Automobile emissions control

Automobile emissions control covers all the technologies that are employed to reduce the air pollution-causing emissions produced by automobiles. Exhaust emissions control systems were first required on 1966 model year vehicles produced for sale in the state of California, followed by the United States as a whole in model year 1968. Their use became widespread in the following decades and now they are ubiquitous in industrialised nations and common in most others.

Related Topics:
Air pollution - Automobile - California - United States

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Emissions controls have been highly successful in reducing the emissions produced by motor vehicles in terms of quantity per distance travelled. However, substantial increases in the distance travelled by each vehicle, and equally substantial increases in the number of vehicles in use, have meant that the overall reduction in pollution has been much slower.

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The emissions produced by a vehicle fall into two basic categories:

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• Tailpipe emissions: This is what most people think of when they think of vehicle air pollution; the products of burning fuel in the vehicle's engine, emitted from the vehicle's exhaust system. The major pollutants emitted include:
• Hydrocarbons: this class is made up of unburned or partially burned fuel, and is a major contributor to urban smog, as well as being toxic.
• Nitrogen oxides (NOx): These are generated when nitrogen in the air reacts with oxygen under the high temperature and pressure conditions inside the engine. NOx emissions contribute to both smog and acid rain.
• Carbon monoxide (CO): a product of incomplete combustion, carbon monoxide reduces the blood's ability to carry oxygen and are dangerous to people with heart disease.
• Carbon dioxide (CO₂): although this is a product of the complete combustion of hydrocarbons, is plentiful in the atmosphere, has no immediate harmful effects to humans and is essential to plant life, emissions of carbon dioxide are considered a pollutant because it is a significant greenhouse gas and increasing its levels in the atmosphere is thought by many to be a contributor to global warming.
• Evaporative emissions: These are produced from the evaporation of fuel, and are a large contributor to urban smog, since these heavier molecules stay closer to ground level. Fuel tends to evaporate in these ways:
• Gas tank venting: the heating of the vehicle as the temperature rises from the night-time temperature to the hottest temperatures of the day mean that gasoline in the tank evaporates, increasing the pressure inside the tank above atmospheric pressure. This pressure must be relieved, and before emissions control it was simply vented into the atmosphere.
• Running losses: the escape of gasoline vapors from the hot engine.
• Refuelling losses: these can cause a lot of hydrocarbon vapor emission. The empty space inside a vehicle's tank is filled with hydrocarbon gases, and as the tank is filled, these gases are forced out into the atmosphere. In addition, there is loss from further evaporation and fuel spillage.
 
Automobile emissions control: Tailpipe emissions control ►