WHAT IS A CATALYTIC CONVERTER?
Put simply, these are large steel cans that are fitted to the front exhaust pipe. Crammed within them is a ‘brick’ that provides a very large surface area – most commonly, this tends to be a honeycomb ceramic. What makes them especially attractive to scrap metal thieves is the thin layers of rare and precious metals (typically platinum, palladium and rhodium) that coat the brick. When these elements reach around 300°C (experts use the term ‘light off temperature’), they promote a chemical reaction with the spent emissions, while remaining unchanged themselves.
This is why catalytic converters have a very long life. Two-way catalysers combine oxygen with carbon monoxide and unburned hydrocarbons to create the less harmful carbon dioxide and water. More advanced three-way catalytic converters are more popular in modern petrol cars and deal with nitrous oxide (NOX) emissions as well, by turning them into the inert nitrogen, although exhaust gas recirculation (EGR) valves and coolers have helped to reduce NOX even further to comply with ever-tightening regulations. These types tend to be regulated with one, or more, oxygen sensors.
Popular production diesel cars have used unregulated catalytic converters since the 1990s, the main purpose of which is to oxidise unburned hydrocarbons and the tiny levels of carbon monoxide to carbon dioxide and water. Later cars, fitted with DPFS, are also equipped with catalytic converters and, on many models, they are incorporated within the same canister and cannot be removed separately. Some catalysts are intended to produce nitrogen dioxide, which, despite being a prominent air pollutant, is desirable because it produces an exothermic reaction within the DPF, thus raising the temperature. However, exhaust-mounted diesel catalysers are far less able to deal with NOX (consisting of NO and NO2) emissions, which is why some cars are fitted with NOX traps (which are maintenance-free and are controlled by the engine management systems) or Adblue/selective Catalyst Reduction (SCR) technology.
The Lambda Sond (oxygen sensor) was a vital invention to permit correct functioning of the three-way catalytic convertor on petrol engines, by maintaining the narrow operating mixture of 14.3:1 air/fuel. Modern cars can have several of these sensors...