Among other parameters they collected data on the mass and number of particles in the exhaust gas and with the help of a “Scanning Mobility Particle Sizer System“ they determined their sizes as well. “We wanted to see if the retrofitted filter also removed the smallest particles with diameters under 20 nanometers, which are believed to be particularly harmful for to health,” explains Ruedy.
The good news is that the retrofitted filter which they tested did actually work. Both the total mass and number of particles in the exhaust gas were reduced by about 40 per cent during the “New European Driving Cycle” (defined by the legislature to be used for emissions measurement) as well as during a driving cycle which more closely simulates actual driving behavior. When the vehicle was traveling at simulated constant speeds of 50, 80 and 120 kmph the retrofitted filter removed between 20 and 50 per cent of the particles from the exhaust gas, independently of the particle of size. “We were rather astonished that the filter also removed particles as small as 10 nanometers,” commented Ruedy. He had expected that above all the larger particles would be removed.
On the other hand, emissions of nitrogen oxides – the very pollutants which are already produced by diesel engines in relatively large quantities – increased by between one to eleven per cent. And at constant speed and under motorway conditions simulated in the “real” driving cycle the filter caused an increase in fuel consumption of up to 3 per cent. At 120 kmph this represents a not insignificant 0.2 litres of diesel per 100 kilometres. “Retrofitted filters are therefore not quite CO2 neutral,” observes Ruedy.