Catalytic Converter: Description and Operation

Exhaust System And Catalytic Converter:

Two three-way catalytic converters (TWC) are used downstream from the exhaust manifolds on each side to reduce oxides of nitrogen (NOx) and oxidize hydrocarbons (HC) and carbon monoxide (CO). The converter contains the catalyst elements platinum, rhodium and palladium, which promote oxidation and reduction reactions at high temperatures. Emissions are converted into carbon dioxide, water vapor and nitrogen.

A pre-converter, which is a two way oxidizing catalyst, is installed in the exhaust pipe between manifold and main three-way converter. This pre-converter heats up rapidly to oxidize excess hydrocarbons that occur during cold engine operation. It also helps heat the main converter to operating temperature more quickly.

OXIDATION REACTION
In order to change HC and CO into harmless materials, the catalytic elements (platinum and palladium) start an oxidation, or burning, reaction in the catalytic converter. Oxidation is the addition of oxygen to an element or compound. During the oxidation process, Oxygen (O2) mixes with the excess HC and CO to form H2O and CO2. During the oxidation reaction process, considerable heat (900~F to 1,600~F) is generated. Caution should be taken while working on or around the catalytic converter and other exhaust system components.

REDUCTION REACTION
Reduction is the opposite of oxidation, or the chemical removal of oxygen from a material. The reduction reaction changes NOx to harmless nitrogen (N) and free oxygen, which is then consumed in the oxidation reaction with other pollutants. The catalytic elements required for the reduction process are platinum and rhodium.

The Three-Way Catalytic Converter is designed to react with all three major exhaust pollutants, but works best to reduce NOx when the CO level of the exhaust is between 0.8 and 1.5 percent. As the CO level increases or decreases, converter efficiency decreases. Exhaust gas first passes through the catalyst, where oxides of nitrogen are reduced, then through the catalyst where HC and CO are changed to H2O and CO2.