Three-way catalyst - Description of components - GF49.10-P-2010BK

ENGINE 113.967 in MODEL 219 

ENGINE 113.964 in MODEL 164.1 

ENGINE 113.971 in MODEL 251 

Catalytic converter design (schematic) 

Fig 1: Identifying Catalytic Converter Design

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Location 

A catalytic convertor is located in the front part of the exhaust system for each cylinder bank, model 219 also has an underfloor catalytic convertor.

Ceramic monoliths are ceramic bodies through which pass several thousand small passages. The exhaust gas flows through these passages. The ceramic consists of high temperature-resistant magnesium aluminum silicate.

The monolith, which is extremely sensitive to voltages, is embedded in an elastic wire mesh made of high-alloy steel wires and fitted in a double-walled stainless steel housing.

Ceramic monoliths require a substrate (washcoat) of aluminum oxide (Al₂ O₃ ) that expands the active surface of the catalytic converter by an approximate factor of 7000.

The active catalytic layer coated on the substrate is available in three-way catalysts primarily out of platinum and rhodium. Platinum accelerates the oxidation of hydrocarbons (HC) and carbon monoxide (CO), whereas rhodium accelerates the reduction of nitrogen oxide (NO_x ).

The stainless steel content in a three way catalytic converter is approximately 3 g.

Exhaust gas components: 

Fig 2: Exhaust Gases Flow Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Function 

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

• Due to oxidation, carbon monoxide (CO) is converted into carbon dioxide (CO₂ ) and hydrocarbons (HC) into water (H₂ O) + carbon dioxide (CO₂ ).
• 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.

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 beyond 1400°C due to a malfunctioning of the engine such as misfiring etc. Such temperatures destroy the catalytic converter by melting the ceramic monolith.

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.

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