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 temperature after a cold start:
- Ignition system
- Idle speed control [ISC]
- Transmission shift delay
- Secondary air injection
- Insulation of exhaust manifold and front exhaust pipes.

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 prevent the exhaust gases from coming into contact with the catalytic layer.

Note: In view of its property of simultaneously reducing three pollutant components, it is known as a three-way catalytic converter.