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Octane Numbers

  1. Jan 24, 2008 #1
    I just learned octane numbers in organic chemistry a few days ago, and I'm extremely confused about it. From what I understand, the octane number indicates resistance of a motor fuel to knock, and "knock" is the tendency for the fuel to detonate before hit by spark. Is there an easier way to understand this? Right now I'm not getting it at all.

    Also, from our class example, we were told that n-heptane has an octane number of 0 while 2,2-trimethylpentane is 100. Other than being told this, how do we judge a compound's octane rating if we're only given the structure or formula? Are there criteria that make something's rating increase/decrease?

    Thanks in advance. =)
     
  2. jcsd
  3. Jan 24, 2008 #2

    chemisttree

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    Octane number is a measurement made in a special engine. It corresponds to the speed of the fuel's flame front (or the speed of ignition) relative to the position of the piston in the 'special' engine. If the flame front is too fast, the pressure in the combustion chamber peaks too early and pushes down on the piston before it achieves it's top dead center position. This condition manifests as a 'knock' in the engine. Slowing down the flame front speed allows the max pressure in the cylinder to occur later in the stroke (perhaps at or beyond top dead center) and results in a smooth ignition cycle... no knock. During the octane determination, the engine's stroke is changed to vary the compression ratio until the timing of the max pressure just begins to cause that knock. The octane is determined by comparing that timing measurement to a series of standard fuels containing increasing amounts of isooctane (80 octane = 80% isooctane and 20% heptane, 100 octane = pure isooctane).

    A lot of things (structure-activity relationship) can affect the rate of ignition in the 'special' engine. For example, you will notice that there are 5 primary carbons, 1 secondary carbon, 1 tertiary carbon and 1 quaternary carbon in isooctane. n-Heptane has 2 primary carbons and 5 secondary carbons. Remember that flame is a free radical process.
    How might these structural differences affect the rate of reaction in a free radical process?
    How might the presence of aromatic compounds like toluene or benzene affect the speed of the flame front?
     
    Last edited: Jan 24, 2008
  4. Jan 27, 2008 #3
    Hmm... don't know if you intended the same as I know, but said in that way it doesn't seem completely correct to me. In case, you will correct me.
    AFAIK, if the speed of the fuel's flame front (after the spark ignition) is very fast, you only have to reduce the "advance" (hope my translation into english is correct) of the spark. Actually, a faster speed of the fuel's flame front is preferred because it increases the engine performance.

    The knock instead is a real "detonation" that can happen after or even before the spark ignition, due to the explosive decomposition of the peroxides formed during the air/fuel compression. That detonation is an expansion that happens for a very small time and very high pressures, so it doesn't have time to push the piston, but only to send a shock wave, which results on the various components as a very fast hit with a very light hammer; depending on the amount of this effect you can have just a "metallic" sound, or a fast destruction of your engine.
     
    Last edited: Jan 28, 2008
  5. Jan 27, 2008 #4
    It's been a couple of days and the OP hasn't responded and I want ot know the answer to this! Wouldn't the presence of more subsituted carbs greatly speed up the reaction process by allowing the formation of more stable radical intermediates? And an aromatic even more because it would be resonsance stabilized.
     
  6. Jan 28, 2008 #5

    chemisttree

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    Remember that this test is performed with a specially-designed engine used to do one thing... test octane. Performance is never considered and neither are alternative reasons that can cause knocking in other engines like detonation. Both compression ratio changes and engine timing are used to initiate the knock in the test.
     
  7. Jan 28, 2008 #6

    chemisttree

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    Is a radical intermdiate more stable on less substituted carbons or more substituted carbons?

    ...and structure is only part of the picture.
     
  8. Jan 28, 2008 #7
    Of course it's more stable on the more substituted ones, r groups donate electron density
     
  9. Jan 29, 2008 #8
    The flame front's speed is relevant to have or not knocking, when you set up the "wrong" timing; for this reason you can measure the O.N. in that way. That happens because (correct me if you think I'm wrong) if you "anticipate" the timing too much, then with a very fast flame front you have too high pressures inside the air/fuel mix and this increases the probability and the amount of knocking.

    What I mean is that a fast flame front's speed is not bad "on itself" in a car (or whatever) engine; on the contrary, is preferable for performance; clearly they must have the correct timing, or the knocking would be easier.
     
  10. Jan 29, 2008 #9

    chemisttree

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    That's exactly as I see it.
     
  11. Jan 29, 2008 #10

    chemisttree

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    The more stable radical intermediates also suck up the reactive ones from the air-fuel mixture and actually remove them from the process. Think of it as an antioxidant...
     
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