Emission Control Systems: Description and Operation

EMISSION CONTROLS
The introduction of emission controls is a result of excessive high pollutants from tailpipe emissions. Smog is the main bi-product of these emissions and is primarily produced from Hydrocarbons (HC), Oxides of Nitrogen (NOx), and sunlight which creates photochemical smog. Since automobiles are the main producers of these pollutants tighter and more stringent laws have been legislated. Auto manufacturers have developed and produced components to control these emissions. Along with these components, fuel systems and internal engine designs have been changed to aid in the effort. With the advent of electronic fuel injection and computer monitoring systems, not only are these vehicles producing less emissions, but they are also running more efficiently.

EMISSION BI-PRODUCTS
The three types of emission bi-products being controlled in gasoline engines are:

1. Hydrocarbons
These are particles, usually vapors, of gasoline that have not been fully burned. They are present in tailpipe emissions and crankcase vapors. Raw gas that evaporates from the tank or carburetor (throttle body) is considered an HC.

2. Carbon Monoxide
Carbon Monoxide (CO) is a poisonous chemical bi-product from the burning of fuel and air (carbon, in gasoline, and oxygen in the atmosphere). It forms in the engine when the burning of air/fuel (combustion) is less than complete. CO is measured at the tailpipe and is a bi-product of combustion, but traces of CO may also be found in the crankcase.

3. Oxides Of Nitrogen
Various compounds of nitrogen and oxygen, both present in the air, used for combustion, are formed in the combustion chamber during excessively high engine temperatures, and are part of the tailpipe emissions. They become part of the tailpipe emissions if not reduced by the EGR, Spark Timing, or Catalytic Converter systems.

EMISSIONS CONTROL SYSTEMS
These are the system components used on this vehicle to decrease emissions:

Positive Crankcase Ventilation ([1][2]PCV)
First used in the early 1960s, the PCV system removes gases that "blow by" the pistons into the crankcase. These gases (HC, CO, and NOx) were originally vented to the air by a road draft tube. They are now recirculated into the induction system.

Evaporative Control System (EVAP)
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 is running. In the 1970s, the system was refined to a "sealed housing" system to improve the control of HC emissions, and purge the fuel vapors into the intake manifold during specific engine speeds.

Exhaust Gas Recirculation (EGR)
EGR is strictly a control for NOx within the exhaust gases. It reduces NOx by diluting the air/fuel mixture with inert exhaust gases. This reduces peak combustion chamber temperatures to limit NOx formation.