I accidentally stumbled on this article at Autoweek about a racing engine builder named Bruce Crower who is tinkering with a six-cycle engine idea.
What Crower proposes to do is to add an additional compression and power stroke to a standard 4 cycle IC engine. Water is injected, which turns to steam, expands, and drives the piston down again, while also cooling the cylinder. Crower proposes that by alternately hitting the piston with gas and water, he can improve fuel efficiency (as long as you don't count the water as fuel) and eliminate the need for a liquid cooling system.
One big problem is that water is heavy, and according to the article,the engine consumes water at about the same rate as fuel. In order to be truly efficient, a vapor recovery system would have to be installed that wold recondense the wastewater so it could be recycled into the engine.
Another issue is the problem of lubrication--how to keep the oil dry and the metal oily.
Also, the question of power balance--how much torque does a steam shot generate compared with a gasoline or diesel combustion?
Still, it seems like such a simple and ingenious idea, it would be interesting to see if it goes anywhere.