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).
Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW.
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.
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.
> 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.
On The Prize, it's really phenomenal, and that's from someone who disagrees pretty strongly with Yergin on his general cozyness to the petroleum industry and enthusiasm for its future prospects. As a history the book is a brilliant work, there's an accompanying PBS/BBC miniseries, and the wealth of information contained (and number of head-turning new-to-me revelations) can't be briefly described. If you're into that sort of thing, I'd also recommend as much of Vaclav Smil as you can stand, though would suggest starting with Energy and Civilization, a look at human history through the lens of energy.
The octane ratings you give are about what I recall from Yergin's description (if that's where I first heard it, again, somewhat vague decade-plus recollection).
My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Otherwise, appreciate the additional knowledge, it fits pretty well with my own weaker understanding. Interesting especially about cetane. Looking that up, the name comes from Hexadecade, a/k/a C16H34, or a sixteen-chain hydrocarbon (double the carbon-atom count of octane, a/k/a C8H18).
> My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Yes, bunker fuel is very much non-volatile stuff. As an aside, they did eventually figure out that you could run slow-running big diesel engines on that stuff too. Perhaps you've seen pictures of such massive engines big as houses, if not e.g. https://www.youtube.com/watch?v=K30_jf-aA_U
These big diesels have some things in common with the old triple-expansion steam engines, e.g. they are directly connected to the propshaft (and thus they turn slowly, about 100 rpm max or thereabouts), and the engines themselves are reversible, so there's no need for any reduction gearing or gearboxes.
Similarly to steam ships, they need steam lines in the fuel tanks to heat the fuel so it can be pumped, and then further heated to 130C or thereabouts in order to be injected. So they need a small auxiliary steam boiler just for producing the steam to heat the fuel; modern ships often have an 'exhaust heat recovery boiler', which as the name implies utilizes the hot exhaust from the main engine to produce the steam, so that the auxiliary boiler isn't needed when the main engine is running.
Marine powerplants are indeed cool, and yes, I've seen a few prior videos, both in operation and maintenance with workers literally crawling through the engines.
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).
Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW.
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.
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.