WWII aircraft engines injected water (and in some cases, water+methanol, or water+ethanol) to raise the effective octane in the fuel, to prevent pre-detonation when you pushed the engine into super-high boosted intake pressures to maximize performance in the “War Emergency Power” ratings that were good for only about 10 minutes.
There’s some sharp boys at MIT looking into re-creating this idea in conventional gas engines for cars. If you could increase the octane of gasoline by either injecting water, ethanol or methanol, you could increase the compression ratio and thereby increase the efficiency by gaining a longer expansion stroke on the engine to extract more heat/pressure from the combustion.
Right now, gas engines are at about 9.5:1 compression ratio. If we could get that up to about 12.5:1, we’d see about a 25% increase in fuel efficiency. We need higher octane fuel to make this happen.
Oh — and for all the people who are pissed off about “the ethanol scam”?
Putting ethanol into gasoline raises the octane quite nicely.
BTW, you're 100% correct about ethanol's anti-knock capability. However, neither does it produce the BTU's of gasoline.
In effect, one winds up with "High-Octane," low octane gasoline. Because you are displacing gasoline, you quickly reach a point at which efficiency suffers.
I was thinking about the ethanol thing. I seem to remember that ethanol is about 104 octane, which with the proper computer control could allow you to raise compression considerably. Of course ethanol also holds less energy gal for gal than gas so I don’t know if ultimately it becomes a wash.