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Resumen Los sistemas de inyección directa han sido uno de los principales puntos focales de la investigación en motores, particularmente en sistemas Diésel, donde la geometría interna, movimiento de aguja y comportamiento del flujo afectan el spray externo y por tanto determinan completamente el proceso de combustión dentro del motor. Debido a regulaciones medioambientales y al potencial de los (más ineficientes) motores "Otto", grandes esfuerzos se están aportando en investigación sobre sistemas de inyección directa de gasolina. Los motores GDi tienen el potencial de incrementar sustancialmente la economía de combustible y cumplir con las regulaciones de gases contaminantes y de efecto invernadero, aunque aún existen muchos desafíos por delante. Esta tesis estudia en detalle una moderna tobera GDi que fue específicamente diseñada para el grupo de investigación conocido como Engine Combustion Network (ECN). Con metodologías punteras, este inyector ha sido usado en un amplio abanico de instalaciones experimentales para caracterizar el flujo interno y varias características clave de geometría y funcionamiento, y aplicarlo para evaluar cómo se relaciona con los efectos observados del comportamiento del chorro externo. Para la caracterización interna del flujo, el objetivo ha sido determinar la geometría de la tobera y el desplazamiento de aguja, caracterizar la tasa de inyección y el flujo de cantidad de movimiento, y evaluar el flujo cercano. Algunas metodologías nunca antes habían sido empleadas en inyectores GDi, y muchas otras lo han sido solo eventualmente. Para la geometría interna, el levantamiento de aguja y el flujo cercano, varias técnicas avanzadas con rayos-x fueron aplicadas en las instalaciones de Argonne National Laboratory. Para la tasa de inyección y flujo de cantidad de movimiento, las técnicas disponibles en el departamento han sido adaptadas desde Diésel y aplicadas en inyectores GDi multiorificio. Dado lo novedoso de las técnicas aplicadas, las particularidades de las metodologías han sido discutidas en detalle en el documento. Aún con la elevada turbulencia del flujo interno, el inyector se comporta de forma consistente inyección a inyección, incluso cuando el estudio se centra en la variabilidad orificio a orificio. Esto ha sido atribuido al comportamiento repetitivo de la aguja, evaluado en los experimentos. También fue observado que el flujo estabilizado tiene una variación de alta frecuencia que no pude ser explicado por el movimiento de la aguja, sino por el particular diseño de las toberas. El análisis de geometría interna realizado a ocho toberas nominalmente iguales resultó en la obtención de un punto vista único en la construcción de toberas y la variabilidad de dimensiones clave. Las medidas de tasa de inyección permitieron estudiar la respuesta hidráulica del inyector a varias variables como la presión de inyección, presión de descarga, temperatura de combustible y la duración de la señal de comando. Estas medidas fueron combinadas con medidas de flujo de cantidad de movimiento para estudiar el bajo valor del coeficiente de descarga, el cual fue atribuido al bajo levantamiento de aguja y coeficiente L/D de los orificios. Por otro lado, el estudio del spray externo resultó en la identificación de un importante fenómeno específico a este particular hardware, el colapso del spray. Las extensivas campañas experimentales, utilizando Schlieren e iluminación trasera difusa (DBI) permitieron identificar y describir las características macroscópicas del spray y las condiciones bajo las que el colapso ocurre. El colapso del spray se forma por una combinación de interacción de las diferentes plumas (causado por el flujo interno) y determinadas condiciones ambiente que promueven evaporación y entrada de aire. Fue determinado que a niveles de densidad y temperatura moderados se desarrolla el colapso, modificando completamente el comportamiento espera / Abstract Fuel injection systems have been one of the main focal points of engine research, particularly in Diesel engines, where the internal geometry, needle lift and flow behavior are known to affect the external spray an in turn completely determine the combustion process inside engines. Because of environmental regulation and the potential development of the more inefficient Otto engines, a lot of research efforts are currently focused into gasoline direct injection systems. GDi engines have the potential to greatly increase fuel economy and comply with pollutant and greenhouse gases emissions limits, although many challenges still remain. The current thesis studies in detail a modern type of GDi nozzle that was specifically developed for the international research group known as the Engine Combustion Network (ECN). With the objective of employing state-of-the-art techniques, this hardware has been used in a wide range of experimental facilities in order to characterize the internal flow and several geometrical and constructive aspects like needle lift; and assess how it relates to the effects seen external spray. For the internal flow characterization, the goal was to determine the nozzle geometry and needle displacement, to characterize the rate of injection and rate of momentum, and evaluate the near-nozzle flow. Some methodologies applied here have never been applied to a GDi injector before, and many have only been applied rarely. For the internal geometry, needle lift and near-nozzle flow, several advanced x-rays techniques were used at Argonne National Laboratory. For the rate of injection and rate of momentum measurements, the techniques available in CMT-Motores Térmicos have been adapted from Diesel spray research and brought to multi-hole GDi injectors. Given the novelty of the techniques used, the particular methodologies and setups are discussed in detail. Despite the high turbulence of the flow, it was seen that the injector behaves consistently injection to injection, even when studying variation in individual holes. This is attributed to the repetitive behavior of the needle that was observed in the experiments. It was also observed that the stabilized flow has a high frequency variability that could not be explained by random movement of the needle, but rather by the particular design of the nozzle. The geometrical analysis done to eight, nominally equal nozzles, allowed a unique view into the construction of the nozzle and provided insights about the variability of key dimensions. The rate of injection measurements allowed to study the hydraulic response of the injector to the main variables like rail pressure, discharge pressure, fuel temperature and command signal duration. These measurements were combined with the rate of momentum measurements to study the low value of the discharge coefficient, that ultimately was attributed to the low needle lift and low L/D ratio of the orifices. On the other hand, the study of the external spray yielded the identification of very important phenomena specific to this particular hardware, the spray collapse. The extensive experimental campaigns featuring shadowgraph (Schlieren) and Diffused Back Illumination (DBI) visualization techniques allowed identifying and describing the macroscopic characteristics of the spray and the conditions under which the collapse occurs. The spray collapse engenders from a combination of the internal flow that creates plume interaction, and ambient conditions that promote air entrainment and evaporation. At moderate density and temperature levels the collapse develops, completely modifying the expected trends in the behavior of the plumes. / Resum Els sistemes d'injecció directa han sigut un dels principals punts focals de la investigació en motors, particularment en sistemes dièsel, en què la geometria interna, el moviment de l'agulla i el comportament del flux afecten l'esprai extern i per tant determinen completament el procés de combustió dins del motor. Degut a regulacions mediambientals i al potencial dels (més ineficients) motors "Otto", grans esforços s'estan aportant en investigació sobre sistemes d'injecció directa de gasolina. Els motors GDi tenen el potencial d'incrementar substancialment l'economia del combustible i complir les regulacions de gasos contaminants i d'efecte hivernacle, encara que existeixen molts desafiaments per davant. Esta tesi estudia en detall una moderna tovera GDi que va ser especialment dissenyada per al grup d'investigació conegut com a ECN. Amb l'objectiu de desenvolupar metodologies punteres, este injector ha sigut usat en un ampli ventall d'instal·lacions experimentals per tal de caracteritzar el flux intern i diverses característiques clau de la seua geometria i funcionament, per tal d'avaluar com es relacionen amb els efectes observats del comportament de l'esprai extern. Per a la caracterització interna del flux, l'objectiu ha sigut determinar la geometria de la tovera i el desplaçament de l'agulla, caracteritzar la taxa d'injecció i el flux de quantitat de moviment, i avaluar el flux proper. Algunes metodologies no s'havien empleat abans en injectors GDi, i moltes altres ho han sigut únicament de manera eventual. Per a la geometria interna, l'alçament de l'agulla i el flux proper, s'han aplicat diverses tècniques avançades amb raigsx a les instal·lacions d'Argonne National Laboratory. Per a la taxa d'injecció i el flux de quantitat de moviment, les tècniques disponibles al departament han sigut adaptades des de Dièsel i aplicades a injectors GDi multi-orifici. Considerant la novetat de les tècniques aplicades, les particularitats de les metodologies es discuteixen en detall al document. A pesar de l'elevada turbulència del flux intern, l'injector es comporta de manera consistent injecció a injecció, inclús quan l'estudi se centra en la variabilitat orifici a orifici. Aquest fet s'ha atribuït al comportament repetitiu de l'agulla, avaluat als experiments. També es va observar que el flux estabilitzat té una variació d'altra freqüència que no pot ser explicat pel moviment de l'agulla, sinó pel particular disseny de les toveres. L'anàlisi de la geometria interna realitzat a vuit toveres nominalment iguals va permetre obtenir un punt de vista únic en la construcció de toveres i la variabilitat de dimensions clau. Les mesures de taxa d'injecció van permetre estudiar la resposta hidràulica de l'injector a diverses variables com la pressió d'injecció, la pressió de descàrrega, la temperatura del combustible i la duració de la senyal de comandament. Estes mesures van ser combinades amb mesures de flux de quantitat de moviment per tal d'estudiar el baix valor del coeficient de descàrrega, el qual va ser atribuït al baix alçament de l'agulla i al coeficient L/D dels orificis. D'altra banda, l'estudi de l'esprai extern va permetre identificar un important fenomen específic d'aquest hardware particular: el col·lapse de l'esprai. Les extensives campanyes experimentals, utilitzant Schlieren i il·luminació darrera difusa (DBI) van permetre identificar i descriure les característiques macroscòpiques de l'esprai i les condicions sota les quals el col·lapse té lloc. El col·lapse de l'esprai es forma per una combinació d'interacció de les diverses plomes (causat pel flux intern) i determinades condicions ambient que promouen evaporació i entrada d'aire. Es va determinar a quins nivells de densitat i temperatura moderats es desenvolupa el col·lapse, modificant completament el comportament esperat de l'esprai. / Vaquerizo Sánchez, D. (2018). Study on Advanced Spray-Guided Gasoline Direct Injection Systems [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/99568

Controlled Auto Ignition (CAI) combustion is a new combustion principle in internal combustion engines which has in recent years attracted increased attention. In CAI combustion, which combines features of spark ignition (SI) and compression ignition (CI) principles, air/fuel mixture is premixed, as in SI combustion and auto-ignited by piston compression as in CI combustion. Ignition is provided in multiple points, and thus the charge gives a simultaneous energy release. This results in uniform and simultaneous auto-ignition and chemical reaction throughout the whole charge without flame propagation. CAI combustion is controlled by the chemical kinetics of air/fuel mixture with no influence of turbulence. The CAI engine offers benefits in comparison to spark ignited and compression ignited engines in higher efficiency due to elimination of throttling losses at part and idle loads. There is a possibility to use high compression ratios since it is not knock limited, and in significant lower NOx emission (≈90%) and particle matter emission (≈50%), due to much lower combustion temperature and elimination of fuel rich zones. However, there are several disadvantages of the CAI engine that limits its practical application, such as high level of hydrocarbon and carbon monoxide emissions, high peak pressures, high rates of heat release, reduced power per displacement and difficulties in starting and controlling the engine. Controlling the operation over a wide range of loads and speeds is probably the major difficulty facing CAI engines. Controlling is actually two-components as it consists of auto-ignition phasing and controlling the rates of heat release. As CAI combustion is controlled by chemical kinetics of air/fuel mixture, the auto-ignition timing and heat release rate are determined by the charge properties such as temperature, composition and pressure. Therefore, changes in engine operational parameters or in types of fuel, results in changing of the charge properties. Hence, the auto-ignition timing and the rate of heat release. The Thesis investigates a controlled auto-ignition (CAI) combustion in internal combustion engines suitable for transport applications. The CAI engine environment is simulated by using a single-zone, homogeneous reactor model with a time variable volume according to the slider-crank relationship. The model uses detailed chemical kinetics and distributed heat transfer losses according to Woschini's correlation [1]. The fundamentals of chemical kinetics, and their relationship with combustion related problems are presented. The phenomenology and principles of auto-ignition process itself and its characteristics in CAI combustion are explained. The simulation model for representing CAI engine environment is established and calibrated with respect to the experimental data. The influences of fuel composition on the auto-ignition timing and the rate of heat release in a CAI engine are investigated. The effects of engine parameters on CAI combustion in different engine concepts fuelled with various fuels are analysed. The effects of internal gas recirculation (IEGR) in controlling the auto-ignition timing and the heat release rate in a CAI engine fuelled with different fuels are investigated. The effects of variable valve timings strategy on gas exchange process in CAI engine fuelled with commercial gasoline (95RON) are analysed.

Autoignition characteristics of pre-vaporized iso-octane, primary reference fuels, gasolines, and dimethyl ether (DME) have been investigated experimentally in a coflow with elevated temperature of air. With the coflow air at relatively low initial temperatures below autoignition temperature Tauto, an external ignition source was required to stabilize the flame. Non-autoignited lifted flames had tribrachial edge structures and their liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, indicating the importance of the edge propagation speed on flame stabilization balanced with local flow velocity. At high initial temperatures over Tauto, the autoignited flames were stabilized without requiring an external ignition source. The autoignited lifted flames exhibited either tribrachial edge structures or Mild combustion behaviors depending on the level of fuel dilution. For the iso-octane and n-heptane fuels, two distinct transition behaviors were observed in the autoignition regime from a nozzle-attached flame to a lifted tribrachial-edge flame and then a sudden transition to lifted Mild combustion as the jet velocity increased at a certain fuel dilution level. The liftoff data of the autoignited flames with tribrachial edges were analyzed based on calculated ignition delay times for the pre-vaporized fuels. Analysis of the experimental data suggested that ignition delay time may be much less sensitive to initial temperature under atmospheric pressure conditions as compared with predictions. For the gasoline fuels for advanced combustion engines (FACEs), and primary reference fuels (PRFs), autoignited liftoff data were correlated with Research Octane Number and Cetane Number. For the DME fuel, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and CH* chemiluminescence were visualized qualitatively. In the autoignition regime for both tribrachial structure and mild combustion, formaldehyde were found mainly between the fuel nozzle and the lifted flame edge. On the other hand, they were formed just prior to the flame edge for the non-autoignited lifted flames. The effect of fuel pyrolysis and partial oxidation were found to be important in explaining autoignited liftoff heights, especially in the Mild combustion regime. Flame structures of autoignited flames were investigated numerically for syngas (CO/H2) and methane fuels. The simulations of syngas fuel accounting for the differential diffusion have been performed by adopting several kinetic mechanisms to test the models ability in predicting the flame behaviors observed previously. The results agreed well with the observed nozzle-attached flame characteristics in case of non-autoignited flames. For autoignited lifted flames in high temperature regime, a unique autoignition behavior can be predicted having HO2 and H2O2 radicals in a broad region between the nozzle and stabilized lifted flame edge. Autoignition characteristics of laminar nonpremixed methane jet flames in high- temperature coflow air were studied numerically. Several flame configurations were investigated by varying the initial temperature and fuel mole fraction. Characteristics of chemical kinetics structures for autoignited lifted flames were discussed based on the kinetic structures of homogeneous autoignition and flame propagation of premixed mixtures. Results showed that for autoignited lifted flame with tribrachial structure, a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. Characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to a nozzle-attached flame was also investigated by increasing the fuel mole fraction.