What is often missed is that ultra low sulphur diesel (ULSD) with 10 parts per million (ppm) sulphur is not clean fuel per se . But that the low sulphur content at that level allows manufacturers to instal particulate trap to control emissions of tiny and deadly particulates. If the sulphur level is higher, the efficiency of the particulate trap is very low, and it has to be regularly cleaned, at great cost to the bus operator and the regulator. There is little evidence that traps work with more than 50 ppm sulphur. The US government has mandated a limit of sulphur content of 15 ppm in diesel, to be implemented from 2006 because of the treatment devices needed to meet its emission standards to fully come into force by 2007. Most other countries are experimenting with 10-50 ppm of sulphur.

Aftertreatment of diesel exhaust is difficult because of its high particulate load and high nitrogen oxide levels. So far the widest commercial application of diesel aftertreatment devices has been that of diesel oxidation catalysts. This device is more effective in controlling hydrocarbons and carbon monoxide than particulates and nitrogen oxides, which are the chief emissions from diesel vehicles. The other devices are:

l CATALYTIC SOOT FILTER (CSF): Manufactured by Engelhard Corporation and also known as DPX