Emission Control Systems: Description and Operation
PURPOSEThe 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:
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.
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.
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 Exhaust Gas Recirculation (EGR), spark timing, or catalytic converter systems.
OPERATION
These are some of the systems used to decrease emissions:
Positive Crankcase Ventilation (PCV)
First used in the early 1960's, 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. Now these gases are mixed with fresh air from the air cleaner and recirculated into the intake manifold after passing through the PCV valve.
Three Way Catalytic Converter
The three way catalytic converter is an emission control device added to the gasoline engine exhaust system to effectively reduce hydrocarbon (HC), carbon monoxide (CO) and Oxides Of Nitrogen (NOx) emissions in the exhaust gas stream. The converter contains a honeycomb network coated with catalytic material containing platinum, palladium and rhodium. The catalytic material promotes burning or "oxidation" of the pollutants as the exhaust gases pass through the converter on their way to the tailpipe. The three way (reduction) catalyst is coated with platinum and rhodium which lowers levels of oxides of nitrogen (NOx), in addition to the carbon monoxide and hydrocarbon levels. This catalytic material (catalyst) in the three way converter is not serviceable.
Evaporative Control System (EVAP)
The basic evaporative emission control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is running the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.
Exhaust Gas Recirculation (EGR)
The EGR system is used to lower NOx emissions levels caused by high combustion temperature. It reduces NOx by diluting the air / fuel mixture with inert exhaust gases which reducing peak combustion chamber temperatures to limit NOx formation.