Emission Control Systems: Description and Operation

The introduction of Emission Controls is the result of the rapidly decreasing quaility in the air we breathe. While it is true that a single car gives off only a microscopic amount of pollutants, remember that there are more than 150 million vehicles in use in the United States. Multiplied by each vehicles contribution, the potential for air pollution is staggering. Photochemical-Smog is the main by-product of tailpipe emissions, and is produced when Hydrocarbons and Oxides of Nitrogen are mixed and exposed to sunlight. California initiated the control on Smog by becoming the first state to place smog controlling devices on motor vehicles. As Smog began to appear in other parts of the country, the Federal Government became active in the area of regulations and control.
Vehicle manufacturers are now developing and producing more sophisticated components to help in reducing harmful tailpipe emissions. Fuel and Ignition systems have also been modified to aid in this effort. Since the introduction of electronic fuel injection and computerized engine controls, today's vehicles are producing less emissions and better fuel economy, while maintaining performance and power.

HYDROCARBONS OR HC.
A chemical compound made up of Hydrogen and Carbon molecules. Hydrocarbons, a major pollutant given off by an automobile, are mostly unburned portions of fuel. Gasoline is a Hydrocarbon compound. Hydrocarbons are the only major automotive air pollutant that can come from sources other than the engine's exhaust. While majority of the Hydrocarbon pollutants come from the fuel system's evaporation (20%) and the engine's exhaust (60%), other sources are oil and gasoline fumes, vapors or blow-by gases within the crankcase (20%).

CARBON MONOXIDE OR CO.
Is a chemical compound of gasoline and oxygen and is an odorless colorless poisonous gas. It is formed when the combustion process is less than complete, due to a rich air/fuel mixture. CO is measured primarily at the tailpipe, but may also escape into the crankcase in the form of blow-by gases.

OXIDES OF NITROGEN OR NOx
Is a chemical compound of Nitrogen and Oxygen, both found in the atmospheric air. NOx forms during the combustion process when engine temperatures become excessively high, and is measurable at of the tailpipe when not reduced through the exhaust system. NOx is one of the major pollutants which is found in Photochemical Smog.

CRANKCASE EMISSION CONTROLS

Beginning with 1961 new model vehicles, California required control over crankcase emissions. This control became standard for the rest of the United States with 1963 new model vehicles. The control was accomplished through the use of a Positive Crankcase Ventilation (PCV) Valve. The valve is designed to eliminate the emission of fumes, vapors, and blowby gases from the crankcase by directing these fumes back through the combustion process. The crankcase draws in fresh air from the air cleaner to be mixed with the blow-by gases in the crankcase, the mixture is then routed to the intake manifold through a PCV Valve.

EVAPORATIVE EMISSION CONTROLS

California's stringent emission laws brought the first evaporative emission controls (EEC or EVAP) on 1970 model cars sold in that state. The use of evaporative emission control systems was extended to all 1971 vehicles, regardless of where they were sold. The purpose of the EEC system is to trap gasoline vapors that would otherwise escape into the atmosphere while the engine is not running. The vapors are vented from the fuel tank into a charcoal canister for storage. Once the engine is running and has reached operating temperature, the vapors are purged from the canister and routed into the intake manifold. The fuel vapors become part of the intake air/fuel mixture and are consumed during the combustion process.

EXHAUST EMISSION CONTROLS

The Exhaust Gas Recirculation (EGR) System is designed to prevent the formation of Oxides of Nitrogen (NOx). During specified engine operating conditions, the MPI Control Unit will recirculate a small amount of the exhaust gases back into the induction system. Since exhaust gases are inert (inactive) and do not contain the needed oxygen for the burning process, the air/fuel ratio is less by volume. This lowers combustion temperatures which prevents the formation of NOx. If too much exhaust gas is recirculated or recirculation occurs at the wrong time, such as during engine warm-up, at idle or at Wide Open Throttle, power and engine performance will be affected.

Secondary Air Injection Systems originated in the mid 1960's. Their function is to add fresh air into the exhaust system to reduce HC and CO to water vapor and carbon dioxide by chemical reaction with oxygen in the air. In the 1970's catalytic converters were introduced to help this process. Three-way catalyst will reduce NOx as well as HC and CO.