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Gasoline fuel vaporization

  1. Aug 11, 2006 #1
    I'm trying to learn about how differing distillation curves affect gasoline's performance. I'm coming at this from the perspective of a performance engine enthusiast who uses racing fuels. My research thus far indicates typical pump fuels have a 90% distillation temp in excess of 300 degrees F. 100LL avgas has a 90% figure of 262 deg F. Many racing fuels have a corresponding dist temp below 230 deg F.

    In trying to understand how these values change the deflagration process of the fuel in an operating gasoline engine I realized I don't know exactly when fuel vaporizes within the engine. I suspect that some components of the fuel (lighter molecules) vaporize shortly after the carburetor atomizes the fuel and if begins to enter the hot engine. Heavier molecules may not vaporize until combustion has actually started--the heavy molecules need more heat to vaporize.

    I understand that the heavy hydrocarbons vaporize at higher temps and the lighter hydrocarbons vaporize at lower temps? Is that right? Do fuels with more light hydrocarbons have faster flame speeds because they vaporize at lower temps (and therefore more quickly--not having to wait to be heated up)??

    I found one engine builder who says this about fuel (in reference to the use of avgas):
    "...Normal ground level race fuels are made up of gas molecules that have a "light end" and a "heavy end". The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process..."

    Is that right? By light end and heavy end is he talking about heavy and light hydrocarbons? If he is, then why do many racing fuels have dist curves indicating far fewer heavy molecules that 100LL avgas?

    Sorry for all the questions...I'll appreciate any insight into these matters you may be able to provide....
  2. jcsd
  3. Aug 11, 2006 #2


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    I cannot help you too much here. Anyway, don't talk about carburettors. No modern car has carburettor. The injector is located either inside the chamber (Direct Injection) or in the inlet manifold after the bifurcation to the cylinders. In this last case, the injectors are designed such that the gasoline is ejected towards the admision valve surface, which turns out to be really hot, and helping for the vaporization of fuel.
  4. Aug 11, 2006 #3
    Thanks for the reply. However, carburetors are relevent to me. Nearly all off-road motorcycles still use carburetors. All of the engines in which I am making specific fuel choices use carburetors.
  5. Aug 11, 2006 #4


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    "Light" and "heavy" ends of the distillation process; the "light" end being the early, low temp fraction taken off the column, and the "heavy" end being the later, higher temp fractions. Gasoline is blended from the various cuts, or fractions, which hopefully have reasonably consistent compositions and properties from one feedstock to the next (which is a vain hope), and as of ten years ago, the number of identified compounds had hit the thousand mark, or thereabouts, but no one's tried to do a total assay --- variations from one feedstock to the next, or from one period of a single well's production history to the next, make such an exercise a waste of time. The blends are "designed" to satisfy various end uses; seasonal performance (extra "light" ends for winter use), military use (avgas --- don't care if the engine is burnt to a crisp at 100 hrs.), agricultural use (if the engine runs forever at a quarter the hp to weight ratio, so what), automotive, racing, industrial ----
  6. Aug 11, 2006 #5
    I see. "Light" and "heavy" ends of the distillation process, not "light" and "heavy" components of individual hydrocarbons.

    Is it accurate that heavy hydrocarbons burn less easily? If so, does that hold true even when both varieties have been fully vaporized? In other words, do heavy hydrocarbons burn less easily soley as a function of their greater resistance to vaporization?

    Are heavy hydrocarbons responsible for "...the hotter, more powerful part of the combustion process..."?

    Thanks again...
  7. Aug 11, 2006 #6


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    Yes, and no. You wanta burn a puddle of candle wax, yes. You wanta burn a fine mist in air, no.

    'Nother less than unambiguous answer: assuming complete combustion, you need one O for every two H, and two for every C in the molecule; you pick those up through collisions with oxygen molecules, radicals, and ions in the flame; the number of collisions required for "heavies" is greater than that for "lights;" you've got 1-10 milliseconds to accumulate the necessary number of collisions for each shot. So, "heavies" take longer to burn completely even when fully vaporized.

    Hotter? No. More energy release per molecule? Yes. More energy per pound? Not significantly.

    Longer burning time contributes more power per stroke compared to a short detonation at, or just before, TDC? Yeah --- you get near zero from a dieseling gasoline engine --- so, "heavies" for lower rpm uses (tractor engines), and lights for chain saws, go-karts, motorcycles, and such.
  8. Aug 11, 2006 #7
    Alright, thanks. Let me abosorb that and I'll likely be back soon with more questions.

    Thanks again...
  9. Mar 5, 2011 #8
    I was searching for information concerning the properties of gasoline, when my search
    led me to this forum. I have just begun searching for information which will help me to efficiently vaporize gasoline. I belive that the additives , which the oil companies have engineered,make the vaporization process more difficult, to prevent a process from being developed which would greatly improve the mpg yield. The current price rise could become permanent, we have several years left with the current technology. We will have to make an adaptation to survive the transition years with current technology.I believe good Physics will make that possible.
  10. Mar 6, 2011 #9


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    1) Check the dates of posts before replying. This one is 4.5 years old.

    2) Post something relevant, other than speculation, or to back up your speculation. How you you believe "vaporization" will "improve the mpg yield" and how do you plan on vaporizing the fuel?

    Edit- I guess I didn't initially include your wild speculation that "the oil companies" are directly inhibiting the "vaporization process" with their additives. Personally, I think you post should be reported, but the stipulation under the Report action makes me wonder if it falls under that category. It certainly doesn't meet the required guidelines for posting in this forum.

    3) You don't know what the price [of gasoline] will do, nor how much time we "have left with the current technology."

    4) Believing in something without basis does not make it any more possible.
    Last edited: Mar 6, 2011
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