applied sciences  
A Synergic Application of High-Oxygenated E-Fuels and New Bowl Designs for Low Soot Emissions: An Optical Analysis  
[PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1 and [PERSON] 2  
CMT--Motores Termicos, Universitat Politecnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain; [EMAIL_ADDRESS] (J.V.P. ); [EMAIL_ADDRESS] (F.L. ); [EMAIL_ADDRESS] (F.J.T.) Istituto di Scienze Tecnologie per iFEnergie La Lobilita Sostenbibli (STEMS-CNR), Via Marconi 4, 80125 Naples, Italy; [EMAIL_ADDRESS]  
molecules (known as OMEx), which allows for achieving more similar properties to those of conventional Diesel than using only OMEn single-component molecules. In addition to their molecular structure characteristics, properties such as a high cetane number compared to Diesel improve autoignition performance. For these reasons, OMEx is a good candidate to replace fossil fuels in ICE [13]. However, using neat OMEx as a fuel in ICE still needs to face several challenges related to compatibility with the engine infrastructure, such as filters and other fuel system parts [14] due to their low lubricity and swelling corrosion effect on rubber and plastics. To overcome this limitation, OMEx blended with conventional Diesel has also been proposed in several studies, reporting that these blends can avoid the infrastructure problem while still offering the benefits of OMEx in terms of soot reduction [15]. [PERSON] et al. [16] reported decreased unburned hydrocarbons (HC) at the engine exhaust using these blends. However, other authors reported slightly higher NOx emissions [17]. For this reason, strategies to reduce the combustion chamber temperature, and thus NOx formation, are mandatory. In this way, the use of exhaust gas recirculation (EGR) schemes are typically used in CI [18] and spark ignition (SI) [19] engines. However, these strategies, based on reducing the oxygen content inside the combustion chamber by adding burnt gases from the exhaust, promote soot particle formation and an increase of HC emissions [20], showing the existing soot-NOx trade-off. Another option to reduce pollutant emissions in ICE applications is using innovative hardware designs [21; 22]. In this regard, significant focus has been placed on studying new piston geometries for CI engines, with many designs created and tested over the previous years. The aim has been to modify the turbulent flow structure in-cylinder air-fuel mixing, soot production, and thermal efficiency [23]. One proposal is redirecting the flames toward the piston center using innovative bowl geometries. The aim is to use available oxygen in this zone to oxidize the soot. In addition, this geometry could reduce (or avoid) the direct flame-to-flame interaction [24]. In this way, the reentrant bowl design allows the flames to be redirected towards the piston center after wall impingement, being fed with the oxygen still available in this region during the late stages of the combustion process. This increases late soot oxidation [25]. An evolution of this piston design includes a stepped lip at the upper part of the reentrant bowl to split the sprays, driving some fuel toward the squish region and taking advantage of the oxygen usage there [26; 27]. The most recent modification of the piston geometry includes protrusions at the piston periphery. Volvo Group initially presented the design of these protrusions for heavy-duty engines [25; 28]. Studies reported an improvement in soot reduction and efficiency compared to the previous piston designs [24]. However, an increase in HC emissions has also been reported using this geometry [29], mainly at higher loads. This effect has been related to promoting lean mixture regions and spray impingement on the piston and liner surface. Considering all the above, there is a literature gap related to the study of possible synergies between these two approaches (oxygenated fuels and improved piston geometries) to reach low-soot combustions.