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After many years of study, catapillar engines and waukesha engines use a modified octane calculations for fuel mixtures, all emperical and not related to autoignition
patprimmer (Automotive)
4 Jun 07 20:59
Alcohol being methanol or ethanol will run with quite rich mixture. This rich mixture cools the charge by evaporative cooling so offsetting some of the adiabatic temperature rise.
Also the engine may have less than 100%Ve and as you say, late intake valve closing will reduce effective CR.
Heat added from and lost to the block and head surfaces will also effect charge temperature as in the early part of the cycle the charge will gain heat, but at a point in the compression stroke it will start to lose heat.
I have seen over 20:1 on methanol. I have never pushed ethanol to the verge of detonation.
Regards
patprimmer (Automotive)
8 Jun 07 18:50
Octane rating is simply a measure of knock resistance. The mechanism of the knock is not considered.
There are a whole host of other factors that effect the onset of detonation.
Regards
6.2 ...
Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used.
...
What is the octane rating of E85 compared to gasoline?
Regular unleaded gasoline has an octane rating of 87; E85 has an octane rating ranging from 100-105 making it a high performance fuel. Ford FFVs produce a 5% horsepower gain when using E85.
The researchers solved the knocking problem by injecting into combustion chambers precisely controlled amounts of ethanol at moments when the engine is working hard enough to cause knock. Compared with gasoline, ethanol has higher octane, a rating of how much a fuel can be compressed before it combusts spontaneously, that is, before it causes knocking. The injected ethanol also cools the mixture, so it effectively increases the octane rating of the fuel mix to about 130 -- as good as high-performance racing fuels, Cohn says.