## 3 Results and Discussion
### Definition of Maximum FSR Function of CA50  
In this section, the main results in terms of maximum FSR reached for each of the CA50 in the considered interval and for the two test points considered were discussed. Figure 3 shows the FSR function of the CA50 for the steady state test point at 1500 rpm and 5 bar BMEP. The evaluation was conducted for various levels of combustion phasing, from 4 to 14 deg aTDC, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & \(\mathbf{1500\times 5}\) & \(\mathbf{2000\times 8}\) \\ \hline
**CA50 Range of Variation** & 4–14 deg & 4–14 deg \\ \hline
**Rail Pressure** & 500 bar & 1000 bar & 700 bar & 1000 bar \\ \hline
**Injection Pulses** & Main & Multi-inj. & Multi-inj. \\ \hline
**NOx Target (EGR)** & & 78 ppm & 120 ppm \\ \hline
**Nozzle (\#)** & 7 holes & 9 holes & 7 holes & 9 holes \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Engine parameters variation range sequence. Figure 2: Injection pattern for the tested operating points. as shown in Table 3, with no EGR, only the main injection strategy and a diesel injection pressure of 500 bar. SOI\({}_{\text{Ethanol}}\) was fixed at 360 deg bTDC\({}_{\text{firing}}\) and an injection pressure of 5 bar. To some extent this setting helped to distinguish the single effect of the CA50 variation on FSR in the DF mode keeping constant the other parameters. Higher values of FSR were achieved for more advanced CA50. FSR shows a flat trend (70-74%) in the range of 4-10 deg aTDC. For a higher CA50, 10-12 deg aTDC, the ethanol fraction was reduced to observe the imposed combustion noise and COV\({}_{\text{IMEP}}\) limits. It was necessary to reduce the FSR to keep the pressure rise rate (PRR) constant, as shown in Figure 3.  
To better characterize and understand the constraints in terms of the FSR increment, the combustion process evolutions of significant operating points, heat release rate (HRR) trends with related injection phases, are also depicted in Figure 3. At 4 deg of CA50, the combustion process showed both a premixed and diffusive combustion phases. With a diesel injection near TDC, the consequently retarded combustion phasing led to faster and prominent premixed combustion phase with a less pronounced diffusive phase as HRR profiles show. A retarded combustion phasing also led to a better air-fuel homogenization, which burns rapidly in the premixed combustion phase and consequently a shorter end of combustion phase was established. Moving to more retarded combustion phasing, the HRR pattern observed is typical of an RCCI (Reactivity Controlled Compression Ignition) combustion process [6]. With ethanol injection, the diesel fuel burned in the diffusion mode is now combusting in a mixture of ethanol/air, which burns in the premixed mode [26]. Longer combustion duration was observed, as remarked in Table 5.  
Figure 4 shows the FSR trend as a function of CA50 at \(2000\times 8\). The evaluation was conducted for the same CA50 range, ethanol injection pressure and SOI\({}_{\text{Ethanol}}\) introduced in the previous case (Table 3), with no EGR and a diesel injection pressure of 700 bar. In contrast to the lower load point \(1500\times 5\), this test point employed a multi injection pattern to ensure acceptable combustion stability. The comparative results among combustion phasing show FSR values ranging in the interval 80-85%,  
Figure 3: Maximum fuel substitution ratio (FSR) and pressure rise rate (PRR) function of CA50, with a single injection strategy and no exhaust gas recirculation (EGR) at 1500 \(\times\) 5. Heat release rate (HRR) profiles and injection patterns at 1500 \(\times\) 5, for CA50 4 (a) and 14 (b) deg after top dead centre (aTDC). \begin{table}
\begin{tabular}{c c c} \hline  & **1500 \(\times\) 5** & **2000 \(\times\) 8** \\ \hline
**CA50 (deg aTDC)** & **CA90-10 (deg)** & \\ \hline
4 & 17.6 & 21 \\
6 & 17.2 & 21.6 \\
8 & 17.9 & 21.7 \\
10 & 19.8 & 22.7 \\
12 & 20.4 & 24.6 \\
14 & 21.2 & 26 \\ \hline \end{tabular}
\end{table} 
Table 5: Combustion duration function of combustion phasing.