# Additional Questions Regarding the Auto-Ignition Temperature

I had some followup questions regarding a previous post I made here regarding the auto-ignition temperature and ASTM E659

1. For fuel temperature below AIT, we should still have finite reactants above activation energy, reacting, and heating the remaining mixture so additional reactants are above the activation energy. Theoretically as $t \rightarrow \infty$, could the fuel burn itself out like this, regardless of what temperature it is at (obviously the rate will be very different)? If so, is there an implicit rate requirement in defining the AIT that the above chain reaction process has to happen within a short duration?

2. Given that ignition is defined as when a flash and temperature rise is seen, does this mean that the actual quantity of fuel burnt is unimportant and it is assumed that a significant enough amount is used?

3. Is the reason for using an open flask in ASTM E659 likely just for simplicity, as opposed to a piston-cylinder arrangement that offers more control over air-fuel mixture?

• I don't have time for a proper answer, but in 1, it sounds like you've got the right idea. Think of paper browning; over centuries it will oxidize into brittle, ruined material, essentially burning to completion at room temperature air. Here the time limit they establish for a flame to appear is explicit, 10 minutes. They do state that some compounds have a longer delay at a given temperature before they ignite. The reaction rate you refer to would be different for each substance, since each is releasing a different amount of energy. – Jason Patterson Oct 15 '14 at 12:06

• Regarding (1), given infinite time, every finite kinetic barrier will eventually be overcome at any finite temperature, no matter how high the barrier is, or how close to $0\ \mathrm{K}$. For example, it would take at most approximately $10^{1500}$ years for nuclei to spontaneously fuse into iron. Any chemical reaction would have a much, much lower kinetic barrier, and would happen in a much shorter timescale. Whether a puddle of fuel inside an atmosphere of oxygen at $300\ \mathrm{K}$ could have had enough time to oxidize completely during the current lifetime of the Universe, I don't know. – Nicolau Saker Neto Jun 6 '15 at 3:23