[sammyg2]

How much time does it take the water vapor to absorb the BTUs from combustion and flash to steam?
What temperature does it take to overcome the higher than ambient boiling point? (hint, to develop 180 psig in the power stroke the water vapor would have to get well over 340 degrees to flash to steam).
The only way around that would be to pre-heat the water under high pressure, but then you have given up the cooling capacity except for the BTUs absorbed throuygh the phase change.

Unless this engine has an extremely long stroke and spins at a painfully slow rpm, the water vapor will stay water vapor. There's a reason steam engines spin slow.
Yes it can be built, yes it will run, but no way no how will it make anywhere near as much power as a conventional comperable displacement sized internal combustion engine.