Catalyst operation (BMW 7 Series E32)

All engines are equipped with a catalytic converter to clean exhaust gases.

The catalyst consists of a ceramic body with cells that are covered with a layer. On the active (working) layer are salts of noble metals that facilitate the conversion process. In the body (1), the catalyst is fixed with insulating protective mats (2), which, among other things, compensate for thermal expansion.

Petrol 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 catalytic converter 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 flow. 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 fuel-air mixture. In a fraction of a second, the lambda probe transmits a corresponding signal to the fuel injection system control unit, and thanks 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), on the other hand, optimal afterburning in the catalytic converter only occurs when there is still a sufficient amount of gasoline in the exhaust gases.



In order for afterburning to occur at all in the catalytic converter at temperatures from 300 to 800°C, there must be more fuel in the fuel-air mixture than is needed for "clean" combustion.



The catalyst used is a so-called three-stage catalyst. This means that with such a catalyst, thanks to lambda regulation, the oxidation of carbon monoxide (CO) and hydrocarbons (HC or CH) occurs simultaneously, as well as a reduction in the concentration of nitrogen oxides (NOx).

Diesel engine

The diesel engine also has a catalytic converter as standard. In the case of diesel, it is not possible to adjust the composition. However, the catalytic converter reduces the concentration of toxic carbon monoxide and hydrocarbons to a level that is much lower than in petrol engines with a regulated catalytic converter. In addition, the characteristic noise from diesel operation is reduced. The higher concentration of nitrogen oxides in the exhaust gases of the diesel is maintained at a low level by an additional exhaust gas recirculation system.

[This article is based on information from the website BMWman]