1.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.
翻译：尽管近几年来私人机动车辆排放的有害污染物已有相当程度的减少，但这类车辆的数量却仍在稳定地增长。因此，美国100多个城市仍然存在着超过法律限定的一氧化碳、悬浮颗粒物和臭氧(由于车辆排出气体中碳氢化合物经过光化反应产生)。人们逐渐认识到实现车辆尾气排放更进一步减少的有效方法--大规模消减私人小汽车除外--是用燃烧更为清洁的燃料，如压缩天然气、液化石油气、乙醇或甲醇代替传统的柴油和汽油。
2.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 set of heavy fuel tanks- a serious liability in terms of performance and fuel efficiency-and liquefied petroleum gas faces fundamental limits on supply.
翻译：所有这些替代物都是碳基燃料，它的分子要比汽油的小而简单。这些分子要比汽油燃烧得更为清洁，部分是因为他们碳-碳键较少，而且即使会释放出碳氢化合物，也不太可能会成为臭氧。较大分子的燃烧，由于它们具有多重碳-碳，伴随着更为复杂的一系列反应。这些反应增加了不完全燃烧得可能性并且易于放出未燃烧的、易起光化反应的碳氢化合物释放到大气中。从另一方面来说，替代燃料也有缺点。压缩天然气要求车辆有一套沉重的燃料箱--在外观和燃料效率方面有严重的不利之处--并且液化石油气面临着基本的供应量限制。
3.Ethanol and methanol, on the other hand, have important advantages over other carbon-based alternative fuels: they have 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.
翻译：从另一个方面来看，乙醇和甲醇具有由于其他碳基替代燃料的重要优势：它们在单位容量下有更高的能量含量，并且只需要在现存的配送燃料的网络中做很小的变更。乙醇通常用作汽油的补充燃料，但它现在的价格是甲醇的两倍，后者的低成本是具有吸引力的特征之一。不过，甲醇最吸引人的特征是它能够将形成臭氧的车辆尾气这一严重的城市空气污染减少90%。
4.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 melhanol-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.
翻译：像任何替代燃料一样，甲醇也有对其的批评意见。然而，大部分批评都是针对“克隆汽油车”的，这些车并没有为使用甲醇而做出哪怕最简单的设计上的改进。这些批评意见是对的，举例来说，一定容量的甲醇只能供给汽油和柴油燃料所能供给的能力的一伴。在其他条件相同时，不得不将车辆的燃料箱做的较大和较沉些。然而，因为烧甲醇的车辆可以被设计成比烧甲醇的“克隆汽油车”更有效率的车辆，所以它们只需要比较少的燃料。车辆只要包含为使用甲醇而作的发动机方面最简单的改进，也将为迅速减轻城市空气污染做出贡献。
