Emission Control Systems: Description and Operation

POLLUTANTS:
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.

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

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 EGR, Spark Timing, or Catalytic Converter systems.

CONTROL METHODS:
Here are some of the system components and methods used to decrease emissions:

Positive Crankcase Ventilation (PCV)
During the compression stroke some of the compressed gas "blow by" the piston rings into the crankcase. These gases contain HC, CO, and NOx. The PCV system keeps these gases from escaping into the atmosphere and utilizes engine vacuum to draw the crankcase fumes back into the cylinders for buring. The system also provides a filtered clean air inlet to the crankcase to replace the polluted air being removed.

Catalytic Converter
The catalytic converter is used to reduced emission levels of exhaust gases. A properly designed converter, in good condition can produce significant reductions in the levels of HC and CO and sometimes NOx depending on the which catalyst the converter is using.

A catalyst is a substance (platinum, palladium, or rhodium) that will cause an increase in the rate of a chemical reaction (exhaust gases being burned). Yet, it will not be consumed, or permanently altered by the reaction.

The catalyst causes a rapid rise in the temperature of the exhaust gas, reducing the hydrocarbons and carbon monoxide through an oxidizing process which changes them into harmless water vapor and Carbon Dioxide. Oxides of Nitrogen levels are lowered through a reduction process.

Evaporative Control System (EVAP)
The EVAP system works along side the fuel system making sure the engine has all the fuel it needs but that all excess fuel and vapor are either returned to the fuel tank (liquid) or directed to the EVAP canister (vapor). When computer determines the correct operating conditions exist, engine vacuum will be directed to the canister and the fuel vapors will be "purged" into the intake system and burned in the combustion chamber. This lowers the overall HC pollution.

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)
The internal engine controls incorporate basic engine design (combustion chamber, camshaft, and intake manifold design) as well as controls used to adjust ignition timing, fuel mixture, engine temperature, and other variables. These are designed to offer a more complete buring of the fuel charge within the combustion chamber.