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.
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 mul-
tiple carbon-carbon bonds, involves a more complex series
of reactions. These reactions increase the probability of
incomplete combustion and are more likely to release unco-
mbusted 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 a set of heavy fuel tanks—a serious
liability in terms of performance and fuel efficiency --
and liquefied petroleum gas faces fundamental limits on
supply.
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 changes in the existing network for distributing
motor fuel. Ethanol is commonly used as a gasoline supple-
ment, but it is currently about twice as expensive as meth-
anol, the low cost of which is one of its attractive featu-
res. Methanol's most attractive 30feature, however, is that
it can reduce by about 90 percent the vehicle emissions that
form ozone, the most 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 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 pro-
vides only about one-half of the energy that gasoline and di-
esel fuel do; other things being equal, the fuel tank would
have to be somewhat larger and heavier. However, since
methanol- 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.