**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 e fficiency 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 grea<sup>t</sup> 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 e ffect on nitrogen oxides (NOx) 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 CH3–O–(CH2–O)n–CH3, are the most promising alternatives to replace conventional diesel [8–10].

HVO and GTL are para ffinic 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 para ffinic fuels very attractive. Moreover, several studies in CI engines agree on their potential for reducing the particulate matter in the exhaust without NOx penalization [11–13].

Just as HVO or GTL, OMEx presents a molecular structure without double bonds and can be generated from methanol or formaldehyde by a synthetic process that consumes CO2 and water [14]. Depending on the chain length their physical-chemical properties di ffer, increasing the density, viscosity, oxygen content and cetane number as carbon number increases from 1 to 5 [15]. Although with di fferent molecular structure, HVO and OMEx coincide in their higher cetane number and exhaust emissions benefits, even better than para ffinic fuels in terms of soot emissions [16]. However, the low viscosity and boiling point of the simplest OME (methylal, usually called OME1) 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 e ffects 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.

The objective of this study was to define the di fferences on combustion behavior of four alternative fuels in comparison to conventional diesel fuel: on the one hand, dodecane and HVO with para ffinic structures and, on the other hand, OME1 and OMEx as oxygenated fuels. The experiments were carried out under an operating condition known as "Spray A conditions" settled as standard by the Engine Combustion Network (ECN) [20]. Experimental data about the fundamental behavior of these fuels under these conditions is a novelty of this study, due to the lack of this type of information in the current literature.

Quantitative parameters describing the evolution of diesel-like sprays such as liquid length, spray penetration, ignition delay, lift-off length and flame length as well as the soot formation were determined using schlieren, OH\* chemiluminescence and diffused back-illumination extinction high-speed imaging techniques. The results can provide a useful information to better understanding of factors that dominate the combustion of these alternative fuels and therefore contributing to the upgrade of combustion models. Furthermore, the results will expand the database available in the ECN [20].

#### **2. Experimental and Theoretical Tools**

#### *2.1. High Pressure and High Temperature Rig*

The experiments were carried out in a high pressure and high temperature (HPHT) rig. Parameters such as the composition of the gas, the pressure and the temperature of the ambient gas can be controlled independently to obtain oxygen concentration between 0 and 21%, pressures up to 15 MPa and temperatures up to 1100 K. Thereby, it is possible to replicate the thermodynamic conditions of the cylinder of an internal combustion engine when the fuel is injected. Furthermore, as the temperature field is homogeneous and constant in the area of interest, this reduces the uncertainties that could be associated to engine transients. The HPHT rig has wide optical accesses, which allow the application of different visualization techniques. The installation can operate as an open circuit with air or, in order to reduce O2 concentration, as a closed loop circuit with a mixture of air and Nitrogen. The regulation and control system ensures steady thermodynamic conditions during long time periods with the aim of getting reliable statistical results of many injections and combustion events. Additionally, one injection is performed every 4 s to avoid any temperature transients. The boundary conditions have been detailed widely in [21] where a full description of the facility is given.

The common-rail used is capable to achieve injection pressures up to 230 MPa and is equipped with a solenoid-activated single-hole nozzle injector. It is worth mentioning that the injection system pump used is made by polymerizing tetrafluoroethylene material. This material has shown compatibility with oxymethylene ether fuels according to the investigation reported in [22].

#### *2.2. Fuel and Test Matrix*

#### 2.2.1. Fuel Description and Properties

A set of two different groups of pure alternative fuels were tested and compared to a low sulfur diesel fuel without biodiesel, which was used as reference. First, two paraffinic fuels with different molecular structure: a renewable hydrotreated vegetable oil (HVO) and a single component *n*-paraffinic fuel (dodecane) were tested. The last is used as standard surrogate for diesel by the engine combustion network (ECN) [20] and, in this work, dodecane is tested to compare the results with ECN database and validate the current experiments. Then, two oxymethylene ethers, with the general structure CH3–O–(CH2–O)n–CH3 were tested under the same thermodynamic conditions as HVO and dodecane. The first one was a single component one corresponding to the shortest carbon chain (n = 1) of the family, which will be denoted here as OME1. The second one is a multi-component fuel, which will be denoted here as OMEx, which is a blend of components of different chain lengths. Table 1 compiles the main properties for all five fuels and Table 2 shows the OMEx actual composition.


**Table 1.** Fuel properties.

**Table 2.** Composition of OMEx used in this study.


## 2.2.2. Operating Conditions

For the five tested fuels, a set of parametric studies was performed. Oxygen concentration, ambient temperature and injection pressure were defined as variables. Table 3 shows the test matrix.


**Table 3.** Variation of parameters to evaluate for each fuel.

The nominal condition is based on Spray A specification from the ECN [20]. It corresponds to 900 K as ambient temperature, 15% of oxygen concentration and 1500 bar as injection pressure and air density of 22.8 kg/m3. The whole test matrix is shown in Table 3.

A single-hole nozzle was used, which has been extensively studied in previous ECN studies [23,24]. The injector serial number is 210,675, with an actual nozzle diameter of 89.4 μm. A single injection was used as injection strategy. The energizing time was 2000 μs, which provides an injection event long enough to study the spray evolution and flame development under stabilized mixingcontrolled combustion.
