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NO₂ is much more toxic than NO. The average proportion of NO₂ in exhaust gases of vehicles increases significantly due to the use of oxidation catalysts and catalytic coatings in the exhaust gas systems during the last decades combined with generalization of using low sulfur fuels. Diesel oxidation catalysts (DOC) and Pt-containing DPF coatings are widely used to support the regeneration of particle filters, being a source of strongly increased production of NO₂. The present work shows some examples and summarizes the experiences in this matter performed at the Laboratories for IC-Engines & Exhaust Emissions Control (AFHB) of the University of Applied Sciences Biel-Bienne, Switzerland, during some research activities on engine dynamometers in the years 2010-2012.

During the VERT *) testing of different DPF systems it was remarked, that the oxidation catalyst converts sometimes a big part of NO to NO2, producing on the one hand a more toxic composition of the exhaust gases and causing on the other hand measuring artefacts, which tend to underestimate of NO2 and NOx by the cold NOx - measurement. The present work summarizes the experiences in this matter elaborated at the Laboratories for IC-Engines & Exhaust Emissions Control (AFHB) of the University of Applied Sciences Biel-Bienne, Switzerland, during several VERT activities and didactic projects on engine and chassis dynamometers in the years 2000-2006.

Exhaust gas recirculation (EGR) has long been used in gasoline and light-duty diesel engines as a NOx reduction tool. Recently imposed emission regulations led several heavy-duty diesel engine manufacturers to adopt EGR as part of their strategy to reduce NOx. The effectiveness of this technology has been widely documented, with NOx reduction in the range of 40 to 50 percent having been recorded. An inevitable consequence of this strategy is an increase in particulate emission, especially if EGR was used in high engine load modes. Selective catalytic reduction (SCR), a method for NOx reduction, is widely used in stationary applications. There is growing interest and activity to apply it to mobile fleets equipped with heavy-duty diesel engines. Results of this work indicate that SCR has the potential to dramatically reduce NOx in diesel exhaust. Reductions greater than 70 percent were reported by several including the Institute's previous work (SAE Paper No. 1999-01-3564).