In my opinion, the fuel to watch, Propane....
Also a very interesting technology you may want to google: "Spark Ignited Heavy Fuel"
More later....
I saw something on the science channel about an engine that used water inside the cylinder for cooling but the stream created was used to create another power stroke, after combustion at the end of the exhaust stroke a small amount of water was sprayed into the cylinder the resulting steam created the extra power and increased fuel efficiency
not sure if it was the same thing