Exhaust emissions vary significantly with vehicle operating mode. There are
three operating modes: cold start, hot start, and hot stabilized. The start
modes refer to the first few minutes of operation after a engine has been
started. A cold start and a hot start are differentiated by the duration between
shutting off and restarting the engine. The hot stabilized mode includes all
operation time except for the start mode period. The fuel-air mixture and the
emission control equipment are two primary factors which cause the differences
in emission amounts among operating modes. During cold start mode, the catalytic
emission control systems do not provide full control until the appropriate
operating temperature is reached. Moreover, a richer fuel-air mixture must be
provided to start a "cold" engine. Therefore, VOC and PM emissions are higher in
the cold start mode than in the hot start mode and reach the lowest amounts in
the hot stabilized mode.
Evaporative emissions are composed primarily of volatile organic compounds
and these emissions are highly dependent on temperature. The following are the
six categories of evaporative emissions:
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Hot soak emissions:
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Emissions from the carburetor or fuel injector when the engine is turned off.
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Diurnal emissions:
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Emissions from the "breathing" of the gasoline tank due to temperature
fluctuations during a 24-hour day.
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Running losses:
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Emissions occurring while the vehicle is being operated. These emissions
result when more fuel enters into the emission control canister than can be
purged by it.
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Resting losses:
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Emissions that result from vapor permeating the evaporative emission control
system or from the vehicle fuel tanks.
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Refueling losses:
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Emissions occurring while a vehicle is being refueled. There are two
components: vapor space displacement and spillage. These emissions have been
estimated for the area source - gasoline service stations; they are not included
in the mobile source emissions.
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Crankcase emissions:
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Emissions that result from defective crankcase ventilation valves. They are
not true evaporative emissions. |
Based on the information presented above, there are three potential
approaches for deriving toxic emissions for highway vehicles:
- use toxic emission factors based on activity level;
- use toxic emission factors based on fuel usage; or
- combine total organic gases (TOG) and PM emissions with speciation profiles
for each source category.
These emission estimation approaches are discussed in the following sections.
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