Light-duty natural gas vehicles perform and function much like gasoline-powered vehicles with spark-ignited engines. A CNG fuel system transfers high-pressure natural gas from the vehicle’s storage tank to the engine compartment where pressure is reduced to the operating pressure of the engine's fuel-management system. An LNG fuel system holds natural gas at cryogenic temperatures (~ -260 deg F) as a low-pressure (5-100 psi) liquefied natural gas. When required, LNG is vaporized into a gaseous state and sent to the engine compartment near the operating pressure of the engine's fuel-management system. With both CNG and LNG, the natural gas is injected into the engine intake air the same way gasoline is injected into a gasoline-fueled engine.
Some heavy-duty vehicles use spark-ignited natural gas systems, with a lower torque rating for a given engine displacement than a high-pressure direct injection engines that burns natural gas in a compression-ignition (Diesel) cycle, where diesel fuel is injected into the combustion chamber along with the natural gas. Diesel ignites when the piston nears top dead center (highest pressure) and this combustion ignites the natural gas. As in spark-ignited engines, expanding gases in a compression-ignition engine produce mechanical energy in the form of rotational forces that propel the vehicle.
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