Thread: Octane and Oxygen

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Hey all. Long time no see!

I got a question thats been plaguing me for a while.

I know that the higher gasoline octane rating the more the fuel burns v.s. exploding in the cylinders making it knock resistant. And higher octane fuel does not supply more oxygen.

But does higher octane gasoline require more oxygen to burn? Currently Im living up in the mountains of Arizona for school and comming from sea level to 5300 feet, I can feel my truck is down on power. 06' Nissan Titan (9.8:1 compression, all aluminum) One day I treated her to mid-grade and after some driving around I noticed the ECU worked its magic and she seems happier.

Is my truck really happier or have my engineering classes made me insane?
I do notice going down to Pheonix my truck has her power back due to the increased oxygen at that altitude.

-CJP

There are at least two effects here. First, yes more oxygen helps combustion. The more complicated second part has to do with the rate/speed of the burning of the fuel; slower is better relative to the mechanical speed of the engine parts. The combustion reaction can be much faster than the mechanical motion of the engine parts so it is better to have a slow "wooomf" rather than a sudden "bang". I once saw a Mr. Wizard demonstration of a soft rubber hammer hitting a piston on a rod and crank mockup where a single hit with the soft rubber mallet would get the piston going up and down many times with the crank spinning many revolutions while with a hard metal/plastic mallet hitting the piston the crank would only revolve one time or less. The smooth combustion explosion is better when it is slow and matched to the engine speed. Basically chemical explosions can be much faster than the mechanical parts of an engine can respond, so a slow continuous burn lasting longer is more effective to push that piston down. Probably in your case the oxygen levels at various altitudes is more important but slow burning high octane fuel is better too. Low octane fuels use long straight chain hydrocarbons which burn fast linearly like a fuse. The high octane fuels are highy branched hydrocarbons which burn slower and are more nearly matched to the timing of the events in the mechanical parts of the engine. 6,000 rpm might seem like a fast situation but with only an explosion every two revolutions for an explosion every 1/3000 of a minute which is 60/3000=0.02 seconds, that is "forever" compared to chemical reactions which can occur in less than 0.000001 seconds so slowing the reaction is needed to match the chemistry to the mechanical parts of the engine.

Don Shillady
Retired Scientist/teen rodder

Your altitude difference thing is a function of dynamic compression ratio. Maybe a definition of both static and dynamic will help those that are unfamiliar with the terms.

Static compression ratio is what we are most accustomed to talking about. In simple terms that's the ratio of the cylinder volume at bottom dead center vs top dead center. For the sake of discussion, if at bdc the volume is 10 times larger than at tdc then the static compression ratio would be 10 to 1.

Dynamic compression ratio takes into account the volume of air in the cylinder. The greater the air density (whether caused by altitude, temperature, supercharger, & so on) the higher the dynamic compression ratio. In your example the volume of air (not just oxygen but all components) is greater at sea level than on top of a mountain (and at varying degrees inbetween). If you watch drag racing on the tube you'll hear Dunn talking about calculated altitude and grains of water, these effect the dynamic compression ratio as well as cylinder temperature. Speaking of temperature, that's the basic culprit in detonation (not to be confused with preignition which is usually a mechanical caused ignition of the fuel prior to the plug sparking). As the charge air is comprssed it heats up, adding to the ignition process. As the flame front of burnt fuel expands and progresses across the combustion chamber it further compresses the unburnt charge. That heat of compression could contribute to sudden ignition of that remaining fuel charge (which results in the characteristic ping/knock) rather than a smooth completion of the burn. The higher octane rated fuel has more stable molecules of fuel that resist that sudden heat of compression ignition better than the lower octane rated components.

I love to read explanations like Don's and Bob's because it is obvious that they know their facts. Without getting into detail, it is my understanding that running any higher octane than is necessary to prevent "ping" or pre-ignition knock adds no improvement in power or efficiency, and in fact may be slightly less efficient as the engine is operating at/near peak on lower octane, faster burn. I would expect that at the higher altitudes CJP was getting some amount of ping/clatter at WOT, like climbing in the mountains, and that the higher octane fuel reduced or eliminated that ping. If this is the case then the higher octane fuel is good for the mountains, but may not be necessary if he visits the coast of CA, other than the time the ECU may take to re-learn the ambient parameters.
Am I off base here?

Hey.

Thanks for the VERY informative replies Bob and Don! Higher octane gasoline does not require more O2 to burn. Do you think this is due because 91-93+ octane fuel vaporizes more readily?

-CJP

Originally Posted by dhemi1

Hey.

. Do you think this is due because 91-93+ octane fuel vaporizes more readily?

-CJP

The measure of how "well" it vaporizes is measued by Reid Vapor Pressure (RVP), that's really more value at startup, nothing to do with octane rating level. If anything the higher octane rated fuel has more complex molecules that would likely vaporize less easily, thus resisting ignition "too soon" as only in a vapor state will it burn, not liquid or solid.

This is interesting to me too. Bob has some very good comments above but it is a pretty complicated situation which could involve the temperature of the fuel coming in (cool can?), atmospheric pressure, and a whole lot of chemical variables. I recall a study at Cornell Univ (Prof. R. Schmook) in which the whole system was simplified to burning pure methane (CH4) in pure oxygen under carefully controlled conditions flowing through a cylindrical chimney and it still took over 100 equations to describe the combustion process chemically, what a complicated mess! Sooo, all I can say is that there are a few common sense factors. First the shape and the wall surface of the intake passages can help break up droplets (injection vs. carb is another factor) and probably a little roughness in the intake ports can actually break up droplets in addition to the temperature effect on the vapor pressure of the liquids. Maybe (?) there is a slight difference in the way some molecules wet the surface of the intake walls but temperature is probably more important as to vaporizing droplets. Second, even if there is no spark there will be a Diesel effect when the piston compresses the mixture and that will raise the temperature of the mixture at least several hundred degrees which will help vaporize the droplets. Then there is the amount of water in the air as the relative humidity since it is known that water injection/bubbler can reduce detonation. All of this falls under fine tuning known better to racers than to me. However the reason that the oxygen required is the same is that whether the octane is the low octane form as linear CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3 or the branched high octane form as 2,2'dimethyl-4methyl-pentane, both have the same overall empirical formula as C8H18 so when the burn ocurs, whether fast or slow the reaction still requires the same amount of O2 as in

C8H18 + 25/2 O2 -> 8 CO2 + 9 H2O + Heat

Of course there can be other molecules in the fuel but I am only comparing two isomers of octane for discussion. I am sure that automotive engineers must have modeled this carefully and I know that in the combustion of jet fuel in aircraft engines the whole process has been modeled in detail at Wright-Patterson AFOSR long ago. There are a few simple equations which can tell the overall story but to model say the difference in flame propagation in a hemi-head versus a wedge-head requires a lot of computer time and a bunch of sensors attached to the engine. Another factor is the position of the spark plug relative to the intake valve and the success of the shape of the combustion chamber in the SBC Vortec heads as well as handbooks on "how to port heads" indicates small differences in the shape can make a difference. From my experience I would approach the problem always asking for a big, fast computer and model the heck out of it but often racers can make wise adjustments from experience. Just think how neat it is that using track runs and what is called "hot rod ingenuity" racers optimize these variables for a given race. It has long been true that racers learn practical truths by a lot of trial and error and keeping an open mind about out-of-the-box thinking. On the other hand not every idea works and that is why we see adjustments in the pits! Bob may have more details and I'll bet Tech1 knows a few things I have missed!

Don Shillady
Retired Scientist/teen rodder

Thanks a million guys. Yet another Club HotRod thread added to the bookmark list!

-CJP