Emission Control Systems: Description and Operation

Increasing air pollution prompted laws requiring vehicle emission controls. Photochemical smog from motor vehicles is produced when tailpipe emissions (hydrocarbons and oxides of nitrogen) are exposed to sunlight. California was the first state to place controls on vehicle emissions. The Federal government became active in regulation and control as smog became problematic in other parts of the United States.

Vehicle makers have responded with sophisticated refinements such as electronic fuel injection and ignition systems. Today's vehicles run much cleaner and get better fuel economy than ever before.

HYDROCARBONS (HC)
These are a chemical compound of Hydrogen (H) and Carbon (C) molecules. Hydrocarbons are the only major automotive emission coming from sources other than engine exhaust. Most HC is produced by fuel system evaporation (20%) and engine exhaust (60%). Another source is oil and gasoline fumes/vapors within the crankcase.

CARBON MONOXIDE (CO)
CO is an odorless, colorless, and poisonous gas. It is formed when the combustion process is less than complete due to an improper air/fuel mixture ratio or poorly tuned engine. CO is measured primarily at the tailpipe, but may also escape into the crankcase as blow-by gases.

OXIDES OF NITROGEN (NOx)
NOx is a chemical compound of Nitrogen (N) and Oxygen (O2). It forms during the combustion process when engine temperatures become excessively high. NOx is the major contributor to photochemical smog.

CRANKCASE EMISSION CONTROLS
A Positive Crankcase Ventilation (PCV) Valve is used to regulate the flow of crankcase fumes into the intake system. The system eliminates release of fumes, vapors, and blowby gases into the atmosphere. The crankcase is vented through hoses to the air cleaner or other part of the intake air system.

AIR INJECTION / CATALYTIC CONVERTERS
Air injection systems put fresh air into the exhaust stream to reduce hydrocarbon and carbon monoxide levels. Additional oxygen, combined with exhaust manifold heat, allows the combustion burning process to continue within the exhaust manifold. This causes excess HC and CO to be converted to water vapor (H2O) and Carbon Dioxide (CO2) before reaching the catalytic converter.
Catalytic converters also help reduce HC and CO emissions. Three-Way Catalytic converters reduce NOx as well.

EVAPORATIVE EMISSION CONTROL
The evaporative system traps gasoline vapors that would otherwise escape into the atmosphere. Vapors are vented from the fuel tank into a carbon canister for storage. Once the engine is running and has reached operating temperature, vapors are purged from the canister, routed into the intake manifold, and burned during the combustion process.

EXHAUST GAS RECIRCULATION
The EGR System controls NOx by diluting the air/fuel mixture with exhaust gas. During specific engine operating modes, a small portion of exhaust gases are recirculated into the induction system. Since exhaust gases do not contain oxygen needed for the combustion process, the air/fuel ratio becomes less by volume. This lowers peak combustion temperatures, preventing NOx formation.