T Honeywill, Major players come clean, Professional Engineering, 1 October 2003, p 41

In Europe, the emissions limits for diesel engines have been met by the use of either the common rail fuel system or exhaust particulate filters. However, the Euro IV regulations applying from 1/01/05 set per-kilometre maxima of 0.025 g of particulates and 0.3 g NOx and hydrocarbons for vehicles below 2.5 tonnes. This will require the use of both common rail and filters. The problem remains of removing the ash deposited within filters due to fuel additives. Raising temperatures by burning off the ash with unburnt fuel and the use of catalytic coating are standard procedures with silicon carbide filters, but Bosch is exploring the potential of a sintered metal technology. Johnson Matthey has devised a continuously regenerating trap that injects a urea reductant downstream to handle residual NOx.