Environmental Engineering Reference
In-Depth Information
But if one considers the synthetic fuel penalties involved in preparing hydrogen, both gaseous and
liquid, or methanol, as done in Table 8.5, then the fuel cell vehicles, at least at this stage of their
development, have little or no fuel economy advantage.
There is no question that fuel cell vehicles suffer from the limited availability and high cost of
hydrogen fuel. Commercial-scale hydrogen reformers fueled by natural gas are complex systems,
with the product stored at high pressure in tanks for transfer to vehicles. Hydrogen is especially
prone to burn and explode, raising safety problems even for fleet vehicles that can be more carefully
supervised than individually owned vehicles. On the other hand, on-board reformers eliminate the
hydrogen supply problem, yet they work satisfactorily only with pure fuel input, such as methanol or
ethanol, but not so far with diverse hydrocarbon mixtures like gasoline or diesel fuel. Development
of on-board reformers (such as used in NECAR 3) and a common fuel would make fuel cell vehicles
suitable substitutes for conventional ones.
8.7
VEHICLE EMISSIONS
By the middle of the twentieth century, vehicle exhaust emissions were recognized to be an important
contributor to urban photochemical air pollution, especially in locations like southern California
where high insolation and temperature and poor atmospheric ventilation combined with a rapidly
growing automobile population to produce record levels of ground level ozone concentrations.
Not long thereafter, similar pollution problems appeared in other major cities around the world as
urban vehicle populations blossomed. These problems are more acute in lower latitude locations,
especially in developing countries where vehicle emission controls are not yet stringent.
Of course, vehicles provide only part of ozone precursor emissions. But they are mobile and
more numerous than stationary sources, and they present different problems for abatement. Early
in the pollutant regulatory history of the United States, it became obvious that it was more effective
to require a few vehicle manufacturers to install control equipment on millions of new vehicles
rather than to require millions of vehicle owners to try to reduce their own vehicle's emissions.
This scheme, adopted by all developed nations, replaces all vehicles with new and cleaner ones
every 12-15 years, providing the opportunity to capitalize on the improvements in emission control
technology. In the United States, it has resulted in significant reductions in air pollutant emissions
from vehicles.
In this section we discuss the vehicle technologies that are used to reduce the emissions from
ICE engines in vehicles in response to national regulations.
8.7.1
U.S. Vehicle Emission Standards
Vehicle emissions to the atmosphere are of two kinds: exhaust emissions and evaporative emissions.
The first are the combustion gases emitted while the engine is running, whether or not the vehicle
is moving. The second are emissions of fuel vapors from the fuel supply system and the engine,
when the vehicle is stationary with the engine not operating.
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The federal government regulates
both of these emissions by requiring the manufacturers of new vehicles sold in the United States
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Emission of fuel vapor while refueling may be separately regulated by state agencies under state implemen-
tation plans for conforming with federal air-quality regulations.
