Emission Control Systems: Description and Operation

The introduction of emission controls is a result of excessive high pollutants from tailpipe emissions. Smog is the main by-product of these emissions. This is primarily produced from Hydrocarbons, Oxides of Nitrogen and sunlight creating photochemical smog. Since automobiles are the main producers of these pollutants, tighter and more stringent laws have been legislated to control these pollutants. Auto manufacturers have developed and produced components to control these emissions. Along with these components, fuel systems and internal engine design have been changed to aid in the effort. With the advent of electronic fuel injection and computers monitoring such systems, not only are these vehicles producing less emissions, they are also running more efficient wasting little of the produced power.

The three kinds of emission gases being controlled in gasoline engines are:

HYDROCARBONS OR HC.
These are particles, usually vapors, of gasoline that have not been fully burned. They are present in the exhaust and crankcase vapors. Raw gas that evaporates out of the tank or carburetor is also considered a HC.

CARBON MONOXIDE OR CO.
This is a poisonous chemical compound of carbon (part of gasoline and oxygen from the air). It forms in the engine when the fuel burning (combustion) is less than complete. CO is found in the exhaust and is a by-product of combustion, but may also be in the crankcase.

OXIDES OF NITROGEN OR NOx
Various compounds of nitrogen and oxygen, both present in the air used for combustion, are formed in the cylinders during excessive high engine temperatures and are part of the exhaust gas. They become part of the tailpipe emissions if not reduced in the exhaust system.

These are some of the systems and components used to decrease emissions:

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 a Positive Crankcase Ventilation (PCV) System, and was designed to eliminate the emission of fumes, vapors, and blowby gases from the crankcase by directing these fumes back through the combustion chamber. The crankcase is ventilated using fresh air from the air cleaner. The fresh air is mixed with the blow-by gases in the crankcase, then the mixture is routed to the intake manifold through a PCV Valve.

SECONDARY AIR INJECTION/CATALYST
Air Injection systems originated in the mid 1960's. Their function is to put fresh air into the exhaust to reduce HC and CO to harmless 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.

EVAPORATIVE EMISSION CONTROLS SYSTEM
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 GAS RECIRCULATION
EGR is strictly a control for NOx in the exhaust gases. It reduces NOx by diluting the air/fuel mixture with some exhaust gas, which doesn't burn. This reduces peak combustion chamber temperature, so less NOx is formed.

ENGINE DESIGN/IMPROVED COMBUSTION SYSTEM (IMCO)
Starting in the 1960's. a group of engine modifications were designed to improve combustion and reduce HC and CO in the exhaust. It included a heated primary air system, carburetor design changes, fuel injection, engine "breathing" refinements and spark timing controls.