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VERT: Open letter to Transport & Environment

17 January 2020

This letter is addressed to the authors of the report “New diesels, new problems” and the accompanying press release, published by Transport & Environment on 9th January 2020.

From your videos it seems that you are a very young team, so maybe you did not have the time yet to carefully study what has been investigated and published on diesel particle filter (DPF) technology since its introduction around 1980.

I am an mechanical engineer of an older generation and was involved in research, development and implementation of best available technology to eliminate ultrafine particles from diesel engines (this is the title of one of our books) from the very first moment. I was responsible for the implementation of DPF technology in tunneling in Switzerland from 1993. We have established the nanosize solid PN-criterion for DPF validation (for health- and metrological reasons), the respective metrology, standards and protocols, which became the template on which the EU ten years later started to build what we now know as the Euro 6/VI emission regulation.

That’s why I am taking the freedom to tell you, that nothing in your report is new, absolutely nothing. RICARDO could have told you all this without performing any new emission test. The observations, questions and speculations which are filling the pages have been obvious from the beginning, and many of us have clarified the physics and chemistry involved and contributed to solutions. Also, most of your interpretations are incorrect and most of your conclusions are of little value. What disappoints me even more is that you don’t seem to understand what are the real problems and to notice where an organization like yours can help to make progress.

Yes, today’s diesels with aftertreatment are clean, unbelievably clean compared to engines without aftertreatment. They are using the best combustion process, electronic controls, clean fuels and lubrication oils, excellent oxidation and reduction catalysts and high quality particulate filters—introduced in an incredibly short time for all onroad and offroad vehicles thanks to straightforward legislation and full support of the aftertreatment industry. This technology to eliminate carcinogens is now expanding fast across the globe and we emission engineers are proud to say that we are saving thousands of years of human and animal lives year by year. We have implemented best available technology (BAT) and we progress because BAT of tomorrow will not be BAT of yesterday and we know exactly what we have to do.

At the beginning of your paper, you mention the “Dieselgate scandal”, which was a criminal act of several vehicle manufacturers that involved illegal “defeat devices” and exceeding NOx limit values by orders of magnitude. By putting the Dieselgate scandal in context, you chose to strongly insult the DPF industry. This is entirely without merit, since the particle reduction technology by DPF fulfills the EU limit values under all operation conditions, and sometimes particle emissions fall below these limits by orders of magnitude. Particulate filters are also a durable technology, and their performance improves over time. As the tailpipe concentrations of particles are below the ambient exposures in many cities, diesels with DPFs are even cleaning ambient air polluted by other sources, including petrol engine particles. Filtration efficiency of the filters you tested was probably above 99.99% between regenerations and higher than 90% during regenerations. Not good enough?

You have not even measured the emission upstream of the DPF, so how can you know whether a filter is good or bad? I am afraid this is a clear sign of unprofessional work. And I did not find any indication of ambient air particle pollution levels. Was the particle emission of the selected cars lower than ambient concentrations, and would this provide an indication to define “clean”? Neither did I find toxicity indicators for your solid and volatile species. Some of the volatiles would be just water, and some water-soluble, easy to metabolize material. I am completely missing the discussion of chemistry and toxicity, and with particle levels at the threshold between Euro 6 and 7 this can be only deemed unprofessional.

You randomly selected two cars of similar regeneration strategy and had them investigated in a freshly invented test cycle. Substrate parameters, catalyst parameters, operation parameters like space velocity are not even mentioned. But in today’s DPF vehicle fleet we have different materials, different porosities, different catalysts, different strategies and they all produce different results. How can you draw conclusions on such small basis? And you let the mainstream media to copy-and-paste your press release under sensational headlines, and to present it as another “scandal”—with an innuendo that the industry creates particle emission peaks on purpose, without any consideration for public health. There are many types of DPF systems—we have passive and active regeneration, regulated and non-regulated, catalyzed and not, excellent fuel borne catalyst (FBC) systems and some of them are not producing any of these peaks. Whether these are better systems, is an open question; they may have other drawbacks such as higher CO2 emissions.

The emission peak during regeneration has always been an important topic in DPF research—if you searched the existing body of technical and scientific papers, you would find many interesting ideas to control these emissions. A regeneration peak limitation is of course part of the VERT filter standards, the legal base in Switzerland since 1996, where we test filters clean, loaded and during regeneration under worst case conditions. I can refer you to the Swiss standard SN 277206, which states that the filtration efficiency of solid PN (in the size range of 10-500 nm) must exceed 80% at the regeneration peak. This requires very good filter substrates with perfect small pore size distribution. As soon as the filter starts collecting soot, minutes after regeneration peak, this efficiency typically climbs up to >99.9%. Even though the DPF filtration efficiency may be lower during and immediately after regeneration, the filter is still providing a significant reduction of particle emissions.

You stated that EU legislation has no provisions to control emissions during regeneration, but this assertion is inaccurate. EU regulations limit particle emissions by mass and number, and particle mass is used for the evaluation of “Ki factors” for all emission compounds (although the VERT “grandfather protocol” uses PN).

You made a strong case for the measurement of particles below 23 nm. You must be aware that EU-JRC has been working on this topic, and lowering the cut-off point to 10 nm is expected at the Euro 7 stage. However, this change is not important for diesel engines with DPFs. While the measured particle numbers may be slightly different, the required filtration performance is defined well enough under the current approach. I’d like to emphasize that it never was a measurement problem—VERT has conducted PN measurements since 1996, while others did it even earlier.

I wish I could help you understand filter technology better, ask better questions and promote technical improvement. I would like to invite you to participate in the next VERT Forum on 18 March 2020, where we discuss best available technology implementations, and in the 24th ETH Nanoparticle Conference in Zürich, on 22-25 June 2020, where we expect 400 experts from 30 countries for emission reduction research. The ETH Conference webpage includes a wealth of scientific and technical information on these subjects.

As a final point, I must admit that your title “New diesels, new problems” is actually quite good, but for reasons which are outside of your report. I am really puzzled that you don’t seem to be aware of several important issues that should be the focus of your activity:

In summary, diesel particulate filters are not among the sources of air quality problems—on the contrary, they are a key part of the solution. It seems that your strategy needs a major change of direction.

Please feel free to contact me if you require supporting information for any of my above statements.

Yours Sincerely,

Andreas C.R. Mayer
Dipl. Ing. Dr. med. h.c., SAE Fellow
VERT Scientific Officer

Source: VERT Association