I assume we're talking about reasonable small quantities here ie. equiv to a cup? If so it's even simpler than antifreeze try a capful or two of metho. note the CAP not CUP
Unlike petrol Meth spirits DOES mix with water and also burns. It will take the water into the cylinders and the burn will turn it into steam from whence it will leave via the exhaust. Nice, simple, clean and non-toxic.
I had a few friends in the 80's that actually added water/meth injection kits (as a diffused spray into the carby - yep carbies back then) to their cars.
The theory being the meth/water combo lowers the air temp, giving a cooler, denser mixture which helped combustion, while turning to steam that helped clean the engine of carbon deposits. I don't know if it worked, but I did see a similar article from a US (New York???) Uni researcher recently talking about adding meth spirits to cars to help emissions and fuel economy and he had similar reasoning.
Amazing the injection kit in the 80's was from one of the hobbyist electronics magazines (Electronics Australia or Electronics Today International for the aussies on the list) it's take 20yrs for Uni researchers to reinvent this