## 3 Results and Discussion
### Effects of Injection Pattern on FSR in the DF Mode  
The effects of a multi-injection strategy, characterized by two pilot plus main injection pattern, as reported in Figure 2, in comparison with the reference one (only main) were evaluated. The investigation was conducted in the range 4-14 of CA50, at constant rail pressure (500 bar), SOIethanol, FSR and with no EGR to discriminate the injection pattern effect only. This analysis aimed to replicate the FSR values obtained with the only main and evaluate the effects on parameters.  
Figure 8: Influence of injection pressure on particulate matter (PM).  
Figure 7: Injection pressure effect on combustion duration and ID at \(1500\times 5\) and \(2000\times 8\) without EGR.  
Figure 9 shows the influence of the injection pattern of FSR varying CA50. The use of a multi-injection pattern led to reach the FSR limit imposed in the range 4-12 of CA50, while at 14 deg, the test point was not performed because misfire occurred. For more retarded combustion phasing, the mixture was below the lower flammability limit due to a too low in-cylinder temperature and an excess of air in the cylinder [19].  
The use of a single pulse strategy, as observed in Figure 9, led to an increase in the ignition delay, which allowed better air-fuels mixing with a more significant premixed phase [33]. These aspects justify a completely different HRR, in particular by delaying the combustion phasing [34]. The multi-injection strategy, as shown in Figure 9, guaranteed a reduction of the premixed phase, favoring the diffusive one, also assuring a significant decrease of the maximum rate of pressure rise and therefore of the combustion noise [35].  
Figure 10 shows the influence of the injection pattern on the combustion indicators in terms of combustion noise and combustion efficiency. As expected, a general decrease of the combustion noise was observed, with a minimum difference of 4 dBA at 6 deg and a maximum reduction of 8 dBA (at 14 deg) retarding combustion phasing.  
A trend comparison of combustion efficiency is also depicted in Figure 10. A reduction for the multi-injection strategy is reasonably due to not full vaporization of the PFI ethanol that can lead to  
Figure 10: Influence of the injection pattern on combustion indicators.  
Figure 9: Effect of injection pattern on FSR function of CA50 in 1500 \(\times\) 5.  
ethanol impinging on the cylinder wall and combustion chamber [36] and incomplete combustion since an amount of mixture burns in the expansion stroke [37]. These aspects affect the combustion of the charge and consequently a worsening of HC and CO emissions, which can worsen the combustion efficiency if the diesel flame is extinguished before reaching the combustion chamber wall. For more retarded combustion phasing (12 deg aTDC), splitting the diesel injections helped to maintain higher mean in-cylinder temperature, which improved the oxidation of HC and CO emissions and consequently the combustion efficiency, as depicted in Figure 10.  
No trend analysis was reported for specific indicated PM values due to their values lower than the minimum filter smoke number (FSN) relates to the resolution of smoke meter utilized during the experimental campaigns (Section 2.1). Despite different values, their order of magnitude is typical of a soot-less zone.  
This section shows that the use of a multi-injection strategy also for the lower load case (\(1500\times 5\)) guaranteed the respect of FSR target and led to an improvement in terms of combustion noise and efficiency. For these reasons, the multi-injection strategy was also adopted for the subsequent experimental investigations related to \(1500\times 5\) test point.