Motor vehicle emissions are pollutants produced by cars，trucks，buses andmotorcycles. The basic sources of the majority of vehicle emission are engine crankcaseblowby fumes，fuel vapors and engine exhaust gases. There are four basic types of vehicleexhaust emissions：hydrocarbons，carbon monoxide, oxides of nitrogen and particulates.
Hydrocarbons are produced by incomplete combustion or by fuel evaporation.Hydrocarbons are produced when unburned fuel escapes from the exhaust system of apoorly running engine. Some of the additives used in gasoline are extremely reactive，which allows the unburned fuel vapors to easily combine photo-chemically with otherelements in the air to form smog.
Carbon monoxide (CO)
Carbon monoxide is an extremely toxic emission resulting from the release of partiallyburned fuel. It is a result of incomplete combustion of a petroleum-base fuel.
Carbon monoxides is a colorless, odorless，and deadly gas. CO prevents human bloodcells from carrying oxygen to body tissues. Any factor that reduces the amount of oxygenpresent during combustion increase CO emissions. For example，a rich air-fuel mixturewould increase CO. As the mixture is leaned, CO emissions are reduced.
Oxides of Nitrogen (NO,)
Oxides of nitrogen are emissions produced by extremely high temperatures duringcombustion. Air consists of 79 percent nitrogen and 21 percent oxygen. With enoughheat, nitrogen and oxygen in the air-fuel mixture combine to form NO, emissions.
An engine with a high compression ratio，lean air-fuel mixture，and high-temperaturethermostat will produce high combustion heat, resulting in the formation of NO_ Thisposes a problem, as these same factors tend to improve gas mileage and reduce HC and COemissions. As a result, emission control systems must interact to lower each form of pollution.
Particulates are solid particles of carbon sot and fuel additives that blow out avehicle's tailpipe. Carbon particles make up the largest percentage of these emissions. Therest of the particulates consist of other additives sometimes used to make gasoline anddiesel fuel.
While particulate emissions is rarely a problem with gasoline engines, it is a seriousproblem with diesel engines. About 30 percent of all particulate emissions are heavyenough to settle out to the air in a relatively short period of time. The other 70 percent canfloat in the air for extended periods.