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Motorcycle Myth #2 Summoning the gods of combustion speed

Discussion in 'Dirt Bike Mods & Maintenance' started by Rich Rohrich, Jun 17, 2014.

  1. Rich Rohrich

    Rich Rohrich BioHazard
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    Myth # 2 Octane boosters, and combustion speed

    It’s common practice for people to summon the gods of combustion speed to try and explain the magic powers of toluene, acetone and any number of Home Depot speed secrets they are trying to foist off on an unknowing audience. They’ll tell you that these paint store solvents raise the octane of the fuel and as a result slow down combustion and fend off detonation as a result. It’s just more pseudo-science BS from people who would have done well to pay more attention in Chemistry 101 in high school. Real combustion events follow the real laws of chemistry, and while it’s a pretty complex set of interactions there are a few things that are easy to explain and understand.

    It's a commonly held misconception that higher Octane fuel slows down the flame speed, which keeps the engine from knocking. What a load of crap.
    A fuel's laminar Flame speed (basically fuel burning without the influence of combustion swirl or tumble) is a function of fuel chemistry (specifically Hydrogen/Carbon ratio), not the Octane rating.

    The Hydrogen/Carbon make up of the fuel will determine it’s flame speed and the energy release whether it's a high-octane fuel or not. You can have fast and slow burning high octane fuels, but in most cases the differences are so slight as to not be an issue.



    The heat of combustion at a simple level is basically a function of combustion efficiency and the calorific (energy) value of a fuel. There isn't a lot of energy difference between most conventional fuels, but combustion efficiency can sometimes be improved with race fuels so a bit more heat release is possible. Overall it's pretty much a wash in most cases though.

    In other words, you aren't going to change the energy release or the speed of combustion in any significant way by switching from 87 to 110 octane gasoline, or anything in between . If you lose or gain power by switching fuels there are other things at work.

    The way octane influences knock has to do with the way fuel is burned (well it’s really REACTED in the chamber) in the combustion chamber. The components in higher-octane fuels are less likely to have weak molecular bonds that can break easily and form active radicals. Higher-octane fuel is just made up of components that are (for lack of a better term) stouter under high temperatures and high cylinder pressures. Things like our long lost friend lead and metallic additives like MMT (common in octane boosters) tend to work as an anti-catalyst and block the formation of the active radicals that lead to auto-ignition of the fuel and ultimately detonation. The longer it takes to react all the fuel the greater the chances are that heat will accelerate the reactions beyond what the fuel components can bear. The idea that purposely SLOWING things down in the chamber is somehow beneficial in light of the reactions taking place is pretty funny really

    Heat , pressure, and time tend to be the driving force in all of this. The more heat and pressure you subject the fuel to and the longer you keep the temps elevated the better and more stable the fuel you need . That's why engines that run at high rpm and have small bores can run very low octane fuels even with high compression ratios.

    It's just simple chemistry really. If you look at the chemistry of explosives you can see a lot of similar reactions taking place, and it can give you some real insights into what's happening inside the combustion chamber when things go wrong. It's a fine line between engine tuner and explosives engineer.


     
  2. Rich Rohrich

    Rich Rohrich BioHazard
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    Now that the season is in full swing lets push a few threads back to the top and see if people want to contribute to the discussion.
     

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