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Question about Auto-Ignition Temperature

  1. Sep 23, 2014 #1

    I was wondering about explanation of the auto ignition temperature (AIT). From the definition in ASTM E659:
    What I get from the last sentence is that fuel-air reaction at some level occurs at any temperature, but the number of particles reacting is not large enough to produce enough energy for sustained combustion until fuel temperature is above AIT. Is any of the two following reasons for this correct:

    1) Higher temperature means higher reaction rate from Arrhenius's equation

    2) Higher vapour pressure due to the higher liquid temperature

    Is there an assumption behind what the oxidizer temperature for fuel AIT? In ASTM E659, the experiment involves heating a small quantity of fuel inside a large flask of air at a pre-determined temperature, but since they are heating the flask I'm not sure how it is possible to control the temperature of the air.

    Thank you very much
  2. jcsd
  3. Sep 23, 2014 #2


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    Gold Member

    You should check out
    Flash Point - lowest temperature at which the vapours of a fuel will ignite from the introduction of an ignition source.
    Flame point - lowest temperature at which the fuel vapours will continue to burn after being ignited.
    Auto Ignition Temperature - lowest temperture at which the fuel will spontanously combust without introduction of an ignition source.

    Flash point and Flame point can be different temperatures, in which case the vapours at flash point will cease combustion once the ignition source is removed; or similar temperatures in which case the vapours will continue combustion.

    As an example, gasoline and air mixture is compressed in the cylinder of the automobile engine. The compression heats the mixture to a temperature somewhere above the flash point but below the AIT. The ignition source is the spark from the spark plug.
  4. Sep 24, 2014 #3
    I have a couple of questions

    1. Are all of these quantities empirical (i.e. we can't calculate the Flash Point or AIT etc.)?
    2. For Flash and Flame Point, why is the ignition temperature irrelevant, as a higher ignition temperature would add more energy to the system?
    3. When we do tests to determine these quantities, is there an assumption of thermal equilibrium between the fuel and oxidizer/air?
    4. Was my hunch in my OP about AIT correct?

    Thank you
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