Auto ign point of petroleum gases

[AntarcticPilot]

Apologies for stretching this thread to a very obscure place, but forumites might be interested to know that in 1945, when the RAF was removing a number of Luftwaffe jet fighters and prototypes from Germany to Farnborough for evaluation, there was no jet fuel available, so they were initially flown to the Dutch coast on "good quality diesel fuel"!

Jet fuel is, in fact, very close to paraffin. The air-side of airports usually smells just like a Tilley lamp or primus stove! We once refueled a bunch of skidoos with two-stroke engines with jet fuel - the drums we were getting it from weren't clearly marked. The skidoos ran OK, but trailed a cloud of white smoke! As most diesels could run on paraffin, they should do fine on Jet fuel, unless the ECO of a common rail engine decides it doesn't like it.

 

[lpdsn]

I think there is a degree of confusion here between flash point and autoignition point.

Not on my part.

 

[Ian_Edwards]

Thanks to all, I found this thread very informative. I didn't have a clear understanding of the difference between, flash point, flame point and autoignition.
Does the lowering of the autoignition temperature with the increase in the length of the hydrocarbon chain, explain why a box of rags soaked in waste oil and grease will spontaneously ignite?

 

[Heckler]

That seems very likely.

Since the autoignition temp is lower than diesel but no where near as low as ether, it should make it safer than ether esp if the problem is due to some fuel system deficiency. Ether perhaps really only to be considered when the compression is so low that nothing else is going to help.

Indeed.
Stu

 

[Hydrozoan]

... I remember that the auto-ignition temperature for hydrocarbon/air mixtures does decrease with increasing molecular weight and increasing chain length. Pretty certain that it is also higher for branched chain hydrocarbons than straight chain hydrocarbons.

I don't think that I ever got a straight answer about the mechanism, don't think they really knew for certain at that time. I think it related to ease of radical formation by breaking the chain. Longer chain, more places to break and large number of resulting shorter chains with radicals which probably reduce the activation energy for further reaction. I think it was actually rather a complex subject and I didn't follow it up at the time, not much application for type of chemistry I was paid to look into. ...

It’s not my field, but does this paper (see Section 4.1.2 especially) help explain the mechanism? http://www.sciencedirect.com/science/article/pii/S0360128516300570

‘At low to intermediate temperatures, alkyl radical isomerizations ... are important precursors to subsequent chain branching reactions. Only transition state rings, sized within a certain range, namely five-, six-, seven-membered, have an adequately low strain energy that is needed in order for RO2 to form QOOH.

Note that a hydrocarbon with a longer paraffinic chain will have more possible sites for having such isomerization reactions, which explains why longer chains are generally more reactive and have commensurately short ignition delay times. The same logic holds for highly branched paraffins, the comparatively slow reaction rates of which being the result of an intrinsic drop in average chain length for every branch.’