I thought of these ideas prior to becoming a member here, and since reading a lot and learning loads, I'm thinking these ideas are more myth than logical.
First one... I have always been told that fuel injectors are less efficent compaired to carbrator in the department of fuel atomization. Since then it seems the best MPG will be found on small engine powered cars which are fuel injected. Down side of carb was the fact you can't control it as well as fuel injection. This made me think fuel vaporization could be a real fuel saver. I have done some small tests with lawnmower engines and was able to get an engine to run on only fuel vapors, but it would die out after starting since it was sucking more at the higher rpm than starting so less air contact time with the fuel to get it to evaporate. Also pulling the intake though a liquid would restrict the air flow, which I would suspect would make the engine low rpm only.
Expanding on the idea after a while, I came across plans online (don't have link atm) of a chamber the designer called the vapor carbrator. The very basics is that it had a small pump which would cycle air though the chamber to get as much fuel in the air as possible. Then the top side of the chamber ran to the intake of the engine, along the way having a exhaust exchange to heat the fuel (and maybe the chamber?), a placed to add fresh air, regulate vacuum, etc. It seems to be an interesting idea, but the claims are reallllllyyyyy out there. Something like 200mpg in the 1950s using a 1930s v8.
Here is one of the cars I came across after the 200mpg v8 thing.... 376mpg car which has a chain drive and ran on vaporized fuel in 1976. Interesting read, but I think it is another one of these unicorns.
Big Oil conspiracy! 376 mpg Opel uncovered! | Hemmings Blog: Classic and collectible cars and parts
Second one... Based on racing mojo... nitro boosts car power by tons which basically adds more oxygen allowing the engine to burn more fuel per power cycle, but this largely adds to temp which I didn't think much about before. The idea was to equip a car with an oxygen tank and run it as a boost to engine power expecting better MPG, and back then thinking the savings would bearly cover the oxygen tank refills. My new thinking is that MPG would be nearly the same giving no other factors are changed, just adding abilities for the engine to burn more fuel at a given rpm would lower rpm, but at a huge cost of having the intake butterfly closed more which of what I read is less efficent (basically having a v6 vs having a v4 in a small car). Only good thing I can think of is for using an extremely small engine and maxing out what it can do within reasion (including turbo) and the benefit could be viewed similar to a normal turbo setup but have further gains? Also note that the oxygen used would be huge, and wouldn't last a tank of fuel I suspect.
Just for giggles I played around with a tourch with LP + Oxygen setup to "fuel" a lawnmower engine. Basically adjusted for the best flame, knocked it out and stuck the nosel in to the carb area and pull started it with my other hand. In about 3 tries, the engine did fire up the gas, but it was a backfire which riped the pull rope out of my hands and puffed out of the carb. That was enough of that for me
. Also to note, I think it was out of gas when I tried it and orignal plans was to just start it on it for a few secs just to see the thing run on it, no revs etc.
Just to make sure I'm clear, I view the above as 99.5% invalid myth with an extremely slim chance of having any type of benefit in any form, but comments should make things funny