As a three-way catalyser, the converter should, in the case of the petrol engine, reduce nitrogen oxides (NOX) and at the same time oxidise hydrocarbons (HC) and carbon monoxide. As an oxidation catalyser in the Diesel engine, it reduces the hydrocarbon portions in the soot particles with the help of the air/oxygen in the diesel exhaust gas.
Function
The 'catalyser' should actually be called a catalytic converter. However, like many other unsuitable expressions, it has become embedded in the jargon of motor technology. The expression actually only applies to the coating, which accelerates a chemical reaction or indeed, allows it to take place at all, without itself being used up. The channels visible in the above pictures represent an area of 2 - 3 m². For these (up to now, not entirely researched) processes in the converter however, the 7000-fold surface area is necessary. This is taken care of by wash-coating the ceramic base. Further coating materials like platinum, palladium or rhodium may be applied in an extremely thin layer (one thousandth of a millimeter), thus, a few grams on the surface area of approx. 3 - 4 football fields is sufficient, and which, through refining can even be recovered to the tune of about 98 percent.
Price of a few grams of platinum in normal catalyst: approx. 100 Euro (2010)
The wash-coat layer is applied to very fine sheet-metal honeycombs in the metal catalysers. Before it is carefully pressed into the sheet-metal casing of the exhaust system, it is wrapped in a protective layer which keeps it in position, even at extreme temperatures of over 1000°C. Two wire-mesh rings (see picture 1) strengthen and protect the ends. Only asymmetric tapered ends are welded together with the catalyser-casing. In concentric tapered ends, the metal casing is heated through induction and then tapered by automatically working machines. The process is completed by a compressed-air test.
From approx. 0,1 bar back-pressure upwards the consumption rises slightly.
The catalyser is installed as near as possible to the engine, because it only begins to do it's job at above approx. 450°C. The near vicinity to the engine is, e.g., in V- and boxer engines, only possible by using two catalysers. To prevent congestion losses, two or more catalysers - placed one after the other - are also found in sports car engines with a high exhaust gas transfer rate.
Recycling is becoming increasingly important in the possibly more highly fluctuating prices for platinum.
Important
The surface of the catalytic converter can be covered by certain petrol additives, e.g., tetraethyl lead, causing the
function to be adversely affected. 09/10
