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[EN] This paper evaluates the potential advantages of introducing a new injector concept with a dual set of orifices in a 2-stroke engine operating with gasoline partially premixed combustion (PPC gasoline), specially in terms of combustion noise reduction. The injector is based on a configuration with two concentric needle valves and dual actuators, which allows to switch among two independent crowns of holes. The first set of orifices is controlled by the primary needle valve while an additional needle valve manages the secondary holes crown. Moreover, both valves are coupled with the two actuators separately, providing enhanced selective control over injection with an extra degree of freedom. In this investigation, a combination of Computational Fluid Dynamics (CFD) simulations and the Design of Experiments (DoE) technique is proposed with the specific purpose of optimizing this new injector configuration. In addition to determining the most convenient design, this method is extremely useful to establish cause/effect relationships between the injection design parameters and their impact on the engine performance and emissions. Results show how this solution could increase the operating range of the PPC gasoline concept by improving the combustion stability while acoustic emissions are reduced, The equipment used in this work has been partially supported by FEDER project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT), (FEDER-ICTS-2012-06)" from the operational program of unique scientific and technical infrastructure of the Spanish Ministerio de Economia y Competitividad.

[EN] In the present investigation the Spray A reference configuration defined in the framework of the Engine Combustion Network (ECN) has been modeled by means of an Unsteady Flamelet Model (USFM) including detailed parametric studies to evaluate the impact of ambient temperature, oxygen concentration and density. The study focuses on the analysis of the spray ignition delay, the flame lift-off length and the internal structure of the spray and flame according to the experimental information nowadays available for validating the results provided by the model. Promising results are obtained for the nominal case and also for the parametric variations (temperature, oxygen...) in terms of liquid and vapor penetration, ignition delay (ID) and lift-off length (LOL). The model permits to predict the ID and the LOL which constitute two parameters of key importance for describing the characteristics of transient reacting sprays. Valuable insight on the details of the combustion process is obtained from the analysis of formaldehyde (CH2O), acetylene (C2H2) and hydroxide (OH) species in spatial coordinates and also in the so-called phi-T maps. Important differences arise in the inner structure of the flame in the quasi-steady regime, which is closely linked to soot formation, when varying the ambient boundary conditions. Additionally, the auto-ignition process is investigated in order to describe in detail the spatial onset and propagation of combustion. Results confirm the impact of the ambient conditions on the regions of the spray where start of combustion takes place, so the relation between the local scalar dissipation rate and mixture fraction variance is also discussed. This investigation provides an insight of the potential of the USFM combustion model to describe the physical and chemical processes involved in transient spray combustion., Authors acknowledge that this work was possible thanks to the Ayuda para la Formacion de Profesorado Universitario (FPU 14/03278) belonging to the Subprogramas de Formacion y de Movilidad del Ministerio de Educacion, Cultura y Deporte from Spain. Also this study was partially funded by the Ministerio de Economia y Competitividad from Spain in the frame of the COMEFF (TRA2014-59483-R) national project.

[EN] In this paper, a computational study was performed using a combination of several numerical tools to better understand the limiting aspects of combustion in a passive pre-chamber ignition system when operating at lean conditions. A specific methodology was developed to analyze in detail the scavenging and combustion processes of this ignition concept. Results show how the scavenging of passive pre-chambers is primarily dependent on the force that the piston makes on the gas during the compression stroke, being independent of the pre-chamber geometry as along as the ratio between the total cross sectional area of the pre-chamber holes and the prechamber volume is kept within a suitable range. Moreover, a successful lean combustion, with an air-to-fuel ratio around 2, cannot be achieved as the burning rates inside the pre-chamber significantly decrease due to the low laminar flame speeds, that results in low quality jets. Further results show that increasing the flow temperature can help to recover competitive combustion rates when knocking combustion is not a limiting factor. The contribution of the heat losses through the pre-chamber walls to the overall energy balance of the pre-chamber has been estimated, showing that their impact is negligible (< 5%). Alternatives for increasing the laminar flame speed were proposed in order to improve combustion inside the pre-chamber. Although the pre-chamber combustion profile was successfully improved, none of the proposed solutions were able to completely burn the main chamber charge with the current pre-chamber design., The work has been partially supported by the Spanish Ministerio de Economia y Competitividad through Grant No. TRA2017-89139-C2-1-R. J. Gomez-Soriano is partially supported through the Programa de Apoyo para la Investigacion y Desarrollo (PAID) of Universitat Politecnica de Valencia [Grant No. FPI-S2-2018-17367].

An experimental investigation has been performed on the modification of in-cylinder gas thermodynamic conditions by advancing the intake valve closing angle in a HD diesel engine. The consequences on the diffusion-controlled combustion process have been analysed in detail, including the evolution of exhaust emissions and engine efficiency. This research has been carried out at full load (100%) and low engine speed (1200 rpm) with the aim of generating a long and stable diffusion-controlled combustion process. The intake oxygen mass concentration was kept at 17.4% to obtain low NOx levels in all cases. The required flexibility on intake valve motion has been attained by means of an electro-hydraulic variable valve actuation system. The results obtained from advancing the intake valve closing angle (IVC) have shown an important reduction on in-cylinder gas pressure and density, whereas the gas temperature showed less sensitivity. Consequently, the diffusion-controlled combustion process is slowed down mainly due to the lower in-cylinder gas density and oxygen availability. Important effects of advancing IVC have also been observed on pollutant emissions and engine efficiency. Where NOx production decreases, soot emissions increase. Finally, the results of pollutant emissions and engine efficiency have been compared with those obtained retarding the start of injection.