One intriguing idea I've come across is the six cycle engine. Basically a combination of a normal ICE and an expansion steam engine... at the end of the exhaust (4th) stroke, a small amount of exhaust is retained, and a drop of water injected, which instantly flashes to steam, and powers another expansion stroke. Hasn't really been tried on a large scale, but there's no theoretical reason why it shouldn't work, just technical details (changing cam designs, dealing with water in the cylinder, etc).