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Although, recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such vehicles has been steadily increasing, consequently, more than 100 cities in the United States still have levels of carbon monoxide, particulate matter, and ozone (generated by photochemical reactions with hydrocarbons from vehicle exhaust) that exceed legally established limits. There is a growing realization that the only effective way to achieve further reductions in vehicle emissions—short of a massive shift away from the private automobile—is to replace conventional diesel fuel and gasoline with cleaner-burning fuels such as compressed natural gas, liquefied petroleum gas, ethanol, or methanol.
All of these alternatives are carbon-based fuels whose molecules are smaller and simpler than those of gasoline. These molecules burn more cleanly than gasoline, in part because they have fewer, if any, carbon-carbon bonds, and the hydrocarbons they do emit are less likely to generate ozone. The combustion of larger molecules, which have multiple carbon-carbon bonds, involves a more complex series of reactions. These reactions increase the probability of incomplete combustion and are more likely to release uncombusted and photochemically active hydrocarbon compounds into the atmosphere. On the other hand, alternative fuels do have drawbacks. Compressed natural gas would require that vehicles have a set of heavy fuel tanks—a serious liability in terms of performance and fuel efficiency—and liquefied petroleum gas faces fundamental limits on supply.
Ethanol and methanol, on the other hand, have important advantages over other carbon-based alternative fuels: they have a higher energy content per volume and would require minimal changes in the existing network for distributing motor fuel. Ethanol is commonly used as a gasoline supplement, but it is currently about twice as expensive as methanol, the low cost of which is one of its attractive features. Methanol’s most attractive feature, however, is that it can reduce by about 90 percent the vehicle emissions that form ozone, the most serious urban air pollutant.
Like any alternative fuel, methanol has its critics. Yet much of the criticism is based on the use of “gasoline clone” vehicles that do not incorporate even the simplest design improvements that are made possible with the use of methanol. It is true, for example, that a given volume of methanol provides only about one-half of the energy that gasoline and diesel fuel do; other things being equal, the fuel tank would have to be somewhat larger and heavier. However, since methanol-fueled vehicles could be designed to be much more efficient than “gasoline clone” vehicles fueled with methanol, they would need comparatively less fuel. Vehicles incorporating only the simplest of the engine improvements that methanol makes feasible would still contribute to an immediate lessening of urban air pollution.
[填空 ]冲分救命题 -9341
One of the virtues of the essay collection is that it presents_____ views side by side, thereby highlighting the contentious nature of many of the arguments within the discipline.
[阅读&逻辑 ]冲分救命题 -9340
The highlighted sentence serves primarily to
[阅读&逻辑 ]冲分救命题 -9339
The author would most likely agree with which of the following assertions about the environmental impact of hydroelectric power plants?
[阅读&逻辑 ]冲分救命题 -9338
The passage suggests that which of the following contributed to the erosion after the First World War of the progress previously realized by the labor movement in the United States?
[阅读&逻辑 ]冲分救命题 -9337
According to the passage, which of the following statements about the National War Labor Board is true?
[阅读&逻辑 ]冲分救命题 -9336
It can be inferred from the passage that which of the following was one of Local 36’s grievances against the Little Rock laundry owners?
[阅读&逻辑 ]冲分救命题 -9335
The passage is primarily concerned with discussing
[阅读&逻辑 ]冲分救命题 -9334
According to the passage, Appalachian square dancing differs from English country dancing in that Appalachian square dancing
[阅读&逻辑 ]冲分救命题 -9333
The passage suggests that which of the following was true of
[阅读&逻辑 ]冲分救命题 -9332
The statements given are structured to lead to which of the following conclusions?
[阅读&逻辑 ]冲分救命题 -9331
Consider each of the choices separately and select all that apply. It can be inferred that historical demographers generally believe which of the following about communities of fewer than 10,000 people in early-modern Germany?
[阅读&逻辑 ]冲分救命题 -9330
The author suggests that according to historical demographers, the birth rate in early-modem German cities
[阅读&逻辑 ]冲分救命题 -9329
Which of the following most logically completes the argument?
[阅读&逻辑 ]冲分救命题 -9328
The highlighted portion of the passage serves primarily to
[阅读&逻辑 ]冲分救命题 -9327
According to the passage, certain African literatures have
[阅读&逻辑 ]冲分救命题 -9326
It can be inferred from the passage that the Tea Cess Committee reorganized as the Indian Tea Market Expansion Board in order to
[阅读&逻辑 ]冲分救命题 -9325
The passage suggests that "small packets of leaf fragments" were
[阅读&逻辑 ]冲分救命题 -9324
In the argument in the editorial, the part that is highlighted plays which of the following roles?
[阅读&逻辑 ]冲分救命题 -9323
Consider each of the choices separately and select all that apply.Which of the following can be inferred from the passage about the"period of relative warmth"
[阅读&逻辑 ]冲分救命题 -9322
Which of the following best describes the organization of the passage?