Catalytic Converter: Description and Operation
The dual catalytic converters used on 6 cylinder and V8 engines, decrease HC and CO emissions or NOx emissions, or all three of these exhaust
pollutants. The converters used in this system are tandem mounted.
The oxidation catalytic converter contains a platinum coated, ceramic, honeycombed structure. Through a chemical reaction, the platinum oxidizes HC
and CO and converts them to carbon dioxide and water vapor. Effective operation of this type of catalyst requires temperatures of 600° F or higher as
well as an adequate supply of oxygen in the exhaust system. Oxidation catalysts will normally ``light off'' (start oxidation) within two minutes after the
first start of a cold engine.
The three-way catalytic converter contains rhodium which reduces or separates oxides of nitrogen (NOx) into nitrogen and oxygen. This action
provides better exhaust emission control than is obtainable by using only exhaust gas recirculation, an oxidation catalyst or engine modifications. Its use
allows richer air-fuel mixtures, more spark advance and less exhaust gas recirculation. All three improve both driveability and fuel economy.
Effective catalytic control of all three pollutants is possible when the correct balance of excess CO is reached for reduction and excess oxygen is
reduced. Therefore, it is necessary to maintain precise control of the air-fuel mixture entering the engine, keeping it very close to the stoichiometric range
(chemically correct for theoretical complete combustion).
The downstream catalyst, along with oxygen supplied by an air pump is used to remove the remaining HC and CO left after the exhaust gases have
passed through the three-way catalyst.