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3 Responses

  1. Marcus
    Marcus 2014/08/28 at 11:24 am | | Reply

    I think the math is off in your example. You are dividing MON by 2 before adding RON, when you should be adding RON with MON and dividing by 2. You should have 105 octane in the 1:1 example, not 151.5.


  2. Sam
    Sam 2014/10/08 at 7:36 pm | | Reply

    Your math appears to be fine but I want to clarify a few things. The octane rating is really a measure of rate of reaction. It should not be confused with the fuel having more energy (“insane horsepower”). Octane and energy have nothing to do with each other.

    Ethanol has significantly lower energy than gasoline but has an octane rating of 118. It burns much slower. The same it true for Xylene. Xylene has fewer hydrogen atoms and more double bonds and thus lower heat of combustion. Its branching makes it difficult for random collision with oxygen to access some of the more protected bonds is less prone to rapid reaction.

    You can definitely go too far with Octane and you WILL lose power because your timing will be off. Ideally you want your flame front to be fully developed at about 15 degrees after TDC. To do this, ignition can take place as much at 40 degrees BTDC. Increasing Octane to a point where the computer runs out of timing could easily make it impossible to realize your maximum pressures and cost you power as well as leave uncombusted hydrocarbons in your exhaust.

    You would need to advance your timing more to make use of the slower reaction as the flame front may not develop sufficiently soon after TDC. Most vehicle computers keep two sets of tables for spark timing and interpolate between those tables to eliminate knock (a supersonic flame front). The high octane table in your PCM is only looking for about 91-93 octane. Any more than that and you have a problem. Your fuel is simply burning too slowly.

    Now if you have a turbo application you are likely either messing with those numbers or your turbo is causing things to burn more quickly anyway because your intake air temperatures are high. Temperature can also affect rate of reaction (higher temperature = higher reaction kinetics).

    The effect of temperature on reaction kinetics is well characterized by the Arrhenius equation. In that case, having high octane can definitely be used to allow you to burn more fuel and push the engine much harder.

    All in all, I would probably avoid Xylene. It’s too expensive. I would go for Ethanol or Methanol. Yes, they have lower energy but who cares, just shoot more in and they are a lot cheaper. E-85 blends try to do their best to reduce their octane by using n-octane as opposed to ISO-octanes or more branched hydrocarbons in an attempt to keep from having an octane rating that is too high.

    For the average person reading this email, please know that if your octane is too high, you may well end up with more uncombusted hydrocarbons that make it to your catalytic converter. In some cases, this can actually melt your cat. I’ve seen them glow red when too much uncombusted fuel makes it to them.

    Chemical Engineer.

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