## 2 Experimental and Theoretical Tools
### Diagnostic Techniques  
Aiming to analyze the spray development, the ignition delay, the combustion behavior and the soot formation for each fuel, four visualization techniques have been implemented using the two opposed optical access windows in the installation. The measurements were carried out in two sets with the slightly different setups shown in Figure 1. The change from one setup to the other was made in few seconds, thanks to the use of an automated translational stage. Consequently, it can be  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Characteristic [unit]** & **Diesel** & **Dodecane** & **HVO** & **OME\({}_{1}\)** & **OME\({}_{\mathbf{x}}\)** \\ \hline Density [kg/m\({}^{3}\)] (T = 15 \({}^{\circ}\)C) & 835.20 & 751.20 & 779.10 & 866.70 & 1057.10 \\ Viscosity [mm\({}^{2}\)/s] (T= 40 \({}^{\circ}\)C) & 2.80 & 1.44 & 2.70 & 0.36 & 1.08 \\ Cetane number [–] & 54.18 & 74 & 75.5 & 28 & 68.6 \\ Lubticity [\(\upmu\)m] & 386 & 563 & 316 & 747 & 320 \\ Flash point [\({}^{\circ}\)C] & - & 83 & 70 & \textless{}40 & 65 \\ Lower heating value [M]/kg] & 39.79 & 44.20 & 43.90 & 19.25 & 19.21 \\ Initial Boiling Point [\({}^{\circ}\)C] & 155.10 & 214.00 & 185.50 & 37.40 & 144.90 \\ Final Boiling Point [\({}^{\circ}\)C] & 363.1 & 218 & 302 & 38 & 242.4 \\ Total contamination [mg/kg] & \textless{}24 & - & 6.0 & \textless{}1 & \textless{}1 \\ Carbon content [\% m/m] & 85.3 & 84 & 85.7 & 48.4 & 44.2 \\ Hydrogen Content [\% m/m] & 13.4 & 16 & 14.3 & 10.4 & 8.8 \\ Oxygen content [\% m/m] & 0 & 0 & 0 & 42.1 & 45 \\ (A/F)\({}_{\text{st}}\) at 21\% of O\({}_{2}\) & 14.39 & 14.92:1 & 14.55:1 & 7.22:1 & 5.89:1 \\ (A/F)\({}_{\text{st}}\) at 15\% of O\({}_{2}\) & 19.98 & 20.72:1 & 20.20:1- & 10.03:1 & 8.18:1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Fuel properties.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Molecule** & **Content (wt\%)** \\ \hline OME\({}_{1}\) & 0.01 \\ OME\({}_{2}\) & \textless{}0.01 \\ OME\({}_{3}\) & 57.90 \\ OME\({}_{4}\) & 28.87 \\ OME\({}_{5}\) & 10.08 \\ OME\({}_{6}\) & 1.91 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Composition of OME\({}_{\mathbf{x}}\) used in this study.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Oxygen Concentration (\%)** & **Temperature [K]** & **Injection Pressure [bar]** \\ \hline
15 & 800 & 500 \\
21 & 900 & 1000 \\  & 1000 & 1500 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Variation of parameters to evaluate for each fuel.  
assumed that measurements will all the techniques were performed nearly simultaneously for any given condition of the test matrix.  
In the first set of measurements, the sketch shown at the top of Figure 1 was used for simultaneous recording of images from schlieren, and OH* chemiluminescence optical techniques. A minimum of 20 injection events were recorded with each camera in this first set of measurements. Immediately after recording and saving these images for each operating conditions, an illumination device based on LEDs was placed in front of the vessel's window in the illumination side using a motorized translational stage (sketch at the bottom of Figure 1). Then, in this second set, images from OH* chemiluminescence and diffused back illumination (DBI) optical technique were recorded for 100 injections in the nominal case and 40 injections for the other operating conditions. The OH* images were recorded in both cases and these results were used to verify consistency between both measurement sets.  
The configuration used for the application of each optical technique will be described below.