**3. Results**

## *3.1. Exhaust Flow*

Figure 2 plots the exhaust flow rate of the motorcycle as estimated by the three methods (Equations (10)–(12)). There is a very good agreemen<sup>t</sup> between the different methods. Even though not shown here, the carbon balance method without the CO2 dilution air correction would slightly overestimate the exhaust flow rate. The CO2 tracer method was considered the basis for further evaluations.

**Figure 2.** Exhaust flow rate as determined by three methods for the first 600 s of the Worldwide harmonized Motorcycle Test Cycle (WMTC) (closed transfer tube) (Equations (10)–(12)). The extracted flow (0.019 m<sup>3</sup>/min) should also be added.

At a next step, the calculated exhaust flow rate with the CO2 tracer method was plotted for the two configurations with closed and open transfer tube (Figure 3) during the first 250 s of a cold start WMTC. After the first minute, there is a generally good agreemen<sup>t</sup> within the experimental uncertainty. However, there is a significant difference at the beginning of the cycle: The exhaust flow rate with the transfer tube closed starts much later. The reason is that there is a big "dead" volume in the transfer tube that delays the CO2 signal at the dilution tunnel. The gas transport delays, mixing and dispersion phenomena, are responsible for smoothing and distortion of the original emission signal [36,38,39]. Furthermore, even though the transfer tube is heated, there may still be colder spots where condensation can take place. This condensation will be different with the open and closed configurations, resulting in different actual CO2 concentrations. However, the dry-to-wet corrections of the CO2 signal do not take this into account [42]. We cannot exclude completely the possibility that the moped behaved differently with open and closed configurations. However, the CO2 tailpipe signals (and the results later) did not show any difference. This figure clearly shows the importance of having short transfer lines. In the following results, the first 50 s of the calculated exhaust flow rate were corrected using the exhaust flow rate of the open configuration; the effect of this correction will also be shown.

**Figure 3.** Comparison of the exhaust flow estimated with the CO2 tracer method during cold start in open/closed configuration. The speed profile is from the open configuration test. The extracted flow (0.019 m<sup>3</sup>/min) should also be added.

#### *3.2. CO2 and Gaseous Pollutants*

Figure 4 plots the CO2 emissions over the WMTC as determined at the tailpipe and the dilution tunnel with integration of the gas analyzers signals with the transfer tube open and closed. The agreemen<sup>t</sup> is very good within the experimental uncertainty. For the tailpipe measurements, the improvement of the exhaust flow determination at the first minute of cold start (as discussed in Figure 3) increased the CO2 emissions by 2.3%. However, single factor analysis of variance (ANOVA) showed that the CO2 differences were not statistically significant with or without the correction for the first 50 s of the of exhaust flow (p = 0.23, N = 10). The dilution tunnel results were increased 24% to 31% from the addition of the mass of the extracted exhaust from the tailpipe sampling (Equation (7)). This percentage was similar to the mean extracted flow rate; the extracted 19–23 l/min corresponded to 23–28% of the mean total exhaust flow rate.

**Figure 4.** CO2 emissions at the tailpipe (TP) or the dilution tunnel (Constant Volume Sampling (CVS)) with open or closed transfer tube over an entire WMTC test (blue areas). The corrections due to the extracted tailpipe flow are also given (red areas). The additional emissions by correcting the first 50 s of the exhaust flow rate based on the open configuration (Figure 3) is given for the TP closed case (green dashed area). Error bars show one standard deviation of 2 (open) to 3 (closed) repetitions.

For the rest gases, the results were similar: The extracted flow rate from the tailpipe contributed 19% to 30%, while the first 50 s erroneous exhaust flow rate determination affected 3.5% (CO), 23% (NOx), and 51% (hydrocarbons) of the rest gases. Thus, the contribution was pollutant specific.

#### *3.3. Solid Particle Number (SPN)*

Figure 5 plots the SPN real time emissions over the tested cycle (WMTC) for the two configurations (open and closed transfer tube). With the open configuration (Figure 5, upper panel), the SPN at the CVS and tailpipe (TP) agree very well, with the exception of the first minute, where the CPC of the tailpipe particle number system was saturated (it measured around 60,000 p/cm<sup>3</sup> where the maximum is around 10,000 p/cm3). The 10 nm was saturated for approximately 25–30 s, while the 23 nm CPC for 10–15 s. The good agreemen<sup>t</sup> between tailpipe and dilution tunnel results indicates that there were no artefacts (i.e., formation of particles after the tailpipe). This was expected because the exhaust gas temperature was low (<200 ◦C) and the instruments dilutions were high (>1000:1).

**Figure 5.** Real time solid particle number emissions (SPN) >10 nm with (**a**) open (upper panel) or (**b**) closed (lower panel) transfer tube. Measurements at the tailpipe (TP) and the dilution tunnel (CVS) are separately plotted for each case.

The picture is very different with the closed configuration (Figure 5, lower panel). The tailpipe results are similar to the open configuration (compare blue dashed TP lines of lower and upper panels), as expected, because the sampling location was the same. However, the SPN at the dilution tunnel is smoother and in some cases the particle peaks appear later due to the variable residence time in the transfer tube. The concentrations seem lower, especially in the first minute. The emissions are very high at cold start, leading to considerable particle losses mainly due to agglomeration. These losses yield decreased particle concentrations at the dilution tunnel.

Figure 6 summarizes the results of all tests conducted: Tailpipe and dilution tunnel measurements with open and closed configurations. The results are given separately for the three CPCs with lower particle sizes of 4 nm, 10 nm, and 23 nm. The CVS results were corrected for the extracted flow rate from the tailpipe (Equation (7)) (bleed off), while for the TP results the extracted flow rate was taken into account by adding it to the exhaust flow rate. The "bleed off" correction was 19–26% for the open transfer tube case and 39–73% for the closed one. The percentages depend on the absolute

emission levels, and also on when the emissions take place. The percentages are higher with the closed configuration because the absolute levels are lower. The tailpipe results with the first 50 s corrected exhaust flow rate for the cold start (Figure 3) were 69% to 149% higher than without the correction due to the high SPN emissions at the cold start. After correction, the tailpipe measurements with the closed configuration were close to the tailpipe measurements with the open configuration.

**Figure 6.** Solid particle number emissions (SPN) at the tailpipe (TP) or the dilution tunnel (CVS) with open or closed transfer tube (blue areas). The corrections due to the extracted tailpipe flow are also given (red area). The additional emissions by correcting the first 50 s of the exhaust flow rate based on the open configuration (Figure 3) is given for the TP closed case (green area). Error bars show one standard deviation of 2–3 repetitions.

After corrections, and assuming that the tailpipe results with the open configuration are the reference values as best available option, the differences become –7% to +4% for the tailpipe with the closed configurations, –2% to +19% for the dilution tunnel with the open configuration, but –19% to –41% for the dilution tunnel with the closed configuration. The lower closed configuration results indicate 20–40% particle losses in the exhaust transfer line. Theoretical calculations using the 4 nm concentrations and a mean size of 20 nm estimated the contribution of agglomeration around 30% [44]. The contribution of thermophoresis was estimated to be <2%, and of diffusion <5%. As the corrected results were in good agreemen<sup>t</sup> to each other, the possibility of sub-23 nm artefacts is unlikely in our tests.

Based on the open transfer tube results, the specific moped had SPN23 emission levels below the light-duty vehicles limit set for particles >23 nm (6 × 10<sup>11</sup> p/km), but higher when considering the particles below 23 nm. Based on the increasing emissions with decreasing lower size, the solid particle size distribution peaked at a size close to 10 nm.
