In most cities across the globe, the personal automobile is the single greatest polluter. Emissions from a billion vehicles running each day add up to a planet-wide problem. Breathing is fairly important for all of us and driving cars is our biggest single air polluting activity. You can be a part of the solution! First you need to learn a tad more...

Adapted from Cars and Pollution EPA Fact Sheet OMS-5

The Combustion Process

Gasoline and diesel fuels are mixtures of hydrocarbons, compounds which contain hydrogen and carbon atoms. Hydrocarbons and burned by combining them with oxygen. Nitrogen and sulphur atoms are also present and are combined with oxygen to produce gases.. Automotive engines emit several types of pollutants.

Typical Engine Combustion:

FUEL + AIR ==>> UNBURNED HYDROCARBONS + NITROGEN OXIDES

+ CARBON MONOXIDE + CARBON DIOXIDE + water

HYDROCARBONS

Hydrocarbon emissions result when fuel molecules in the engine 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.

Catalytic converters, the part of car exhaust systems designed to break down nitrogen gases are actually forming nitrous oxide - 300 times more potent than carbon dioxide as a greenhouse gas. Nitrous oxide makes up about 7.2 percent of the gases cited in global warming, the USA EPA said in a study published spring 1998. Vehicles fitted with catalytic converters produced nearly half of that nitrous oxide. Nitrous oxide also comes from nitrogen-based fertilizer and manure from farm animals.

CARBON MONOXIDE

Carbon monoxide (CO) is a colorless, odorless, poisonous gas. A product of incomplete burning of hydrocarbon-based fuels, carbon monoxide consists of a carbon atom and an oxygen atom linked together. In 1992, carbon monoxide levels exceeded the Federal air quality standard in 20 U.S. cities, home to more than 14 million people.

Carbon monoxide results from incomplete combustion of fuel and is emitted directly from vehicle tailpipes. Incomplete combustion is most likely to occur at low air-to-fuel ratios in the engine. These conditions are common during vehicle starting when air supply is restricted ("choked"), when cars are not tuned properly, and at altitude, where "thin" air effectively reduces the amount of oxygen available for combustion (except in cars that are designed or adjusted to compensate for altitude).

Two-thirds of the carbon monoxide emissions come from transportation sources, with the largest contribution coming from highway motor vehicles. In urban areas, the motor vehicle contribution to carbon monoxide pollution can exceed 90 percent.

Carbon Monoxide

Carbon Dioxide

U.S. Environmental Protection Agency (EPA) originally viewed carbon dioxide as a product of "perfect" combustion, but now views CO2 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 - 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.