A fuel's octane rating is it's ability to resist pre-ignition - often called "knock." The higher the octane number, the less propensity there is for a fuel to pre-ignite under a given set of conditions. This standard is for gasoline. It isn't necessarily true that a fuel with a higher octane number will do anything for your engine, unless it's designed to make use of it. Like if it's turbo- or super-charged, or it has a high compression ratio.
No argument from me with your statements above – all pretty basic stuff so far.
Diesel fuel has a theoretical octane number somewhere north of 3000,
Wow, I had no idea. Sure would like to learn more about this fact. Got any reference you could point me to?
A molecule's "carbon chain length" is only indirectly related to a fuel's octane number.
Agreed, but do longer chains increase knocking tendency or not?
According to
Internal Combustion Engines, McGraw Hill, 1939, they do. And they show that in the case of, say, straight chain saturated hydrocarbons, knocking tendency is pretty linearly related to chain length. Same holds for other structures. Granted, an old text, but it’d be surprising if this volume, in it’s 5th edition has this relationship backwards…
That said, their presentation is a little fuzzy, as they don’t exactly define what is meant by “knocking tendency”.
I would suspect that what you are hearing is not actually "knock" (pre-ignition), but something else. Possibly detonation. Possibly your engine's death rattle, as it burns a fuel at temperatures and pressures far in excess of those it was designed for... but hey, I'm not there, maybe it's something else.
It was knocking, as I stated. I know the sound, and it occurred under heavy load. With pure gasoline, the engine does not make this sound. My experience with this knocking was based on a air cooled single cylinder lawnmower engine.
Adding middle distillates (diesel, kerosene, jet, furnace) to gasoline for use in a gasoline engine would actually increase the octane rating (resistance to pre-ignition) of the fuel....
I still disagree with this, but look forward to your evidence backing it up...