AFR, BSFC, Stioch, Max Power
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AFR, BSFC, Stioch, Max Power

  1. #1
    Or Seth, either one Budweiser's Avatar
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    Default AFR, BSFC, Stioch, Max Power

    Have had this thought rolling around in my mind for a bit. I think we touched on the subject years ago. Can't find the thread though.

    Anyway, if:
    an AFR is stoichiometric, then presumably all the oxygen in the air is reacted with all the fuel in the air and there's none of either left over.
    and
    Any AFR richer OR leaner than stoich is actually cooler in the cylinder than it would be at stoich...

    Then:
    Why is max power achieved at AFR's slightly richer than stoich? In thermodynamics, for a given mass of gasses in a given volume, more heat is more pressure.

    I'm curious what your thoughts are.

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    Senior Member wagspe208's Avatar
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    Because you never get complete fuel ignition.... due to tons of factors.
    Surface areas being one.
    the amount of air and the amount of fuel would be in a perfect world with 100% combustion of ALL fuel.... every single molecule
    Last edited by wagspe208; 12-31-2016 at 01:32 PM.

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by wagspe208 View Post
    Because you never get complete fuel ignition.... due to tons of factors.
    Surface areas being one.
    the amount of air and the amount of fuel would be in a perfect world with 100% combustion of ALL fuel.... every single molecule
    Hey Wags,

    So, speaking in loose terms (there are tons of factors), are you saying that there has to be more fuel in the cylinder to actually react all the oxygen? Or at least maximize the available oxygen?

    Edit to add: So, when a factory car is tuned for stoich (at whatever conditions) it's not actually stoich when the O2 sensor says it is?


    Happy New Year Wags.
    Last edited by Budweiser; 12-31-2016 at 02:18 PM.

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    LP-25.com Infomaniac's Avatar
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    Throw this into the mix of the discussion.

    The factor that most people do not consider is nitrogen is 71% of the "air" that enters the engine. The nitrogen is what expands from the heat of combustion. Oxygen only supports combustion.

    Fuel oxygen mixtures at different ratios react differently during combustion. Burn faster or slower and also when combustion temps or pressures reach a specific point it can release all of the remaining energy instantly (detonation) instead of a steady burn. Starts off as a smooth burn but at some point explodes the rest of it so to speak.

    The reason boosted engines make more power. More oxygen for combustion and more nitrogen to expand. Nitrous wouldn't work at all if nitrogen wasn't released during the process to expand and absorb heat.

    The rate of the release of energy from the fuel oxygen mixture is best for power at a richer percent. << My final answer
    If For Some Reason I Do Something Worthy Of Recognition. God Provided The Ability And Deserves The Credit.


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    Quote Originally Posted by Infomaniac View Post
    The rate of the release of energy from the fuel oxygen mixture is best for power at a richer percent. << My final answer
    Hadn't considered that angle. I'll roll that around the mind shop a little.

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    Senior Member wagspe208's Avatar
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    Quote Originally Posted by Budweiser View Post
    Hey Wags,

    So, speaking in loose terms (there are tons of factors), are you saying that there has to be more fuel in the cylinder to actually react all the oxygen? Or at least maximize the available oxygen?

    Edit to add: So, when a factory car is tuned for stoich (at whatever conditions) it's not actually stoich when the O2 sensor says it is?


    Happy New Year Wags.
    I'm only guessing.... I agree with your original premise.
    Fuel molecules that are "laying" for lack of a beter term.... against the chamber, piston top, around top ring land area.... does not burn efficiently. SMall chamgers are more efficient... rolled overheads require less timing.....
    SO.... AFR is a theoretical number. It is the ration of fuel to air... duh.... but that does not account for the fuel that reminas unburned due to contact with surfaces, poorly atomized, moon position, etc.
    HA

    I am not good t explaining stuff.... so the fuel that dit not burn at all... or burn effieiently... is made up for... by the overly rich mixture.
    I am tired and know I have a shit load of typos... I might fix them tomorrow.

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by wagspe208 View Post
    I'm only guessing.... I agree with your original premise.
    Fuel molecules that are "laying" for lack of a beter term.... against the chamber, piston top, around top ring land area.... does not burn efficiently. SMall chamgers are more efficient... rolled overheads require less timing.....
    SO.... AFR is a theoretical number. It is the ration of fuel to air... duh.... but that does not account for the fuel that reminas unburned due to contact with surfaces, poorly atomized, moon position, etc.
    HA

    I am not good t explaining stuff.... so the fuel that dit not burn at all... or burn effieiently... is made up for... by the overly rich mixture.
    I am tired and know I have a shit load of typos... I might fix them tomorrow.
    Moon position is key I find waning gibbous to be nearly optimal.

    Pretty sure we're all guessing. It's kind of what makes it an interesting topic to discuss.

    Still rolling Info's input around.


    Here's what I'm thinking... Additional fuel, that does not burn, is heated and expands at a rate greater than the other gasses. However, too much extra fuel cools the process too much and the benefits start to tip into diminishing returns. Short version anyway.
    Last edited by Budweiser; 01-01-2017 at 08:40 PM.

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    I'm impressed ...... You guys have given me my first headache in 2017.



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    The discussion is going to be quite different when comparing gasoline to alcohol.
    Gasoline only needs to be atomized to burn. You can start a car on gas at below 0 temps.
    Below 50 degrees and I can throw lit matches in a can of ethanol and it won't light.

    Alcohol fuels need to be vaporized to burn. There has to be enough heat present to make the phase change.
    150 degrees for Methanol and 173 degrees for Ethanol. Less heat under vacuum, more heat under boost.

    The tuning principal is to have sufficient fuel to consume all of the available oxygen.
    That's the reason for the O2 sensor, to read remaining oxygen % after combustion.

    There is a "rich best tq" # for fuels. Gasoline is 12.5: 1
    In a boosted application most will run it richer like 11.7: 1 as the added fuel gives some combustion cooling at the expense of some lost power.

    Fuel .......................... AFR
    Gas stoich ................. 14.7
    Gas max power rich .... 12.5
    Gas max power lean .... 13.23
    E85 stoich .................. 9.765
    E85 max power rich ..... 6.975
    E85 max power lean .... 8.4687
    E100 stoich ................. 9.0078
    E100 max power rich .... 6.429
    E100 max power lean ... 7.8

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by Mark55 View Post
    The discussion is going to be quite different when comparing gasoline to alcohol.
    Gasoline only needs to be atomized to burn. You can start a car on gas at below 0 temps.
    Below 50 degrees and I can throw lit matches in a can of ethanol and it won't light.

    Alcohol fuels need to be vaporized to burn. There has to be enough heat present to make the phase change.
    150 degrees for Methanol and 173 degrees for Ethanol. Less heat under vacuum, more heat under boost.

    The tuning principal is to have sufficient fuel to consume all of the available oxygen.
    That's the reason for the O2 sensor, to read remaining oxygen % after combustion.

    There is a "rich best tq" # for fuels. Gasoline is 12.5: 1
    In a boosted application most will run it richer like 11.7: 1 as the added fuel gives some combustion cooling at the expense of some lost power.

    Fuel .......................... AFR
    Gas stoich ................. 14.7
    Gas max power rich .... 12.5
    Gas max power lean .... 13.23
    E85 stoich .................. 9.765
    E85 max power rich ..... 6.975
    E85 max power lean .... 8.4687
    E100 stoich ................. 9.0078
    E100 max power rich .... 6.429
    E100 max power lean ... 7.8
    Granted every fuel has it's own unique characteristics, the discussion is the same with any fuel, just different numbers. Isn't it?
    Why does richer than stoich produce more power than stoich? Is your thinking similar to Wags, in that there has to be more to react all the oxygen?

    Something interesting in the numbers you posted. Some of them are very very specific, out to a ten-thousandth. They must be calculated numbers. What are they calculated from, I wonder?

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    Quote Originally Posted by Nice Pair View Post
    I'm impressed ...... You guys have given me my first headache in 2017.
    We're just getting started. Are you sure it's not a hang over?

    Happy New Year Steve

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    Quote Originally Posted by Budweiser View Post
    Granted every fuel has it's own unique characteristics, the discussion is the same with any fuel, just different numbers. Isn't it?
    Why does richer than stoich produce more power than stoich? Is your thinking similar to Wags, in that there has to be more to react all the oxygen?

    Something interesting in the numbers you posted. Some of them are very very specific, out to a ten-thousandth. They must be calculated numbers. What are they calculated from, I wonder?
    All of them are are calculated..... hence your difference of theoretical vs reality.

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    Quote Originally Posted by wagspe208 View Post
    All of them are are calculated..... hence your difference of theoretical vs reality.
    Okay... So, what's theoretical compared to actual? Actual to theoretical? Theoretical stoich vs. actual stoich? Theoretical max power AFR vs. actual max power AFR?

    Is the theoretical max power AFR really anything different in reality?

    If they're ALL calculated, why does slightly rich calculate out to make max power when it's "calculated" that all the oxygen is already used up? Calculated, it's gone. Why is richer beneficial?

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    Senior Member propless's Avatar
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    Love a good "theory" discussion !!!

    The way it was explained to me, not saying its right, just how it was explained to my limited brain, is.... Stoich is the "perfect" mixture for complete burn, but its burn rate is to fast with the RPM range we are in and the engine under a heavy load. You need the right burn rate to match the RPM and load of the engine. As you go rich the burn rate slows down. Its kinda like "timing the fuel burn rate".

    Mark55 posted =
    There is a "rich best tq" # for fuels. Gasoline is 12.5: 1
    In a boosted application most will run it richer like 11.7: 1 as the added fuel gives some combustion cooling at the expense of some lost power.

    Fuel .......................... AFR
    Gas stoich ................. 14.7
    Gas max power rich .... 12.5
    Gas max power lean .... 13.23...............

    Best TQ is 12.5, because its doing it in the lower RPM range. Slows the burn rate to better match the cycle time available at lower RPM.

    Best HP is 13.2, because its doing it at higher RPM and less time is available in the cycle. Have to speed up the burn rate.

    Stoich would eventually be the best HP ratio if the RPM climbed high enough for the burn rate to match the time available in the cycle.

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