Function of the catalytic converter
A catalyst as a concept from the field of chemistry is a substance that controls a chemical reaction or accelerates it. The composition and structure of the catalyst remain unchanged. In cars, it is part of the exhaust system.
Vectra has a ceramic catalyst. This is a ceramic body covered, together with its shell, with noble metals - platinum and rhodium. The ceramic part, located in a wire mesh, is penetrated by several thousand parallel channels. An intermediate layer is applied to the walls of the channels, which serves to increase the external surface (so-called wash-coat). It increases the active surface area of the catalyst to approximately the size of a football field.
The substances acting as a catalyst in both designs are platinum and rhodium. These noble metals are contained in the catalytic converter at 2-3 g, with platinum supporting oxidation and rhodium reducing the level of nitrogen oxide.
With the help of a catalytic converter, harmful substances such as carbon monoxide, hydrocarbons and nitrogen oxides are converted (hence the name - triple-action catalyst).
- Carbon monoxide and hydrocarbons are converted by oxidation with oxygen into carbon dioxide (CO ₂).
- To break down nitrogen oxides, a substance is used that takes away their oxygen. This substance can be a harmful substance - carbon monoxide. In this case, nitrogen (N ₂) and again CO ₂ are formed. Both are not dangerous.
Tips: Individually, the reduction in the amount of harmful substances in a regulated catalytic converter is: carbon monoxide by 85%, hydrocarbons by 80%, nitrogen oxides by 70%. It is taken into account that as the mileage increases, the catalytic converter loses some of its effectiveness.
In combination with the catalytic converter, other concepts often appear: "nested" catalytic converter means that the ceramic body of the catalytic converter is embedded in a closed housing (with metal wool lining). A "dual-purpose" catalytic converter can neutralize only two harmful substances in exhaust gases, while a "triple-purpose" catalytic converter (installed in Vectra), on the contrary, three.
Range of action
The catalytic converter functions with a high degree of efficiency only in a narrow range: at a "stoichiometric" ratio of fuel and air in the combustible mixture (λ=1). The greatest difficulty in catalytic converter technology is to maintain this ratio under all vehicle operating conditions by means of lambda regulation. A deviation of even 1% has a significant impact on the efficiency of the catalytic converter.
For the catalytic converter to start working, the initial temperature must reach approximately 300°C. This usually happens after 25-80 seconds; in urban conditions it may take up to three minutes before the required temperature is reached. At the same time, the ceramic catalyst is sensitive to overheating. If the temperature in the catalytic converter rises above 900°C, the device begins to age rapidly. From 1200°C its efficiency is destroyed for a long time.
The catalytic converter requires unleaded gasoline. Lead would quickly clog the surface of the catalytic converter, and the exhaust gases would no longer be able to come into contact with substances that have a catalytic effect. Experiments have shown that even after one fill of the fuel tank with leaded gasoline, carbon monoxide is almost no longer broken down. After 2-3 fills, the remaining harmful substances are no longer broken down. The catalyst is poisoned.
Tip: The precious metals in the catalytic converter can be recovered after its service life. The German Society for the Recycling of Automotive Catalytic Converters (DAR) collects old catalytic converters from car repair shops and other companies in order to prepare them for other purposes, such as making jewelry. Some repair shops pay a small reward for used catalytic converters.
Material copied from this website www.opelbook.ru
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