I confirm the high autoignition temperature of hydrogen, higher than most hydrocarbons.
Yes, autoignition is measured with a mix. Often given as a function of the proportion; generally with air, but sometimes with oxygen, where hydrocarbons ignite just a few 10K earlier. The proportion easiest to light is generally near the stoechio in air, that is, making CO2
, and tends to be the chosen condition if not told. And yes, the mix catches fire when reaching this autoignition temperature (air heated by compression in a Diesel engine), or if ignited by some other means (spark in a gasoline engine).
Application: in a gasoline engine, the premix shall not ignite before the spark, so you want a high autoignition temperature, related with a high octane number; highly branched alkanes like isooctane achieve it, aromatics too. In a Diesel engine, the oil shall light easily when injected in the cylinder's hot air, so you want a low autoignition temperature, related with a high cetene number; long straight alkanes achieve it. And with a similar shape, longer alkanes are less volatile (determines much the flash point) but have a lower autoignition, so flame stability in a jet engine results from a mix of molecule sizes, kerosene being one good compromise (bringing also a low melting point and some more).
I haven't checked the detailed reasons for hydrogen's autoignition temperature, but:
- H-H holds with 436kJ/mol, versus variable 400 for C-H and variable 350 for C-C;
- Shocks with the too light H2 could be inefficient.
- Possibly (I'd need to think more at that one) longer molecules with more degrees of freedom can concentrate from time to time more heat energy in few atoms.