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93-04 section 4
Although, recent years have seen
substantial reductions in noxious
pollutants from individual motor
vehicles, the number of such vehicles
(5) has been steadily increasing conse-
quently, more than 100 cities in the
United States still have levels of
carbon monoxide, particulate matter,
and ozone (generated by photochemical,
(10) 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
(15) 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
(20) 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
(25) gasoline, in part because they have
fewer, if and , carbon-carbon bonds,
and the hydrocarbons they do emit are
less likely to generate ozone. The
combustion of larger molecules, which
(30) have multiple carbon-carbon bonds,
involves a more complex series of
reactions. These reactions increase
the probability of incomplete com-
bustion and are more likely to release
(35) uncombusted and photochemically active
hydrocarbon compounds into the atmos-
phere. On the other hand, alternative
fuels do have drawbacks. Compressed
natural gas would require that vehicles
(40) have a set of heavy fuel tanks-a
serious liability in terms of perfor-
mance and fuel efficiency and liquefied
petroleum gas faces fundamental limits
on supply.
(45) 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
(50) 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
(55) 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
(60) 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
(65) 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
(70) 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-
(75) 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
(80) simplest of the engine improvements
that methanol makes feasible would
still contribute to an immediate
lessening of urban air pollution.
The author of the passage is primarily
concerned with
a) countering a flawed argument
that dismisses a possible solution
to a problem
b) reconciling contradictory points
of view about the nature of a problem
c) identifying the strengths of
possible solutions to a problem.
d) discussing a problem and arguing
in favor of one solution to it
e) outlining a plan of action to solve
a problem and discussing the obstacles
blocking that plan.
————————————————————————
答案：(C)
尽管最近几年中已目睹了各别的机动车辆有害污染物质的大量减少，但这类机
动车辆的数量却有增无减。因此，美国100多个城市仍存在着超过法定限度的一氧
化碳、微粒物质、以及臭氧（由涉及车辆废气中碳氢化合物的光化反应所致）含量。
人们日趋强烈地意识到，要取得车辆尾气排放的进一步减少，唯一有效的手段——
除了大规模摆脱私人轿车——是不再使用传统的柴油机燃产和汽油，而代之发燃烧
得更为干净的燃料，例如压缩天然气、液化石油气、乙醇、或甲醇。
所有这些可供选择的替代手段均是碳基（carbon-based）燃料，其分子要比汽
油的分子来得更小，更简单。这些分子要比汽油燃烧得更为干净，部分程度上是因
为它们具有为数较少的（即使真有的话）碳—碳键，并且它们即使真的会释放出碳
氢化合物，亦不太可能会产生臭氧。较大分子的燃烧，由于具有多重碳—碳键，涉
及到一系列更为复杂的反应。这些反应会增加不完全燃烧的可能性，并更有可能将
未被燃烧的和光化活跃的碳氢化合物释入到大气中。从另一方面来说，可供选择的
替代燃料也有其缺陷。压缩天然气将要求车辆备有一整套笨重的燃料箱——这在车
辆的表现和燃料效率方面不啻是一个严重的不利因素。而液化石油气所面临的是根
本上的供应量限制。
从另一方面来看，乙醇和甲醇具有某些优于碳基替代燃料的重要有利因素：它
们每一容量中具有更高的能量含量，并几乎不需要在现存的配送发动机燃料的网络
中作太大的变更。乙醇普遍用作汽油的补充物，但目前它的价格是甲醇的二倍，而
就甲醇来说，低成本是它吸引人的特点之一。然则，甲醇最吸引人的特点是，它能
将形成臭氧的车辆尾气这一最严重的城市空气污染物数量降低90%。
与其它任何可供选择的替代燃料一样，甲醇也不乏其批评者。然而，在部分批
评都以“汽油系列”车辆的使用为基础，而这些车辆却不包含由于甲醇的使用而得
以成为可能的最简单的设计改进。诚然，举例来说，一特定容量的甲醇仅可提供汽
油和柴油燃料所能提供的能量的约一半；在其它条件相等的情况下，燃料箱将不得
不做得稍大加稍沉些。然则，既然在甲醇为燃料的车辆可被设计成比以甲醇为燃料
的“汽油系列”车辆更省油得多，故它们所需的燃料相对较少。车辆一旦包含由于
甲醇的使用而得以成为可行的仅仅是最简单的引擎改进，将仍能对迅速减轻城市空
气污染作出贡献。
[此贴子已经被pear于2003-1-12 13:45:50编辑过] |
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