As previously mentioned, a considerable reduction in diesel particulate matter has already been achieved by modifications to the engine.
Since the introduction by the EU Commission in 1989 of the first emission standard for diesel passenger vehicles, the limit for diesel particulates has been reduced from 1.1 g/km by a factor of 22 to only 0.05 g/km today (Emission Standard III).
With regard to Emission Standard IV (0.025 g/km) it is becoming clear, however, that the means by which diesel particulate emissions can be reduced through engine modifications have been virtually exhausted.
A further incentive for achieving a reduction is increasing environmental awareness and the fact that the residual diesel particulate matter has a harmful effect on the human body.
Diesel particulates are composed mainly of a chain of carbon particles (soot) with a very large specific surface area.
The noxious effect of diesel particulate matter is a result of adsorption of unburned or partially burned HC. In addition, fuel and lubricant oil aerosols (solid or liquid substances finely distributed in gases) and sulphates
(depending on the sulphur content of the fuel) bind with the soot.