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WARNING:  There will be a quiz at the end of this article!


    Lets talk about gasoline and additives to increase octane ratings.
MYTH:  HI-Test (high octane) gasoline burns faster and is more powerful
than regular.

Gasoline is composed of a series of straight chain hydrocarbons.  Usually
these are in the 7 to 9 carbon atom length chains.  

FACT:  Straight chain molecules burn faster than branched chain ones.
          In fact the higher the degree of branching the slower the molecule
burns.  Lets look at the specific molecules now.

 The standard for an octane rating of zero is n-heptane (normal heptane).

and it looks like  C-C-C-C-C-C-C. (I leave it for the interested student to fill
in all  the hydrogen atoms.  This consists of 7 carbon atoms in a straight line.
This burns very fast and thus detonates under compression BEFORE the  spark plug
has a chance to fire.  This is PRE-IGNITION and is the cause of valve ping.
To reduce the burning time and ingition point to the place where
the spark plug will fire the gas (instead of the compression), we add branched
chain molecules.

The standard for 100 octane rating is 2,2,4,4-tetramethylpentane, and it looks
like:

             C   C
             |   |
           C-C-C-C-C
             |   |
             C   C
Notice that it is the most highly branched (substituted) molecule you can get
in this chain length range.

NOW THE PROBLEM:  Crude oil consists of mainly straight chain molecules,
few branched chain ones, and  fewer aromatic ones. SOLUTION: During the refining
process, make more branched chain molecules.  This is called "cracking" because
it makes smaller branched chain molecules out of bigger ones.  Unfortunately
this uses special catalysts (sp?) and is  therefore EXPENSIVE. CHEAPER SOLUTION:
Add a compound that will  retard the flame front like tetraethyl lead.

                C-C
                |
           C-C-Pb-C-C
                |
              C-C

 Now we see that "high test" burns slower than regular to give a more
controlled flame propagation velocity and prevent detonation.
Low octane gas burns too fast a must be retared by the addition of
compounds like tetraethyl lead or branched chain hydrocarbons.
Problem: The EPA has now virtually abolished the use of lead.  Thus we no longer
have the really high octane levels of the "muscle car" days.  Remember that
pre-ignition occurrance increases with increasing compression, so the high
compression levels of  the GTOs etc. required high-octane-rated gas.

Now, with the gas-oil crisis of the mid 70s we reduced the compression levels
to allow cars to run on cheaper (low octane rated) gas.  Very little
high octane rated gas is being produced today.

THIS IS THE QUIZ:

     TRUE or FALSE :  Adding a few gallons of high octane rated gas to
a tank full of  regular unleaded will increase the octane to a level higher
than with either one alone.

     T ____     F ____

ANSWER:
    Since the use of lead is out of the question, what has to  be done is to
increase the total number of branched chain molecules to the point where
there is a high octane rating.  Adding just a few gallons of high test
to a tank of regular will not add enough molecules to raise the octane
rating significantly.  The effective octane rating can be calculated by
an algebraic summation of the amount of gas.
If I add 15 gallons of 88 octane to 5 gallons of 92 octane, the new
rating is (15*88 + 5*92)/20 = 89.  Not much improvement is it?
So,  the answer is false.  The other alternative is to drastically reduce
the flame front propagation velocity by adding really slow burning compounds
like aromatic molecules.  ie. benzene and naphthalene.  We will save that
for a future article on gasoline, diesel fuel, and aromatics.  

Hope this helps clear up a lot of the confusion over how gasoline burns
and how we get high octane rated gasoline.

                    From the resident chemist at: Dan's English Car Garage
                                                  akgua!dlp

 Contrary to popular belief, Lucas was not the inventor of darkness.