单项选择题

Natural gas Vehicles 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. Natural Gas Basics 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 (水生的) 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 (沼气) 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. Like gasoline, natural gas is combustible (易燃的), 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. Throughout the 19th century, the use of natural gas remained localized because there was ho way to transport large quantities of gas over long distances. In 1890, the invention of a Teak-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 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. Natural-gas Vehicle Design Natural-gas vehicles use the same basic principles as gasoline-powered vehicles. In other words, the fuel (natural gas in this case) is mixed with air in the cylinder of a four-stroke engine and then ignited by a spark plug to move a piston(活塞) up and down. Although there are some differences between natural gas and gasoline in terms of flammability and ignition temperatures. NGVs themselves operate on the same fundamental concepts as gasoline-powered vehicles. Still, some modifications are required to make an NGV work efficiently. These changes are primarily in the fuel storage tank, the engine and the chassis (底盘). Advantages The biggest advantage of NGVs is that they reduce environmentally harmful emissions. Natural-gas vehicles can achieve up to a 93 percent reduction in carbon monoxide emissions, 33 percent reduction in emissions of various oxides of nitrogen and a 50 percent reduction in reactive hydrocarbons when compared to gasoline vehicles. NGVs also rate higher in particulate matter 10 (PM10) emissions. PM10 particles transport and deposit toxic materials through the air. NGVs that operate in diesel applications can reduce PM10 emissions by a factor of 10. Natural-gas vehicles also offer these benefits: NGVs are safer. The fuel storage tanks on an NGV are thicker and stronger than gasoline or diesel tanks. There has not been an NGV fuel-tank rupture in more than two years in the United States. Natural gas costs are lower than gasoline. On average, natural gas costs one-third less than gasoline at the pump. Natural gas is convenient and abundant. A well-established pipeline infrastructure exists in the United States to deliver natural gas to almost every urban area and most suburban areas. There are more than 1 ,300 NGV fueling stations in the United States, and more are being added every day. Natural gas prices have exhibited significant stability compared to oil prices. Historically, natural gas prices have exhibited significant price stability compared to the prices of petroleum-based fuels. This stability makes it easier to plan accurately for long-term costs. NGVs have lower maintenance costs. Because natural gas burns so cleanly, it results in less wear and tear on the engine and extends the time between tune-ups and oil changes. Disadvantages One of the biggest complaints about NGVs is that they aren’’t as roomy as gasoline cars. This is because NGVs have to give up precious cargo and trunk space to accommodate the fuel storage cylinders. Not only that, these cylinders can be expensive to design and build—a contributing factor to the higher overall costs of a Natural-gas vehicle compared to a gasoline-powered car. Another drawback is the limited driving range of NGVs, which is typically about half that of a gasoline-powered vehicle. For example, Honda’’s natural gas Civic, the Civic GX, can go up to 220 miles without refueling. A typical gasoline-powered Civic can go approximately 350 miles without refueling. If a dedicated NGV ran out of fuel on the road, it would have to be towed to the owner’’s home or to a local natural gas refueling station, which might be harder to find than a "regular" gas station. Finally, it should be noted that natural gas, like gasoline, is a fossil fuel and cannot be considered a renewable resource. While natural gas reserves in the United States are still considerable, they are not inexhaustible. Some predict that there are enough natural gas reserves remaining to last another 67.1 years, assuming that the 2003 level of production continues. This passage explains the basic idea of natural gas and compares the performances of different types of NGVs.