Catalytic Converter: Description and Operation

The Catalytic Converter is mounted under the vehicles body within the exhaust system, and is designed to convert harmful exhaust gas pollutants, Carbon Monoxide (CO), Hydrocarbons (HC), and Oxides of Nitrogen (NOx) to harmless by-products of Nitrogen (N), Carbon Dioxide (CO2) and water vapor (H2O) through a catalytic chemical reaction.


Two types of converters in use today are the Oxidation Catalytic Converter (OC) and Oxidation-Reduction Converter. Both consists of two stamped aluminum or stainless steel pieces welded together to form a round or oval housing.

Some oval-type converters contain tiny pellets that are impregnated with the catalyzing element(s). Although this type of converter is known to create a fair amount of exhaust restriction, it is less expensive to manufacture and in some vehicle models contaminated pellets may be replaced.

Single Brick Catalytic Converter:

The round-type converters may contain one or two ceramic monolithic honeycomb substrates. The substrate bricks or beds are impregnated with the catalyzing element(s), then covered by a wire mesh sleeve for insulation and protection from shock and severe jolts. A diffuser inside the shell allows the exhaust gases to flow uniformly over the entire area of the substrate.

Dual Brick Catalytic Converter:

THE 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. If there is not already enough oxygen in the exhaust, a secondary air injection system is incorporated into the vehicle's design. This system supplies additional air to the oxidation catalyst. During the oxidation process, Oxygen (O2) mixes with the excess HC and CO to form H2O and CO2.
Caution should be taken while working on or around catalytic converters and other exhaust system components, due to the considerable heat (900~F to 1,600~F) that is generated during the oxidation process.

THE 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 carbon dioxide (CO2) by chemically moving the oxygen from the NOx compound to the CO compound. The catalytic element required for the reduction process is rhodium.

The Three-Way Catalytic Converter (TWC) is designed to reduce all three 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 from that percentage, the TWC converters efficiency decreases. Exhaust gas first passes through the catalyst, where NOx is reduced to N2 and CO2; then through the catalyst where HC and CO are changed to H2O and CO2.

The Warm-Up Oxidation Catalytic Converter (WU-OC) is a single bed oxidation-type converter, and is mounted to the exhaust manifold ahead of the main converter. The WU-OC primarily used in controlling HC and CO emissions during cold engine warm-up, while the main converter has not yet reached operating temperature. Heat from the exhaust manifold rapidly warms the WU-OC in order to start the Oxidation Process, which is then continued at the main converter.
The Warm-Up Oxidation Catalytic Converter is also referred to as a Pre-Cat or a Mini-Cat by other vehicle manufacturers.