Exhaust gases from diesel cars contain soot particles, which are the result of combustion (pyrolysis) in the diesel engine. Soot particles in the (sub)micron range appear to have such a detrimental effect on human health that the European Commission decided to restrict their emission. For example, the Euro V standard came into effect as of 1 September 2009 for new cars fitted with a diesel engine. This standard states that only 5 mg of soot may be emitted for each kilometer travelled. Incorporating a soot filter in diesel cars is a practical implementation of this standard.
The operation of such a filter is quite simple: a porous metal or ceramic wall allows exhaust gases to pass and rejects solid soot particles which are burned off the filter at a later stage. This burning step is necessary because the soot particles on the filter form a layer that gets thicker and thicker, so that the exhaust gases experience an increasing resistance to pass through the pores of the filter. In practice, this burning step or ‘thermal regeneration’ occurs once every 1000 kilometers of driving. The aim is that the soot particles are completely burned into carbon dioxide and water vapour, which can leave the exhaust through the pores of the filter.