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| 文章出处：学生大考试站 发布时间：2005-10-12 |
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
emi
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