Automobile Emissions: an Overview

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U.S. ENVIRONMENTAL PROTECTION AGENCY OFFICE OF MOBILE SOURCES

EPA 400-F-92-007

Automobile Emissions: An Overview
Cars and Pollution
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.

Sources 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.

• Evaporative Emissions • Refueling Losses

• Exhaust Emissions

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. FACT SHEET OMS-5 August, 1994

Automobile Emissions: An Overview

“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

Evaporative Emissions
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:

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Automobile Emissions: An Overview

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.

What Has Been Done to Control Automobile Emissions?
The...
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