Photo courtesy Honda A Honda Civic GX being refueled using a home refueling appliance.  See more pictures of alternative fuel and alternative fuel cars.

Kermit the Frog once said, "It's not that easy bein' green." Although he wasn't referring to cars, his observation seems particularly appropriate for the auto industry today: Designing, developing and marketing "green" cars has not been an easy task, which is why gasoline-powered vehicles still rule the road and fossil fuels still account for almost 75 percent of the world's energy consumption. As gasoline prices soar and concern over harmful emission mounts, however, cars that run on alternate fuel sources will become increasingly important. A natural-gas vehicle, or NGV, is the perfect example of such a car -- it's fuel-efficient, environmentally friendly and offers a relatively low cost of ownership.

This article will explain the basic idea of NGVs, look at the unique design challenges involved and examine the advantages and benefits of this technology.

Thank You Thank you to Jeff Beckman for his assistance with this article.

Natural Gas Basics

Discovery Some scholars speculate that people living in the Middle East discovered this property of natural gas between 6000 and 2000 BC when lightning strikes ignited natural gas seeping from the ground. The Chinese burned natural gas as early as 211 BC to dry rock salt found in limestone rock. They also drilled the first known natural gas wells, using bamboo poles and primitive bits, but they were only able to reach depths of 500 feet.

The word "gas" is a confusing term because it is used to describe many different substances that are similar but not exactly the same. For example, the "gas" you put in your car is gasoline, one component of crude oil, or petroleum. Petroleum is a dark, sticky liquid mixture of compounds formed underground by the decay of ancient marine animals.

Natural gas also comes from the decay of ancient organisms, but it naturally takes a gaseous form instead of a liquid form. Natural gas commonly occurs in association with crude oil. It is derived from both land plants and aquatic organic matter and forms above or below oil deposits. It is often dissolved in crude oil at the high pressures existing in a reservoir. There are also reservoirs of natural gas, known as non-associated gas, that contain only gas and no oil.

Natural gas consists primarily of methane and other hydrocarbon gases. Hydrocarbons are organic compounds composed only of the elements carbon and hydrogen. The hydrocarbons in natural gas are called saturated hydrocarbons because they contain hydrogen and carbon bound together by single bonds. As the diagrams show, methane is the simplest saturated hydrocarbon.

Like gasoline, natural gas is combustible, which means it can be used in a combustion engine like gasoline. But cars that could burn natural gas didn't appear on the scene until the 1930s.

From Field to Ford
We extract natural gas trapped in underground reservoirs by drilling wells into the earth. A modern well, equipped with diamond-studded drill bits, can drill to depths approaching 25,000 feet.

Extraction and Processing Most modern wells extract both crude oil and natural gas at the same time. Some natural gas can be used as it comes from the well without any refining, but most requires processing. Natural gas processing consists of separating all the various hydrocarbons and fluids from the "wet" natural gas until it is "dry." Dry natural gas is pure methane, which is the fuel of choice for many consumer applications, including natural-gas vehicles.

Throughout the 19th century, the use of natural gas remained localized because there was no way to transport large quantities of gas over long distances. In 1890, the invention of a leak-proof pipeline coupling made it possible to transport gas miles from the source. Improvements in pipeline technology continued over the next two decades until long-distance gas transmission became practical. From 1927 to 1931, laborers constructed more than 10 major natural gas transmission systems in the United States, making natural gas a viable energy source for many applications. The oil shortages of the late 1960s and early 1970s brought renewed interest in natural gas as a fuel source, especially for automobiles.

Today, owners of natural-gas vehicles can fill up their cars at one of 1,300 fueling stations located in the United States. Honda also offers a personal natural gas pump to people who purchase its natural-gas-powered Civic. The pump uses a home's existing natural gas lines and can be installed for $500 to $1500.

In the next section we'll discuss how natural-gas vehicles are designed.

Bio-gas Natural gas is not the only source of methane. Methane can also be obtained by fermenting organic matter, such as manure, in low levels of oxygen. In such conditions, bacteria will use the nutrients in manure as a food source and will release methane and carbon dioxide as waste products. This methane, known as bio-gas, can be collected and used as a fuel source.