## 3 Results and Discussion
### Trade-Offs by Exhaust Gas Recirculation (EGR) Sweep  
The effect of diesel-LPG blends on the PM reduction effectiveness was carried out employing EGR sweeps at different engine loads and speeds. The PM emissions of all key-points plotted against the NOx values are shown in Figure 9. The engine operating conditions (intake pressure, rail pressure, exhaust backpressure, intake temperature) were the same during the tests. Dashed lines, representative of the post-Euro6 NOx target, are displayed and used as a reference for the specific comparison. As shown in Figure 9, the trade-off based on diesel, showing high levels of PM and NOx for all tested points, is drastically improved, employing the LPG blend fuel, in the range 90-95% in PM, depending on the load. By adopting the highest EGR rate in the case of blends, characterized by lower cetane numbers and lower evaporation temperatures, both NOx and PM were reduced simultaneously below reference targets.  
In particular, significant benefits are shown at a higher load, of about 95% in terms of PM is observed. This result is significant considering that, these points are particularly critical for PM engine control, and is one of the operating points with the greatest contributing factor in applying an emission estimation procedure of the test emission cycle [17, 18]. Therefore, the PM emission index improvement in these points appear significantly favorable for the engine performance over the WLTP procedure when the LPG blend is used.  
In deeper detail, Figure 10 shows the detailed trend in the post-Euro6 region, as well as the zoom of the area highlighted in the previous figure. The PM benefit by adopting the highest EGR ratio is explored, and corresponds to the last trade-off point of Figure 9. The test points of \(1500\times 5\) and \(2000\times 2\) are reported. Starting from the diesel reference, adopting the DPLG20 a PM reduction of 70%  
Figure 8: Specific emissions comparison among all fuels and operating points.  
in 1500 \(\times\) 5 and 93% for 2000 \(\times\) 2 is observed at the same engine calibration setup. By increasing the LPG percentage up to 35%, a further reduction of 93% and 50% is obtained for 1500 \(\times\) 5 and 2000 \(\times\) 2, respectively. In general, soot-less levels can be achieved without deterioration of thermal efficiency, indicated in the box (Figure 10) for each tested point, thanks to the promotion of the premixing thanks to the LPG blends. The effectiveness shown allows reaching very low PM emissions in the whole operating range. The LPG blend guarantees significant benefits even in the engine operating areas where the NOx values are very low, and soot control particularly sensitive. It is proper to specify the PM results obtained by using the LPG blends are close to the limit-sensitive thresholding of the instrument used. For this reason, future characterization with appropriate tools will be performed, such as a fast particle analyzer or micro-soot sensor.  
Figure 10: NOx-PM by varying the fuel at the highest EGR rate at 1500 \(\times\) 5 and 2000 \(\times\) 2.  
Figure 9: Specific trade-offs NOx-particulate matter (PM) by exhaust gas recirculation (EGR) sweep for all operating points and fuels. The dashed lines are indicative of post-Euro6 levels.