Yes, there is octane in gasoline, by volume the greatest constituent. Yes, you are also correct the an "octane rating" is a measure of it's resistance to detonatioin, but it is a standard using octane as the metric. But C8H18 is the primary molecule of gasoline at the pump.

Thanks Ralph!! I was looking for a way to understand the chemistry of internal combustion, given my limited backround, and you gave me enough to answer most of my initial question plus enough more...I know it took you about an hour or more to write all that and man is it appreciated. Albeit I'll have to re-read it 5 more times before it all sinks in but I am getting it.
For many years I read the RM/2 rating at the pumps and wondered...still do...and still will. I figured that the Carbons...chain ends aside...just bonded with 2 other C's and 2 other H's, left right, top bottom respectively but I never considered the bond angles...they must fluctuate but average out at 90 degrees except at the chain terminus, termini rather. To my new thinking the 87% Octane represents the fire while the 13% represents the water as a certain balance must be maintained for reliable ignition over a long duration...I mean pure alcohol burns great but building an engine that can last 100,000 miles on pure alcohol would be difficult...so dilute your fuel to a managable point where power output is reasonable and the ability to cool the internal parts is do-able and you have a gasoline engine. ???? My lame attempt at reverse engineering but it makes sense. Plus you want it to run at 14.7psi on a regular basis...and add in a turbo when convenient...awesome chemistry lecture and man do I appreciate it! Thanks and if you have the time please write a bit more as I'm eating this up and I get it...just wish my last chem prof had the inclination...and I'm a biodiesel nut so am needing all the free education I can get. I wonder where Cetane rating and Octane rating meet. Given a static compression ratio obviously there is a meeting point somewhere as one promotes ignition via pressure while the other retards it. It's all in understanding the chemistry and of that I am certain...add 80% Nitrogen, 18% Oxygen and 2% other and your engine runs...GOOD STUFF!! Please feel free to practice your dissertation here...Graci`...thanks Ralph! and Pmari and 240D and FI

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1983 300SD... 269,000 miles, nearly 2,500 on my B-100, Faded Grey, Ugly in an elegant sort of way...Duh-Benz


If any of this has been a blasphemy to you, then good, because it's been a blast for me to...A.Whitney Brown

Hi ElktonJohn,
I'm glad I was somewhat helpful. The "octane rating" though does not mean, say it is rated as 87 octane, that it is 87% octane with the other 13% being inert (non-reactive) molecules. I don't recall the molecule, but it is set as the standard of 100 octane rating (though the molecule isn't octane). I'm pretty sure n-heptane has an octane rating of 0, which give's two points on the standard curve (with the 100 octane rated molecule being the other point on the curve) (years ago Sunoco sold a premium blend rated at 120 octane; it was the only fuel that wouldn't pre-ignite badly in my 11:1 compression 383). So, what I'm saying is that a fuel with an octane rating of 87 behaves in a certain manner as regards it's resistance to ignition. In addition to the simple carbon chains of the form CnH((n*2)+2), there are many additives the fuel companie's put in gasolines, such as detergents, other volatile molecules, etc. I don't have the forum member's name in front of me who provided the information on some of these more complex molecules, especially the aromatic hydrocarbons, but read that post and you'll see further the complexity of it all. Lead-ethyl was the old favorite before the pollution control hit heavy in the late 60's. It was a good heat absorber, so raised the octane rating, yet it is not a true hydrocarbon at all. Though you can mix ethanol & water and burn it in a gasoline engine (because the ethanol binds with water, preventing it from globulizing (if that's a word, lol), gasoline contains no water, per se, and your fuel filters are designed to not let water pass. As regards the bonding angles of the hydrogens the organic chemists can quantify these, but now we're delving into quantumn mechanics, a world completely non-intuitive. Just a quick example, in a single hydrogen atom with one proton as the nucleus and one electron in orbit about the proton, just where in a given instant is the electron, keeping the electron in it's ground state (lowest energy orbit)? Well, the answer is "no place, everyplace". But if we take a measurement to determine it's localization then it has a localization, but only if we take the measurement, "collapses the probability wave function" (at least according to what is known as the Copenhagen Interpretation of QM. Do a search on "quantumn mechanics probability wave collapse" if interested. Further, as Heisenberg figured out in the late 20's, the more exact we measure the electrons localization, the less we know about it's energy (and vice versa). So, if we had infinite precision as to the electrons energy (momentum) then as far as we can know is that the localization of the electron could be anyplace in the entire Universe. In addition to the angular values of the hydrogen speaking of a single CH chain, we have these chains nearing & repelling one another as they vibrate and all of that is far beyond my understanding, as even quantumn mechanics struggles with it beyond a few molecules as the computational power required quickly exceeds all the computational power of the world's computers combined. The "where do the octane/cetane curves cross", I'll just be guessing the correct answer here: It's not where they have the same defined value (a certain #). They, I think, are best thought of as inverse functions relative to one another; but, cetane rating is applied to diesel and octane to gasoline. In the intuitive sense Cetane=(1/octane), just meaning as one goes up the other goes down. But, since the two functions are derived by using different standards, it is not arithmetically correct to say if you know either the cetane or the octane rating, but not both, you cannot, for example say that if cetane = 100, then octane=0.01. Not exactly, but it's something like saying where do temperature and texture measurements meet? Though, the cetane/octane thing isn't quite so ridiculous.

ElktonJohn,
I noticed your interest in "just what happens" in the environment of an internal combustion engine at the point of ignition, we have the same interest. One thing that fascinated me about diesel at the point of ignition by compression is the manner in which the injected fuel cloud burns. The seemingly best picture I could come up with as to reliability of the source was some old Mercedes Manuals. This is their abbreviated picture: We're coming up the compression stroke, greatly raising the temperature; at some point BTDC the injector sprays it's cloud of pure diesel; the cloud is elongated (and obviously changing across the whole of the fuel release). This cloud has the highest density in the middle of the elongated stream and becomes less and less dense as we move outwards. So, the inner cloud is initially too rich to even ignite, so, the initial ignition takes place in multiple spots at the outer edges of the cloud. That initial ignition is totally a function of the high temps created by the compression. Some of the middle section of the cloud also ignites by compression from the rising piston and the high pressure (so temps) created by the outer cloud igniting from multiple spots. There is a precise time frame in which all this is happening, but I don't have those #'s in front of me. So, after the ignition sequence I've described up to this point, compression no longer dominates as the fuel igniter. What dominates now is the radiative heat from the burn pattern of the outer cloud and more and more of the inner cloud. The fascinating thing to me is that I had always thought of diesel engines (even though I was a diesel mechanic in the Service and went through courses at Aberdeen Proving Ground, and the civilian Detroit Diesel / Allison Automatic in Iowa) as being totally compression dominant devices. And, most certainly, a diesel with compression problems runs poorly to not at all. But, it is the engineering of the pistons and combustion chamber and fuel injector that results in the engine utilizing the two components of complete ignition, that is: 1) compression 2) radiant heat from multiple compression initiated burn sights at the outer, lean edges of the cloud. (Though the piston & initial compression mediated ignition produces, well, more compression, it is the radiant heat from burning diesel that dominates as the igniter of the initially rich inner cloud which becomes leaner and leaner as we step through the cycle).
This makes it very easy to see why we want a diesel fuel of proper cetane rating; one can go too high and one can go too low. If one is to use #2 diesel or #1 diesel or bio-diesel, or mixes, the engine sounds different dependent on which fuel it is burning, so what is going on in the combustion chamber is altered; not necessarily for better or for worse, but that same engine has something different enough going on from beginning to end of fuel burn as a function of fuel type. In light of this discussion.