REFLECTOR: The definitive answer on octane and av-gas

[Donald Royer]

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 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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