The catalyst consists of a ceramic body with cells that are covered with a layer. On the active (working) layer contains salts of noble metals, which contribute to the conversion process. In case (1) the catalyst is fixed with insulating protective mats (2), which, among other things, compensate for thermal expansion.
Gasoline engines
Thanks to the electronic fuel injection system and the lambda probe, the amount of fuel for combustion in the cylinders is precisely dosed so that the catalyst can effectively reduce the concentration of harmful substances. The lambda probe is located in the exhaust pipe in front of the catalytic converter and is flowed around by the exhaust gas stream. The lambda probe is an electrical sensor that measures the concentration of oxygen in the exhaust gases and provides this information in the form of voltage fluctuations, providing feedback on the correction of the composition of the fuel-air mixture. In a fraction of a second, the lambda probe transmits the appropriate signal to the fuel injection system control unit, and due to this, the air-fuel ratio is constantly changing. On the one hand, this is necessary, since the operating conditions of the engine are constantly changing (idle, full load), and on the other hand, optimal afterburning in the catalyst occurs only when there is still a sufficient amount of gasoline in the exhaust gases.
In order for afterburning to occur in the catalyst at temperatures between 300 and 800°C, there must be more fuel in the air-fuel mixture than is necessary for «pure» combustion.
The catalyst used belongs to the so-called three-stage catalysts. This means that with such a catalyst, thanks to lambda regulation, carbon monoxide is simultaneously oxidized (SO) and hydrocarbons (HC or CH), as well as a decrease in the concentration of nitrogen oxides (NOx).
Diesel engine
The catalytic converter is also fitted as standard on the diesel engine. In the case of a diesel engine, composition correction is not possible. The catalyst reduces, however, the concentration of toxic carbon monoxide and hydrocarbons to a level that is much lower than in gasoline engines with a variable catalyst. In addition, the characteristic noise from the diesel operation is reduced. The higher concentration of nitrogen oxides in diesel exhaust gases is kept low by an additional exhaust gas recirculation system.