Diesel NO/NO2/NOX Emissions - New Experiences and Challenges

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 NO_x by the cold NO_x - 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.

In general it can be stated:

• With a strong oxidation catalyst in the exhaust system the hot NO_x-measurement allows to recognize the NO_x-values, which are in average of the investigated cases 27% higher, than with cold NO_x-measurement,
• A supplementary use of a Nafionpermeability sample dryer (still in case of a strong catalyst) enables to measure in average 5% higher NO_x-values,
• With a weak catalytic influence in the exhaust system the hot NO_x-measurement yields higher values until
• 3% and the use of Nafion dryer is not necessary,
• The present configuration of Nafion dryer is not appropriate for transient measurements,
• With a coated catalyst (Ox.Cat), or with catalytic surface filter (CSF) there is a maximum of NO₂ / NO_x-ratio typically in the exhaust gas temperature range of 300°C - 350°C,
• With fuel borne catalyst (FBC), or with noncatalyzed DPF + FBC there is no NO - NO₂ conversion, nevertheless FBC can affect (increase) the conversion activity of an Ox.Cat, or DPF in particulary due to the long term deposits,
• The above tendencies were confirmed on different engines.