## 2 Materials and Methods
### Experimental Methodology  
The operating points were performed using a Euro 6 Diesel engine calibration (e.g., boost, backpressure, injection parameters, etc.) to ensure a practical parameters migration on real light-duty engine applications. The investigations were conducted at two operating points under a steady state engine speed of 1500 rpm, and 2000 rpm and 5 and 8 bar of brake mean effective pressure (BMEP) respectively. The level of premixed ethanol, quantified by the fuel substitution ratio (FSR), can be defined through the following equation:  
\[\text{FSR}=\frac{\text{m}_{\text{p}}\cdot\text{LHV}_{\text{p}}}{\text{m}_{ \text{p}}\cdot\text{LHV}_{\text{p}}\ +\ \text{m}_{\text{d}}\cdot\text{LHV}_{\text{d}}} \tag{1}\]  
\(\text{LHV}_{\text{p}}\) and \(\text{LHV}_{\text{d}}\) indicate the lower heating value respectively of premixed ethanol and direct-injected diesel, while \(\text{m}_{\text{p}}\) and \(\text{m}_{\text{d}}\) their respective mass flow rate. The combustion duration is defined as the crank angle duration between CA10 and CA90, where the 10% and 90% of the total heat release occurred, respectively. The ignition delay (ID) is defined as the crank angle difference between CA10 and the start of injection (SOI). The start of direct diesel injection (SOI\({}_{\text{diesel}}\)) and the injection durations for both fuels were varied to achieve the reference combustion phasing (CA50) and BMEP values, respectively. The first step of the test procedure consisted of an engine warm-up procedure, which was carried out in the diesel mode to reach the loads' targets. Switching to the DF mode, the quantity of diesel injected was reduced while ethanol fraction was proportionally increased to reach the maximum FSR at the same load. Ethanol fuel was injected via PFI at 5 bar at the constant start of injection (SOI\({}_{\text{Ethanol}}\)), fixed at 360 deg bTDC. The intake air temperature was kept constant in all test conditions and equal to 60 \({}^{\circ}\)C. The experimental conditions are summarized in Table 3.  
To achieve the maximum FSR, which was the main target of the investigation, a parametric analysis of different parameters was performed. First, a CA50 sweep was made through the SOI variation of the direct-injected diesel fuel, in the interval 4-14 CA after top dead centre (aTDC). The FSR value assessed was considered maximum when the noise limit bound to 90 dBA and a stable engine conditions quantified by a COV\({}_{\text{IMEP}}\) up to 3% were achieved. The noise level was determined through \begin{table}
\begin{tabular}{c c c} \hline \hline
**Properties** & **Diesel** & **Ethanol** \\ \hline
**Research Octane Number (-)** & - & 108–109 \\
**Molar Mass (g/mol)** & 170 & 46.07 \\
**H/C (-)** & 1.8 & 3 \\
**O/C (-)** & 0 & 0.5 \\
**LHV (MJ)/kg)** & 42.9 & 26.9 \\
**(A/F)**s (-)** & 14.5 & 9.0 \\
**Viscosity (mPa/s)** & 3.91 & 1.36 \\
**Density (kg/m\({}^{3}\))** & 837 & 785 \\
**Boiling Point (\({}^{\circ}\)C)** & 184–362 & 78 \\
**Heat of Vaporization (kJ/kg)** & 270 & 840 \\
**Specific Heat (kJ/kgK)** & 2.2 & 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fuel properties [2; 24]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Pint** & **Pexh** & **IMEP** & **PFI\({}_{\text{pressure}}\)** & **SOI\({}_{\text{ethanol}}\)** & **T\({}_{\text{ethanol}}\)** \\ \cline{2-7}  & **(barG)** & **(barG)** & **(bar)** & **(bar)** & **(deg bTDC)** & **(K)** \\ \hline
**1500 x 5** & 0.3 & 0.6 & 6.2 & 5 & 360 & 333 \\
**2000 x 8** & 0.6 & 1.0 & 9.2 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Test conditions. a transfer function using the peak release rate (PRR) as input. These constraints were maintained in all experimental tests performed in this study. The FSR values assessed with CA50 variation were considered as a reference in the next sweeps. At the operating point \(1500\times 5\), the experimental assessments were conducted at constant diesel injection pressure (500 bar).