Component description of the three-way catalytic converter - GF49.10-P-2010SM

ENGINE 113.986 in MODELS 215, 220 

ENGINE 113.987 in MODEL 209 

ENGINE 113.988 in MODEL 203 

ENGINE 113.989 in MODEL 171 

Illustrated on model 171 

Design (schematic) 

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 fixed in a double-walled stainless steel housing.

Ceramic monoliths require a substrate of aluminum oxide (Al₂ O₃ ), the "washcoat", which increases the active surface of the three way catalytic converter by a factor of about 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 about 3 g.

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 (N_X ) + carbon dioxide (CO₂ O).

The critical factors for the conversion of the pollutants is the residual oxygen content in the exhaust. It is determined by means of 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 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.

It is called a "three-way catalytic converter" because of its ability to reduce three polluting components simultaneously.