## 2 Materials and Methods
The CNG engines were positive ignition with three-way catalyst (TWC) and with closed crankcase ventilation. All OEMs tested the regulated cycles for on-road heavy-duty engines WHTC (World harmonised transient cycle) with cold and hot start, and the WHSC (World harmonised stationary cycle). Additional tests included hot start cycles such as the US type-approval cycle FTP (Federal test procedure), the off-road engines cycle NRTC (Non-road transient cycle) and the CO\({}_{2}\) mapping, in order to cover as many as possible transient cases and a big part of the engine map. Additional investigations included forced regeneration events and connection of the crankcase ventilation to the tailpipe. Since Euro VI regulation both regeneration events and crankcase ventilation emissions have to be included in the emissions of the engines. The objective of these tests though was to see differences of the systems with and without challenging aerosol (unburnt fuel and oil particles). Some additional tests with different length of the sampling line of the \"Golden\" system targeted in better defining the sampling conditions for possible future regulations. Some OEMs measured exchanging the position of the external \"Golden\" 10 nm and \"Golden\" 23 nm CPCs in order to compare them with the internal CPCs with the same cut-off sizes (parallel checks) (see results in Appendix A). The test protocol is summarised in Table 1.  
The JRC (Joint Research Centre) of the European Commission provided and checked the \"Golden\" instruments at the beginning of the testing campaign, at the end, and twice in between the testing to confirm their proper operation. These tests and the parallel checks of the OEMs are discussed in Appendix A. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Investigated** & & & & & & & & & \\
**Conditions** & **DPF \#1** & **DPF \#2** & **DPF \#3** & **DPF \#4** & **DPF \#5** & **DPF \#6** & **DPF \#7** & **CNG \#1** & **CNG \#2** \\ \hline Extra cycles & Y & - & Y & Y & Y & Y & Y & - \\ Crankcase & Y & - & - & Y & - & - & Y & - & - \\ Regeneration & Y & - & Y & Y & Y & - & - & - & - \\ Biofuel & - & - & Y & - & - & - & - & - & - \\ Sampling line & - & Y & Y & - & - & - & - & - & Y \\ Extra tailpipe & - & - & Y & - & Y & Y & - & Y \\ Parallel check & - & Y & Y & - & Y & Y & - & - & Y \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test protocol. Y = Yes. The SPN emissions measured with the systems connected to the proportional partial flow dilution systems were calculated according to the regulation. For the systems at the tailpipe the equation given in the RDE (real-driving emissions) regulation of light-duty vehicles was used replacing distance driven with engine work [27]. This calculation needs correct time alignment of the particles signal with the exhaust flow rate. Misalignment of \(\pm 1\) second in our measurements resulted in differences of \(\pm 5\%\) (up to \(\pm 10\%\) in some cases), in agreement with others [28]. For all calculations the mean PCRF (particle number concentration reduction factor) as defined in current regulations was used (i.e., average of 30 nm, 50 nm, and 100 nm). This means that the 10 nm emission levels are underestimated, as losses below 30 nm are not considered in the calculations. The losses should be similar for all systems examined, thus the conclusions should be the same even correcting separately for the losses below 30 nm. If losses would be taken into account, the results of the 10-23 nm levels would need a correction of approximately 1.7, as explained elsewhere [16; 27].