## 3 Results and Discussion
### Combustion Analysis  
In this section, the assessment of the fuel blends' performance on combustion characteristics and emissions is regarded at 1500 rpm. For each DLPG blend, and each operating point, the EGR and the SOI levels were varied to achieve the NOx and combustion center targets. Figure 5 shows the in-cylinder pressure, rate of heat release (HRR), and the injection pattern for operating 1500 \(\times\) 2 and 1500 \(\times\) 5 at constant MBF50 and NOx emissions. Advancement of the injection pattern is needed for the blends to achieve the MBF50 reference value, increasing the premixed combustion phase, combustion peaks and burning rates as illustrated in Figure 5. At higher loads, the LPG effect is even more evident. Indeed, the lower cetane rating of the blends increases the chemical delay and confirms an overall longer ignition delay of 3-4 degrees and 5-8 degrees (Figure 6), for DLPG20 and DLPG35, respectively. Figure 5 provides a further representation of the combustion duration (MBF10-90) for diesel and LPG blends. Moreover, the effect of varying the engine load on the angular ignition delay can also be drawn. It increases with the load due to the cooling effect of LPG, which in turn lowers the in-cylinder charge temperature proportional to the fuel amount. The LPG cooling effect is detectable in the interval \(-\)10 to 5 CA, as also demonstrated in Figure 6, which distinguishes a reduction of the in-cylinder temperature up to a maximum of 30 and 60 \({}^{\circ}\)C for DLPG20 and DLPG35, respectively. In the case, the low-temperature heat release phase related to the pilot injection, practically disappear, being involved almost entirely into the main combustion phase. As the engine load increases, the higher ignition delay and then the premixed combustion phase reduce the combustion duration with advantages in terms of cycle conversion efficiency, as shown in Figure 6. The efficiency benefits are confirmed with an indicated specific fuel consumption (ISFC) reduction, as shown in Figure 6, of about 1-2% for DLPG20 and 3% for DLPG35. Slight differences in terms of combustion are observed at 2 bar of load, while at 5 bar, the prominent premixed phase, in comparison to only diesel case, causes an increase of the maximum pressure rise rate (MPRR) and noise combustion. For both blends, the combustion noise values are over the acceptable \"comfort\" limit of 90 dBA [10], and to reduce it, a more complex  
Figure 4: Mass flow rate (**c**,**d**), energizing current (**a**,**b**), and fuel amount (table) for all fuels. multi-injection pattern [4], out of the scope of this study, is required. The covariance of indicated mean effective pressure (COVIMEP), as an indicator of the cycle-to-cycle combustion process stability, shows any significant drift in comparison to diesel. Figure 6 also displays combustion efficiencies (\(\eta_{\text{comb}}\)). Despite the consistent differences at lower loads, at higher loads, they are not significant, and in the range of variation of 0.5%. The lower \(\eta_{\text{comb}}\)  
Figure 5: Pressure, rate of heat release (HRR), injection energizing pattern, and combustion duration at \(1500\times 2\) (**a**) and \(1500\times 5\) (**b**) at constant engine-out NOx. Figure 6: Covariance of indicated mean effective pressure (COVIMEP), maximum pressure rise rate (MPPR), ignition delay, and noise (**left**) and efficiency performances (**right**) for all test points. is a consequence of the higher CO and HC emissions explicable by the worsening of the air-fuel spray interaction inside the combustion chamber and linked to the presence of over-lean regions and fuel trapped into the crevices [14]. HC and CO emissions will be further discussed in the following paragraphs. Indeed, at higher loads, the higher adiabatic in-cylinder conditions temperatures convert more CO into CO\({}_{2}\) while comprehensively oxidizing the fuel.