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.