Availability of materials could present problems, especially for the makers of the honeycomb-shaped ceramic solids made from silicon carbide. Soot particles settle on the platinum-coated surface of the filter.
The base material is extremely fragile, said Wolfgang Hatz, head of aggregate engineering at Audi AG. Suppliers must cope with a high level of rejects and with demand rising sharply availability could be a problem, he added.
BMW said that currently suppliers can meet only 30 percent of German demand. They will not reach full supply capacity until 2007.
Japanese supplier Ibiden, which manufactures particulate filters primarily for PSA in a joint venture with Saint-Gobain in Courtenay, France, is planning a new manufacturing plant in eastern Europe by 2006. The search is currently going on for a location.
Faurecia is also substantially expanding its capacities, while Bosch plans to start production of particulate filters in 2007.
Audi plans to achieve full availability of filters for all Audi model series by the end of 2005, said Hatz, a move that will cost the company a lot of money -- not all of which can be passed onto consumers.
An Audi internal paper confirms that retro-fitting the A4 alone costs the group more than 10 million euros a year, excluding engineering costs. Of the purchasing costs of 462-491 euros, only 300-425 euros can be passed on to the customer under EU competition rules.
All Audi diesel cars already comply with the Euro 4 exhaust emission standards that come into effect from 2005.
Mercedes-Benz cars boss Juergen Hubbert said that the filter debate was "very much a German subject."
He said: "More than 80 percent of diesel vehicles sold in Germany are fitted with particulate filters. In France it is only 0.2 percent and in Italy 0.4 percent."
Mercedes now offers filters for all four- and six-cylinder diesel models.
The German government plans to grant tax relief for cars with particle emissions of less than 8.5 milligrams per kilometer and 200 milligrams of nitrogen oxide per kilometer starting in 2005 to speed up the spread of clean diesel engines. A filter is not required to qualify for the tax benefits.