Abstract: A fuel supply system for an internal combustion engine utilizing hydrogen, natural gas or other alternate fuel is described which includes a pressure vessel for containing liquid fuel in a first compartment and gaseous fuel in a second compartment, the compartments separated by a movable piston configured to maintain the fuel in liquid and gaseous phases by maintaining constant, but different pressures, respectively, in the two compartments regardless of fuel quantity in either compartment. A thermal expansion system interconnecting the two compartments draws fuel from the first compartment and expands the fuel to a superheated gas into the second compartment for transfer to the engine.