E 320 4MATIC Wagon (210.282) V6-3.2L (112.941) (1998)
Substrate
Ceramic monoliths are ceramic bodies through which pass several thousand small passages. The exhaust gas flows through these passages. The ceramic
consists of a 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 is fitted in a stainless
steel housing.
Coating
In three-way catalysts, the active catalytic layer applied to the substrate consists
Three-way catalyst mainly out of platinum and rhodium (modified non-metallic coating e. g. on model 124.066 as of 1/97, model 140 (USA) as of 11/96
or at EURO3/4).
Platinum promotes the oxidation of hydrocarbons (HC) and carbon monoxide (CO), rhodium the reduction of nitrogen oxides (NOX). The content of
rare metals contained in a catalytic converter is approx. 1 up to 3 grams, depending on size and design.
Three-Way Catalytic Converter, Function
Three-way Catalytic Converter, Design
A
Exhaust upstream of catalytic converter
B
Exhaust downstream of catalytic converter
C
Pollutant emission
D
Lambda (fuel-air ratio)
F
Rich mixture
M
Lean mixture
The exhaust gases flow through the catalytic converter and, in so doing, come into contact with the rare metals (platinum, rhodium). Due to oxidation,
carbon monoxide (CO) is converted into carbon dioxide (CO2) and hydrocarbons (HC) are converted into water (H2O) and carbon dioxide. As a result
of reduction the nitrogen oxides (NOx) are converted into nitrogen (N2) and carbon dioxide. The critical factors for the conversion of the pollutants is
the residual oxygen content in the exhaust. This is determined by the lambda control.
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. The following functions interact in order to rapidly reach this