Bruce Crower has an interesting idea for a six stroke engine that uses both gasoline and steam. His motor uses the standard four strokes of a modern gasoline engine, but adds two more steam strokes in an attempt to scavenge more useful work from the engines waste heat. On the fifth stroke, he injects water into the cylinder which then flashes to steam upon contact with the hot metal. The sixth stroke exhausts the steam to a condenser track where it would be reused.
The problems? Well flashing water to steam using the surfaces in the cylinder sounds like a bad idea to me. It just seems like a good way to promote oxidation and corrosion. More importantly though, you're unlikely to get a perfect scavenge from the transitions between the gasoline and steam cycles. This means the steam stroke is going to pickup exhaust components from the gasoline cycle and vice versa. The steam side will need to try to filter these components out or they'll break down and cause problems. Having a little steam in the fuel to air mixture probably won't cause too much trouble but you can't run a closed steam system if you are constantly exhausting steam like that.
If you wanted to do a gas-steam hybrid, you probably want to use the coolant and exhaust systems of the internal combustion engine to provide heat instead of going directly for the combustion chamber itself. Then use a separate steam motor to keep the loop closed. But of course then you run into problems with the additional weight of a motor, etc.