How does a cryogenic engine work?

The cryogenic engine gets its name from the extremely cold temperature at which liquid nitrogen is stored. Air moving around the vehicle is used to heat liquid nitrogen to a boil. Once it boils, it turns to gas in the same way that heated water forms steam in a steam engine. A rocket like the Ariane 5 uses oxygen and hydrogen, both stored as a cryogenic liquid, to produce its power. The liquid nitrogen, stored at -320 degrees Fahrenheit, is vaporized by the heat exchanger. Nitrogen gas formed in the heat exchanger expands to about 700 times the volume of its liquid form. This highly pressurised gas is then fed to the expander, where the force of the nitrogen gas is converted into mechanical power.