## 1 Introduction  
Pollutant emission regulations are becoming more stringent every year and strategies to reduce them in compression ignition (CI) engines are being constantly investigated. Among them, active and passive solutions can be found. These latter strategies or after-treatment systems, such as diesel particle filters (DPF), catalytic oxidizers and selective catalytic reduction systems (SCR), are focused on retaining the pollutant emissions prior being expelled through the tail pipe [1]. However, the efficiency and life cycle of the after-treatment systems are deeply related to the fuel behavior inside the combustion chamber. In this sense, active strategies to avoid pollutant formation, as the redesign of the combustion chamber or the study of new combustion concepts [2], the improvement of the mixing formation and injection systems [3], the study of new exhaust gas recirculation schemes as well as the development of cleaner alternative fuels, have great interest and importance to the automotive manufacturers [4]. Biodiesel is the most used biofuel in conventional compression ignition engines because of its potential for reducing the particulate matter due to its oxygenated nature without aromatic compounds [5]. However, lower heating value and higher viscosity, as well as a negative effect on nitrogen oxides (NO\({}_{x}\)) are important drawbacks compared to conventional diesel fuel [6; 7]. These have promoted the study of more appropriate alternative fuels. Among them, hydrotreated vegetable oil (HVO), gas-to-liquid (GTL) and new oxygenated fuels like the oxymethylene ethers with the general structure CH\({}_{3}\)-O-(CH\({}_{2}\)-O)\({}_{n}\)-CH\({}_{3}\), are the most promising alternatives to replace conventional diesel [8; 9; 10]. HVO and GTL are paraffinic fuels without oxygen on their composition and similar chain length. They are obtained by a hydrotreatment of vegetable oils at controlled temperature or by a Fischer-Tropsch process of natural gas or gasified biomass, respectively. Due to their analogous molecular structure, their physical-chemical properties are also similar. However, while viscosity and heating value are comparable to conventional diesel fuel, their higher cetane number, lower cold filter plugging point (CFPP) and the absence of aromatic compounds make these paraffinic fuels very attractive. Moreover, several studies in CI engines agree on their potential for reducing the particulate matter in the exhaust without NOx penalization [11; 12; 13]. Just as HVO or GTL, OME\({}_{x}\) presents a molecular structure without double bonds and can be generated from methanol or formaldehyde by a synthetic process that consumes CO\({}_{2}\) and water [14]. Depending on the chain length their physical-chemical properties differ, increasing the density, viscosity, oxygen content and cetane number as carbon number increases from 1 to 5 [15]. Although with different molecular structure, HVO and OME\({}_{x}\) coincide in their higher cetane number and exhaust emissions benefits, even better than paraffinic fuels in terms of soot emissions [16]. However, the low viscosity and boiling point of the simplest OME (methylal, usually called OME\({}_{1}\)) limits its use in diesel engines without injection system and storage modifications [15; 17]. Even though, these alternative fuels have proved to be environmentally friendly, the study of their spray characteristics and combustion behavior is of importance for a complete understanding of their effects on the exhaust emissions [18]. In this sense, optical techniques are very helpful and effective tools to evaluate combustion progress and soot formation and oxidation [7; 19]. Therefore, in this work, an exhaustive study of spray and combustion behavior of alternative fuels has been carried out in a high pressure and high temperature combustion chamber under diesel-like conditions.