Catalytic Converter: Description and Operation

Fig. 3 Fuel tank filler nozzle:

Catalytic converters are used to convert carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx) into water vapor (H2O), carbon dioxide (CO2) and nitrogen (N2). The catalysts used to create these conversions are platinum, palladium and rhodium (depending on type of converter).
During engine operation, all of the exhaust gases flow through the converter where a chemical change takes place. This change causes the temperature inside the converter to be higher than the temperature of the exhaust gases when they leave the engine. Due to this increase in heat, the converter is insulated so that its outside temperature is about the same temperature as the muffler. However, due to its solid mass, the converter remains hot much longer than the muffler.
The body of the catalytic converter is made of stainless steel designed to last the life of the vehicle. Excessive heat can bulge or distort the converter. Since excessive heat build up is not the fault of the converter, the carburetor or ignition system should be checked whenever a converter is damaged by overheating.
Although all vehicles with catalytic converter must use unleaded fuel, small amounts of leaded fuel can be used in case of an emergency. To prevent adding leaded fuel, the fuel tank filler nozzle has a built-in restrictor, Fig. 3.

Fig. 4 Pellet type catalytic converter:

CONVENTIONAL OXIDIZING CATALYTIC (COC) PELLET TYPE
This type of converter, Fig. 4, changes carbon monoxide (CO) and hydrocarbon (HC) into water vapor (H2O) and carbon dioxide (CO2). The catalysts in the converter which produce the chemical change are platinum, and palladium included as a fine coating on the substrate (beads of alumina).

Fig. 5 Three-Way catalytic converter:

THREE-WAY CATALYTIC (TWC) PELLET TYPE
This type of converter, Fig. 5, contains platinum and palladium as conventional oxidizing agents in addition to rhodium. The addition of rhodium allows the converter to convert carbon monoxide (CO) and oxides of nitrogen (NOx) to carbon dioxide (CO2) and nitrogen (Ne02).
During operation, as the air fuel mixture is leaned, the converter's efficiency for converting HC and CO increases but decreases for NOx. If the air fuel mixture were increased, NOx formation would be reduced, but HC and CO would be increased. In order to optimize the simultaneous reduction of all these pollutants, the carburetor must be able to provide an air fuel mixture of approximately 14.7:1. In order to provide this air fuel mixture, a feedback carburetor system is used along with this type of converter.

Fig. 6 Monolithic type catalytic converter:

CONVENTIONAL OXIDIZING CATALYTIC (COC) MONOLITHIC TYPE CONVERTER
This type of converter, Fig. 6, operates on the same chemical principles as the COC pellet type converter. The catalyst in the converter is a mixture of platinum and palladium, coated to an extruded material resembling a honeycomb. This converter is integral with the front exhaust pipe.

Fig. 7 Dual bed monolithic type catalytic converter:

DUAL BED (TWC & COC) MONOLITHIC TYPE CONVERTER
This type of converter, Fig. 7, is two converters in one container. The front half is a three-way catalytic (TWC) converter while the rear half is a conventional oxidizing catalytic (COC) converter. The rear half of the converter has a provision for ``downstream'' air injection.
As with the three-way catalytic (TWC) converter, this converter must be used in conjunction with a feedback carburetor system in order to maintain high efficiency.