Emission Control Systems: Description and Operation

A BRIEF HISTORY OF US EMISSION CONTROLS
The introduction of automotive Emission Controls was a reaction to the rapidly decreasing quality of the air we breathe. While the average car emits only a small amount of pollution, this amount multiplied by the more than 150 million vehicles in use in the United States adds up rapidly.

California was the first state to require emission control devices on motor vehicles. As Smog worsened across the country, the Federal Government became involved in emission control regulations and standards as well.

Vehicle manufacturers are continually producing more sophisticated emission control components and systems to meet more stringent tailpipe emission standards. Fuel, Ignition and Exhaust systems have all been modified to aid in this effort. Utilizing electronic fuel injection and computerized engine control systems, todays vehicles produce less emissions, get better fuel economy, and deliver better performance and power, than those produced only a few years ago.

VEHICLE EMISSIONS

Smog
Smog or "Photochemical-Smog", the main by-product of tailpipe emissions, is produced when Hydrocarbons (HC) and Oxides of Nitrogen (NOx) mix while being exposed to sunlight.

Hydrocarbons Or HC.
Hydrocarbons, formed by combining Hydrogen and Carbon molecules, make up a major portion of the pollution emitted by automobiles. Hydrocarbons are basically unburned fuel, (gasoline is a Hydrocarbon compound). Hydrocarbons are the only major automotive air pollutant that comes from sources other than engine exhaust. Hydrocarbon pollution comes from fuel system evaporation (20%), engine exhaust (60%), and vapors or blow-by gases from the crankcase (20%).

Carbon Monoxide Or CO.
Carbon Monoxide (CO), a chemical compound of gasoline and oxygen, is a colorless, odorless, poisonous gas. CO is formed when the combustion process is less than complete, usually due to a rich air/fuel mixture. CO is measured primarily at the tailpipe, but may also escape into the crankcase in blow-by gases.

Oxides Of Nitrogen Or NOx
Oxides of Nitrogen or NOx are chemical compounds of Nitrogen and Oxygen, (both common in atmospheric air). NOx form during the combustion process when engine temperatures exceed 2500° F. NOx mix with hydrocarbons in sunlight to produce Photochemical Smog.


EMISSION CONTROL SYSTEMS

Fuel Injection
The primary emission control is fuel injection. Fuel injection offers precise fuel control over a wide range of conditions. Feedback supplied by the exhaust gas oxygen sensor and various engine sensors allows the injection system to compensate for changes in atmospheric conditions (temperature, altitude, etc.) as well as mechanical conditions (such as state of tune, etc.).

Catalytic Convertor
The catalytic convertor effectively changes harmful pollutants into carbon dioxide and water.

Air Injection
The Air Injection system adds air to the already burning exhaust gases, oxidizing CO and HC's, and creating additional heat which accelerates the catalytic converter operation.

Fill Pipe Restrictor
The fillpipe restrictor prevents the larger diameter fuel nozzles used for leaded fuels from entering the fillpipe. Leaded fuel will permanently contaminate the oxygen sensor, and the catalytic converter.

Positive Crankcase Ventilation
The PCV system prevents blowby (emissions produced by the engine combustion chambers which escape into the crankcase) from entering the atmosphere. By recirculating the gases back into the intake system, blowby gases are burned along with the incoming air fuel mixture.

Evaporative Control System
The emissions created by the evaporation of raw fuel is controlled by the use of a closed ventilation system for the fuel tank, and a canister filled with activated charcoal to capture the fuel vapors. The charcoal canister captures and stores the vapors as they expand. When the engine is started, a valve is opened allowing the engine to draw air through the charcoal canister thereby also drawing in and burning the stored fuel vapors.

Exhaust Gas Recirculation
The EGR system recirculates exhaust gases back into the intake system. The introduction of these gases lowers the temperature in the combustion chambers which reduces oxides of nitrogen produced.

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