Abstract: An engine operable using flash steam expansion comprising one or more cylinders each having a piston and one or more water injection ports, the pistons driven by flash steam expansion when injected water rapidly change state from liquid to gas. Another engine operable in an internal combustion mode and a flash steam expansion mode comprising one or more cylinders having a piston, an air intake port and valve, and first and second exhaust ports and valves, first, second, and third cam shafts, wherein the engine is selectably operable in a four-cycle mode or a two-cycle mode. A method of operating an engine in a four-cycle internal combustion mode and a two-cycle flash steam expansion mode comprising the steps of using internal combustion in a four-cycle sequence, detecting when a temperature of the engine reaches a predetermined temperature; and terminating internal combustion and using flash steam expansion in a two-cycle sequence.