热门搜索:如何备考GRE GRE填空 GRE数学机经 GRE模考
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.
Amaketh
[阅读&逻辑 ]新版冲分救命题 -11894
Select the sentence that presents a conventional requirement that is rejected by the author of the passage.
[阅读&逻辑 ]精选题 -11861
Which of the following, if true of the cave, most weakens the argument?
[阅读&逻辑 ]精选题 -11860
It can be inferred that the “sediments
[阅读&逻辑 ]精选题 -11859
Which of the following best describes the issue raised by the question in the highlighted portion of the passage?
[阅读&逻辑 ]精选题 -11858
It can be inferred that which of the following is true of the “geologic evidence”?
[阅读&逻辑 ]精选题 -11857
Which of the following elements in the debate over the origin of Varanus is NOT provided in the passage?
[阅读&逻辑 ]精选题 -11856
It can be inferred from the passage that the geographical distribution of the Komodo dragon is
[阅读&逻辑 ]精选题 -11855
Which of the following, if discovered to be true, would most tend to suggest that the “correlation” mentioned in the passage held true for the people of the West Branch site?
[阅读&逻辑 ]精选题 -11854
The final sentence of the passage primarily serves to do which of the following?
[阅读&逻辑 ]精选题 -11853
The passage implies which of the following about beavers?
[阅读&逻辑 ]精选题 -11852
The passage provides support for which of the following statements about beaver dams?
[阅读&逻辑 ]精选题 -11851
According to the passage, which of the following best accounts for the apparent bias in antelope-migration direction?
[阅读&逻辑 ]精选题 -11850
The author of the passage implies that during the
[阅读&逻辑 ]精选题 -11849
According to the passage, which of the following is true of an Eurasia-Africa land bridge during ice ages?
[填空 ]精选题 -11843
The political upheaval caught most people by surprise: despite the ______ warning of some commentators, it had never seemed that imminent.
[填空 ]精选题 -11842
Even if the story now seems a surprisingly innocuous overture to the author’s later, more fully developed narrations, it ____ some of the key traits of those bleaker tales.
[填空 ]精选题 -11841
William Perkins, his ______ speaking style notwithstanding, has long been seen as the moderate face of his political party.
[填空 ]精选题 -11840
Just because, as a photographer, Friedlander (i)____ places that most people consider ugly does not mean that he is out to prove they are beautiful. Instead, his work suggests that the photographer simply cannot ignore so much of the built American landscape but is obligated to (ii)_____ what we pass through day in and day out, regardless of (iii)_____.
[填空 ]精选题 -11839
In the search for truth, the knowledge gained by scientists consists of approximations with varying degrees of certainty. Such (i)_____ truth can be highly (ii)_____, as the rapid and relatively steady progress of medical knowledge well exemplifies.
[填空 ]精选题 -11838
History teaches us that science is not _________ enterprise, indeed, it is quite the opposite, a motley assortment of tools designed to safeguard researchers against their own biases.
微信公众号:greonline
高效冲分,预见你想象的目标分!
1.使用雷哥网GRE的时间?
2.通过何种渠道了解到雷哥网GRE?
3.使用雷哥GRE的用途?(可多选)
4.使用雷哥网GRE曾遇到的问题?(可多选)
5.你对雷哥网GRE的优化建议(可多选)
6.向朋友推荐雷哥网GRE的可能性?
不会
不一定
可能
很可能
一定