Who will be the cleanest?

Mercedes, Opel and BMW are introducing diesel particulate filters

Frankfurt/Main. They are late but they are coming. Beginning in October, German carmakers will be fitting diesel particulate filters to their vehicles.

Mercedes-Benz will be the first: from October the four-cylinder diesel engines of the C class and E class will be fitted with particulate filters, but only on special request and for an extra cost of 580 euros.

From January, there will also be a filter option for the six-cylinder CDI engines used in the E class and S class.

And from the beginning of 2004, Opel will fit filters into Vectra and Signum models with the new 1.9-liter CDTI diesel engines.

Within the coming year, BMW will supply the six- and eight-cylinder diesel engines used in the 5 series and 7 series with filters.

All of those new systems are maintenance-free, comply with the Euro 4 emissions norms and will allow their owners to claim tax relief.

Due to the catalyzer being close to the engine and innovations specific to the individual manufacturer -- Opel, for example, is using a special metal coating of the filter substrate plus multiple injections -- the systems do not require any fuel additives.

Ford, however will fit its C-Max compact minivan with an additive system from this autumn. The reason: the new diesels the C-Max uses were developed in a joint venture with PSA/Peugeot-Citroen. Therefore the filters, which are favored by the French manufacturers, were the quickest solution for Ford.

"Nevertheless, we are developing our own system that will function without additives," said Rudolf Kunze, director at the Ford research center in Aachen.

The VW group's particle filter will also need additives. Audi will introduce a "soot trap" for use in A4 models at September's IAA auto show in Frankfurt. The VW brand will present a similar solution for large models such as the Phaeton and the Touareg.

What are the advantages and disadvantages of the two competing systems?

Liquid additives that contain iron and the heavy metal cerium facilitate the burning-off of particles that have collected in the filter. Normally, this process requires high temperatures of up to 600 degree Celsius, which can hardly be reached in suburban driving conditions. By using additives, the carbon particles can be burned at temperatures of between 400 and 500 degree Celsius.

However, the additives remain as soot in the microscopically small filter channels, increasing the exhaust emission pressure.

"Engine power loss and increased fuel consumption are the results," said Joachim Schommers, design engineering director for automobile diesel engines at DaimlerChrysler.

In 1985, Mercedes introduced the first-ever particulate filter in a diesel automobile. However, Mercedes was disappointed with the performance of the filter, which was available on 300 D and SD turbo models that were only sold in North America. When other, more advanced particulate filter systems were introduced in 1988, Mercedes stopped using filters.

In addition, additive systems still need to be serviced. The reservoir attached to the car, which holds several liters of additive, and the filter has to be exchanged or cleaned on a regular basis.

In November 2002, the service period for Peugeot models with diesel particulate filter technology was extended from 80,000km to 120,000km.

Peugeot will present a V-6 engine at the IAA that has a maintenance-free particulate filter system, which will be used as a standard from 2004.

The filter with modified "Octosquare" geometry and larger entrance passages uses the redeveloped additive Eolys 2, of which smaller doses will be needed than previously. In addition, fewer soot deposits will be left on the filter walls.

But how clean are those filters really? The previous PSA filters did not comply with Euro 4 norms, which will be compulsory from January 2005. The problems were not particulate matter emissions but nitrogen oxide emissions.

Nonetheless, there will soon be diesel cars on the market that comply with the Euro 4 norms. Beside the new models by the German carmakers mentioned earlier, other OEMs will also be launching environmentally friendly engines and models in 2003: Peugeot (1.6- and 2.0-liter engines), Renault (Vel Satis 2.2-liter dCi) and Toyota (Avensis D-Cat). The Renault and Toyota filters do not need any additives.

But there is disagreement among carmakers over the percentage of soot particles remaining in the new filters. Some additive-free systems are thought not to be as efficient as PSA’s technology regarding the degree of purification.

The Euro 4 norms specify the limit value of particles emitted per kilometer as 0.025 grams. While the Peugeot system emits hardly any particles into the air, the German systems are not quite as effective.

Opel said that 95 percent of particles remain in their filter. The system by Mercedes-Benz supposedly filters 91 percent of particle emissions.

The dispute over who is the cleanest will continue until independent tests bring more clarity.

With German OEMs increasingly providing particulate filters, the VDA (German Automotive Industry Association) is now also slowly reducing its opposition.

Managing Director Kunibert Schmidt said: "Particulate filters will be fitted if the technical requirements are met and if it is sensible, but also when customers request them."