Re: Premium Fuel

From: JT McBride (James.McBride@GDEsystems.COM)
Date: Tue Oct 01 1996 - 18:41:21 EDT


>Yes. Actually 'octane' is a type of hydrocarbon molecule. Others are
...
>100% pure octane has an octane rating of 100. An octane rating
>of 89 is equivalent to a mixture containing 89% octane. Pretty

Ahh, chemistry. As I'm sure most of us are aware, the 'octane rating' on pump
gas is the average of "research octane" and "motor octane". Octane is eight
carbon atoms with single chemical bonds, but there are many possible isomers
(arrangements of the carbon-carbon bonds) possible, which varies the number of
hydrogen atoms in the molecule. Maybe someone can clarify whether there is a
particular isomer of octane with a research octane of 100, or whether there
is a mixture of isomers that most commonly occurs or is standardized to achieve
this value -- at any rate, I believe 100 research octane has a less than 100
motor octane rating.

Stuff from that excellent FAQ (see: www.smartpages.com/faqs/autos/gasoline-faq/top.html ).

The way the hydrogen and carbons hold hands determines which hydrocarbon
family they belong to. If they only hold one hand they are called
"saturated hydrocarbons" because they can not absorb additional hydrogen.
If the carbons hold two hands they are called "unsaturated hydrocarbons"
because they can be converted into "saturated hydrocarbons" by the
addition of hydrogen to the double bond. Hydrogens are omitted from the
following, but if you remember C = 4 hands, H = 1 hand, and O = 2 hands,
you can draw the full structures of most HCs.
 
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. Detailed descriptions of structures
can be found in any chemical or petroleum text discussing gasolines [11].
 
4.4.1 Saturated hydrocarbons ( aka paraffins, alkanes )
 
- stable, the major component of leaded gasolines.
- tend to burn in air with a clean flame.
- octane ratings depend on branching and number of carbon atoms.
 
alkanes
  normal = continuous chain of carbons ( Cn H2n+2 )
  - low octane ratings, decreasing with carbon chain length.
 
    normal heptane C-C-C-C-C-C-C C7H16
 
  iso = branched chain of carbons ( Cn H2n+2 )
  - higher octane ratings, increasing with carbon chain branching.
 
    iso octane = C C
    ( aka 2,2,4-trimethylpentane ) | |
                                     C-C-C-C-C C8H18
                                       |
                                       C
 

-- This is a cool FAQ, thanks for the pointer.

Jim

Need start speak like Dole, Bob. Next President, United... of America, of America!

 



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