## 4 Discussion  
Current heavy-duty regulations for on-road and off-road engines require particulate measurements from the full dilution tunnel or partial flow dilution systems that take a sample proportional to the exhaust flow. This is necessary for the gravimetric method where a filter is taken for the whole test cycle, but not for real time instruments, such as the particle counters [6]. The main objective of this study was to assess whether particle number measurements directly from the tailpipe with fixed (constant) dilution are equivalent to the measurements with proportional dilution. There are two direct concepts: the first one is a \"cold\" dilution at ambient temperature; the second is \"hot\" dilution, typically \(>\)100 \({}^{\circ}\)C. The first one is closer to the proportional partial flow diluters currently used in the regulation. The second one avoids any condensation and nucleation to take place. Theoretically, the effect should be small because the particle number systems have a thermal pre-treatment unit that dilutes the sample at 150 \({}^{\circ}\)C and then evaporates the remaining semi-volatile particles in an evaporation tube at 350 \({}^{\circ}\)C. Thus, only solid (i.e., non-volatile) particles should be counted. The evaporation tube does not remove the volatile species; it only evaporates them. Thus, some condensation on existing solid particles can take place downstream of the evaporation tube and grow their size [19]. With high dilutions the concentration of the volatile species is reduced significantly and the growth is negligible. An experimental study estimated the growth to be \(<\)0.5 nm downstream of an evaporation tube [29]. Replacing the evaporation tube with (or adding) a hot catalytic stripper avoids this growth because volatiles are oxidised [25]. In addition, the risk for re-nucleation is minimised. This growth was irrelevant for older technologies with size distributions with count median diameters at 50 nm or higher, because the majority of the particles was already counted by the counters with cut-off size at 23 nm. However, for size distributions close to the 50% detection efficiency of the instruments, the effect can be significant. In this study, in order to check the biggest possible differences, the reference system that was chosen had fixed \"hot\" dilution and included a catalytic stripper. Additional measurements with another system with fixed \"cold\" dilution were also conducted. According to our knowledge, this is the first study to address the topic in depth. Some engine studies have conducted comparisons but only with one of the two additional fixed dilution concepts and only with one engine in most cases [8; 10]. No study so far has examined particles \(<\) 23 nm. The first investigation was the evaluation of the length of the sampling line before the first dilution. The length varied from 0.5 m up to 4 m, but the wall temperature was kept in all cases at 150 \({}^{\circ}\)C. The increase of the length resulted in losses of 10-20% for particles \(>\)23 nm and 20-50% for particles  
Figure 9: Comparison of \(>\)23 nm emissions measured with the PMP system with evaporation tube and a cold pre-diluter at the tailpipe TP(cET’) or at the proportional partial flow dilution system without pre-diluter PD(ET’) with the PMP system of the laboratory with evaporation tube connected to the tailpipe TP(ET) or the “Golden” system with catalytic stripper connected to the tailpipe TP(CS). Error bars show one standard deviation of 2–9 repetitions (given by the number in the bars). \(>\)10 nm. The expected diffusion losses of 10 nm for an increase of 3.5 m are 10%, thus the extremely high losses with the viton tube have to do either with the material or other unknown experimental uncertainties [30]. Based on these findings, stainless steel heated at 150 \({}^{\circ}\)C with a maximum length of 1 m should be allowed. This requirement is in line with the maximum length permitted for PD systems (1 m). The second investigation was the crankcase emissions.