I have some questions about the auto-ignition temperature for fuels. Per ASTM E659:
AIT (auto-ignition temperature) is the lowest temperature at which the substance will produce hot-flame ignition in air at atmospheric pressure without the aid of an external energy source such as spark or flame. It is the lowest temperature to which a combustible mixture must be raised so that the rate of heat evolved by the exothermic oxidation reaction will overbalance the rate at which heat is lost to the surroundings and cause ignition.
The last sentence seems to say that AIT is the temperature where enough of the particles are above the activation energy such that enough heat released goes to perpetuate the remaining fuel to combust (cause ignition). Is this correct?
Per ASTM E659, ignition is subjectively defined as the appearance of a flame with a sharp rise in temperature. Why it necessary that we observe the appearance of a flame (in other words, can we have combustion without an observed flame)?
Is there an assumption of thermal equilibrium between fuel and oxidizer when defining AIT? The testing method above says air at predetermined temperature will be used, but I'm not sure how they plan to control air temperature when heating begins.