## 3 Results and Discussion  
First, the DMS500 without CS was used to analyse PSDs on the GDI engine. The digital filtering approach by [PERSON] [35] (1) was then implemented on the raw PSDs to obtain F1-PN \(>\) 23 [35]. Subsequently, PSDs were measured again, this time using the catalytic stripper. Equation (2) by [PERSON] et al. [37] was applied to obtain F3-PN \(>\) 10 [37] from the new raw PSDs. F1-PN \(>\) 23 and F3-PN \(>\) 10 for the baseline, LoLo and HiHi oils are shown in Figure 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Variable** & **Value** \\ \hline Max. Torque & 170 Nm @ 1400–4500 rpm \\ Max. Power & 92 kW @ 6000 rpm \\ Stroke & 82.0 mm \\ Bore & 71.9 mm \\ Compression ratio (CR) & 10.5:1 \\ Volume & 999 cm\({}^{3}\) \\ Injector & 5-hole solenoid \\ Max. Injection Pressure & 200 bar (common rail) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: GDI Ford Ecoboost specifications. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**ASTM** & **Parameter** & **LoLo (0-16 W)** & **Baseline (0-20 W)** & **HiHi (0-16 W)** \\ \hline D2270 [57] & KV at 40 \({}^{\circ}\)C mm\({}^{2}\)/s & 39.2 & 41 & 38 \\ D4683 [58] & KV at 100 \({}^{\circ}\)C mm\({}^{2}\)/s & 7.7 & 8.1 & 7.7 \\ D5293 [59] & HTHS (cP) & 2.5 & 2.6 & 2.5 \\ D5800 [60] & Noack (\%) & 12 & 11.2 & 30.2 \\ Calculated & Sulfated ash (\%) & 0.6 & 0.8 & 1.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Oil formulation summary. Both LoLo and HiHi exhibit F1-PN > 23 concentration levels that are generally lower than baseline oil, especially at medium-high loads for all speed ranges. In contrast, they are higher at all speeds of idling. A similar outcome is found in F3-PN > 10 with LoLo and HiHi emissions only exceeding baseline at fast idle and at 2250 rpm-40 Nm. This result is better emphasised and represented in the comparison contour graphs in Figure 3, where the percentage difference in filtered PNs of baseline to LoLo and to HiHi is directly plotted by overlapping Figure 2 images. This indicates that the baseline has a greater PN relative to either LoLo or HiHi when a red shade colour is present (positive percentage), whereas a green shade colour indicates the opposite. Also, in the figure, the tables display the percentage differences in the contour plots at each of the selected test matrix points. Figure 2: Baseline, LoLo, and HiHi SPN with F1-PN > 23 and F3-PN > 10. These plots make it easier to see how the two oils have lower F1-PN > 23 and F3-PN > 10 than baseline at multiple points. The baseline has reduced emissions mostly at fast idle and 2250 rpm-40 Nm. Furthermore, a direct percentage difference of HiHi relative to LoLo is also performed and depicted in Figure 4. HiHi confirms a higher F1-PN > 23 level at mid-high speeds for all loads. This changes for F3-PN > 10, where HiHi has a lower count in the region between 2000 rpm-fast idle and 2250 rpm-20 Nm, except at 2250 rpm-fast idle. Variation ranges from \(-\)22% to +289% in F1-PN > 23 and from \(-\)34% to 104% in F3-PN > 10. To condense the PN differences in a single value, a weighted arithmetic mean (WAM) was calculated while accounting for the confidence interval (CI) correction. According to this, HiHi's overall F1-PN > 23 was 9.7% higher than LoLo, and its F3-PN > 10 was 3.6% higher than LoLo. Figure 3: F1-PN > 23 and F3-PN > 10 Baseline to LoLo and HiHi comparison plots. High ash and volatility emitting more PN have also been detected in other research work. [PERSON] et al. [50] show that higher ash content is expected to decrease the oxidation reactivity of soot particles, leading to a higher particle count due to less soot being lost through oxidation. [PERSON] et al. [51] found not only slightly higher emissions with higher ash but also a higher sub-23 nm proportion being formed. [PERSON] et al. [49] observed a 5% increase in particle number emission from exhaust (measured using DMS) with Noack volatility from 8% to 19% over eight medium power points. [PERSON] et al. [45] learnt similar results using three different oil formulations.