Environmental Engineering Reference
In-Depth Information
Hybrid vehicles have enjoyed greater acceptance, but they will not be a
significant part of the vehicle fleet for many years to come.
5
At a consider-
able distance over the commercial horizon are vehicles powered by fuel
cells that, theoretically at least, emit nothing but water vapor.
6
Although
prototype fuel-cell vehicles are currently being tested by a number of
manufacturers, they are not likely to be a commercial reality until well into
the second decade of the twenty-first century, if even then.
TECHNOLOGIES FOR CLEANER AIR
With alternatives to the internal-combustion engine many years away from
practical application, the motor vehicle industry has had to develop a num-
ber of technologies to reduce the tailpipe emissions produced by conven-
tional engines. The centerpiece of these efforts has been the catalytic
converter. California was the first place to require its use, mandating that all
1975 model cars be so equipped.A catalytic converter has an internal struc-
ture made of tiny ceramic pellets or a ceramic honeycomb that gives the
interior of the converter a surface area the size of a football field.The con-
verter's internal surfaces are coated with metals that catalyze chemical reac-
tions: palladium, rhodium, and platinum. The first catalytic converters
supported only an oxidation process that turned unburned hydrocarbons
and carbon monoxide into water vapor and carbon dioxide. Within a few
years cars began to be equipped with three-way catalytic converters that, in
addition to the first two functions, support a reduction process that turns
oxides of nitrogen into free nitrogen and oxygen.
A car produces the most emissions when a cold engine is started, as cat-
alytic converters work effectively only at operating temperatures of
250-300˚C (480-570˚F).Tests conducted by the Environmental Protection
Agency have shown that in the course of a 10-mile trip made by a catalytic
converter-equipped car 80 percent of hydrocarbon emissions occur during
a warm-up period of 2
1
/
2
minutes.
7
To counter this problem, it is likely that
the next generation of catalytic converters will be kept at operating tem-
perature by the car's electrical system.
Catalytic converters require just the right amount of oxygen admitted
into an engine's combustion chamber. If there is too little, the unburned
hydrocarbons and carbon monoxide are not oxidized; if there is too much,
the NOx will not be reduced to free oxygen and nitrogen. Consequently,
