Thread: Octane and Oxygen

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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