Three-way catalytic converter, component description

P49.10-2681-04

1 Near-engine mounted catalytic converter

2 Underfloor catalytic converter

Location

A near-engine mounted catalytic converter is located in the front section of the exhaust system directly downstream of the turbocharger and an underfloor catalytic converter located behind the front axle.

P49.10-2419-76

Catalytic converter design (schematic)

1 ceramic monolith

2 Wire mesh (embedded)

3 Double-walled housing (insulation)

4 Substrate (washcoat) with a coating of platinum and rhodium

Design

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 made out of alumina 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 catalytic converters primarily out of platinum and rhodium.

Platinum accelerates oxidation of the hydrocarbon and carbon monoxide.

Rhodium accelerates reduction of nitrogen oxides.

P49.10-2420-76

Function

A Unpurified exhaust

B Purified exhaust

F Rich mixture

M Lean mixture

λ Lambda (air/fuel ratio)

CO Carbon monoxide

CO2 Carbon dioxide

HC Hydrocarbon

H2O Water

N 2 nitrogen

NOX Nitrogen oxides

Function

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

 Oxidation causes carbon monoxide to be converted to carbon dioxide and hydrocarbon in water and carbon dioxide.

 Reduction causes the nitrogen oxides to be converted into nitrogen and carbon dioxide.

The remaining oxygen content in the exhaust is a crucial factor in the conversion of pollutants.

This is determined by the lambda control and maintained at λ=1 in normal operating conditions.

 Because of its ability to reduce three polluting components simultaneously, it is called a "three-way catalytic converter".