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1. Althoughrecent yearshave seensubstantial reductions innoxious pollutantsfromindividual motor vehicles, the number of such vehicles has been steadily increasing. (完成时进行式) Consequently, more than 100 citiesin theUnited Statesstill havelevels of carbonmonoxide, particulate matter, andozone(generated by photochemical reactions with hydrocarbons fromvehicle exhaust)that exceed legally established limits.There is a growing realization that the onlyeffectiveway to achieve furtherreductions in vehicleemissions- short of amassiveshiftaway from theprivate automobile- isto replace conventional diesel fuel and gasoline with cleaner-burning fuels such as compressed natural gas, liquefied petroleum gas,ethanol, ormethanol.
1.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(have)超过法律限定的（that exceed legally established limits）一氧化碳、悬浮颗粒物和臭氧（由车辆排出气体中碳氢化合物(hydrocarbons)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(to achieve further reductions invehicle emissions)唯一有效方法--大规模消减私人小汽车除外（short of）--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。(replace A with B 用B替代A)(下文分别有侧重地讲述这四种碳基替代燃料)
2.All of these alternativesare carbon-basedfuels whosemoleculesaresmaller and simplerthanthose ofgasoline.These molecules burnmore cleanly than gasoline, in partbecausetheyhave fewer, if any, carbon-carbonbondsand thehydrocarbons theydoemit are lesslikely to generate ozone. The combustion oflargermolecules,which havemultiple carbon-carbon bonds involves a more complex series of reactions. These reactions increase theprobabilityof incomplete combustionand are morelikely to releaseuncombusted and photochemically active hydrocarboncompounds intothe atmosphere.On the other hand, alternative fuels dohave drawbacks. (1) Compressed natural gaswould require thatvehicles havea set of heavy fuel tanks- a seriousliability in terms ofperformance and fuel efficiency-and (2) liquefied petroleum gas facesfundamentallimits onsupply.
2.所有这些替代物都是碳基燃料(carbon-basedfuels)，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(are lesslikely to)会成为臭氧。较大分子的燃烧，由于它们具有多重碳-碳键，伴随着(involves)更为复杂的一系列反应（a more complex series of reaction）。这些反应增加了不完全燃烧得可能性，并且易于将未燃烧的、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，替代燃料也还有缺点。（1）压缩天然气要求车辆有一套沉重的燃料箱（a setof heavy fuel tanks）-在外观和燃料效率方面有严重的不利之处(a serious liability)--并且(2)液化石油气面临着基本的供应量限制。
3. (3)Ethanoland (4) methanol, on the other hand, have importantadvantagesoverother carbon-based alternative fuels: they have higher energy content per volumeand would require minimalchangesin theexisting networkfor distributingmotor fuel. Ethanoliscommonlyusedasa gasolinesupplement, but it is currently abouttwice as expensive asmethanol, the low cost ofwhichis one of its attractive features. Methanol's most attractivefeature, however, is that it canreduce byabout 90 percentthe vehicle emissions thatform ozone, themost serious urban air pollutant.
3.从另一个方面来看，(3)乙醇和(4)甲醇具有优于其他碳基替代燃料的重要优势(have important advantages over)：(a)它们在单位容量下有更高的能量含量(have higher energy content pervolume)，(b)并且只需要在现存的配送燃料的网络中（in the existing network for distributing motor fuel）做很小的变更。通常用作(be used as)汽油的补充燃料(supplement)，但它现在的价格是甲醇的两倍，后者的低成本(the low cost of which)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.Likeany alternative fuel, methanolhas its critics. Yet much ofthe criticismis based onthe use of "gasolineclone" vehicles thatdonot incorporateeven the simplest designimprovementsthat are made possiblewith the use of methanol. It is true, for example, thata given volume of methanolprovides only aboutone-half of the energy thatgasolineand diesel fuel do; other things being equal, the fuel tank would have to be somewhatlarger and heavier. However, since methanol-fueledvehicles couldbe designed to be muchmore efficient than"gasoline clone" vehicles fueledwithmethanol(so)they would need comparatively less fuel. Vehiclesincorporatingonly the simplest of the engine improvements that methanol makesfeasible wouldstill contribute to an immediatelesseningof urban air pollution.
4.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(that are made possible with theuse of methanol)哪怕最简单的设计上的改进。这些批评意见是对的(这是事实)，举例来说，一定容量的甲醇(a given volume of methanol)只能供给汽油和柴油燃料所供给的能力(that gasoline and diesel fuel do)的一半(one-half)；在其他条件相同时(other things being equal)，不得不将车辆的燃料箱做的较大和较沉些（somewhat larger and heavier）。然而，因为(since)烧甲醇的车辆(methanol-fueled vehicles)可以被设计成比烧甲醇的“克隆汽油车”(gasoline clone vehicles fueledwith methanol)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(that methanol makes feasible)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。
