Three-way catalytic converter, component description - GF49.10-P-2010V

ENGINE 272.967 in MODEL 164, 251 

ENGINE 273.923 in MODEL 164 

ENGINE 273.943 in MODEL 164 

ENGINE 273.963 in MODEL 164, 251 

Shown on engine 272.963, right cylinder bank 

Location 

Near the engine in the front section of the exhaust system.

Task 

Reduction of the following pollutants in the exhaust:

• Carbon monoxide (CO)
• Hydrocarbon (HC)
• Nitrogen oxide (NO_X ).
  Fig 1: Identifying Three-Way Catalytic Converter Components - Shown On Engine 272.963, Right Cylinder Bank

  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Catalytic converter design (schematic) 

Fig 2: Identifying Three-Way Catalytic Converter Design (Schematic) Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

In each catalytic converter, there are two ceramic monoliths with each approx. 600 cells. The exhaust gas flows through these passages. The ceramic consists of high temperature-resistant magnesium aluminum silicate.

The monoliths are extremely sensitive to warping and are embedded in an elastic wire mesh of high-carbon steel wire and mounted inside a double-wall housing of stainless steel.

Ceramic monoliths require a support layer of alumina (Al₂ O₃ ) (the "washcoat"), which increases the effective surface of the catalytic converter by a factor of 7000.

The active catalytic layer coated on the substrate is available in three-way catalysts primarily out of platinum and Rhodium.

Platinum promotes the oxidation of hydrocarbons (HC) and carbon monoxide (CO) and Rhodium the reduction of nitrogen oxides (NO_X ).

Owing to its property of simultaneously reducing three pollutant components, it is called a "three-way catalyst".

Function 

Fig 3: Identifying Three-Way Catalytic Converter Function Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The exhaust gases flow through the three way catalytic converter and hence come into contact with the rare metals, platinum and Rhodium.

• The carbon monoxide (CO) is converted to carbon dioxide (CO₂ ) and hydrocarbons (HC) converted to water (H₂ O) + carbon dioxide (CO₂ ) by oxidation.
• Due to reduction, nitrogen oxides (NO_X ) are converted into nitrogen (N₂ ) + carbon dioxide (CO₂ ).

The critical factors for the conversion of the pollutants is the residual oxygen content in the exhaust. It is determined through lambda control and maintained at λ=1 in normal operation.

Operating conditions 

As is the case for the O₂ sensor, the operating temperature also plays a very important role in the case of the catalytic converter. Appreciable conversion of the pollutants does not commence until an operating temperature of approx. 250°C. This temperature is reached quickly after the engine is started, due to the near-engine location, the air-gap insulation on the exhaust manifold and mixture enrichment.

Ideal operating conditions for high conversion rates and a long life prevail at temperatures between around 400 to 800°C.

The temperature of the three way catalytic converter can increase

The temperature of the three way catalytic converter can increase beyond 1400 °C due to a malfunctioning of the engine such as misfiring etc. These high temperatures can lead to destruction of the catalytic converter, by melting the ceramic monoliths.

Another requirement for reliable long-term operation is that only unleaded fuel be used. Lead compounds form a deposit on the active surface and as a result prevent the exhaust gases from coming into contact with the catalytic layer.

If the vehicle is driven during direct injection in stratified charge operation with excess air, the catalytic converter cannot convert the nitrogen oxide sufficiently. Additional NOX storage catalytic converters are necessary.