So I know absolutely nothing about chemistry, so can someone explain to me why lead was added to gasoline in the first place? Disregarding its toxicity, what benefit does lead buy you?
> It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that allowed engine compression to be raised substantially. This in turn increased vehicle performance and fuel economy.
If you want a fascinating story telling video with a lot of sciency stuff for the relative layman, here's an excellent one by Veritasium on how it happened.
"The Man Who Accidentally Killed The Most People In History"
https://en.m.wikipedia.org/wiki/Thomas_Midgley_Jr. He was a fascinating man, relatively non-famous. Probably singlehandedly contributed to more environmental damage than you can imagine. Died in a strange way too.
My understanding (and I'm no authority in this subject) is that adding lead to gasoline allows achieving better octane ratings (ie lower chance of self-ignition). This reduces engine knock, which makes the engine live longer, also allows pushing more fuel into a cylinder per ignition cycle more confidently.
These things are possible without lead as well, it just costs more money to do so.
It's about increasing the compression ratio without self-ignition. High compression ratios mean higher power for the same weight, and airplanes are all about minimizing weight. You also get higher efficiency. Again, lower fuel weight = good.
You can dump as much fuel in the cylinder as you want (in fact, most airplane operating handbooks call for a very rich mixture on takeoff because of the cooling effect).
Half the story is the lead acts as a octane booster, but raising the octane without lead chemicals is trivial. MTBE works, so does just selling pure toluene. The issue is that in addition to raising the octane, some metal lead deposits around the valves which maybe makes them seal better and switching away from lead could cause engine failures unrelated to octane.
It allows the fuel to withstand higher pressures without detonating in an uncontrolled manner. This is important in aircraft engines, which run at a significant fraction of their maximum power output for most of their working lives.
People have been looking for alternatives for a long time, but it's difficult because leaded aviation gasoline is not a very large market, and there are substantial regulatory obstacles.
So higher octane means it needs higher temp/ pressure to burn? I've completely misunderstood octane my whole life.. i always thought it meant "high octane make engine brrrrrr fast"
Higher octane allows it to get compressed more before it goes boom, which means you can make a more powerful engine.
But adding high-octane gas to a normal engine doesn't make it go any more boom, in fact, it goes less boom because it doesn't burn as much when it booms.
Modern engines can be designed to advance or retard timing based on knock, so they can run on different octanes and provide different power curves.
>But adding high-octane gas to a normal engine doesn't make it go any more boom, in fact, it goes less boom because it doesn't burn as much when it booms.
It’s probably not your fault. It always annoyed me when old (20-30 years ago) Shell gas commercials would imply that high-octane (read: much higher profit margin) gasoline would cause your engine to make more power. Now, they didn’t actually say that outright, because it’s not true.[0] But if you put Shell gasoline in your car, you were told that you’d “feel the power your car is capable of”, or some weasely bullshit. The word “power” was used a lot in that commercial, just never next to the word “more”.
I was a professional mechanic around that time, and I had coworkers that thought that not only would high-octane gas give more power, would also swear it increased fuel mileage. Neither is true. High-octane fuel has less energy per gallon than lower octane fuel, it can’t give better mileage. (Now, “mechanic” != “engineer”, but c’mon, guys.)
[0] Yes, Captain Pedantic, I’m aware of knock sensors and computerized ignition timing. It was a quarter century ago.
> I’m aware of knock sensors and computerized ignition timing. It was a quarter century ago.
This stuff started to show up in the late 1970's-- the 1981 280ZX Turbo my dad had was equipped with a knock sensor and would retard timing and add fuel.
And just to highlight and explicitly explained the weird edge case that GP mentioned...
On many modern cars, high octane gas gives you more boom. They are designed for high octane gas, but the engine management computer is able to detect low octane gas and prevent damage from predetonation-- giving up some power.
This is probably not true of modern naturally aspirated cars. Maybe they could lean out the mixture a little to get a bit more efficiency on higher octane, but they largely can't leverage the higher octane fuel as well as a car with an electronically controlled turbocharger system.
>They are designed for high octane gas
The vast majority of modern engines are designed for 87 octane, and not downrated to 87 octane, because that would give you poorer performance than a clean sheet 87 octane design, and single digit percentages matter here.
> This is probably not true of modern naturally aspirated cars.
There's a whole lot of cars that say "Use premium fuel only" with compression ratios of >10.5:1, but rely upon anti-knock sensors to retard timing (and adjust mixture) to avoid detonation with 87 octane fuel.
Great comments on the subject here - pardon the gross over simplifications please. I'll add what I know. "Octane" can be thought of as "Resistance To Pre-ignition" where the higher the octane number, the more resistant the fuel mixture is to ignition/burning. Lower octane fuel can have a tendency to pre-ignite via cylinder compression (compression on the upstroke raises cylinder temperatures to the point of combustion) rather than ignite when the spark plug fires. This is whats called "knocking" as in the benign case it cause the engine to make a knocky-rattling sound ( not unlike a diesel which in fact relies on compression ignition as a feature of its design ). In the bad case, pre-ignition can cause catastrophic engine damage where engines "grenade" themselves.
More or less, if you want to build a light weight/high power engine you want a higher compression ratio. As the piston travels upward on the compression stroke higher pressures are achieved and this allows designers to take advantage of different crank/cylinder characteristics to make more power in a smaller, lighter package.
We used lead in car gas because engine timing technology and fuel delivery technology were analog/mechanical and really very limited in terms of tuning potential. The cheap hack was just to use a fuel that was more resistant to burning in the first place. Just add a hotter spark/ignition system and it was good to go design wise. Given the old tech running in most GA planes, this was a natural thing to do at the time and the problem was well understood. To point here, modern non-lead additives have been around for a long time and the problem can be adequately solved with a non-lead substitute.
If you wanna get deep with this, take your old normally aspirated car and bolt a turbo on it!
