**4. Discussion**

This study, based on transient tests of a moped, quantified the uncertainties in emissions measurements using gaseous analyzers and solid particle number (SPN) instruments with cut-off sizes of 4 nm, 10 nm, and 23 nm at the tailpipe and the dilution tunnel. Two configurations were tested: Closed and open transfer tube to the dilution tunnel; both allowed in the current EU regulations (for gaseous pollutants). The results of this study can be used to draft a list of issues to be discussed in current and future legislation development and research activities. This is the first study to discuss uncertainties of this type for L-category vehicles. We are also not aware of similar discussions for passenger car and trucks. Although the results cannot be directly generalized to other mopeds and motorcycles (relative magnitude of the uncertainties may differ), Table 1 identifies common issues which will be of significance.


**Table 1.** Contribution of various parameters on results.

The results had to be corrected for the extracted flow rate from instruments sampling from the tailpipe. This is typically done by the laboratory automation software for gaseous pollutants, but not for SPN. The typical approach is to have two instruments measuring simultaneously at tailpipe and dilution tunnel and add to dilution tunnel results the particles that are extracted from the tailpipe. Using the CVS real time signal and correcting with the "true" CVS dilution factor based on the differential method (Equation (9)) would be another approach when no tailpipe measurements are taken. This correction was on the order of 19–73% in this study due to the low exhaust flow rate of the moped (extracted flow 23–28% of mean flow). Thus, the instrument used (SPN instrument and gas analyzers) were taking a significant part of the moped's exhaust flow. The Global Technical Regulation (GTR) 15 for light-duty vehicles allows a maximum 0.5% of the exhaust flow to be extracted and not returned in order to validate a type-approval test. Practically, for this moped it would require instruments with flow rates of <0.25 L/min, which do not exist. Thus, the extracted flow correction might be necessary if in the future measurements from the tailpipe (e.g., for additional pollutants or PEMS) will be required.

The tailpipe measurements need the exhaust flow rate for the calculation of the emissions, which can introduce an error. We used three methods, all applicable only in the laboratory (not on-road): Difference of CVS total and dilution air flow rates, CVS total flow rate, and dilution factors based on CO2 tracer or carbon balance. The agreemen<sup>t</sup> of the three methods was excellent (5%). For diesel vehicles, big errors have been reported with the CO2 tracer method during fuel cut-offs [48], but for stoichiometric gasoline engines, the error should be small because the CO2 concentration remains constant over most operating conditions. More studies are needed to better determine the exhaust flow associated uncertainty, e.g., with other methods, such as intake air and fuel consumption or the lambda measurement. This will be even more important when direct methods (i.e., exhaust flow meters) will appear in the market. The time misalignment of the analyzers' signals and the exhaust flow rate signals for the tailpipe measurements can have an effect. For SPN, the effect of ±1 s misalignment was ±5%, reaching ±10% in a few cases, which was in agreemen<sup>t</sup> with another study for light-duty vehicles [18]. Another study found up to 20% effect on the estimated catalyst efficiency for a ±2 s misalignment [48]. The time misalignment of the CO2 could also introduce an error on the exhaust flow determination with the CO2 tracer method. An important issue was that with closed configuration the exhaust flow of the first 50 s was erroneously calculated to be zero due to the "dead" volume of the transfer tube and the associated dispersion and diffusion phenomena. This led to an underestimation of around 69% to 149% in the SPN results, but less than 2.3% for CO2. The error was significant for SPN because the majority of the SPN emissions take place during cold start. Thus, this error will be important for engines from which the majority of the emissions take place at the beginning of the test (cold start). It could be avoided with better determination of the exhaust flow (e.g., exhaust flow meter or measurement of intake air and fuel flow). For SPN measurements, the instrument dilution (2000:1) that was used was not enough for the 23 nm and 10 nm CPCs during cold start (10–30 s). This resulted in an uncertainty in the tailpipe SPN results. Comparing with the concentrations of the CPCs at the CVS with the open configuration for those seconds, the underestimation of the emissions was negligible for the 23 nm CPC but around 10% for the 10 nm CPC. Higher dilutions can solve this issue, but higher dilutions have higher uncertainty [24]. Thus, the tailpipe method needs further investigation in the future before introducing it in the regulation.

For the specific moped and tests, the applied corrections increased the emissions significantly. For example, for >10 nm, the tailpipe and dilution tunnel with closed configuration emissions were around 4 × 10<sup>11</sup> p/km; with the corrections, the emissions were approximately 112% (tailpipe) and 39% (dilution tunnel) higher. The tailpipe results with the two configurations (open and closed transfer tube) and the dilution tunnel results with the open configuration were within ±10% for the 23 nm and 10 nm CPCs and ±20% for the 4 nm CPCs. The results of the closed configuration were 30% (23 nm, 10 nm) to 47% (4 nm) lower due to particle losses in the transfer tube (30% from agglomeration, <5% from diffusion, and <2% from thermophoresis). Decrease of particle levels of 10–40% have been reported for passenger cars [18,43]. The open configuration decreased the agglomeration and thermophoretic losses due to the immediate cooling dilution at the tailpipe and due to the decreased residence time in the exhaust transfer line. There were no indications that the open or closed configurations formed particles, but this needs more detailed studies as the partitioning of the semi-volatile particles will be different for the two configurations [30]. Tests with heavy-duty diesel and CNG (compressed natural gas) engines with cold and hot dilution showed similar results with 23 nm and 10 nm CPCs. Thus, we expect small effect if any also for mopeds and motorcycles [49]. Dedicated studies have shown that particles may be formed or grow to the measurement range of instruments from the silicone, Teflon parts, or desorbed material of the transfer tube when the exhaust gas temperature is high. These phenomena typically lead to quite large errors that in some cases reach an order of magnitude difference [14,16]. The open configuration seems to minimize such artefacts, because the exhaust gas temperature drops to low levels (<150 ◦C) [14]. It should be mentioned though that the uncertainty of the SPN instruments are in the 10–15% range [3], and thus the errors associated with the extracted flow, the cold start, and the particle losses are significant. The lower results from the dilution tunnel with the closed configuration shows that L-category studies so far might have been underestimating the emissions [10,13]. The exact magnitude depends on the exhaust gas temperatures, the size of particles, and the SPN concentration levels. The major contributor seems to be the agglomeration losses during cold start where the concentration levels are high. One of the key messages of this comparison is that for future SPN regulations the open transfer tube configuration is more appropriate. In order to avoid the uncertainties related to the open configuration (i.e., how much open, the ambient air might not be filtered, etc.), it is the authors' opinion that the open configuration could be replaced with a mixing tee; a commonly used approach for gasoline light-duty vehicles. In this case, the flow of the dilution air entering is well controlled and, in addition, the air is filtered minimizing the contribution of ambient particles to the results. Special attention should be given to the material of the tube after the mixing tee in order to minimize particle losses.

Closing, it should be mentioned that the SPN23 emissions after corrections were around 5 × 10<sup>11</sup> p/km; lower than the current light-duty vehicle limit of 6 × 10<sup>11</sup> p/km. However, the SPN10 and SPN4 emissions were 8.3 × 10<sup>11</sup> p/km and 14 × 10<sup>11</sup> p/km, respectively, indicating that the current methodology captures only a small percentage of the emitted particles. This is in agreemen<sup>t</sup> with other studies for mopeds and motorcycles [10,13] and references therein. It also raises concerns when comparing SPN emissions using instruments with different lower sizes. The most important message though, based also on the previous studies [10,13], is that future regulations for mopeds and motorcycles should consider a size lower than the current of 23 nm. Similar discussion is on-going for light-duty and heavy-duty vehicles, where high sub-23 nm particle fractions (>100%) have been measured in some cases, for instance, motorcycles [10], port-fuel injection engines [6], compressed natural gas engines [18], and heavy-duty engines [50,51].
