Environmental Engineering Reference
In-Depth Information
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C. During the minute or two that is required to reach converter light-off conditions, high
engine-out pollutant levels are emitted from the tailpipe. Indeed, more than half of the emissions in
the Federal Test Procedure may be emitted during this warm-up period. When engines are started
in cold winter weather, excess fuel must be injected to achieve sufficient fuel evaporation to start
the engine, giving greatly increased CO and HC emissions. Preconverters of low heat capacity,
located close to the engine or electrically heated, are required to achieve low emission levels during
cold starts.
Catalyst surfaces may be damaged by overheating if the exhaust gas contains excessive un-
burned fuel, which might occur if the air/fuel control system fails. Also, fuel impurities that leave
surface deposits may destroy the catalytic function. Lead additives to gasoline have been phased
out to protect converters, and current U.S. fuel regulations will require removal of nearly all sulfur
in the near future. Ultraclean fuels will ensure that converters will not deteriorate over the useful
life of the vehicle.
8.7.2.3 Evaporative Emissions
Exhaust emissions of nitrogen oxides and incompletely burned fuel contribute to the formation of
ground-level ozone in the atmosphere. But the inadvertent escape to the atmosphere of vehicle fuel
vapor also can be an important contributor to the reactive organic compounds that participate in
the formation of ozone. As a consequence, evaporative emission control systems are required on
new U.S. vehicles.
There are several sources of vehicle evaporative emissions. Fuel stored in the fuel tank emits
vapor into the air space above the fuel surface within the tank. This vapor can leak to the atmosphere
during fuel refilling operations and during diurnal atmospheric temperature and pressure changes. At
engine shutdown, unburned fuel remains in the engine and can subsequently leak to the atmosphere
from the air intake or exhaust.
Fuel vapor has a different chemical composition than does the fuel because the vapor con-
stituents are much richer in higher-vapor-pressure, lower-molecular-weight components than is
the liquid fuel. Some of these components are also more chemically reactive in ozone formation,
making it all the more important to prevent their release to the atmosphere. Fuel vapor pressure in-
creases rapidly with temperature, so that uncontrolled evaporative emissions are higher in summer
than in winter. Because ozone formation is inherently higher in summer than in winter, escape of
fuel vapors exacerbates the summertime smog problem.
Fuel tank vapor emissions are controlled by placing a vapor adsorbing filter in the vent line
between the fuel tank air space and the atmosphere. If the fuel and tank air space warm up from
solar heating, expelling some vapor-air mixture through the vent line, the filter will retain the vapor
molecules on its adsorbing surface. To prevent the adsorbing surface from becoming saturated with
fuel molecules, and thereby ineffective for further filtering, air is drawn inward through the filter
when the engine is running, cleaning it of adsorbed vapor molecules. This inflow is ducted into the
engine intake system so as to incinerate the desorbed vapor in the running engine.
When a fuel tank is filled, the vapor-air mixture in the tank air space is displaced by the
incoming fuel. This mixture preferentially escapes through the fuel fill opening; and if it is not
collected during the filling process by a vapor control system at the filling station, it will be emitted
into the atmosphere. U.S. states with ozone exceedence problems usually require the installation
of such equipment by service stations.
