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WALKER® EXHAUST SYSTEMS: Evolution of the Catalytic Converter
TWO-WAY CATALYTIC CONVERTERS
  • Allows oxidation of CO (Carbon Monoxide) to less-harmful CO2 (Carbon Dioxide)
  • Allows oxidation of HC (Unburned Hydrocarbons) to CO2 (Carbon Dioxide) and H2O (Water)

In this design, exhaust gases are directed to flow through the substrate containing precious metals platinum and palladium, which allow the chemical reaction to occur. The exhaust gases increase in temperature as the conversion process takes place.

Because of the intense heat created by this process, exhaust gases leaving the converter should be hotter than the gases entering the converter. This also explains why heat shields are required on most units.

Two-way converters operate relatively efficiently with a lean fuel mixture. Ineffectiveness in controlling NOx led to the introduction of three-way converters.

THREE-WAY PLUS AIR CATALYTIC CONVERTERS
  • Allows reduction of NOx (Nitrogen Oxides) to N2 (Nitrogen) and O2 (Oxygen)
  • Allows oxidation of CO (Carbon Monoxide) to less-harmful CO2 (Carbon Dioxide)
  • Allows oxidation of HC (Unburned Hydrocarbons) to CO2 (Carbon Dioxide) and H2O (Water)

Three-way plus air converters were used in vehicle emissions systems in North America during the late '70s and early '80s.

Inside this converter there are two substrates. The front, coated with the precious metal rhodium, is used to reduce NOx emissions into simple N2 and O2. This process is most effective when little O2 is present (rich mixture). That is why it is located upstream of the air tube.

Since a rich mixture is high in HC and CO, an air pump and tube supply additional O2 to this mixture before it enters the second substrate.

The second substrate, coated with the precious metals palladium and platinum, allows oxidation of HC and CO to less harmful emissions CO2 and H2O.

This system was not very efficient and was phased out in the early '80s, when the current three-way converter was introduced.

THREE-WAY CATALYTIC CONVERTERS
  • Allows reduction of NOx (Nitrogen Oxides) to N2 (Nitrogen) and O2 (Oxygen)
  • Allows oxidation of CO (Carbon Monoxide) to less-harmful CO2 (Carbon Dioxide)
  • Allows oxidation of HC (Unburned Hydrocarbons) to CO2 (Carbon Dioxide) and H2O (Water)

Three-way converters have been used in vehicle emissions control systems in North America - and many other countries - since 1981.

The three-way without air uses advanced catalyst chemistry to store and release O2, in conjunction with an O2 monitoring and control system.

This system utilizes one or more O2 sensors to oscillate the fuel mixture between lean and rich conditions. This oscillation, combined with the O2 storage and release on the catalyst surface, allows for optimum reduction of all three emissions.

Three-way converters are used in conjunction with OBDII diagnostic systems on today's vehicles. This system alerts the driver when the converter is not working at peak efficiency.