REFLECTOR: The definitive answer on octane and av-gas

[Jim Sower]

I STAND CORRECTED. 
The "long strands" I alluded to was more of analogy than detailed 
science and I should have been more explicit around that.  Am I correct 
in drawing from your response that the more complex chains do burn 
slower, and that slower burning equates to higher Octane? 

I'm looking forward to hearing more details about the relationship 
between the numbers we see on Autogas pumps and Avgas ratings.  This is 
IMO important in making a decision

Donald Royer wrote:

> I don't want to get between the two Jims here, and I claim no 
> expertise in the details of the determination of octane ratings. 
> However, as a chemist, I can comment on a few things that Jim Sower 
> has said. Some of what he has said makes no chemical sense at all.
>  
> As a Franklin driver, I personally have no intention of ever using 
> mogas in a Franklin considering the unfortunate incident at Little 
> Rock a few years ago.

I believe I was pretty explicit around Mogas in high compression engines 
like Franklin.

That said, Embry Riddle, who are operating what is arguably the largest 
fleet of recip aircraft in the world, and are in all respects clearly in 
a position to know, have used Mogas exclusively for years and report 
savings as much as $10/hr in Cessna 150s to $35/hr on their 250-300 hp 
engines.  These include large savings in maintenance and engine life.

>  
> I have capitalized my comments.
>  
> Don
>
>
>
>     As for lead and octane, I'm told that Octane is a long string
>     molecule, Heptane not so long, Hextane shorter, etc. and that
>     "octane" rating reflected the percentage of octane in the fuel -
>     100 octane was defined as just that - 100% octane, none of the
>     shorter molecules in the mix.  The molecules "burn" from both ends
>     toward the middle of the molecule, so the longer octane molecule
>     takes longer to "burn" than the shorter molecules so the shorter
>     ones are consumed much faster.  Octane concentration results in
>     slower burning fuel, a slower flame front.
>     FIRST THE OCTANE USED IN THE OCTANE RATING IS NOT THE LONG
>     STRAIGHT CHAIN NORMAL OCTANE, IT IS THE HIGHLY BRANCHED ISOOCTANE
>     ACTUALLY THE CORRECT CHEMICAL NAME IS 2,2,4-TRIMETHYLPENTANE.
>     HISTORICALLY THE UNBRANCHED CHAIN NORMAL HEPTANE WAS ASSIGNED AN
>     OCTANE RATING OF ZERO. TYPICALLY THE HIGHLY BRANCHED CHAIN
>     HYDROCARBONS BURN MORE SLOWLY AND HAVE HIGHER OCTANE RATINGS THAT
>     THE LONG UNBRANCHED CHAINS. THAT IS WHAT THE REFINING PROCESS
>     CALLED REFORMING IS ALL ABOUT. IT CONVERTS LONG UNBRANCHED CHAINS
>     TO SHORTER BRANCHED CHAINS IN ORDER TO RAISE THE OCTANE RATING OF
>     THE FUEL. THE IDEA THAT HYDROCARBON CHAINS "BURN" FROM THE ENDS IS
>     SIMPLY NOT TRUE.
>
>     Lead raises "Octane" number because the Lead in Tetraethel-Lead
>     has four receptors that the fuel molecules bind to.  Since they're
>     bound to the lead molecule at one end, they can't be consumed as
>     fast, burning from "free" end toward the end that's bound to the
>     lead so burning a "bound" fuel molecule takes longer than burning
>     "free" one from both ends toward the middle. 
>
>     All fuel molecules bound to lead sort of "double" their burn time
>     and thus raise the octane of the fuel.  Octane molecules can bind
>     to the lead too, and like the others take longer to burn, and
>     that's how you get numbers over 100 octane.
>
>     I AM NOT AN EXPERT ON THE MECHANISM BY WHICH TETRAETHYL LEAD
>     RAISES THE OCTANE RATING OF GASOLINE, BUT I CAN ASSURE YOU THAT
>     THE DISCUSSION IN THE TWO PREVIOUS PARAGRAPHS IS NOT CORRECT. THE
>     REAL SITUATION IS WAY MORE COMPLICATED THAN THAT. IN THE FIRST
>     PLACE THERE IS NOT NEARLY ENOUGH LEAD ADDED FOR SUCH A ! MECHANISM
>     TO WORK.
>      
>     I CAN ASSURE YOU THAT INTACT FUEL MOLECULES DO NOT BECOME BOUND TO
>     THE LEAD ATOMS BEFORE THEY "BURN". THAT WOULD BE A VERY SLOW
>     PROCESS THAT WOULD REQUIRE A CONSIDERABLE AMOUNT OF ENERGY. THE
>     FOUR "RECEPTORS" ON THE LEAD ATOM ARE ALREADY FIRMLY BOUND TO THE
>     ETHYL GROUPS. THE REAL MECHANISM PROBABLY INVOLVES SOME FORM OF
>     RADICAL CHAIN REACTION TERMINATION. 
>
>     In any event, if I'm misinformed I need to get the straight skinny
>     ... Jim S.
>
>
>
>      
>
>
>------------------------------------------------------------------------
>
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