Three-way catalytic converters

High-quality three-way catalysts contain at least two of the three precious metals platinum, palladium and rhodium. In addition to the precious metals, a material is also required that can temporarily store oxygen atoms and release them again. Cerium oxide is used as a storage material. Special formulations of the precious metal coating make it possible for high-quality catalytic converters to function permanently even at temperatures of up to 1,000 °C.

For a reduction of the three harmful gases to be possible with a three-way catalytic converter, the oxygen content in the exhaust gas must be very low. This is referred to as operating the engine at lambda 1. Today, this mode of operation has only been widely implemented in petrol and gas engines.

At very short intervals, the engine control unit adjusts the combustion so that there are slight fluctuations in the oxygen content in the exhaust gas and lambda 1 is maintained on average. The oxygen content is measured with a lambda sensor. As the measured value is used by the engine control unit for regulation, a three-way catalytic converter is also referred to as a regulated system or catalytic converter. When the engine is running slightly rich (oxygen deficiency), nitrogen oxides are reduced particularly well, while hydrocarbons and carbon monoxide are reduced particularly well when the engine is running slightly lean (excess oxygen). The oxygen reservoir in the catalytic converter buffers this rapid change in engine operating mode so that all three harmful gases can be broken down in the catalytic converter with a high degree of efficiency.

Area of application

By far the largest use of three-way catalytic converters is in gasoline and gas engines in passenger cars. Engines with three-way catalytic converters do not require an SCR catalytic converter. Retrofitting is possible, in which case an additional control unit and a lambda probe are required.