I'm glad somebody remembers something from Organic Chem.

There is no octane in fuel, please READ my previous post. In my previous post, it tells how the "RESEARCH" octane number is established (by comparing the fuel to a mixture of isooctane which has an octane number of 100, and heptane, which is assigned an octane number of 0), then on Lycoming-8's post it explains how the MOTOR number is established, and the average of both of these numbers is how the "Octane" number posted on the pumps is derived.
By increasing the ratio of isooctane in a combination of isooctane and heptane, the "octane rating" of that mixture is higher than if the ratio of isooctane is lower, that's all part of the "research method", but they don't actually PUT the isooctane into the fuel, this is only the mixture used in comparison with gasoline.
They used to use tetraethyl lead to increase octane, now they have to use aromatics or some other stuff, it's all explained in the first link I put up if you care to read it.

Gilly

Great thermo Eric, you a ChE? Believe that a lot of the confusion about "octane" in the general public and possibly some not heard from on this board is the way octane and "Octane Number" are so freely interchanged. Also of interest to us dieselers is that in a gas engine besides the compression heat and spark, there is the hot glowing carbon from a not clean combustion chamber. Hence the tendency of hi-compression engines to "diesel" on, even though they have been turned off at the Ign. switch. That said, does anyone remember how the "Cetane Number" is calculated or derived for diesel fuel?

If you must know . . .
http://www.chevron.com/prodserv/fuels/bulletin/diesel/L1_toc_fs.htm
A pretty good website loaded with information on just what is "standard" diesel fuel.

There is octane - that is, molecules made up of chains of eight single bonded carbon atoms attached "end to end" and surrounded by hydrogen atoms - in gasoline, there's no doubt about it. For the reasons Eric all ready has mentioned, the distillation process is skewed to produce arrangements of 8 carbon atoms other than "end to end" like isooctane (which is a string of 5 carbon atoms attached "end to end" and the remaining 3 carbon atoms are attached to other carbon atoms in the string). The point is that while the process is designed to mainly create the branched compounds like isooctane, there still will be a certain, fairly small number of "straight-line" carbon chains like octane - perhaps 4-5% percent - so it isn't correct to say there is NO octane is gasoline. There is very little unbranched octane, and it is not added to gasoline to increase the "octane rating" which as you correctly point out is a different thing altogether. This is a very rough, and imprecise explanation, but I hope it gets the point across.

Gee, I go away for a few days and an organic debate breaks out. I guess I need to run a gas sample on the GC to be sure, but I would surely think gasoline has octane in it. Now, if I only had a gas vehicle from which to take a sample and an explanation for my supervisor when she asks me why I am running gasoline on the GC...

Gilly:

There is plenty of octane (in various isomers) in gasoline. Exact amounts and ratios will vary from mix to mix and source to source, as will as by manufacturer. Gasoline normally ranges from C1 to C8, with the largest amounts in the C4-C5 range. Hot weather blends will be heavier, cold weather blends lighter.

There shouldn't be much if any octane (of any isomer) in diesel fuel, it ranges from C12 to C32, more or less, again depending on source and manufacture.

Octane and cetane ratings are an index of knock resistance (octane) or ignitability (cetane) RELATIVE to 100% octane or cetane.

That means a gasoline blend with an 89 octane rating has equivalent knock resistance to a straight distallate blend containing 98% pure octane (dont' remember if it is n-octane or a specific isooctane). You can have fuels with considerably more knock resistance -- aviation gas is a prime example, often rated 120/140 octane for sea level non-turbo/high altitude turbocharge use. Gets complicated, as there are several methods for determining "octane rating" -- caluclation (the R in R+M ratings), open combustion, or quartz window engine (the M in the R+M ratings), and actual operation in standard vehicles. A number of additives can be used to raise the octane rating (tetra ethyl lead was the cheapest and most common), so this isn't pure applied organic chemistry by any stretch.

The usual pump rating (required by law in Indiana) is the average of the research and motor octane ratings.

Cetane ratings are similar -- running characteristics of the fuel compared to straight cetane vs cetane blends of a specific synthetic fuel mix (synthetic in that the composition is known). I'm not sure how cetane ratings are done, probably mostly by boiling point range and combustion engine tests.

The plate is right, no octane to speak of, even with kerosine in the tank -- wide cut kerosene may have some.

JetA and JetB will have considerable octane. JP4 (wide cut gasoline) certainly would, but I don't know if JP4 is produced any longer (I've heard various reports on this, but my standards supplier can't make it for me).

Peter

OK, thanks for the clarification.

I also read they no longer make JP4, and that originally they were using the wide-cut fuels like that because they thought they would be readily available, plus for the jet aircraft they were intended to be used in, they though gelling would be less of a problem. One of the big stoppers on JP4 I heard was becuase the Navy thought it was a little too hazardous for aircraft carrier use, had a few accidents I guess, burns pretty good when that happens.

Gilly:

I spent about a month a few years back attempting to identify an unknown fuel like material found floating after a flood. Had a very strange hydrocarbon distribution (may have been lighter fluid, never though to check that), but I did look at all the readily available fuels including the jet fuels at the airport.

In spite of strong insistance that it HAD to be JP4 because the local independent outfit at the airport had a military refueling contract, I couldn't find any JP4, just Jet A.

Anyway, JP4 is "wide cut gasoline" -- straight distillate extending to about C24. Nasty stuff, since the light fraction makes it just as flamable as gasoline. There were a number of accidents with commerical aircraft for which there was strong evidence that a less volatile fuel would have gone a long way to prevent (Pan Am crash in Maryland where an internal tank explosion blew a wing off a Boeing 707, a hanger explosion at KLM were a DC8 had been fueled on a previous flight with JP4 -- sparks from a live wire during a fuel pump change caused an explosion and fire). Navy stopped using it years ago, very dangerous, Air Force used it for a long time (cheaper), some commerical outfits used it till the 70's. It does, however, not suffer from gelling until about -40C or colder, so I guess it might still be around for Arctic use.

No one would use it now, burns too dirty on top of being an uneccessary hazard.

Peter

My understanding is that JP4 is a "wide cut" somewhere between gasoline and kerosene or diesel fuel, not really either one, just in between.

Gilly

Gilly:

I believer "wide cut" means wide boiling point range -- the smaller the hydrocarbon molecule, the lower the boiling point -- therefore "wide cut gasoline" in the 1950's meaning would be all the distillable "top cut" down to the boiling point of C24 or so. No other refining to speak of. A better description would be a mix of gasoline, kerosene, and diesel fuel.

Just like gasoline in the 50's was mostly just what boiled off crude oil to the boiling point of hexane (C6) or octane (C8) with some additives (mostly tetraethyl lead). Modern engines wouldn't run on the stuff.

Modern fuels are much more "refined" in that the composition is controlled, the hydrocarbons are "cracked" to give more branched and unsaturated compounds (double bonds instead of single bonds, less hydrogen).

Except US diesel fuel, that is. US diesel fuel is just C12 to C32 boiling point range distillate, plus some additives (I got this information confirmed by the chemist at the local refinery --Shell if that means anything). Pretty crude, and one of the reasons US deisel engines smoke so much. Probably will be changed in the future, since taking the sulfur out requires considerably more chemical manipulation of the hydrocarbons. Need a couple tank cars of molten sulfur?


Peter

I'll pass on the sulphur, trying to quit;) .
What I had read refers to "wide cut" as in the distillation process, you start at the bottom with the heavier fluids, like bitumous tar and bunker oil, then work on up through the heavier stuff, and the JP4 came out of the distillation process somewhere between gasoline and kerosene/diesel fuel. They thought it would be more readily available in time of war, ie it's not gasoline, so not taking away from gasoline production, and not diesel fuel.

Gilly