Emission Control Systems: Description and Operation

Emission control systems are designed to reduce excessive pollution from motor vehicles. These systems work in conjunction with computerized fuel systems and engine design modifications, resulting in lower emissions and more efficient vehicles.

HYDROCARBONS OR HC

Hydrocarbons are vapors of gasoline that have not been fully burned. They are present in exhaust and crankcase vapors. Raw gas that evaporates out of the tank or carburetor (throttle body) is considered HC.

CARBON MONOXIDE OR CO

Carbon monoxide is a poisonous carbon compound formed when combustion is incomplete. CO is a bi-product of combustion found in the exhaust and may also be in the crankcase.

OXIDES OF NITROGEN OR NOx

Various compounds of nitrogen and oxygen that are formed during excessively high engine temperatures become emissions if not reduced in the exhaust system.

These are some EMISSION CONTROL systems and components:

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system removes gases that "blow by" the pistons into the crankcase. Formerly vented to the air by a road draft tube, these gases (HC, CO and NOx) are now recirculated into the induction system.

EVAPORATIVE EMISSION CONTROLS (EVAP)

In the 1960's evaporative control systems were used to trap raw gas vapors from the fuel tank and 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)

EGR is a control for NOx in the exhaust gases. It lowers NOx by diluting the air/fuel mixture with non-burning exhaust gas. This reduces peak combustion chamber temperatures and thus NOx formation.

THREE-WAY CATALYTIC CONVERTER (TWC)

By utilizing platinum and rhodium metals to induce chemical reactions, catalytic converters neutralize CO, HC and NOx. These products of combustion are reduced to carbon dioxide, nitrogen and water.