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Re: Fuel question

On 15 Jul 99, at 8:52, Michael Geurink wrote:

> With my aircooled aircraft mechanics hat on I fully concur with Jim.  Take
> the octane number on the pump as directly related to flashpoint.  The lower
> the number, eg 87, the lower the temperature and/or pressure the fuel will
> spontaneously ignite.  That's why we flyguys use 100/130 & occasionally 140
> octane fuel. 

It is my understanding that avgas uses a completely different rating 
system for octane, so you cannot compare the octane numbers. I have 
NO idea why this is so.

> Actually it's usually not even the octane content that is
> referred to with the number.  It used to be but it's more efficiently done
> by other chemicals now.  Pumps by us here in no-VW-show-country (Indiana)
> declare the number on the pump to be an anti-knock rating rather than the
> popular octane nomenclature.

Under the automotive system the octane rating is measured by 
comparing the resistance to knock of a particular fuel to that of 
pure Octane (straight chain 8 carbons, 18 hydrogens.) If they are the 
same then the fuel is given an octane rating of 100.

It turns out that there are 2 popular ways to make this comparison 
and they end up giving different numbers. The 2 are called the RON 
(Research Octane Number) and the MON (Motor Octane Number.) The 
number that is usually put on the pump in the US is the average of 
the 2 = (RON + MON)/2, but the number that VW specifies as required 
is the smaller of the 2, so this explains why we can almost always 
"get away with" a couple of % less than what VW seems to ask for.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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