Catalytic Converter: Description and Operation

Fig. 20 - Single Bed Monolithic Catalytic Converter:

Fig. 21 - Dual Bed Monolithic Catalytic Converter:

Fig. 22 - Single Bed Pellet Type Catalytic Converter:

Fig. 23 - Dual Bed Pellet Type Catalytic Converter:

CAUTION: Use unleaded fuels only in catalytic converter equipped vehicles. Leaded fuel renders catalyst ineffective.

PURPOSE
The catalytic converter reduces the levels of carbon monoxide, hydrocarbons, and in some cases oxides of nitrogen, in exhaust emissions.

Catalyst Design
GM uses four different converter designs in conjunction with two types of catalysts.

The four converter designs are:
- Single Bed Monolith
- Dual Bed Monolith
- Single Bed Pellet
- Dual Bed Pellet

The two types of catalysts used are;
- Oxidation Catalyst
- Three Way (reduction) Catalyst (TWC)

Oxidation Catalyst
An oxidation catalyst is coated with material containing platinum and palladium which lowers levels of carbon monoxide and hydrocarbons by adding oxygen to form CO2 and H2O.

Three Way Catalyst
The Three Way (reduction) Catalyst is coated with platinum and rhodium which lowers levels of oxides of nitrogen (NOx), as well as carbon monoxide and hydrocarbons.

Phase II Converter--Three Way Catalyst
This converter contains rhodium and platinum to aid in the control of oxides of nitrogen (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO).

In order for this converter to operate effectively, it is essential that the exhaust gases contain a certain amount of oxygen. Therefore the air-fuel mixture must be maintained as close to the ideal level as possible.

Dual Bed Converters
All dual bed converters--monolith and pellet type--contain both oxidation and three way catalysts.

Fill Pipe Restrictor
A small diameter fuel tank filler tube is used on catalytic converter equipped vehicles to prevent the larger diameter--leaded fuel--pump nozzles from being inserted into the filler tube.

Heat Shields
The use of heat shields is sometimes required to protect chassis components, the passenger compartment and other areas from heat related damage.