Emission Control Systems: Description and Operation

While providing good driveability and improved fuel efficiency, the exhaust emission control system must meet the stringent emission levels set forth by the Federal Government.
The introduction of emission controls is the result of excessively high tailpipe emissions. Photochemical smog, a main by-product of these emissions, is produced when Hydrocarbons and Oxides of Nitrogen are combined and exposed to sunlight. Since automobiles are the main producers of these pollutants, tighter and more stringent laws have been legislated to control exhaust emissions. Vehicle manufacturers are developing and producing more sophisticated components to aid in the reduction of these 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, better fuel efficiency while maintaining performance and power.

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

HYDROCARBONS (HC)
These are particles, usually in vapor form, 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 (throttle body) is considered a HC.

CARBON MONOXIDE (CO)
This is a poisonous chemical compound of carbon (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 (NOx)
Various compounds of nitrogen and oxygen which form in the cylinders under excessively high engine temperatures. NOx becomes measureable at the tailpipe if not reduced in the catalyst process.


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

POSITIVE CRANKCASE VENTILATION
First used in the early 1960's, the PCV system removes the gases that "blow by" the pistons into the crankcase. These gases (HC and CO) were originally vented to the atmosphere by a road draft tube. They are now recirculated into the induction system.

SECONDARY AIR INJECTION/CATALYST
Air Injection systems originated in the mid 1960's. Its function is to induce fresh air into the exhaust system. This aided in the conversion of HC and CO to water vapor and carbon dioxide by chemical reaction with the 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 CONTROL SYSTEM
In the 1960's. evaporative control systems were used to trap raw gas vapors in the fuel tank (and later carburetor bowl) and route them to the air cleaner when the engine ran. In the 1970's the system was refined to a "sealed housing" system to control emissions better and purge them to the intake manifold during specific engine modes.

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.

Some of the components within the system form their own sub-systems. Some of these components may also be controlled by or interact with the electronic control module, as with the Exhaust Gas Recirculation