Number Measurements of Diesel Exhaust Particles - Influence of Dilution and Fuel Sulphur Content

A volatility tandem differential mobility analyzer (v-TDMA) in parallel mode with the heated section at 350 °C has been used to study the number size distribution of exhaust particles from a heavy-duty Diesel engine equipped with a continuously regenerating trap (CRT). Total number concentrations have also been measured by use of a stand-alone CPC preceded by a heater at 350 °C. The sample was extracted directly from the exhaust pipe and conditioned by use of a rotating disc diluter. Two different dilution factors were applied (86 and 386 times) showing that the higher dilution reduces the number of small particles as particle formation is partly suppressed. Two different Diesel fuel qualities have been used showing that the 400 ppm sulphur fuel generates higher numbers of nucleation mode particles than the 3.5 ppm sulphur fuel. Particles formed with the 400 ppm sulphur fuel are present also after heating the aerosol to 350 °C which is not the case for equally sized particles formed when using the 3.5 ppm sulphur fuel. The different shapes of the particle size distributions for each of the fuels indicate that a minor change in cut-off size of the particle counter for number concentration measurements will correspond to a large difference in number of particles. The current study points out some of the difficulties encountered when trying to state an integrated value for particle emissions.