Catalytic converter in Cars
The catalytic converter (commonly referred to as the catalyst) in the exhaust system is designed to reduce the emission of harmful substances into the atmosphere with exhaust gases by converting them into harmless components. The catalytic converter is used both on gasoline and diesel engines.
Three-way Catalytic Converter
It is used in gasoline engines running on the stoichiometric composition of the mixture (the mixture provides complete combustion of fuel). The design of the three-way catalytic converter includes a carrier block, thermal insulation and housing.
The main element of the catalytic converter is a carrier block, which serves as the basis for the catalysts. The carrier block is made of special refractory ceramics. Structurally, the carrier block consists of many longitudinal hundred cells, which significantly increase the area of contact with the exhaust gases.
The surface of the cells is a thin layer of applied catalyst substances, which include three components: platinum, palladium and rhodium. Catalysts accelerate the flow of chemical reactions in the catalyst.
Platinum and palladium are oxidative catalysts. They contribute to the oxidation of unburned hydrocarbons (CH) to water vapour, carbon monoxide (carbon monoxide, CO) to carbon dioxide. Rhodium is a reducing catalyst. It reduces nitrogen oxides (NOx) to harmless nitrogen. Thus, three catalysts reduce the content of three harmful substances in the exhaust gas.
The carrier block is placed in a metal case. Between them is usually a layer of thermal insulation. An oxygen sensor is installed in the converter housing.
The prerequisite for starting the catalytic converter is to reach a temperature of 300 ° C. The ideal temperature range is from 400 to 800 ° C. At this temperature, 90% of harmful substances are retained. Temperature above 800 ° C causes sintering of metal catalysts and cell cells of the carrier block.
The catalytic converter is usually installed directly behind the exhaust manifold or in front of the muffler. The first installation of the converter contributes to its rapid heating, but then the device is subjected to high thermal loads. In the second case, additional measures are required for the quick warm-up of the converter, which increases the temperature of the exhaust gases:
- regulation of the ignition moment in the direction of delay;
- increase in idling;
- variable valve timing;
- several fuel injections per cycle;
- air supply to the exhaust system.
To increase the efficiency, a three-component catalytic catalyst installation scheme is used, divided into two parts: the primary converter (located behind the exhaust manifold), the main converter (located under the bottom of the car).
Diesel Catalytic Converter
The diesel engine catalyst (DOC) provides for the oxidation of the individual components of the exhaust gas with oxygen, which is present in sufficient quantities in the diesel exhaust.
When passing through a catalytic converter, harmful substances (carbon monoxide, hydrocarbons) are oxidized to harmless products (carbon dioxide and water vapour). In addition, the catalyst almost completely eliminates the unpleasant odour of diesel exhaust gases.
Oxidative reactions in the catalyst also create undesirable products. So, sulphur dioxide is oxidized to sulphur trioxide with the subsequent formation of sulfuric acid. Sulfuric acid gas combines with water molecules, resulting in the formation of solid particles – sulphates. They accumulate in the converter and reduce its performance.
To remove sulphates from the converter, the engine management system starts the desulfation process, in which the catalytic converter is heated to temperatures above 650 ° C and purged with rich exhaust gases (with a lack of air, up to its complete absence).
The catalytic converter of a diesel engine is not used to reduce emissions of nitrogen oxides in exhaust gases. This function in a diesel engine is performed by the exhaust gas recirculation system or a more advanced selective catalytic neutralization system.
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