[Diy_efi] Timing Advance Curve?

[Erik Jacobs]

> Octane has everything to do with the speed of burn.  Higher octane
actually
> controls ping by slowing down the burn.  This is why a very low HP/cubic
> inch engine doesn't necessarily gain power with higher octane.

Now this is something I have a hard time with over and over and over again
in my mind, and we really need a chemist to answer it, but I'll provide as
much insight as I can.

Octane is simply a number that represents a measure which is a ratio of the
different lengths of the hydrocarbon chains present in your fuel.

America uses like RON or MON rating, and Japan uses the other one (wichever
one America doesn't use).  In any case, the number comes from some formula
which looks liks (m + n)/2 or something, whatever, it's unimportant.

Anyway, there is a certain type of hydrocarbon chain (i guess octane, the
longer chain) which takes more initial energy to cause a combustion
reaction.  The reason that higher octane fuels resist ping is not that they
take longer to burn, but that they take more input energy to start the
reaction.  I don't believe that octane has anything to do with burn rate.  I
used to think it did, but when I talked to a friend about it, we realized it
might not.

There is a higher initial energy to START combustion, this is true.  But now
why would the combustion take LONGER?  Unless the entire combustion reaction
(breakdown of C8Hx + all the other funky chains) takes LONGER with the
higher octane number, then there's no reason why the burn should be any
slower.  The flame front will propagate as fast as it can, assuming that all
the reactants are present.  Given a perfectly stoich mixture in the
cylinder, with perfect atomization and uniform blend yadda yadda (a perfect
combustion chamber with a perfect mixture, ideal), then the flame front
should propagate at the same speed regardless of whether or not you had c8hx
or c4hx -- once the reaction is STARTED the process is constant, and I don't
think that c8hx takes any LONGER to react given the right amount of o2 than
c4hx.

Now I could be totally wrong.  If the actual reaction takes longer, then
yes, the burn is effectively slower.  The only way to know if the reaction
takes longer would be to talk to a chemist who could tell you based on lots
of fancy chemistry and thermodynamic things like gibbs energy and entropy
and enthalpy and all that jazz.

Shrug... any thermodynamicists or chemists on the list?? =)


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