Catalytic Converter Operation

GENERAL INFORMATION

All engines are equipped with one or more catalytic converters for exhaust gas purification. The catalytic converter has a central ceramic honeycomb substrate –3–, coated with a washcoat layer. The washcoat layer contains precious metal salts that act as catalysts. The catalytic converter is secured in the housing –1– by means of an insulating support mat –2–, which also compensates for thermal expansion of the catalytic converter.

Catalytic converter 1 housing 2 gasket 3 catalytic converter honeycomb substrate

Petrol engines

To reduce harmful emissions with the catalytic converter, the fuel injection system must precisely meter the amount of fuel injected for combustion. The oxygen sensor is located in the exhaust pipe upstream of the catalytic converter and is washed by the exhaust gas flow. The oxygen sensor is an electronic sensor that measures the oxygen content in the exhaust gases and outputs it as a voltage signal. The oxygen sensor signal is sent to the fuel injection control unit. The received value is used to adjust the air-fuel mixture composition. This is necessary because operating conditions constantly change (idle, full throttle), and because optimal afterburning of fuel in the catalytic converter occurs only when the exhaust gases contain a sufficient amount of fuel.

For fuel afterburning to take place in the catalytic converter, a temperature of 300 – 800 °C is required. For this, the mixture must contain more fuel than is needed for complete combustion.

Catalytic converters used on gasoline engines are so-called three-way devices. In the controlled conversion process, carbon monoxide (CO) and hydrocarbons (HC) are oxidized, and the concentration of nitrogen oxides (NOx) is reduced.

Gases at the catalytic converter inlet and products at its outlet

Catalytic Converter and Exhaust Gas Recirculation System of Diesel Engine

Exhaust gases of the diesel engine are also purified in a catalytic converter. This is an unregulated oxidation catalytic converter. Such a catalytic converter converts the carbon monoxide and hydrocarbon compounds contained in the exhaust gases into CO2 and H2O.

The reduction of nitrogen oxide (NOx) concentration in diesel engine exhaust gases is achieved by an additional exhaust gas recirculation system (EGR system).

A vacuum-controlled recirculation valve is located on the engine exhaust manifold. Its function is to direct part of the exhaust gases into the engine combustion chambers, which helps lower the exhaust gas temperature and thereby reduce the concentration of harmful substances.