What is high octane gas composed of?

[Elite Engineer]

I'm trying to figure out exactly how high octane gas works, and how it can remain stable under higher pressure in the cylinder. I assumed high octane gas (93+) has more benzene-like molecules in it to retain stability. So, I looked up a list of octane ratings of different fuels. At first I thought I was correct, because the two molecules with the second of thirds highest R+M/2 ratings were toluene (114) and xylene (116.5). I thought it makes sense because they were alkylbenzene molecules, so of course they'll have higher stability than a linear octane molecule and such.

 

But then I saw that non-benzene molecules had fairly close octane ratings, like" isopropanol (118), propane (112), i-butane (102). And the most shocking was methane, with the highest octane rating of 120! So now I'm very confused. How do these molecules have higher stability/ higher octane rating if they're not benzene derivatives?

 

(Correct me if I'm wrong on understanding the correlation of "higher molecular stability + higher compression ratio = more horsepower"...my thought is this: You use a fuel with a high stability, so that when it's sprayed into the cylinder it can withstand the high compression (i.e. 1:12), and then combust with the spark igniting the fuel... instead of a less stable fuel that will inherently combust due to the high compression, before the spark ignition) <---- am I right?

 

~EE

Edited August 21, 2016 by Elite Engineer

[Endy0816]

Iso-octane isn't the best, so if we're using iso-octane as our arbitrary 100 rating on our scale other substances can have higher ratings.

 

https://en.wikipedia.org/wiki/Octane_rating#Isooctane_as_a_reference_standard

 

My experience is that knocking is typically caused by unburnt fuel. Anything that burns cleanly is going to do great in that respect.

Edited August 21, 2016 by Endy0816

[Elite Engineer]

Iso-octane isn't the best, so if we're using iso-octane as our arbitrary 100 rating on our scale other substances can have higher ratings.

 

https://en.wikipedia.org/wiki/Octane_rating#Isooctane_as_a_reference_standard

 

My experience is that knocking is typically caused by unburnt fuel. Anything that burns cleanly is going to do great in that respect.

Yes, but why does isoproanol and propane have relatively close ratings to toluene and xylene? Should the latter be wayyyy higher in rating due to benzene ring stability? That was the crux of the question.

[MigL]

I was a bit of a 'gear-head' in my younger days...

 

The only reason higher octane fuel produces more horsepower is because it can take advantage of higher initial timing, or how many degrees before top-dead-center the spark can fire the fuel-air mixture. Alternately it allows a higher compression ratio or turbo/super charging. This puts the explosion/expansion at a more advantageous position to produce power and other cylinders waste less.

It does this by reducing the speed that the flame front travels through the air-fuel mixture, in effect by being LESS explosive.

That pinging you hear at wide-open-throttle while going up-hill in high gear is pre-ignition. The flame front explodes too fast and slams against the aluminum piston dome. It can actually blow a hole in it, ruining your engine.

 

Believe it or not, in North America the highest octane consumer gasoline at a rating of 94 is obtained by mixing up to 20 % methanol with gasoline .

And some higher performing vehicles have pre-ignition detection. If a sensor detects the pinging vibration, the computerized ignition lowers the initial timing to eliminate it. Still, a lot of high performance vehicles, especially turbocharged or supercharged, require the use of 91 or higher octane.

Edited August 21, 2016 by MigL

[Elite Engineer]

I was a bit of a 'gear-head' in my younger days...

 

The only reason higher octane fuel produces more horsepower is because it can take advantage of higher initial timing, or how many degrees before top-dead-center the spark can fire the fuel-air mixture. Alternately it allows a higher compression ratio or turbo/super charging. This puts the explosion/expansion at a more advantageous position to produce power and other cylinders waste less.

It does this by reducing the speed that the flame front travels through the air-fuel mixture, in effect by being LESS explosive.

That pinging you hear at wide-open-throttle while going up-hill in high gear is pre-ignition. The flame front explodes too fast and slams against the aluminum piston dome. It can actually blow a hole in it, ruining your engine.

 

Believe it or not, in North America the highest octane consumer gasoline at a rating of 94 is obtained by mixing up to 20 % methanol with gasoline .

And some higher performing vehicles have pre-ignition detection. If a sensor detects the pinging vibration, the computerized ignition lowers the initial timing to eliminate it. Still, a lot of high performance vehicles, especially turbocharged or supercharged, require the use of 91 or higher octane.

ah thanks, that makes more sense. However, i still need a chemistry explanation about the fuel.

[Elite Engineer]

As no one replied, I had to be a big boy and really dig for this answer. It turns out octane rating is proportional to activation energy. Hence 93 octane takes more energy to detonate in the cylinder (via higher compression ratio) than 87 octane.

 

This higher activation energy correlates with the molecular stability of the molecule. So for example, toluene is more stable than butanol because of its benzene ring and the two alkyl groups that aid in stabilization. I also guess that methane is somehow more stable than toluene/ xylene because it has an octane rating of 120...not quite sure of that mechanism though.

 

~EE

[John Cuthber]

It looks to me as if the molecules with high octane ratings are ones which produce stable free radicals easily.

That's going to influence the combustion process which is a radical reaction, but I'm not sure of how.

[Elite Engineer]

It looks to me as if the molecules with high octane ratings are ones which produce stable free radicals easily.

That's going to influence the combustion process which is a radical reaction, but I'm not sure of how.

I didn't know combustion yielded free radicals. I thought radicals only came about via halogenation.

[John Cuthber]

Oxygen is a di-radical species - essentially all of its reactions are radical reactions.

The Antiknock agents (TEL, TBME) all form even more stable radicals which can act as quenching agents in free radical chains reactions.

Both effects would tend to retard the reaction + prevent pre ignition detonation.