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New Diesel exhaust gas aftertreatment systems, with combined DPF*) and deNOx (mostly SCR) systems represent a very important step towards zero emission Diesel fleet. These combined systems are already offered today by several suppliers for retrofitting of HD vehicles. Reliable quality standards for those quite complex systems are urgently needed to enable decisions of several authorities. The present report informs about the international network project VERT *) dePN (de-activation, de-contamination, disposal of particles and NOx), which was started in Nov. 2006 with the objective to introduce the SCR-, or combined DPF+SCR-systems in the VERT verification procedure. Examples of results for some of the investigated systems are given. These investigations included parameters, which are important for the VERT quality testing: besides the regulated gaseous emissions several unregulated components such as NH3, NO2 and N2O were measured.

Most particulate traps efficiently retain soot of diesel engine exhaust but the potential hazard to form secondary emissions has to be controlled. The Diesel Particle Filter (DPF) regeneration is mainly supported by metal additives or metallic coatings. Certain noble or transition metals can support the formation of toxic secondary emissions such as Dioxins, Polycyclic Aromatic Hydrocarbons (PAH), Nitro-PAH or other volatile components. Furthermore, particulate trap associated with additive metals can penetrate through the filter system or coating metals can be released from coated systems. The VERT test procedure was especially developed to assess the potential risks of a formation of secondary pollutants in the trap. The present study gives an overview to the VERT test procedure. Aspects of suitability of different fuel additives and coating metals will be discussed and examples of trap and additive induced formation of toxic secondary emissions will be presented.

Based on the emission inventory Fig. 1, the Swiss 1998 Ordinance on Air Pollution Control (OAPC) mandates curtailment of carcinogenic diesel particle emissions at type B construction sites [1]. Moreover, particle traps are compulsory at underground workplaces [2]. In compliance, more than 6,000 Diesel engines were retrofitted with various particle trap systems. Many traps surpassed 99% filtration efficiency and secondary emissions were mostly prevented. However, trap failure due to mechanical and thermal damage was initially rather high at about 10%. By the year 2000 the failure rate was halved to about 6%. Thanks to focussed improvements, the year 2003 statistics show yearly failures of “only” about 2%. The Swiss target is to retrofit 15,000 construction machines with traps, fully compliant with environmental directives, having 5,000 operating hours durability and failure rates below 1%. Traps must pass the VERT suitability test before deployment.

Fine pored hot gas traps have filtration efficiencies exceeding 99% of the solid particles in the diesel exhaust gas. There is a favorable trend to deploy this technology ex-factory and retrofitting on-road and off-road engines. The trap system however functions as a chemical reactor. The filter has a large effective area and the engine exhaust gas has plenty of reactants, which can promote undesirable chemical reactions that release toxic secondary emissions. These effects may be amplified when traps have catalytic influence, e.g. due to surface coatings or fuel-borne catalysts. The VERT suitability tests for particle trap systems therefore include a detailed test procedure for verifying the presence of over 200 toxic substances. These include PAH, nitro-PAH, chlorinated dioxins, furans as well as metals. The paper describes test procedures, test reporting, sample extraction and analysis.

1 A methodology for correctly matching trap systems to the vehicle types was developed within the scope of a feasibility study to retrofit the entire Swiss fleet of on-road HDV. Representative test vehicles from 11 vehicle categories were equipped with high capacity data loggers during a period of 4-6 weeks. Statistical evaluation of exhaust temperatures indicate that data on averages, peaks and frequency distributions alone can be misleading, because these tend to over-estimate the available exhaust enthalpy. Analysis of dwell time intervals, at certain temperature levels, is a better method to assess the energy available for the regeneration. Such verification of duty cycles is indispensable before retrofitting traps and choosing either active or passive regeneration systems.

1 Switzerland is enforcing the use of particulate traps for offroad applications like construction as well as for occupational health applications like tunneling. This decision is based on the results of the VERT-project (1994-1999), which included basic aerosol research, bench screening and field testing of promising solutions as well as the development of implementation tools like trap specification, certification scheems and field control measures. On the other hand there is no corresponding regulation for city-buses yet although PM 10 is about 2× above limit in most Swiss cities. Public pressure however is growing and city transport authorities have reacted by retrofitting Diesel city-buses instead of waiting for cleaner engine technology or CNG-conversions. The favored trap system with about 200 retrofits so far is the CRT.

Apart from crash simulation, theoretical simulation of the static and dynamic structure behavior is another integral part of vehicle body development. Due to the complex boundary conditions, however, this tool has not been used before as a development aid in this field. With respect to further reduction of development time, a reliable simulation technique for the time-consuming structure durability test runs is desirable. The presented technique allows the relative evaluation of the individual development steps regarding component strength. Thus only the most essential variants need to be tested, and future body development could be possible by designing two prototype generations only.

1 The VERT project aimed at curtailing the construction site diesel emissions of ultra-fine particles to 1% of the raw emissions. Thus, compliance with occupational health legislation should be achieved. Particulate traps have attained this target. In contrast, engine tuning, reformulated fuels and oxidation catalytic converters are almost ineffective. This paper reports on the concluding project stage in which 10 traps were field tested during 2 years. Subsequent detailed measurements confirmed the excellent results: > 99% filtration rate was achieved in the nano-particulate range. The PAH, too, were very efficiently eliminated. Trap deployment becomes therefore imperative to fulfill VERT-targets.

Microfibers with high specific micro-surface can be knitted into two-dimensional structures with large internal porosity. Catalytically active metals can be deposited on the fibers with high dispersion by wet-impregnation, sol-gel or CVD, respectively. These microfiber knits may be used for exhaust gas treatment systems with a triple function: particle filtration, gas conversion and muffling. The total oxidation of propane on Pd and Pt coated fibers has been studied as a test reaction. Conversion temperature could be remarkably reduced compared to cellular structures. For a bimetallic (Pt-Pd) coating, the activity is independent of humidity or oxygen concentration. Thus a catalytic converter based on micro-fiber knits appears feasible. Its high mass and heat transfer prevent hot spots. And it functions as submicron filter for combustion aerosols. Integrated electric heating can also be provided in case of low gas temperatures. First tests on engines show promising results.

Knitted fiber particulate traps facilitate deep-bed structures. These have excellent filtration properties, particularly for ultra-fine particulates. They are also suitable as substrate for catalytic processes. The two characteristics are: high total surface area of the filaments, and good mass transfer. These are prerequisites for intense catalytic activity. The deposited soot is uniformly distributed. Therefore, temperature peaks are avoided during regeneration. The tested coatings lower the regeneration temperature by about 200°C to burn-off temperatures below 350°C. Further improvements seem attainable. Thus, a purely passive regeneration appears feasible for most applications. The system is autonomous and cost effective. However, in extreme low load situations, e.g. city bus services, the necessary exhaust temperatures are not attained. Hence, burners or electrical heating is necessary for trap regeneration.

The development of particulate-traps for big engines is more difficult than for automobile applications. The usual placement, after the turbocharger, necessitates complex solutions to challenges in size, flow distribution and regeneration. The placement of the particulate trap ahead of the turbocharger has technical and financial advantages, and has previously been extensively investigated, but did not prevail because of poor reliability of the monolithic traps. This paper investigates the knitted fiber trap, a mechanically and thermically dependable unit, developed for integration into the engine. A modular design makes the trap very compact. Filtration rate and pressure loss are satisfactory. The filter element has not shown any weakness. A typical deficiency of this application, that needs further investigation, is worsening of the engine's transient response by the thermal inertia of the filter material.

Primary measures, to reduce the NOx emissions from diesel engines, penalize the fuel consumption and aggravate the CO2 problem. Instead, an after-treatment system is proposed that permits optimum combustion and yet reduces the NOx by more than 95%. Such installations are in operation for more than five years. Successful deployment on a short-haul ferry, subject to highly cyclic operation, began in Spring 1992. The chief features are high space-velocity (25,000 1/h), urea as non-toxic reactant and rapid transient response. The attained results counter the misgivings about the SCR catalysis. Development aims at further halving the size thus facilitating service in off-highway vehicles such as locomotives and earth-movers. The integration of particulate traps using knitted micro-fibers is under development.

Ceramic fibers, in a knitted structure, offer an elastic deep-filter medium having a very high specific surface. The robustness of this trap, and its invulnerability to thermo-shock, was demonstrated during a further year of development and tests. By using new manufacturing techniques, the filtration efficiency was further improved, pressure losses reduced, and the required volume diminished. New insight was obtained regarding the employment of the fiber medium for catalysis. The filter concept permits regeneration either electrically or by fuel-additives. The layout versatility facilitates deployment on vehicular and stationary engines, in the pre-turbo position, too.

Ceramic fibers with high specific surface area and adequate high-temperature strength are commercially available for filtration of diesel particulates and in-situ hot regeneration. The manufacturing of a deep bed filtration medium, using such brittle fibers, became possible after a special knitting technique was developed which forms the loops with minimum friction and pretension. Within this structure, the fibers are very little constrained and expose their active surface almost completely. Hence, high filtration efficiencies in the range of 95% could be demonstrated with favorable back-pressure characteristics. Blow-off phenomena were never observed. Endurance testing on engines, with full-flow burner regeneration, proved the high robustness to mechanical and thermo-mechanical loading. This is one of the particular advantages of the new concept.

Close collaboration between ABB and NGK enabled the development of the rotor for the Comprex(R) pressure-wave supercharger (PWS) as an extruded component in pressureless sintered silicon nitride. The ceramic rotor is lighter than the steel rotor, permitting a free-running PWS. And, the lower thermal expansion of ceramic improves the supercharging efficiency. Prototypes have been road tested since 1984 and fulfil the major engineering requirements: burst speed thrice nominal, probability of fracture ≤ 10-5, maximum temperature well above operating and low risk of damage from foreign objects. When the ceramic rotor becomes cost competitive and can be mass produced, it will considerably expand the range of applications of the PWS.

The Comprex(R) is a pressure-wave supercharger (PWS) for passenger car diesel engines. It has many features which ideally suit it for the continually increasing demands on driveability, fuel economy and reduction of exhaust pollutants. To counter the disadvantages of the previously required belt drive, a free-running machine was developed. It is self driven by the kinetic energy of the exhaust gas; made possible by employing a rotor having reduced inertia. In addition to the well known Comprex features, this advanced development offers advantages such as rapid response, high efficiency, compactness and freedom in placement. The paper discusses the design of the free-running PWS, its construction, supercharging characteristics and preliminary experience.