## 4 Discussion
They increased the emission levels of the diesel engines to 2-3\(\times\)10\({}^{11}\) p/kWh, but above these levels their contribution to the engine emissions was not evident, i.e., any contribution was within the repeatability of the measurements. It is known that the emission levels depend on the DPF fill state. The first tests with DPF #4 without the crankcase ventilation connected were high because the DPF was relatively empty. At the next tests the emission levels were lower but the contribution of crankcase emissions brought them to similar levels. Interestingly, the levels \(>\)23 nm and the sub-23 nm fraction measured by the two concepts of proportional and fixed dilution were the same with and without the crankcase ventilation connected to the tailpipe. This can be explained by the size distributions typically found at the crankcase ventilation, which have the majority of particles \(>\)10 nm, and in most case even above 30 nm [31; 32; 33]. The third investigation was the regeneration. In most cases the SPN emissions increased when regeneration took place, in agreement with the literature, for instance References [34; 35]. In one case the emissions remained at the same levels. This has also been shown before [14], or it could be that the crankcase emissions had a higher contribution. The comparisons of the two concepts were similar, even though the absolute levels of the emissions were increasing. In one test though, it was evident that the sub-23 nm levels increased more with the proportional diluter and the evaporation tube. As discussed above, it is possible that due to the low dilution (100:1) used re-nucleation occurred downstream of the evaporation tube or small solid pre-existing particles grew to the \(>\)10 nm range for that system, but remained \(<\)10 nm for the catalytic stripper one (dilution 2000:1). Regeneration tests have shown high concentrations of volatile particles [36; 37; 38] and even high molecular weight volatile organic compounds from the dilution tunnel [39]. In one case it was also shown that the dilution measurements can have differences compared to the tailpipe results for sub-23 nm measurements [40]. The results for the rest test cycles were promising: the differences between the \"hot\" fixed dilution and the proportional systems of six OEMs (with nine engines in nine different engine dynamometers) were mostly within \(-\)20% and \(+\)25%. There were some cases that reached \(+\)40%. The average difference of the two concepts for all laboratories for the official cycle was 9% for the 23 nm and 1% for the 10 nm counters. The ratio of 10 nm and 23 nm was also similar between the two concepts and remained within \(\pm\)15% for a wide range of sub-23 nm fractions (up to seven times higher concentrations of particles below 23 nm to particles above 23 nm). Only in one case the difference was higher: for this case issue with the 23 nm counter in one of the systems is suspected. The results are in good agreement with the limited number of studies for heavy-duty engines (only \(>\)23 nm) [8; 10] and light-duty vehicles (both \(>\)23 nm and \(>\)10 nm) [27]. The results with the fixed \"cold\" dilution were more difficult to interpret. There was an overestimation of the emissions of around 26% (average of five engines). In order to confirm that this was not due the concept, dedicated tests were conducted with the system at the tailpipe or the proportional dilution system. The overestimation remained similar regardless of the position confirming that the concept of fixed \"cold\" dilution is an acceptable alternative. It highlighted though the importance of correct calibration procedures. The CPC of the overestimating system was calibrated with soot particles, against a 23 nm reference CPC (same model) that was calibrated with emery oil. Although being similar CPC models, slight differences in the slope of the detection efficiency near the cut-off regime can lead to some deviations.