder, and that higher pressures are present at a rich condition.

    There's no way of knowing what the actual AF:R is inside the cylinder is. It could be that the chamber shape works well and produces a better distribution and burns more efficiently, but the majority AF:R produced in the chamber is closer to 12.5:1 than 13.6:1.
    I don't know, but do you see what I'm saying?

    Below shows a carbed engine, I think its a SBC (from here http://speedtalk.com/forum/viewtopic.php?f=15&t=29451&start=45)

    If you're just using one O2 sensor on each bank there's going to be some discrepancy, and O2 sensor readings are affected by temperature, pressure and calibration as well.
  6. Jan 26, 2012 #5
    Don't forget that when you've liberated the oxygen, you've also liberated the carbon and hydrogen which will be more than happy to recombine without involving any extra fuel. It's been a while since I took Chem but I suspect that the liberated C and H will be more active than any long chain hydrocarbon molecules.

    I think they already answered that. Production of NOx requires energy and reduces the net energy released; if something more reactive than N2 is present (like C and H from more fuel) less NOx forms and more net energy is available to be converted into power.
    Last edited: Jan 26, 2012
  7. Jan 26, 2012 #6
    Well, that's real life for you; it doesn't always track the theory properly.:smile:

    The chart you posted (that's a 622 big block by the way) has an average of 12.17 A/F on one bank and 12.47 on the other. The big issue however is the presence of cylinders running in the high 13's; to keep the engine in one piece they're having to overfuel the other cylinders. Getting the lean cylinders down into the low 13's would allow the other cylinders to get out of the high 11's and make more power in the high 12's to low 13's (I'm assuming a few things based on my years of race engine building).

    Note that I didn't say the engine would make peak power at stoichiometric, just that as the conditions in the engine become more consistent a clearer picture of what the engine really wants for mixture will appear. In other words, recent development is revealing the 12.5:1 "standard" was needed to "crutch" the engine to keep it alive at full throttle and a higher A/F ratio makes more power when the crutch isn't needed.

    I think so, but we can only go by the average until we have more detail. Still, as I said the indication is that with better fuel distribution and A/F ratio control there is less need to compromise the other cylinders to keep some cylinders out of trouble, the power peak will be trending toward stoichiometric. How close remains to be seen but some reputable builders are using 14.0:1 as their target. I suspect that the trend for peak power at leaner mixtures reflects better mixture control inside the combustion chamber as well as between cylinders.

    Don't forget that the first article you linked isn't actual testing but is a simulation to show the difference between dissociation and no dissociation. If the modeler used 12.5:1 as the mixture that represents the peak power it should come as no surprise that the results reflect that.

    That's my take on this after a bit of reading and thinking and I could be wrong. I try to combine equal amounts of theory and testing but I believe that if the theory doesn't match the data it's the theory that needs to be changed.
    Last edited: Jan 26, 2012
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