TOEFL Listening: ETS-TOEFL听力机经 - U53N9GGH0L48X43DQ$

Lecture: Transportation fuel: Narrator: Listen to part of a lecture in an environmental science class. Professor: In research years, there's been a lot of attention paid to the use of ethanol as a supplement for gasoline in the United States. And for those of you who don't know what ethanol is, let me just explain. Ethanol is simply a kind of alcohol. And the ethanol added to transportation fuel in the United States comes primarily from corn. Basically, you harvest the corn, ferment it to turn the silvergenic into alcohol, and then distill the alcohol to concentrate and purify it. As I was saying, ethanol is used extensively in the US today as an additive to gasoline. Ethanol supplies energy in the same way gasoline does in an internal combustion engine. Mixing some ethanol into gasoline means that you don't burn as much gasoline to run your car because some of what is burning is ethanol instead. So, what are some benefits of using ethanol as a transportation fuel? Well, it's renewable. It's not a fossil fuel where you pump it out of the ground and sooner or later it's all gone. Also, there's much less emission of carbon dioxide when you burn ethanol as a fuel versus as burning gasoline. However, the current US program of using ethanol as a gasoline supplement doesn't really make a lot of sense from the point of view of environmental impact and energy efficiency. First, a liter of ethanol doesn't have as much energy as a liter of gasoline. It's got about one third less energy in fact. So you need more ethanol to go the same distance in your car as gasoline. Also, although ethanol is a green fuel because corn could be replanted year after year, creating ethanol from corn is actually quite energy intensive. I mean, if you know anything about agriculture in this country, you know it's not just sowing some seeds in the spring, waiting then harvesting the results in the fall. Farming takes fertilizer, lots of fertilizer and currently it takes up fossil fuel, natural gas, to produce that fertilizer and to refine the ethanol, too. Industrial scale ethanol production, you have to heat giant vast of mass corn using natural gas first to prevent the corn and then to distill and purify the ethanol and you have to haul it in tanker trucks which run primarily on fossil fuel and oil. You can transport oil through pipelines. But ethanol cannot be sent through pipelines because if it accidentally gets mixed with water, it's useless. You'd have to distill it all over again. And it's impossible to make a long pipeline that doesn't leak a little bit of water. There's various calculations out there but there's a general agreement that a lot of energy goes into the production and distribution of corn based ethanol in this country. It probably takes about three quarters of a unit of fossil fuel energy to get one unit of ethanol energy. The other thing about ethanol production as it's done here is that it uses only the corn kernels, the part we eat so the more corn used to produce ethanol, the less food you have. Now, at the moment you know, we've decide that this is okay in this country. But there could come a time where we're no longer comfortable doing that; using corn that could be feeding people to you know, fuel our vehicles instead. So does this all mean that trying to produce ethanol as a fuel is never a good idea? No. That's not what I intend to say here. In fact, there are other parts of the world like Brazil especially. They use ethanol in Brazil, too. But their ethanol is made from sugar cane. It's just so much less energy intensive to grow sugar cane in the tropics than it is to grow corn in the American Midwest. Yeah, there are some environmental tradeoffs to sugar cane cultivation, too. But that's a topic for another day.