> What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Early gasoline was more or less output straight from the refinery distillation tower, yes. Octane rating varied a lot depending on the quality of the crude oil, but usually something in the range of 50-70. Thus necessitating the low compression ratios on those early gasoline engines. But the volatility of that gasoline was approximately similar to modern day gasoline.
What was then developed were various further processing steps to improve the octane rating of gasoline (and as the demand for gasoline increased, to increase the fraction of gasoline that you could get from a given amount of crude oil), like dehydrogenation, catalytic cracking, alkylation etc. First these were used for producing high octane aviation gasoline, but after WWII these processes were also put into use to produce automotive gasoline, enabling higher compression ratios in cars. Anti-knock additives helped a bit as well.
> This information I'm remembering from Yergin's The Prize, FWIW.
A pretty good book, I hear. I should read it.
> Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
Diesels inject ALL of the fuel during the combustion stroke. During the compression stroke, they only compress air. Which is why they can have so high compression ratios, there's no fuel vapor mixed with the air that may ignite and cause knock or detonation. Due to the high temperature and pressure in the air caused by the compression, the fuel ignites by itself more or less immediately as it's injected. No spark plug needed.
If you think about it, diesels want something which is sort-of the opposite of an anti-knock (octane) rating. You want the fuel to ignite by itself as soon as it's injected, not resist ignition. For diesel fuel this scale is called the 'cetane' rating, FWIW.
> I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
I believe you're sort-of right here. Diesel fuel is injected under high pressure, modern common-rail injection systems reach injection pressures of up to 2000 bar FWIW, which causes the fuel to be atomized into small droplets. The actual burn process AFAIU is sort-of a liquid burn process where fuel vaporizes from the droplets and immediately ignites.