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A Student's Guide to Alternative Fuel Vehicles

Liquefied Natural Gas (LNG) - natural gas that is very, very cold

Natural gas can be made into three forms. One kind is the low-pressure form you use to cook or heat your home. It comes from the underground pipe from the gas company. Another form is compressed natural gas (CNG). This form is compressed into high-pressure fuel cylinders to power a car or truck. It comes from special CNG fuel stations. The third form is liquefied natural gas (LNG). LNG is made by refrigerating natural gas to condense it into a liquid. The liquid form is much more dense than natural gas or CNG. It has much more energy for the amount of space it takes up. So, much more energy can be stored in the same amount of space on a car or truck. That means LNG is good for large trucks that need to go a long distance before they stop for more fuel.

Liquefied natural gas is made by refrigerating natural gas to minus 260 degrees Fahrenheit (260 degrees below zero!) to condense it into a liquid. This is called liquefaction. The liquefaction process removes most of the water vapor, butane, propane, and other trace gases, that are usually included in ordinary natural gas. The resulting LNG is usually more than 98 percent pure methane. As this was first written (in 1999), Caterpillar, Cummins, Detroit Diesel, Mack and Navistar sell heavy-duty natural gas engines that can operate on LNG.

Liquefied Petroleum Gas (LPG) - hydrocarbon gases under low pressure

Most people call it propane. Quantum LPG Van

Most people call liquefied petroleum gas (LPG) "propane." That is because LPG is mostly made up of propane. Actually, LPG is made of a mixture of propane and other similar types of hydrocarbon gases. Different batches of LPG have slightly different amounts of the different kinds of hydrocarbon molecules. These hydrocarbons are gases at room temperature, but turn to liquid when they are compressed. LPG is stored in special tanks that keep it under pressure, so it stays a liquid. The pressure of these tanks is usually about 200 pounds per square inch (abbreviated "psi").

Most LPG produced in the U.S. comes from natural gas wellhead processing. That is because natural gas has LPG gases and water vapor in it, which have to be removed before the natural gas can be sent away in pipelines. Most of the LPG produced in California comes from petroleum refining.

The LPG used in vehicles, such as the Quantum / ProCom's Propane Van pictured above, the same as that used in gas barbecues and camper appliances. LPG is also used in many homes in the country, where there are no natural gas pipelines. These homes use LPG for heating, cooking, hot water and other energy needs.

LPG fueled engines can pollute less than gasoline and diesel engines. LPG usually costs less than gasoline for the same amount of energy. In some countries LPG is used much more for vehicle fuel than in California. In the Netherlands over 10 percent of the motor fuel used is LPG.

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Page updated: April 22, 2002
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