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Introduction:-
Emissions
from an individual car are generally low, relative to the
smokestack image many people associate with air pollution.
But in numerous cities across the country, the personal
automobile is the single greatest polluter, as emissions
from millions of vehicles on the road add up. Driving a
private car is probably a typical citizen's most "polluting"
daily activity.
ces
of Auto Emissions
The
power to move a car comes from burning fuel in an engine.
Pollution from cars comes from by-products of this combustion
process (exhaust) and from evaporation of the fuel itself.
The Combustion Process
 Gasoline
and diesel fuels are mixtures of hydrocarbons, compounds
which contain hydrogen and carbon atoms. In a "perfect"
engine, oxygen in the air would convert all the hydrogen
in the fuel to water and all the carbon in the fuel to carbon
dioxide. Nitrogen in the air would remain unaffected. In
reality, the combustion process cannot be "perfect,"
and automotive engines emit several types of pollutants.
"Perfect" Combustion:
FUEL (hydrocarbons) + AIR (oxygen and nitrogen) ==>>
CARBON DIOXIDE + Water + Unaffected nitrogen
Typical Engine Combustion:
FUEL + AIR ==>> UNBURNED HYDROCARBONS
+ NITROGEN OXIDES
+ CARBON MONOXIDE + CARBON DIOXIDE + Water
Exhaust
Pollutants
HYDROCARBONS
Hydrocarbon
emissions result when fuel molecules in the engine do not
burn or burn only partially. Hydrocarbons react in the presence
of nitrogen oxides and sunlight to form ground-level ozone,
a major component of smog. Ozone irritates the eyes, damages
the lungs, and aggravates respiratory problems. It is our
most widespread and intractable urban air pollution problem.
A number of exhaust hydrocarbons are also toxic, with the
potential to cause cancer.
NITROGEN OXIDES (NOx)
Under the high pressure and temperature conditions in an engine,
nitrogen and oxygen atoms in the air react to form various
nitrogen oxides, collectively known as NOx. Nitrogen oxides,
like hydrocarbons, are precursors to the formation of ozone.
They also contribute to the formation of acid rain.
CARBON MONOXIDE
Carbon monoxide (CO) is a product of incomplete combustion
and occurs when carbon in the fuel is partially oxidized rather
than fully oxidized to carbon dioxide (CO ). Carbon monoxide
reduces the flow of oxygen in the bloodstream and is particularly
dangerous to persons with heart disease.
CARBON DIOXIDE
In recent years, the U.S. Environmental Protection Agency
(EPA) has started to view carbon dioxide, a product of "perfect"
combustion, as a pollution concern. Carbon dioxide does not
directly impair human health, but it is a "greenhouse
gas" that traps the earth's heat and contributes to the
potential for global warming
Hydrocarbon pollutants also escape into the air through fuel
evaporation. With today's efficient exhaust emission controls
and today's gasoline formulations, evaporative losses can
account for a majority of the total hydrocarbon pollution
from current model cars on hot days when ozone levels are
highest.
Evaporative emissions occur several ways:
DIURNAL: Gasoline evaporation increases as
the temperature rises during the day, heating the fuel tank
and venting gasoline vapors.
RUNNING LOSSES: The hot engine and exhaust
system can vaporize gasoline when the car is running.
HOT SOAK: The engine remains hot for a period
of time after the car is turned off, and gasoline evaporation
continues when the car is parked.
REFUELING: Gasoline vapors are always present
in fuel tanks. These vapors are forced out when the tank is
filled with liquid fuel.
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