On gasoline engines, there are three sources of automotive pollutants: crankcase fumes, exhaust gasses, and gasoline evaporation. The pollutants formed from these substances fall into three categories: unburned hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The equipment used to limit these pollutants is called emission control equipment. This equipment ranges from the inherent internal design of the engine (combustion chamber, heads, valves, camshaft, etc) to external additive pieces such as temperature activated vacuum valves, solenoids, relays and computers. As the emission laws of the U.S. and other nations become stiffer, emission control systems change year-to-year to maintain the required balance of vehicle performance and driveability as well as reduced emissions.
Due to varying state, federal, and provincial regulations, specific emission control equipment has been devised for different pollution sources. For the years covered by this guide, U.S. emission equipment is divided into two categories: California and 49 State. In this section, the term: "California" applies only to cars originally built to be sold in California. In most cases, California emissions equipment is not shared with equipment installed on cars built to be sold in the other 49 States. In 1986, Volvo began phasing out the California designation; in 1987, all cars sold were "50 state" certified, although some models built to be sold in Canada remained different from U.S. versions.
Since diesel engines use a completely different principle of combustion (compression fired rather than spark fired), almost all of the emission control is designed into the construction of the engine. While diesel exhaust is more visible, it actually contains fewer atmospheric pollutants than a gasoline engine's exhaust. The great bulk of diesel exhaust is particulate carbon-soot-which quickly settles out of the air.
The only serviceable emission control on the diesel engine is the positive crankcase ventilation hose located between the valve cover and the intake manifold.