The elimination of diesel particulate emissions is now considered as a priority in Europe where the 2005 regulatory directives (passenger vehicle and LDV directives have already been adopted and the HDV directives are in the final stages of adoption) are considered as enforcing the general use of diesel particulate traps (DPF's). Moreover, the German Minister of Environment took in early August 1999 a strong position requiring DPF installation on all diesel vehicles.
The introduction of diesel engines on SUV's in the USA appears to be an attractive way to meet the CAFE requirements for large vehicles but diesel vehicles will still need to meet strict PM limits within Tier 2 regulations to reach the market.
In the meantime, integrated systems for retrofitting buses are being distributed commercially but require low-sulfur fuel which is not widely available.
The recent announcement by PSA Peugeot Citroen of an integrated system using the Eolys™ fuel-borne catalyst (FBC), with regeneration secured by precise electronic management of direct fuel injection by common rail and oxidation catalyst, is a significant step forward in the development of DPF's which can function with today's relatively high-level sulfur fuels.
It has been established for more than a decade that metal-containing combustion improvers are able to reduce the mass of emitted particulate matter (PM) in, and the opacity of, diesel exhaust gas without DPF's Reduction by primary additivation could be as high as 50% when the improver is properly dosed. Major environmental Agencies, such as the EPA and the UBA (German EPA), have expressed reservations to such use of metals because of the resulting atmospheric dissemination of their residues. This has effectively frozen the development of such technologies into a potentially very large market.
More recently, the Swiss VERT program (which concentrated on measuring PM emissions by number and not mass) supported the view of these Environmental agencies by demonstrating that without a DPF, metallic residues from combustion improvers are emitted into the atmosphere. Furthermore, that they are of very small diameter of 20-30 nm (1 nanometer = 1E-9 meter) which is the size area of greatest concern for deposition in lung alveoli, and thus of potentially high medical risk. [fig.1 (source VERT)]. Cerium was among the most effective combustion improvers with a consequent large market potential. However, based on the results of the first part of the VERT study, Rhodia stopped all its promotional activity in the field of primary additivation.
Fig 1: Impact of 50 pppm Cerium [Eolys™] on raw emission (upper curves - without trap) and with trap (lower lines - with a sinter metal trap); source VERT, ref .
NB: this graph demonstrates clearly that adding metal compounds in fuels without trap (similar results were obtained with all other tested additives) creates a new population of ultra fines particles in the range of 16-30 nm, while the combination of trap + Eolys™ eliminate 99.9 % of this category of particles, therefore nearly all the residues of cerium.
Fortunately, the second part of the VERT study was very positive for the use of Eolys™ in combination with a variety of DPF's: not only were all the residues of the cerium retained by the filters, but the addition of Eolys™ to the fuel significantly improved (by nearly one order of magnitude) the filtration efficiency of PM of diameter smaller than 100 nm. Filtration efficiencies of 99.9% where recorded in this size range [fig.1 (source VERT)] .
Based on these results, the UBA (German EPA)  and the BUWAL (Swiss EPA)  decided to officially recommend the use of such a combination from December 1997. In parallel, Rhodia amended its EPA registration to restrict Eolys™ to use only in combination with efficient DPF's.
Filtration efficiency based on number and size was also recently confirmed when efficiency was expressed by mass during transient FTP cycle and steady state measurements in a demonstration program sponsored by the Manufacturers of Emission Controls Association (MECA)  run at SwRI facilities on a DDC Series 60 engine. Of particular note was that the highest filtration efficiency was obtained when a 500 ppm S fuel was used [fig.2 (source MECA)]. NB DPF C is the only one using an FBC: the mass filtration efficiency was up to 97 % with this filter.
Fig 2: Different Diesel Particulate Filters tested downstream of EGR (source MECA )
In order to confirm that the mass retention of cerium residues was comparable to the number retention measured in the VERT study, SwRI was commissioned by Rhodia to measure cerium in the exhaust when the DPF was on line: it was confirmed  that the mass retention is 99.8%, very close to the number retention of 99.9% measured by VERT.
The benefit of using Eolys™ is then obvious as it improves the filtration efficiency by about an order of magnitude, while the residues of the metal contained are retained in the filter.
In real world operation, Eolys™ alone, like all fuel-borne catalysts, can not achieve DPF regeneration in all driving conditions.
Today, the critical issue relating to DPF use is to achieve a safe regeneration in all driving conditions. All catalyzed filter systems are facing difficulties in achieving full operation in all circumstances. Many different catalytic substances have been tested in the hope that the most active would allow a totally passive regeneration. Some very promising results were published following laboratory and bench tests leading to the belief that the solution was found. But when tested in the real world, the low dosage rates employed in these tests turned out to be insufficient. Even increasing the dosage rate up to 10 times did not allow total safety of regeneration and thus eliminating the dual risk of destroying the filter due to excess heat or of rapidly plugging it with residues.
In one of the only successful demonstration programs up to now involving a large number of vehicles (more than 100) over a long period of time (2-year operation), it was demonstrated in Athens in 1989-1991 that 110 buses, each one driving more than 100,000 km during the period, were able to work with a dosage rate of 50 ppm. 
Bench experiments had demonstrated that 25 ppm was sufficient when throttling-assisted regeneration was used. In order to lower the fuel penalty to a non-detectable level, in real world operation the dosage rate was set at 50 ppm. Taking into account the dirtiness of the engines installed on these Ikarus buses and the fact that the dose of cerium has to be determined as a function of the amount of soot emitted, the same system applied to a modern engine would require only10 to 20 ppm of cerium. This was the first example of an integrated system incorporating a FBC. The safety of the regeneration was achieved by throttling the exhaust in order to increase the temperature to a level where the catalytic effect of ceria provoke the burning of the doped soot.
The concept of a passive-active integrated system was then demonstrated to be effective, but its cost was considered to be too high. As it was demonstrated that throttling was used on very limited occasions, the goal of cost-effectiveness led to research on purely passive regeneration with an optimization of the dose as a function of the soot emission. Experimental bench tests demonstrated that dosage rates as low as 10 ppm could be sufficient for US 1994 engines. At the same time it was obvious that such a low dose could not be applied in the real word without assisted - regeneration to account for those critical driving conditions which always occur in the life of a vehicle, except in very specific applications where high exhaust gas temperatures are regularly achieved in normal service. Such results were not published [fig.3].
Fig 3: Trap inlet and outlet temperatures for four regenerations with 10ppm Ce
Rhône-Poulenc [the older name of Rhodia] invested in some basic programs with AVL   [fig.4 (SAE 942 069)] to demonstrate that a secured regeneration was possible in a multitude of ways when the engine management is used to provide additional heat to the exhaust or when throttling is properly applied.
Fig 4: Effect of different measures on exhaust gas temperature in lower load/speed map
12L DI/TCI Diesel Engine. Modified load / speed map due to: (1) increase of exhaust gas back temperature, (2) increase of intake air temperature, (3) decrease of intake air pressure, (4) retardation of S.O.I.
Meanwhile, the other producers of metal-containing fuel soluble catalysts were continuing to look for higher activity of the catalyst itself by investigating new catalytic phases. Copper, which looked the most active (about 100°C less than cerium by some bench tests), was withdrawn from the market as it catalyzed dioxin formation in the trap . It must be also mentioned that despite years of trials, no publication has ever established that copper can offer a safe passive-regeneration in the real word.
Taking into account the diversity of real world operating conditions, the absence of published results with competitive FBC's and our own expertise, it can be stated that, with the exception of some very specific applications, neither Eolys™ nor any other fuel-borne catalysts can achieve passive regeneration in all real world driving conditions.
Published [7,8] and unpublished works convinced Rhodia that the only viable way of development was an integrated system, either for original equipment or retrofit.
The system promoted by PSA for it new luxury 607 sedan is a perfect example of such an integration. The detail of the system is given in the web site . Looking at it from the perspective of the FBC, the safety of having the exhaust temperature increased up to 450°C is a full guarantee of operation in the most severe service conditions. The dosage rate is then reduced to 25 ppm of cerium.
Another system, targeted mainly for retrofit, is currently proposed by the German company HJS. Regeneration of the sintered metal filter is secured by a small external burner and requires a dose of cerium of 20 ppm  which can be reduced when the soot emission is low.
All integrated systems require the soluble catalyst to be introduced on-board in a dose corresponding to the amount of soot emitted. This on-board dosing system has a double advantage:
When Eolys™ is used in such a way, we consider that the term 'fuel additive' is not accurate; the terminology 'Fuel-Borne Catalyst' is preferred in order to make a clear distinction between primary additivation of fuels distributed at service stations or after-market addition with a dose not optimized for the vehicle and with the risk of misuse in vehicles not equipped with filters.
It is only when a product meets all of these criteria that it can be considered a valuable Fuel-Borne Catalyst.
The potential of an FBC cannot be judged solely by bench-test regeneration temperature.
FBC dosage rate should be adjusted to real-world exhaust composition.
Eolys™ is one of two products meeting the health requirement of the UBA and BUWAL.
Eolys™ was tested against all the above criteria and gave outstanding results.
Special acknowledgements to my colleague Adrian Richards, who helped me in the final preparation of this document, to Andreas Mayer and Ulrich Matter (VERT) who provided fig 1, to Magdi Khair (SwRI) who provided fig 3, and to Paul Zelenka (AVL) who provided fig 4.