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Increasing concern, about the health risk due to solid aerosols from engine combustion, has provoked more stringent imission limits, for soot particles in the range of pulmonary intrusion, at critical work-places (e.g. tunnel sites, see Table 1). Within the scope of the joint European project VERT, these emissions were characterized and their effective curtailment through exhaust gas after-treatment investigated. Diesel engines, irrespective of design and operating point, emit solid particulates in the range of 100 nm, at concentrations above 10 million particulates per cm3. Engine tests showed that a drastic curtailment of pulmonary intruding particulates seems not feasible by further development of the engine combustion, nor by reformulation of fuels, nor by deployment of oxidation catalytic converters. Particulate traps, however, can curtail the total solid particulate count, in the fine particulate range 15-500 nm, by more than two orders of magnitude.

Particulate traps are mechanical devices for trapping soot, ash and mineral particles, to curtail emissions from Diesel engines. The filtration effectiveness of traps can be defined, independent of the pertinent engine, as a function of the particle size, space velocity and operating temperature. This method of assessment lowers cost of certifying traps for large-scale retrofitting projects [1,2]. VERT [3] is a joint project of several European environmental and occupational health agencies. The project established a trap-verification protocol that adapts industrial filtration standards [4] to include the influence of soot burden and trap regeneration phenomena. Moreover, it verifies possible catalytic effects from coating substrates and deposited catalytic active material from engine wear or fuel/ lubricant additives.

By means of catalysts, either coatings or fuel-borne, the temperature level for triggering the combustion of soot stored in particulate traps can be lowered from 600°C to 300°C, in case of CRT even to 250°C; but even that may fail, if in dense traffic application of a city-bus only 150 - 200°C are attained - similar situations of low load duty cycles exist in most other applications too. Mere passive regeneration may then not be sufficient, active support is needed. This paper presents an “active” method applicable to any Diesel engine to increase the exhaust temperature whenever required: load of Diesel engines is controlled by the fuel flow only; consequently, excess of air above stochiometric requirement is increasing from λ = 1.5 to λ = 8 with decreasing load, which is in fact the principal cause of the low temperature at light loads.

Metallic substances, usually added to fuel as organic compounds are, as fuel additives proven to curtail particulate emissions from diesel engines and, as fuel borne catalysts (FBC), to promote regeneration of particle traps. During combustion, these substances form catalytic metal oxides and exit the combustion chamber as ultra-fine solid clusters in the mobility diameter range of 5-30 nm. Particles of this size and composition have a health impact and should not enter the respiratory air. FBC should therefore only be used together with particle traps, which can efficiently collect these metal oxide particles at all operating conditions. This and other requirements are stipulated in the VERT suitability tests for particle trap systems. The approval procedure includes a particle size-specific analysis to verify trap penetration in trace quantities.

Diesel engines are irreplaceable in tunnel construction. The particulate emissions of present day engines are so high that the imission limits valid since 1991 cannot be attained by ventilation alone. This problem had to be solved preparatory to the large tunnel projects in Switzerland, Austria and Germany. Several retro-fitting measures were investigated both in the laboratory and in field tests, within the scope of the Project VERT. Oxidation catalytic converters, exhaust gas recirculation, and the usage of special fuels cannot be recommended. Particulate trap deployment, in different systems, was mostly successful. Particular attention was focused on the dependable filtration of finest particulates < 200 nm. The VERT proved that exhaust gas after-treatment with particulate traps is feasible, cost effective and controllable in the field. Pertinent directives are in discussion.

Vegetable oils blended to Diesel fuel are becoming popular. Economic, ecological and even political reasons are cited to decrease dependence on mineral oil and improve CO2 balance. The chemical composition of these bio fuels is different from mineral fuel, having less carbon and much more oxygen. Hence, internal combustion of Diesel + RME (Rapeseed Methyl Ester) blends was tested with particular focus on nanoparticle emissions, particle filtration characteristics and PAH-emissions. Fuel economy and emissions of bus engines were investigated in traffic, on a test-rig during standardized cycles, and on the chassis dynamometer. Fuel compositions were varied from standard EN 590 Diesel with <50 ppm sulfur to RME blends of 15, 30, and 50%. Also 100 % RME was tested on the test-rig. Emissions were compared with and without CRT traps. The PAH profiles of PM were determined. Particles were counted and analyzed for size, surface, and composition, using SMPS, PAS, DC and Coulometry.

1 Occupational Health Authorities in Germany and Switzerland require the use of particulate traps (PT) on construction machines used in underground and in tunneling since 1994. Swiss EPA has extended this requirement 1998 to all construction sites which are in or close to cities. During the VERT*-project, [1, 2, 3, 4, 5]**, traps systems were evaluated for this purpose and only those providing efficiencies over 95% for ultrafine particles < 200 nm have received official recommendation. 10 trap-systems are very popular now for these application, most of them for retrofitting existing engines. Efficiency data will be given as well as experience during a 2-years authority-controlled field test. LIEBHERR, producing their own Diesel engines in Switzerland and construction machines in Germany is the first company worldwide supplying particulate traps as OEM-feature (Original Equipment Manufacturing) on customers request.