## 4 Conclusions  
In this study, the main spray parameters and combustion characteristics for diesel, HVO, dodecane, OME\({}_{\rm x}\) and OME\({}_{\rm 1}\) were evaluated in an ambient at High Pressure and High Temperature installation, with controlled oxygen concentration. The parameters studied were liquid and spray tip penetration, ignition delay, flame lift-off length, flame length, flame tip penetration and soot production. These were assessed using four high-speed imaging based optical techniques: schlieren, OH* chemiluminescence and diffused back illumination. The operating conditions were defined using as baseline the \"Spray A\" targets recommended by the Engine Combustion Network (ECN). The main conclusions of this study are summarized below.  
Results obtained for the dodecane at the baseline condition (900 K 1500 bar and 15% of O\({}_{\rm 2}\)) for ignition delay, liquid length and flame lift-off length are consistent with those found in the ECN database, which provides confidence in the reliability of the results of this study:  
* The maximum liquid length of all five fuels correlates with the corresponding distillation temperature, stratified in increasing order: OME\({}_{\rm 1}\)-dodecane-OME\({}_{\rm x}\)-HVO-diesel.
* ID trend matches the cetane number rating of paraffinic fuels. HVO and dodecane ignite very close and earlier than diesel. However, OME\({}_{\rm x}\) ignites before the other fuels although its cetane number is not the highest, but its shorter ID is due to the oxygen in its molecular structure, which improves its ignition. The only fuel that ignites after diesel does is OME\({}_{\rm 1}\). However, at 1000 K, OME\({}_{\rm 1}\) shows a similar behavior than diesel.
* The longest flame lift-off length is for OME\({}_{\rm 1}\) and at 800 K and 900 K, which is quite far from OME\({}_{\rm x}\) and paraffinic fuels. Conversely, OME\({}_{\rm x}\) has a closer behavior to paraffinic fuels in all operating conditions.
* During the period until ignition, all fuels show the same behavior in terms of penetration, since the spray tip penetration is known to be governed by the momentum flux conservation at the nozzle orifice, which only depends on the pressure drop across the nozzle and the orifice area.
* The difference of flame penetration between paraffinic fuel and oxygenated fuel is very marked. Oxygenated fuels stabilize early while paraffinic fuels reach the flame stabilization later as a consequence of the fuel's stoichiometry. Furthermore, an important difference between OME\({}_{\rm 1}\) and OME\({}_{\rm x}\) at nominal operating condition exists due to the OME\({}_{\rm 1}\) flame stabilizes at lean air-fuel ratio.
* Regarding soot production, the oxygenated fuels (OME\({}_{\rm x}\) and OME\({}_{\rm 1}\)) did not produce detectable soot at any operating condition tested. This can be explained by the very low equivalence ratio at lift-off owing to the oxygen content in the molecule as well as by the absence of C-C bonds.
* HVO produces less soot than diesel, but more than dodecane. This behavior can be related with the absence of aromatic compounds in HVO formulation.  
Conceptualization, J.M.G.-O.; Data curation, A.A.G.-C.; Formal analysis, A.A.G.-C. and A.G.; Methodology, J.V.P. and C.M.; Project administration, J.M.G.-O.; Resources, J.V.P.; Supervision, J.M.G.-O.; Writing-original draft, A.A.G.-C. and A.G.; Writing-review & editing, J.V.P., J.M.G.-O., C.M., A.A.G.-C. and A.G. All authors have read and agree to the published version of the manuscript.  
This research has been partly funded by the European Union's Horizon 2020 Programme through the ENERXICO project, grant agreement n\({}^{\circ}\) 828947, and from the Mexican Department of Energy, CONACYT-SENER Hidrocarburos grant agreement n\({}^{\circ}\) B-S-69926 and by Universitat Politecnica de Valencia through the Programa de Ayudas de Investigacion y Desarrollo (PAID-01-18).  
The authors declare no conflict of interest.