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1. Althoughrecent yearshave seensubstantial reductions innoxious pollutantsfromindividual motor vehicles, the number of such vehicles has been steadily increasing. (完成時進行式) 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.盡管近幾年來私人機動車輛排放的有害污染物已有相當程度的減少，但這類車輛的數(shù)量卻仍在穩(wěn)定地增長。因此，美國100多個城市仍然存在著(have)超過法律限定的（that exceed legally established limits）一氧化碳、懸浮顆粒物和臭氧（由車輛排出氣體中碳氫化合物(hydrocarbons)經過光化反應產生）。人們逐漸認識到實現(xiàn)車輛尾氣排放更進一步減少的(to achieve further reductions invehicle emissions)唯一有效方法--大規(guī)模消減私人小汽車除外（short of）--是用燃燒更為清潔的燃料，如壓縮天然氣、液化石油氣、乙醇或甲醇代替?zhèn)鹘y(tǒng)的柴油和汽油。(replace A with B 用B替代A)(下文分別有側重地講述這四種碳基替代燃料)
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.所有這些替代物都是碳基燃料(carbon-basedfuels)，它的分子要比汽油的小而簡單。這些分子要比汽油燃燒得更為清潔，部分是因為他們碳-碳鍵較少，而且即使會釋放出碳氫化合物，也不太可能(are lesslikely to)會成為臭氧。較大分子的燃燒，由于它們具有多重碳-碳鍵，伴隨著(involves)更為復雜的一系列反應（a more complex series of reaction）。這些反應增加了不完全燃燒得可能性，并且易于將未燃燒的、易起光化反應的碳氫化合物釋放到大氣中。從另一方面來說，替代燃料也還有缺點。（1）壓縮天然氣要求車輛有一套沉重的燃料箱（a setof heavy fuel tanks）-在外觀和燃料效率方面有嚴重的不利之處(a serious liability)--并且(2)液化石油氣面臨著基本的供應量限制。
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.從另一個方面來看，(3)乙醇和(4)甲醇具有優(yōu)于其他碳基替代燃料的重要優(yōu)勢(have important advantages over)：(a)它們在單位容量下有更高的能量含量(have higher energy content pervolume)，(b)并且只需要在現(xiàn)存的配送燃料的網絡中（in the existing network for distributing motor fuel）做很小的變更。通常用作(be used as)汽油的補充燃料(supplement)，但它現(xiàn)在的價格是甲醇的兩倍，后者的低成本(the low cost of which)是具有吸引力的特征之一。不過，甲醇最吸引人的特征是它能夠將形成臭氧的車輛尾氣這一嚴重的城市空氣污染減少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.像任何替代燃料一樣，甲醇也有對其的批評意見。然而，大部分批評都是針對“克隆汽油車”的，這些車并沒有為使用甲醇而做出(that are made possible with theuse of methanol)哪怕最簡單的設計上的改進。這些批評意見是對的(這是事實)，舉例來說，一定容量的甲醇(a given volume of methanol)只能供給汽油和柴油燃料所供給的能力(that gasoline and diesel fuel do)的一半(one-half)；在其他條件相同時(other things being equal)，不得不將車輛的燃料箱做的較大和較沉些（somewhat larger and heavier）。然而，因為(since)燒甲醇的車輛(methanol-fueled vehicles)可以被設計成比燒甲醇的“克隆汽油車”(gasoline clone vehicles fueledwith methanol)更有效率的車輛，所以它們只需要比較少的燃料。車輛只要包含為使用甲醇而作的(that methanol makes feasible)發(fā)動機方面最簡單的改進，也將為迅速減輕城市空氣污染做出貢獻。
