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UFO,
All liquid fuels depend on vaporization pressure to some degree. Its the vapors that mix with the air exposing it to O2. If the conditions are just right you can put a match out in gasoline (don't try it). Diesel has a much lower vapor pressure than gasoline, and has a much wider range of where you could put a match out in it(still don't try it).
You are correct that its the compressed hot air that causes it to ignite. You are also correct that fuel is injected in a manner to facilitate this process. However if Diesel fuel is cold enough its viscosity rises to the point it doesn't atomize well. The larger droplets have a much greater mass to surface area than a fine mist does. The larger droplets don't heat quickly enough to burn before they hit and stick to a combustion chamber wall. When this happens they are deprived of adequate O2 exposure to burn properly.
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