Hydrocarbons do have a property called 'autoignition temperature' - you can find the value on a material safety data sheet. See section 5 of this MSDS for diesel fuel.
http://www.petrocard.com/Products/MSDS-ULS.pdf
The 'fire triangle' model has been replaced by the 'fire tetraherdon' model in recent years.
http://www.firesafe.org.uk/html/miscellaneous/firetria.htm
This site gives a decent explanation on the difference between 'flash point' (external ignition source required) and (auto)'ignition temperature' (no external ignition source required). The graphic representation of a tetraherdon however is if you cut the seams and laid it flat (2D instead of 3D).
http://www.firefightercentral.com/public/what_is_fire.htm
I don't own diesel powered vehicles, but I have rented and driven a few. Here's my understanding. The glow plugs are heated to the diesel fuel autoignition temperature when you turn the key to the 'run' position. Once the glow plug indicator light goes out, the glow plugs have reached the autoignition temperature and the key can be turned to the 'start' position to start the engine. Once the engine starts (and the glow plugs are off), you are relying on autoignition temperature to keep the diesel engine running - some is supplied by the heat of compression, some is supplied by residual heat from the previous cycle.
I have some experience with industrial diesel engines without glow plugs - such as oil barge liquid transfer pumps. These had a lever where the engine operator had to 'pump up' a reservoir to enough pressure to get the engine to start. This isn't really user-friendly, but can save you some health club fees!
If you are looking at what temperature a puddle of diesel has to reach before you can light the evolved vapors with an ignition source, that is the flash point temperature. Much lower than the autoignition temperature.
Cetane rating for diesel fuel is somewhat directionally opposite octane rating for gasoline fuel (used in spark ignition engines). Higher octane rating indicates greater resistance to ignition before the spark plug fires - engine 'knock' results when the fuel ignites prematurely in the cylinder - during the compression stroke instead of close to TDC to deliver the combustion energy in the power stroke. Higher diesel fuel cetane rating indicates lesser resistance to ignition and increased ease of use in a diesel engine.
http://www.procarcare.com/icarumba/...a/icar_resourcecenter_encyclopedia_fuels1.asp
http://www.familycar.com/Classroom/fuel.htm
http://www.itow.org/fuelguide.htm