Engine & fuel engineering - Smokey's Adiabatic Engine
Hot vapor induction. Doesn't look like anyone has really followed up on it to prove it does or doesn't work.
I also seem to remember a patent on a system that injected fuel around the exhaust valve, vaporizing the gas and cooling the valve at the same time. Nothing ever came of that one either.
We need a hardcore version of mythbusters to try some of this stuff out.