[Diy_efi] Timing Advance Curve?

[Mike]

At 01:29 PM 19/12/2002 -0800, you wrote:

Nice piece of detailled info on the chemistry Brian,
I guess with transparent chambers, high speed camera
and spectral anaylsis these species can be straigthforward
to locate and observe the dynamics...

tah

mike



>> 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I suggest the Gasoline FAQ for anyone interested, it's a tad outdated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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>

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