octane rating vs the fuel itself in a given condition

Turtle TR

boioing boioing boioing..
Feb 15, 2002
This is prolly like poking on a dead toad..
I'm trying to understand the two.
A friend and I are in a debate as to what High Octane Fuel is. I came out saying, it's slow burning which is why there less chance of knock happening in an engine it's being used it. he says it's a numeric value to it's resistance to knock. that it has nothing to do with flame speed. I knew that already. Im in the understanding that higher octane fuels itself, disregarding the numeric value, that it is having a slower burn in a sense. or maybe harder to ignite. all this is making me confused while i'm searching all over the internet trying to understand it. In a sense..he explaining what it is, while I'm trying to understand what it's doing or what's happening to the fuel. maybe you guys can explain to me in "octane for dummies" format. :)
I have learned much about this topic from a local kart racer here in town. He is by far the smartest criminal on the face of this earth when it comes to fuel technology and terminology.

The octane number is nothing more than an indication of the resistance of the fuel to prevent knock. The higher the number octane, the more resistance to knock it will have. That's why a lot of guys on this list run C116.

Now, the properties of the higher octane fuel require the timing curve to be changed due to the slower burn rate of the fuel. I'm not sure if the higher octane fuel is harder to ignite, but it definitely has a slower burn rate.

Hope this makes it seem as clear as mud now. :D
So far everybody is correct.

High octane is the resistance to detonation. But in order for something to resist detonation, it must be harder to ignite when the air and fuel is compressed or else the heat of compression it will ignite it. The fact that C16 is slower burning than 91 pump crud is simply a byproduct of its higher octane and not a determining factor.

It is kinda like saying fast cars get bad gas mileage. It may be true that most do get poor mileage but just because you get bad mileage doesn't make your car fast. Does that help?
I'm going to have to both agree and disagree here :). The definition of octane rating is the resistance to knock. Knock happens when fuel that is in parts of the combustion chamber that the flame front has not reached yet decompose because of the heat, pressure, and light from the fuel that is already burning. As the fuel decomposes it can form species that spontaneously ignite at the temperature the mix has already reached, and so a relatively large volume of fuel mix will simultaneously ignite. This gives a large uncontrolled spike in the pressure in the combustion chamber and can be heard as knock. Preignition or pinging is something else and I'm not going to talk about that now. During normal combustion, as the flame front moves in an expanding circle from the spark plug to the chamber walls, the pressure will rise smoothly and then fall as the mix is consumed and the piston starts going down on the expansion stroke. When knock happens a lot of the fuel mix ignites at the same time when the piston is still very near top dead center, so the pressure can reach over ten times higher than normal, so the forces on the piston, rings, rod, rod bearings, crank, main bearings, chamber, head gasket, and everything else are also ten times normal, which is why knock breaks stuff.

If you take any fuel and simply heat it up rapidly in air there will be some temperature where it will spontaneously ignite. This is called the autoignition temperature and is the property that best correlates with octane rating. As the temperatue rises and the decomposition process begins there are a whole host of chemical reactions that occur that lead to combustion, and anything that slows one of those reactions will slow the overall process and raise the octane rating. Different additives function at different times in this process - oxygenates slow down the low temperature, early reactions, while tetraethyl lead slows down some of the medium-high temperature processes near the end. Neither one is "better", they just do the same job different ways.

As for "how fast high octane fuel burns", I think that people are confusing multiple effects. It is true that higher octane fuel is harder to start burning (the fuel must decompose as part of the combustion process and we just said that high octane means slow decomposition). However, the speed of the flame front as it moves across the chamber is approximately constant for similar gasolines, no matter what the octane rating. So in that sense high octane fuel does not burn "slower" than low octane gasoline. Finally, the flame speed is different for different kinds of gasoline components such as alcohols, aromatics, and ethers such as MTBE, and each of those have their own octane ratings. That means that a gasoline composed purely of branched hydrocarbons will have some octane rating and flame speed, while another gasoline with straight chain hydrocarbons and lots of aromatics and MTBE could have the same octane and much different flame speed. You just can't make any blanket statements about gasoline because there are so many different components in use today (yeah, yeah, that's a blanket statement in itself, sigh :)).

I highly recommend locating a copy of the gasoline FAQ put together by Bruce Hamilton if you want lots more info on this stuff. It's available from several places on the web. He also just posted an article to sci.chem last night that has some good stuff in it; Iwill try to post that here tonight if I remember.
Here's the article by Bruce in sci.chem, subject Re: Anti-knock additives, posted here without permission:

On 25 Nov 2003 13:07:19 -0800, muhammar@hotmail.com (Muhammar) wrote:

>there is a correlation between anti-knock efficiency of
>octane-boosting additives and their capability to form stabilized
>(=lazy) radicals. Tetraethyl-led, branched alkanes, TBME etc.

The octane rating of hydrocarbons is determined by the structure of
the molecule, with long, straight hydrocarbon chains producing large
amounts of easily-autoignitable pre-flame decomposition species, while
branched and aromatic hydrocarbons are more resistant. This also
explains why the octane ratings of paraffins consistently decrease
with carbon number.

In real life, the unburnt "end gases" ahead of the flame front
encounter temperatures up to about 700C due to compression and radiant
and conductive heating, and commence a series of pre-flame reactions.

These reactions occur at different thermal stages, with the initial
stage ( below 400C ) commencing with the addition of molecular oxygen
to alkyl radicals, followed by the internal transfer of hydrogen atoms
within the new radical to form an unsaturated, oxygen-containing

These new species are susceptible to chain branching involving the HO2
radical during the intermediate temperature stage (400-600C), mainly
through the production of OH radicals. Above 600C, the most important
reaction that produces chain branching is the reaction of one hydrogen
atom radical with molecular oxygen to form O and OH radicals.

The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Antiknock
additives work by interfering at different points in the pre-flame
reactions, with the oxygenates retarding undesirable low temperature
reactions, and the alkyl lead compounds react in the intermediate
temperature region to deactivate the major undesirable chain
branching sequence.

MTBE works by retarding the progress of the low temperature or
cool-flame reactions, consuming radical species, particularly
OH radicals and producing isobutene. The isobutene in turn consumes
additional OH radicals and produces unreactive, resonantly stabilised
radicals such as allyl and methyl allyl, as well as stable species
such as allene, which resist further oxidation.

In contrast to oxygenates, the alkyl lead interferes with hydrocarbon
chain branching in the intermediate temperature range where HO2 is the
most important radical species. Lead oxide, either as solid particles,
or in the gas phase, reacts with HO2 and removes it from the available
radical pool, thereby deactivating the major chain branching reaction
sequence that results in undesirable, easily-autoignitable

>If scavenging the radicals in the chain-reaction of the combustion
>process is the way to get octane number iboosted, then one can perhaps
>use common radical scavengers known from organic synthesis, food
>industry, polymer processing.

As you can see from the above, it's not that simple.

> For example, tert-butylated hydroxytoluene is cheap, non-toxic and
> highly efficient as a scavenger. Only a tiny bit would suffice and
> it would not extract into groundwater from leaking fuel storage tanks.

Very unlikely - it would decompose in the flame reaction and wouldn't
be available to scavenge. If the fuel evaporated the BHT would remain
as a residue, blocking jets. etc.

Bruce Hamilton