Catalytic converters- Dual bed and three way catalytic converters

Catalytic Converters 

Catalytic converter is a  way of treating exhaust gases. These convert harmful pollutants into harmless gases. A catalyst is a material that causes chemical change without entering into chemical reaction. In effect the catalyst encourages to react with each other. For example, in the HC/CO catalytic converter, the catalyst encourage5 the HC to unite with oxygen to produce H20 (water).It encourages the CO to change to CO2 (carbon dioxide). The catalyst in the NOx converter splits the nitrogen from the oxygen. The NO therefore becomes harmless nitrogen and oxygen. Figure 31-40 shows that a two way type catalytic converter that converts only HC and CO. Figure 31-40 also shows that the flow of the exhaust gas through the converter. The converter is filled with BB-shot-size metal pellets. They are coated with a thin layer of platinum or similar catalytic metal. The pellets form a matrix through which the exhaust gas must pass. As the exhaust gas flows through the catalyst produces the chemical reaction. Another type of catalytic converter must use non-leaded gasoline. If the gasoline contains lead, the lead will coat the catalyst and the converter will stop working. If this happens to the pellet type converter, there is a way to remove the old pellets and put in a charge of fresh pallets. But in the honey-comb type, the complete catalytic converter must be replaced.


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Dual bed and three way catalytic converters 

There are three general categories of catalytic converters .These are oxidizing, reducing, and three way. The oxidizing converter handles handles HC and CO, using platinum and palladium as the catalyst. To control NOx, rhodium is used as a reducing catalyst. It changes NOx to harmless N2. Instead of having two separate catalytic converters in the exhaust system, one for HC and CO and the other for NOx, most manufacturers use either a dual-bed catalytic converter or a three-way catalytic converter.

A dual bed converter (Fig 31-42) is like two bead type converters in one housing with an air chamber between them. The exhaust gas first passes through the upper bed, reducing the Nox and oxidizing some of the HC an CO. Then the exhaust gas flows through the air chamber to the lower bed, where the air pump is adding sufficient air for final oxidizing of the HC and CO.

A three-way catalyst is a mixture of platinum and rhodium (sometimes mixed with palladium). It acts on all three of the regulated pollutants (HC, CO and Nox), but only when the air-fuel mixture ratio is precisely controlled (Fig 31-43). If the engine is operated with the ideal of stoichiometric air-fuel ratio of 14.7:1, the three-way catalyst is very effective. It strips oxygen away from the Nox to form harmless water (H2O), carbon dioxide (CO2 and Nitrogen (N2). However, the air-fuel mixture must be precisely controlled if thus action is to occur. For this reason, a closed-loop fuel-metering system (either feedback carburetor or fuel injection) must be used. 

There are two types of three way catalytic converters. They have a mesh or honeycomb (Fig 31-44) coated with catalyst. The front section (in the direction of the gas flow) handles NOx and partly handles HC and CO. The partly treated exhaust gas then flows through the air chamber into the rear section of the converter. There the gas mixes with the air being pumped in by the air pump. This is called secondary air. It puts more oxygen in the exhaust gas to that the two way catalyst can take care of the HC and CO.

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