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 bi-product of these emissions and is primarily produced from Hydrocarbons, Oxides of Nitrogen, and sunlight which creates photochemical smog. Since automobiles are the main producers of these pollutants tighter and more stringent laws have been legislated. Auto manufacturer's 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 efficient.

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

1. HYDROCARBONS (HC)
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 (CO)
This 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 (NOx)
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.

These are some of the system components used 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 orginally vented to the air by a road draft tube. They are now re-circulated into the induction system.

SECONDARY AIR INJECTION/CATALYST (AIR/CAT)
Air Injection systems orginated in the mid 1960s. Their function is to supply fresh air into the exhaust system in order to chemically reduce HC and CO into harmless water vapor and carbon dioxide. In the 1970s, catalytic converters were introduced to help this process. Three way catalyst will reduce NOx as well as HC and CO.

EVAPORATIVE CONTROL SYSTEM (EVAP)
In the 1960s 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 with-in 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.

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