[Diy_efi] Timing Advance Curve?

[Brian Dessent]

Mike wrote:

> At 08:35 AM 12/19/02 -0500, you wrote:
> >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.
> 
> Daniel,
> 
> You are wrong on all points, I take it you either havent been on this
> list for long or havent read anything about octane, what it is or how
> its measured...


Correct.  Octane ratings don't correspomd with flame speed or energy
content.  It has everything to do with what happens to the charge that
has not yet been reached by the flame front.  

I suggest the Gasoline FAQ for anyone interested, it's a tad outdated
(mid-90s) but it still has tons of info.  I'll include the relevant
excerpt from it below.

http://www.faqs.org/faqs/autos/gasoline-faq/part1/

6.3  What fuel property does the Octane Rating measure?

The fuel property the octane ratings measure is the ability of the
unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand  
pre-flame conditions without decomposing into species that will
autoignite 
before the flame-front arrives. Different reaction mechanisms, occurring
at
various stages of the pre-flame compression stroke, are responsible for
the 
undesirable, easily-autoignitable, end gases.

During the oxidation of a hydrocarbon fuel, the hydrogen atoms are
removed 
one at a time from the molecule by reactions with small radical species
(such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen
bonds depends on what the carbon is connected to. Straight chain HCs
such as
normal heptane have secondary C-H bonds that are significantly weaker
than
the primary C-H bonds present in branched chain HCs like iso-octane
[21,22].


The octane rating of hydrocarbons is determined by the structure of the 
molecule, with long, straight hydrocarbon chains producing large amounts
of 
easily-autoignitable pre-flame decomposition species, while branched and 
aromatic hydrocarbons are more resistant. This also explains why the
octane
ratings of paraffins consistently decrease with carbon number. In real
life, 
the unburnt "end gases" ahead of the flame front encounter temperatures
up 
to about 700C due to compression and radiant and conductive heating, and 
commence a series of pre-flame reactions. These reactions occur at
different 
thermal stages, with the initial stage ( below 400C ) commencing with
the 
addition of molecular oxygen to alkyl radicals, followed by the internal 
transfer of hydrogen atoms within the new radical to form an
unsaturated, 
oxygen-containing species. These new species are susceptible to chain 
branching involving the HO2 radical during the intermediate temperature 
stage (400-600C), mainly through the production of OH radicals. Above
600C, 
the most important reaction that produces chain branching is the
reaction of 
one hydrogen atom radical with molecular oxygen to form O and OH
radicals.

The addition of additives such as alkyl lead and oxygenates can 
significantly affect the pre-flame reaction pathways. Antiknock
additives 
work by interfering at different points in the pre-flame reactions, with
the oxygenates retarding undesirable low temperature reactions, and the
alkyl lead compounds react in the intermediate temperature region to 
deactivate the major undesirable chain branching sequence [21,22]. 

The antiknock ability is related to the "autoignition temperature" of
the 
hydrocarbons. Antiknock ability is _not_ substantially related to:-
1. The energy content of fuel, this should be obvious, as oxygenates
have 
   lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this should be
   evident from the similarities of the two reference hydrocarbons. 
   Although flame speed does play a minor part, there are many other
factors 
   that are far more important. ( such as compression ratio,
stoichiometry,
   combustion chamber shape, chemical structure of the fuel, presence of 
   antiknock additives, number and position of spark plugs, turbulence
etc.)
   Flame speed does not correlate with octane.

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