For separating solid particles (mostly black carbon or soot) from the exhaust gas of internal combustion engines (ICE), there are basically two very different types of devices commercially available. One type, the FTF is based on turbulent flow along a catalytically coated wall; as soot particles touch the wall they should burn off, which happens if the catalyst is above the activation temperature. Unfortunately, modern ICE exhibit very high efficiencies which means their exhaust temperature has to be pretty low anytime and even lower at part load such that the catalyst does not work at all. It comes even worse: As long as the catalyst is not activated soot is accumulated at the wall, thus isolating thermally the catalyst from the exhaust gas such that the catalyst won't work even if the temperature of the exhaust increases. Finally, at full acceleration the accumulated soot layer is blown off; in practice this happens when cars in city drive start up at intersections blowing their "goodies" just unto the noses of waiting pedestrians. Plenty of brain has been used to overcome said disadvantage of FTF; supporters of FTF claim the simplicity, the low manufacturing costs, however, they agree on a separation efficiency of only 30% to 50% at maximum; independent tests challenge even these numbers.

In contrast to the FTF is the DPF: The separation mechanism is based on the Brownian motion, hence diffusion. Separation efficiency is always better than 99% of ultra-fine (or nano-) particles and better than 97% of fine particles. Usually, the exhaust gas is forced through a porous ceramic wall (therefore the expression wall flow filter); however, the material need not be necessarily ceramics - some sinter-metals and other fabrics may be used too. In tiny micro-cavities accumulated soot may be burned off in intervals or steadily. DPF are more expensive than their counterparts and they require some adjustment of the electronic engine management system.

Public officials at some environmental authority, in charge of clean air or air pollution reduction, face the burden to decide on measures to be taken. These measures may be direct public grants, or of indirect nature in the way of tax reduction or limitation of entry to special areas, like environmental zones. They are burdened with accountability for public health, because everybody needs breathing.

In the following a guideline is presented for checking the overwhelming evidence against FTF and in favour of DPF. A series of papers is presented for download and a commentary to the context of these original papers. For copyright reasons only the first page of pertinent SAE papers, abstract and part of the introduction are taken here as a reference.

Guideline and questionnaire for achieving personal judgement

Is discharge of soot from FTF really that bad?

Above, six compelling reason are brought forward against FTF-technology; but what is wrong, if admission to German environmental zones is based on such FTF devices, why have they been cleared, is there anything wrong with German rules?

Sure, the German testing rules are based on unfortunate engineering, see file "Anlage 26-PKW Verordnung BMU", and in particular for convenience the special passage in question is reproduced separately (2 pages) "Anlage 26 Konditionierung" they provide "conditioning" (in view of reproducibility of measurements) however, this means as well cleaning of the catalytically coated wall-surface prior to each state of engine-operation on the test bed; of course, results obtained this way are even further away from real world performance than test cycles usually.

In view of the importance to public health, was there any reverberation, any reaction, in German authority whether elected or appointed?

There was a parliamentary hearing on March 5th, 2008; at the end the then German Minister of the Environment, Dr. Gabriel, sighed and remarked "Mr. Mayer may have been right" see file "GermanParliament Russfilterskandal 050308" -; however, disappointments were blamed on the criminal energy of one single individual instead of a basic mistake. Conclusion: The sovereign's right is to set the rules while admitting to an error for a U-turn in policy is very difficult for any administrative body.

The alternative to FTF is in DPF-Technology. - Is there any position paper of an impartial professional body with regard to FTF vs. DPF?

There are publications from impartial origin to that effect; as an example the position paper of the German Society for Chemical Engineering, DECHEMA, is presented here; it is entitled "Feinstäube und Umweltzonen", see file "DECHEMA-Stellungnahme-Feinstäube V10a 20090528" (red markings are not original, they have been set by the author of this collation). This position paper broils down to three important statements:

  1. There is no doubt left regarding the efficiency of separation of DPF (99%)
  2. Costs of removing soot per unit mass by means of FTF are about hundred times higher than by means of DPF!
  3. A statement regarding priorities, which is dealt in the following paragraph.

Options left for reducing air pollution. How to find priorities?

Three serious publications in agreement should be sufficient for having confidence with regard to the basis of reasoning:

What to tackle first, passenger cars or heavy duty equipment?

The summary of the DECHEMA position states clearly "heavy duty first", due to long life expectation or such workhorses (20 years HDV, 40 years railway locomotives) retrofit is a necessity - retrofit with DPF! This conclusion is easily corroborated by means of a simple example for illustration from Wang, who claims emission factor of HDV is 20 times that of passenger cars: Assume 20 passenger cars and 1 HDV then emission of each category is 50%. FTF (of 30% efficiency in separation - quite optimistic) on all passenger cars yields an emission reduction of 15% overall, i.e. 85% of the emission is still ambient around; in contrast, retrofitting only one single HDV yields 50% reduction. It is left to the reader to set up examples with different ratios. Further, there arises the question:

Is retrofit only the second best solution compared to OEM?

By no means, see previous references SAE 2007-01-1112 or alternately Austrian Engineering Journal OIAZ vol. 152, no 1-3, pp. 62-69, "MayerEuro3w-o-vs-4und5" a Euro 3 retrofit DPF compared rather with Euro 6 particle emission, leaving Euro 4 and Euro 5 behind.

LDV and passenger cars, have a shorter life time, in the average 8 years; therefore, one may expect that in due course of time manufacturers will follow the example of Peugeot, the company that was first to introduce real DPF, while other big manufacturers have been reluctant to do so.

Of course, there exists the option of traffic restrictions too in defined areas, the environmental zones, the decision to set up such an area is a different topic; it is dealt with in www.airqualitypolicy.co.uk and www.lowemissionzones.eu.

Importance of air pollution abatement and conclusion

An adult human needs about half a kilogram of solid food, 2 kilograms water (liquid) and approximately 20kilograms air for breathing; healthy food and sound water (mineral water in bottles) can be bought, the air cannot be selected, one has to breath wherever we are. Polluted air affects the health of the population with repercussions to individual life expectation, quality of life, and last not least to the public purse (the revenue service suffers a loss of tax revenue). Public officials are burdened to do their best based on reason and experience, they ought to shy rules admitting FTF like pestilence - in their very own interest.

Postscript

In case of further interest, test reports and even views of three manufacturers of FTF tested may be found by means of this link. Reading and considering the manufacturers’ points one is tempted to think: “Our engineering schools are great in educating design- and test engineers for industry; however, they apparently are less good in providing public administration with rule-makers.”