Chevrolet TrailBlazer, Руководство по обслуживанию, Страница: 209 / 677

Automotive Emissions
Before emission controls were mandated on internal combustion engines, other sources of
engine pollutants were discovered along with the exhaust emissions. It was determined
that engine combustion exhaust produced approximately 60 percent of the total emission
pollutants, fuel evaporation from the fuel tank produced 20 percent, with the final 20
percent being produced through the crankcase as a by-product of the combustion process.
Exhaust Gases
The exhaust gases emitted into the atmosphere are a combination of burned and
unburned fuel. To understand the exhaust emission and its composition, we must review
some basic chemistry.
When the air/fuel mixture is introduced into the engine, we are mixing air, composed of
nitrogen (78 percent), oxygen (21 percent) and other gases (1 percent) with the fuel, which
is 100 percent hydrocarbons (HC), in a semi-controlled ratio. As the combustion process is
accomplished, power is produced to move the vehicle while the heat of combustion is
transferred to the cooling system. The exhaust gases are then composed of nitrogen, a
diatomic gas (N
2
), the same as was introduced in the engine, carbon dioxide (CO
2
), the
same gas that is used in beverage carbonation, and water vapor (H
2
O). The nitrogen (N
2
),
for the most part, passes through the engine unchanged, while the oxygen (O
2
) reacts
(burns) with the hydrocarbons (HC) and produces the carbon dioxide (CO
2
) and the water
vapors (H
2
O). If this chemical process would be the only process to take place, the
exhaust emissions would be harmless. However, during the combustion process, other
compounds are formed which are considered dangerous. These pollutants are
hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NO
x
) oxides of sulfur (SO
x
)
and engine particulates.
Hydrocarbons
Hydrocarbons (HC) are essentially fuel which was not burned during the combustion
process or which has escaped into the atmosphere through fuel evaporation. The main
sources of incomplete combustion are rich air/fuel mixtures, low engine temperatures and
improper spark timing. The main sources of hydrocarbon emission through fuel
evaporation on most vehicles used to be the vehicle's fuel tank and carburetor float bowl.
To reduce combustion hydrocarbon emission, engine modifications were made to
minimize dead space and surface area in the combustion chamber. In addition, the air/fuel
mixture was made leaner through the improved control which feedback carburetion and
fuel injection offers and by the addition of external controls to aid in further combustion of
the hydrocarbons outside the engine. Two such methods were the addition of air injection
systems, to inject fresh air into the exhaust manifolds and the installation of catalytic
converters, units that are able to burn traces of hydrocarbons without affecting the internal
combustion process or fuel economy.