Another engine idea
 

Why not make an engine that is a combination of a standard gasoline engine and a STEAM ENGINE COMBINATION. Maybe a 6 cylinder with 4 burning gas and the other 2 working on STEAM. The 4 burning gas will produce heat making steam and then its pumped into the 2 steam cylinders,,,,This would have to save gas wouldnt it?

what you have to remember though is that steam is a very inefficient power source, the energy it takes to make the steam is not even close to the energy put out by the steam... and if your engine is putting out enough extra heat that it can make the steam... that means your engine isnt very efficient... so in theory it may seem like a good idea but in practice it would reduce efficiency and increase emissions