A diesel particulate filter is a system for separating particulate matter from the exhaust gases produced by diesel engines. Two different systems are in use at Volkswagen depending on the model: Diesel particulate filters without an additive are fitted on models where the particulate filter is positioned near to the engine. On models where such a positioning of the particulate filter is not possible, systems with an additive are used. The filter with a catalytic coating works without the need for an additive by using a filter coating containing precious metals which fulfils a dual function. Passive regeneration slowly and gently converts the particulate matter contained in the catalytic converter into CO2. This process takes place at temperatures between 350 and 500 °C and can run continuously without the need for intervention, particularly in cars which are driven mostly on the motorway. Only when vehicles are operated for prolonged periods at low engine loads - such as is the case when driving in city traffic - does additional filter regeneration have to be carried out by actively raising the exhaust temperature to around 600 °C every 1,000 to 1,200 kilometres. At this temperature, the particulate matter trapped in the filter burns off. On additive-based systems, the additive serves to reduce the particulate ignition temperature to around 500 °C. Regeneration needs to be performed every 500 - 700 km, depending on the driving style. The additive is automatically flushed into the fuel tank every time the driver refuels, with one litre of additive lasting for approx. 2,800 litres of fuel.