How does a catalytic converter work?
When the exhaust gases come out from the engine, they are filled with pollutants. At least part of it is removed by a catalytic converter.
Fortunately, cars no longer release their exhaust gases untreated into the air. In the mid-1970s, the catalytic converter made its way into gasoline engines in the USA, and the first gasoline engines with G-cat were approved in Germany in 1985. Later, diesel engines also received an oxidation catalyst. The goal: The exhaust gases should become cleaner.
Both types of engines convert harmful carbon monoxide into carbon dioxide (CO2) and unburnt hydrocarbons, i.e. fuel residues, into CO2 and water vapor. In addition, the "three-way catalyst" in the gasoline engine also converts poisonous nitrogen oxides (NO, NO2, collectively referred to as NOx) into relatively harmless nitrogen and oxygen. This works in the diesel engine due to the high excess air in the exhaust gases, but not without further ado. Therefore, since a while, the SCR-cat takes over the NOx purification.
Reaction to make pollutants in the exhaust gas harmless
The working principle is the same in all cases: The exhaust gas is led through a porous body, which is coated with a material to obtain as large a surface area as possible. Several hundred square meters per gram. In this mostly aluminum-based layer, the catalytically active substances are embedded, usually precious metals such as platinum, palladium or rhodium. These enable the reaction to make the pollutants in the exhaust gas harmless, but they do not participate in the reaction themselves and are therefore not consumed. A small part, however, gets through despite abrasion into the environment.
A special form is the SCR-catalyst, which came to fame in the context of the diesel scandal. Like in it, nitrogen oxides are converted into nitrogen and water in it. However, a catalyst is necessary for this to work: Ammonia. It is usually supplied in the form of an ammonia solution (trade name: AdBlue), which transport much better than ammonia. The urea is injected into the exhaust tract, decomposes at high temperatures and releases the necessary ammonia for gas purification.
Certain temperature required
Catalytic converters require a certain temperature to work optimally. Therefore, the effectiveness can be significantly influenced by the design of the exhaust system as well as the operating strategy of the engine. For example, two SCR-catalysts are used in the Twin dosing system, one directly at the engine, one in the vehicle underbody.
A temperature difference of around 100 degrees Celsius between the two cats results solely from the different distance from the hot engine. Therefore, the respective catalyst is used, in which the exhaust gases have the right temperature at that moment; after starting, it is usually the one closer to the engine, during full-load driving, it is rather the one farther away.
The introduction of catalytic converters in the 1970s and 1980s significantly improved the emission standards for both gasoline and diesel automobiles. Despite the use of catalytic converters, diesel engines continued to emit harmful nitrogen oxides (NOx), leading to the development of the SCR-cat to further purify the exhaust gases in diesel vehicles.
With the advancement in technology, modern cars, whether diesel or gasoline, are equipped with sophisticated exhaust systems and catalytic converters to ensure that they comply with environmental regulations, reducing the harmful emissions coming from their diesel or gasoline engines.
