Dumb Fuel Mixture Question

Discussion in 'Touring Models' started by bobalu, May 30, 2014.

  1. bobalu

    bobalu Active Member

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    Pardon my ignorance (non-mechanic here), but why would a "lean" mixture produce more heat in an engine than a rich mixture? (i.e.: My thought would be that the more fuel in the mixture, the hotter it would burn, no?).
     
  2. gusotto

    gusotto Junior Member

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    Fuel is acually a coolant (might be hard to believe) in the engine.
     
  3. btsom

    btsom Active Member

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    The first answer is quite correct but you might want a bit more. Fuel burning is a chemical reaction of the fuel and oxygen. There is a fixed quantity of each reactant and when one or the other (fuel or oxygen) is used up, any excess of the other reactant goes unused. If all of the fuel is used to make fire (only extra oxygen left over) there is nothing to soak up the heat already made. If there is too much fuel, when all of the oxygen is used up there are droplets of fuel left over and the heat is absorbed as the droplets are changed form liquid to the gaseous state just as your skin gets cooler when water on it evaporates.

    Enriching the mixture is done intentionally under high power situations. When cruising at a steady speed the throttle limits the amount of air going into the engine and the computer meters the fuel for a lean mixture for clean burning and better fuel economy. The amount of heat generated is relatively small when compared to full throttle operation when maximum air and fuel are running through the engine. When the throttle is opened up, more fuel is needed to make use of the increased amount of air. However, even more fuel than needed for proper combustion is added for the cooling described above. Also, as the fuel changes from liquid droplets into gas it expands so there is a slight extra pressure generated to push the piston. This "power enrichment" is usually programed for above 75% of engine output. If you cruise very fast or into a strong headwind, it is possible to run continuously in the power enrichment range. Fuel consumption makes quite a jump at that point. That is one of the reasons that speed is expensive.
     
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  4. bobalu

    bobalu Active Member

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    Thank-you both for these responses, much appreciated.

    btsom, thanks especially for your clear detail (you must be a teacher!). I now have a very good understanding of this. :small3d028: