Exhausts have four main functions: to control noise, to direct exhaust fumes away from passengers, to improve the performance of the engine and to improve fuel consumption.
Since January 1993, all new cars sold in the European Union (EU) have been fitted with a catalytic converter (CAT). This is made up of a very thin layer of platinum group metals on a honeycomb structure. The surface area of a typical 3-way CAT covers the equivalent of two football pitches. As exhaust gasses pass through the catalyst a chemical reaction occurs which converts carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) to less harmful compounds (water, nitrogen and carbon dioxide).
The use of catalytic converters leads to a dramatic reduction in the emissions of CO, HC and NOx. However, they also result in an increase in CO2 emissions, which do not cause a problem for urban air quality, but may contribute to global warming. The efficiency of a CAT can be as high as 90%.