## 3 Results and Discussion
### Effects of Diesel Fuel Injection Pressure on FSR in the DF Mode  
This section shows the experimental results on the influence of diesel injection pressure on the maximum FSR. To discriminate this effect on the FSR, an injection pressure of 1000 bar was considered, and all the other parameters were kept constant. It is worth highlighting that a different injection strategy was used in the two operating points considered. Furthermore, the maximum FSR assessed in the previous section was used as a reference. As seen in Figure 5, in 1500 x 5 test point despite a higher injection pressure, the FSR trend was similar to the reference one over the CA50 range considered. In terms of the combustion process, as HRR profiles show, the higher diesel injection pressure seemed not to produce relevant changes. Important differences can be noticed for 14 deg aTDC, where a lower peak in the HRR trend was observed due to a slight increase in the ethanol fraction. It is also noticed that the change in slope related to higher injection pressure HRR profiles occurred earlier compared to the reference profiles because an increase in injection pressure allows better atomization of diesel injected leading to a better air-fuel mixing [28], the autoignition and the combustion occur earlier due to a shorter ignition delay (ID) [26]. HRR traces also show a long diffusive phase that overtakes the reference traces within CA50 range. Figure 6 shows the effects of the fuel injection pressure in 2000 x 8 test point. With higher diesel injection, the reference trend was replicated, except for 4 deg aTDC due to exceeding the noise and \(\text{COV}_{\text{IMEP}}\) limits. In this case, HRR profiles show more pronounced peaks, with an increase of about 2%/deg, because the rise of diesel injection pressure allowed better mixing of the charge and improved the first phase of the premixed combustion. It is also noticed that the diffusive phase of higher injection HRR profiles overlapped the reference ones. Figure 4: Maximum FSR function of CA50 with a multi-injection strategy, no EGR at 2000 x 8. HRR profiles and injection patterns in 2000 x 8 test point at 4 (a) and 14 deg (b) aTDC. Figure 7 depicts the influence of injection pressure on combustion indicators for both test points in terms of CA10-90, ID. As seen, in \(1500\times 5\), the combustion duration rose due to the prolonged diffusive phase, as shown by the HRR profiles of Figure 5. It is reasonable because after the premixed phase, the remaining charge burned in the diffusion phase in a mixture of ethanol/air, which leads to a slower and prolonged combustion phase [29]. As already stated, with increasing injection pressure, the ignition delay decreased due to smaller particles of atomized fuel, which enhanced the mixing diesel and premixed mixture of ethanol-air hence ID shortened. On average, a shorter ID of about 1 deg CAD could be noticed. For higher loads (\(2000\times 8\)) a shorter combustion duration was observed, on average 2 deg less of the reference case. With the increase of the injection pressure and consequently a shorter ID, the greater fuel amount accumulated in the ID period leads to an increase in the premixed heat release [26], as the higher HRR peaks show in Figure 6. It is reasonable to assume more rapid combustion of the fuel mixture in the first part of the premixed combustion phase, consequently, shorter combustion duration was observed. Related to this point, retarding combustion phasing led to a rise of combustion duration from a minimum of 20 CAD to a maximum of 24 CAD. Figure 5: Effect of diesel injection pressure on FSR varying CA50 in \(1500\times 5\). Figure 6: Effect of diesel injection pressure on FSR varying CA50 in \(2000\times 8\). Figure 8 shows the rail pressure effect on PM, for both test points in the DF mode. The consistent reduction that can be observed between the trends comparison at \(2000\times 8\) was mainly due to two reasons. First, as shown in Figure 6, a longer premixed phase allowed better PM oxidation.