The exhaust of automobiles is one of the major contributors to the world’s air pollution problem. Recent research and development has made major reductions in engine emissions, but a growing population and a greater number of automobiles means that the problem will exist for many years to come.
During the first half of the 1900s,automobile emissions were not recognized as a problem, mainly due to the lower number of vehicles. As the number of automo- biles grew along with more power plants, home furnaces, and population in general, air pollution became an ever-increasing problem. During the 1940s,the problem was first seen in the Los Angeles area due to the high density of people and automo- biles, as well as unique weather conditions. By the 1970s, air pollution was recognized as a major problem in most cities of the United States as well as in many large urban areas around the world.
Laws were passed in the United States and in other industrialized countries which limit the amount of various exhaust emissions that are allowed. This put a major restriction on automobile engine development during the 1980s and 1990s.
Although harmful emissions produced by engines have been reduced by over 90% since the 1940s,they are stilI a major environmental problem.
Four major emissions produced by internal combustion engines are hydrocar- bons (He), carbon monoxide (CO), oxides of nitrogen (NOx), and solid particulates. Hydrocarbons are fuel molecules which did not get burned and smaller nonequilibri- um particles of partially burned fuel. Carbon monoxide occurs when not enough oxygen ispresent to fully react all carbon to CO2 or when incomplete air-fuel mixing occurs due to the very short engine cycle time. Oxides of nitrogen are created in an engine when high combustion temperatures cause some normally stable N 2 to disso- ciate into monatomic nitrogen N, which then combines with reacting oxygen. Solid particulates are formed in compression ignition engines and are seen as black smoke in the exhaust of these engines. Other emissions found in the exhaust of engines include aldehydes, sulfur, lead, and phosphorus.
Two methods are being used to reduce harmful engine emissions. One is to improve the technology of engines and fuels so that better combustion Occurs and fewer emissions are generated. The second method is aftertreatment of the exhaust gases. This is done by using thermal converters or catalytic converters that promote chemical reactions in the exhaust flow. These chemical reactions convert the harm- ful emissions to acceptable CO2, H20, and N2