An experimental study of the effects of fuel properties on diesel spray processes using blends of single-component fuels

[EN] This last few years, the trend in diesel engines has been to use different kinds of fuels to identify their influence and behaviour on the emissions and performance. Among the wide variety of fuels employed are the so called Primary Reference Fuels (PRFs), which represent the behaviour of diesel and gasoline in terms of ignition properties, as they are located at both ends of the octane rating scale and also have very different cetane numbers. One of the disadvantages of using pure gasoline or diesel-gasoline blends in diesel engines is the time needed for the mixture to ignite and to completely burn the fuel. This generally requires working with partial loads or with premixed charges. In order to isolate the fuel effects on the spray processes and to be able to study the characteristic parameters of ignition delay time, lift-off length, vapour and liquid penetration, among others; different experiments under parametric variations of diesel like conditions have been performed. The tests were performed under inert and reactive conditions in a 2-stroke optical engine and a constant-pressure flow (CPF) high-pressure high-temperature vessel using single-hole nozzles, while diverse optical techniques were being employed. To study the influence of the fuel properties, different single-component fuels were employed as well as binary blends and a six-component diesel surrogate, which was also compared to conventional diesel. Additionally, the results have been contrasted with a one-dimensional model in order to further explain the values and trends found. The results presented a strong dependency on the fuel properties for the tests performed under inert and reactive conditions. The difference in physical properties of n-decane and n-hexadecane showed an almost linear reduction of the stabilized liquid penetration down to approximately 60% under some conditions. Additionally, due to the composition of the surrogate fuel, pure n-hexadecane was demonstrated to have almost identical evaporation characteristics, hence proving itself as a good candidate for a single-component surrogate of diesel fuel. In a similar way, the chemical properties of the PRFs n-heptane and iso-octane also proved to be influential on the spray development and radiation emitted. Ignition delay values up to one order of magnitude larger where obtained for both extremes of the blend range, as well as lift-off lengths up to three times longer. The radiation emitted by the soot incandescence presented the highest variations, as some conditions showed a reduction of almost four orders of magnitude among the blend range. Moreover, some cases did not present any radiation corresponding to the soot, and increasing the sensitivity of the camera only caused the chemiluminescence of the OH* radical to be captured. On a different way, the stabilized flame length determined also by the soot radiation did not present much variation as the fuel properties or the air temperature were changed; in fact, the only noticeable differences were caused by the changes in the oxygen composition of the ambient air. In conclusion, the fuel properties proved to have a significant effect on the spray processes. Lighter fuels favoured the evaporation of the spray under a range of conditions, while fuels with lower octane numbers ignited sooner and closer to the spray tip but with more soot luminosity measured.[ES] Estos últimos años, la tendencia en motores diesel ha sido la de emplear distintos tipos de combustibles para identificar su influencia y comportamiento sobre las emisiones y rendimiento. Dentro de la amplia variedad de combustibles empleados están los llamados combustibles de referencia (PRFs ingl. Primary Reference Fuels), los cuales representan el comportamiento del diesel y la gasolina en lo que respecta a propiedades de encendido, ya que se encuentran en ambos extremos de la escala del número de octano y también poseen números de cetano muy distintos. Una de las desventajas de utilizar gasolina pura o mezclas de diesel-gasolina en motores diesel es el tiempo que toma la mezcla en encender y quemar completamente el combustible. Esto generalmente requiere trabajar con cargas parciales o cargas premezcladas. Para poder aislar los efectos del combustibles sobre los procesos de un chorro y que sea capaz estudiar los parámetros característicos de tiempo de retraso de encendido, longitud de despegue de llama, penetración de líquido y vapor, entre otros, se han realizado distintos experimentos bajo variaciones paramétricas de condiciones de motor diesel. Los ensayos han sido realizados bajo condiciones inertes y reactivas en un motor óptico de dos tiempos y una instalación de alta presión y alta temperatura de flujo continuo a presión constante (CPF ingl. Constant-Pressure Flow) empleando toberas mono-orificio, con aplicación de diversas técnicas ópticas. Para estudiar la influencia de las propiedades de los combustibles se utilizaron distintos mono-componentes, así como mezclas binarias y un sustituto de diesel conformado por seis componentes, el cual fuel comparado con diesel convencional. Adicionalmente, los resultados han sido contrastados con un modelo unidimensional para ayudar a explicar los valores y tendencias encontrados. Los resultados presentaron una fuerte dependencia de las propiedades de los combustibles en los ensayos realizados bajo condiciones inertes y reactivas. La diferencia entre las propiedades físicas del n-decano y n-hexadecano mostraron una reducción casi lineal sobre la longitud líquida estabilizada hasta aproximadamente un 60% bajo ciertas condiciones. Adicionalmente, debido a la composición del combustible de sustitución, el n-hexadecano puro demostró tener características de evaporación prácticamente idénticas, probándose a sí mismo como un buen candidato para ser un sustituto mono-componente del diesel convencional. De una manera similar, las propiedades químicas de los PRFs n-heptano e iso-octano también probaron tener influencia sobre el desarrollo del chorro y radiación emitida. Se obtuvieron valores de tiempo de retraso con diferencias de hasta un orden de magnitud entre ambos extremos del rango de las mezclas, así como longitudes de despegue de llama hasta tres veces más largas. La radiación emitida por la incandescencia del hollín presentó las variaciones más altas, ya que algunas condiciones mostraron reducciones de hasta cuatro órdenes de magnitud dentro del rango de mezclas. Es más, algunos casos no presentaron radiación correspondiente al hollín, e incrementar la sensibilidad de la cámara solo ocasionó que la quimioluminiscencia del radical OH* sea detectada. Por otro lado, la longitud estabilizada de llama calculada mediante la radiación del hollín no presentó mucha variación respecto a las propiedades del combustible o la temperatura del aire. De hecho, la única diferencia apreciable fue causada por los cambios en la composición del oxígeno del aire ambiente. En conclusión, las propiedades de los combustibles demostraron tener un efecto significativo en los procesos de un chorro diesel. Los combustibles más ligeros favorecieron la evaporación del chorro en un rango de condiciones, mientras que combustibles con números de octano más bajos encendieron más pronto y cerca de la tobera pero con mayor luminosidad del hollín medida.[CA] En aquests últims anys, la tendència en motors Diesel ha estat la d'emprar diferents tipus de combustibles per a identificar la seva influència i comportament sobre les emissions i rendiment. Dintre de l'àmplia varietat de combustibles emprats estan els anomenats combustibles de referència (PRFs angl. Primary Reference Fuels), els quals representen el comportament del dièsel i la gasolina pel que fa a propietats d'encesa, ja que es troben en ambdós extrems de l'escala del nombre d'octà i també posseeixen nombres de cetà molt diferents. Un dels desavantatges d'utilitzar benzina pura o barreges de Diesel-benzina en motors Diesel és el temps que pren la barreja a encendre i cremar completament el combustible. Això generalment requereix treballar amb càrregues parcials o càrregues premesclades. Per a poder aïllar els efectes del combustibles sobre els processos d'un doll i que sigui capaç estudiar els paràmetres característics de de temps de retard d'encesa, longitud d'enlairament de flama, penetració de líquid i vapor, entre altres, s'han estudiat diferents experiments sota variacions paramètriques de condicions de motor Diesel. Els assajos han estat realitzats sota condicions inertes i reactives en un motor de dos temps i una instal·lació d'alta pressió i alta temperatura de flux continu a pressió constant (CPF angl. Constant-Pressure Flow) emprant toberes mono-orifici, amb aplicació de diverses tècniques òptiques. Per a estudiar la influència de les propietats dels combustibles, van ser utilitzats distints mono-components, així com barreges binàries i un substitut de Diesel conformat per sis components, el qual fuel comparat amb Diesel convencional. Addicionalment, els resultats han estat contrastats amb un model unidimensional per a ajudar a explicar els valors i tendències trobats. Els resultats van presentar una forta dependència de les propietats dels combustibles en els assajos realitzats sota condicions inertes i reactives. La diferència entre les propietats físiques del n-decà i n-hexadecà van mostrar una reducció gairebé lineal sobre la longitud líquida estabilitzada fins a aproximadament un 60% sota certes condicions. Addicionalment, degut a la composició del combustible de substitució, el n-hexadecà pur va demostrar ser tindre característiques d'evaporació pràcticament idèntiques a aquell, demostrant ser un bon candidat per a ser un substitut mono-component del dièsel convencional. D'una manera similar, les propietats químiques dels PRFs n-heptà i iso-octà també provaren tindre influència sobre el desenvolupament del doll i la radiació emesa. Es van obtenir valors de temps de retard amb diferències de fins a un ordre de magnitud entre ambdós extrems del rang de les barreges, així com longituds d'enlairament de flama fins a tres vegades més llargues. La radiació emesa per la incandescència del sutge va presentar les variacions més grans, ja que algunes condicions van mostrar reduccions de fins a quatre ordres de magnitud dintre del rang de barreges. Encara més, alguns casos no van presentar radiació corresponent al sutge, i incrementar la sensibilitat de la càmera solament va ocasionar que la quimioluminiscència del radical OH* sigui detectada. D'altra banda, la longitud estabilitzada de flama calculada mitjançant la radiació del sutge no va presentar molta variació respecte a les propietats del combustible o la temperatura del aire. De fet, la única diferència apreciable va ser causada pels canvis en la composició del oxigen de l'aire ambient. En conclusió, les propietats dels combustibles van demostrar tenir un efecte significatiu en els processos d'un doll dièsel. Els combustibles més lleugers van afavorir l'evaporació del doll en un rang de condicions, mentres que els combustibles amb nombre d'octà més baixos van prendre més aviat i prop de la tovera però amb més lluminositat del sutge mesurat.Vera-Tudela Fajardo, WM. (2015). An experimental study of the effects of fuel properties on diesel spray processes using blends of single-component fuels [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/58865TESI

A study on tip penetration velocity and radial expansion of reacting diesel sprays with different fuels

[EN] The reacting diesel spray structure was investigated using n-dodecane, n-heptane and one binary blend of Primary Reference Fuels (80% n-heptane and 20% iso-octane in mass) based on the existing database from previous experimental results from Schlieren imaging technique in a constant pressure combustion chamber. The spray tip velocity was derived from the derivative of tip penetration versus time. The operating conditions and the injector used (single axially-oriented hole, 89 mm-diameter) were chosen following the guidelines of the Engine Combustion Network. A 1D spray model was also applied here to support the analysis of experimental results. Parametric variations of injection pressure, ambient temperature and oxygen concentration have been performed for each fuel. Analysis of radial expansion and reacting tip velocity was performed in terms of an average spray radial increase (DR) and a constant (k) defining the tip penetration velocity. k values of reacting cases are always bigger than those from inert ones for both experimental and theoretical results. Based upon this parameter, quasi-steady tip penetration under the investigated conditions seems not to be affected by ambient temperature, oxygen content or fuel cetane number. Three cases with different fuels and similar ignition delay and lift-off length were further analyzed, which shows that the reactivity of the mixture has an effect on the transition timing from inert to reacting states, as well as on the initial penetration stages, but not on the quasi-steady phase. Apart from the similar tip velocity during quasi-steady phase, the full transient evolution of the tip is highly similar. The fact that this full overlap does not occur for other operating conditions indicates that early penetration stages are highly affected by the transient chemistry development, which largely depends on fuel cetane number.This study was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF (TRA2014-59483-R) project. Funding for Tiemin Xuan's PhD studies was granted by Universitat Politecnica de Valencia through the Programa de Apoyo para la Investigacion y Desarrollo (PAID) (Grant reference FPI-2015-S2-1068).Desantes J.M.; García-Oliver, JM.; Xuan, T.; Vera-Tudela-Fajardo, WM. (2017). A study on tip penetration velocity and radial expansion of reacting diesel sprays with different fuels. Fuel. 207:323-335. https://doi.org/10.1016/j.fuel.2017.06.108S32333520

Combustion Recession after End of Injection in Diesel Spray

This work contributes to the understanding of physical mechanisms that control flashback, or more appropriately combustion recession, in diesel-like sprays. Combustion recession is the process whereby a lifted flame retreats back towards the injector after end-of-injection under conditions that favor autoignition. The motivation for this study is that failure of combustion recession can result in unburned hydrocarbon emissions. A large dataset, comprising many fuels, injection pressures, ambient temperatures, ambient oxygen concentrations, ambient densities, and nozzle diameters is used to explore experimental trends for the behavior of combustion recession. Then, a reduced-order model, capable of modeling non-reacting and reacting conditions, is used to help interpret the experimental trends. Finally, the reduced-order model is used to predict how a controlled ramp-down rate-ofinjection can enhance the likelihood of combustion recession for conditions that would not normally exhibit combustion recession. In general, fuel, ambient conditions, and the spray rate-of-injection transient during the end-of-injection determine the success or failure of combustion recession. The likelihood of combustion recession increases for higher ambient temperatures and oxygen concentrations as well as for higher reactivity fuels. In the transition between high and low ambient temperature (or oxygen concentration), the behavior of combustion recession changes from spatially sequential ignition to separated, or isolated, ignition sites that eventually merge. In contradistinction to typical diesel ignition delay trends where the autoignition times are longer for increasing injection pressure, the time required for combustion recession increases with injection pressure.Knox, BW.; Genzale, C.; Pickett, L.; García-Oliver, JM.; Vera-Tudela, WM. (2015). Combustion Recession after End of Injection in Diesel Spray. SAE International Journal of Fuel and Lubricants. 8(2):1-17. doi:10.4271/2015-01-0797S1178

Study of ignition delay time and generalization of auto-ignition for PRFs in a RCEM by means of natural chemiluminescence

An investigation of the effects of contour conditions and fuel properties on ignition delay time under Homogeneous Charge Compression Ignition (HCCI) conditions is presented in this study. A parametric variation of initial temperature, intake pressure, compression ratio, oxygen concentration and equivalence ratio has been carried out for Primary Reference Fuels (PRFs) in a Rapid Compression Expansion Machine (RCEM) while applying the optical technique of natural chemiluminescence along with a photo-multiplier. Additionally, the ignition delay time has been calculated from the pressure rise rate and also corresponding numerical simulations with CHEMKIN have been done. The results show that the ignition delay times from the chemical kinetic mechanisms agree with the trends obtained from the experiments. Moreover, the same mechanism proved to yield consistent results for both fuels at a wide range of conditions. On the other hand, the results from natural chemiluminescence also showed agreement with the ignition delay from the pressure signals. A 310 nm interference filter was used in order to detect the chemiluminescence of the OH* radical. In fact, the maximum area and peak intensity of the chemiluminescence measured during the combustion showed that the process of auto-ignition is generalized in the whole chamber. Moreover, the correlation of peak intensity, maximum area and ignition delay time demonstrated that natural chemiluminescence can also be used to calculate ignition delay times under different operating conditions. Finally, the area of chemiluminescence was proved to be more dependant on the fuel and ignition delay time than on the operating conditions. (C) 2015 Elsevier Ltd. All rights reserved.The authors would like to thank different members of the LAV team of the ETH-Zurich for their contribution to this work. The authors are grateful to the Universitat Politecnica de Valencia for financing the Ph.D. studies of Vera-Tudela (FPI SP1 Grant 30/05/2012) and his stay at ETH-Zurich (grant 30/12/2014). Finally, the authors would like to thank the Spanish Ministry of Education for financing the Ph.D. studies of Lopez-Pintor (Grant FPU13/02329) and his stay at ETH-Zurich (Grant EST14/00626).Desantes Fernández, JM.; García Oliver, JM.; Vera-Tudela-Fajardo, WM.; López Pintor, D.; Schneider, B.; Boulouchos, K. (2016). Study of ignition delay time and generalization of auto-ignition for PRFs in a RCEM by means of natural chemiluminescence. Energy Conversion and Management. 111:217-228. https://doi.org/10.1016/j.enconman.2015.12.052S21722811

Experimental study of the ignition of lean methane/air mixtures using inductive and NRPD ignition systems in the pre-chamber and turbulent jet ignition in the main chamber

The optimization of the combustion process in spark ignition engines has led to novel strategies to ignite lean and low reactivity mixtures, such as the application of turbulent jet ignition and Nanosecond Repetitively Pulsed Discharge (NRPD) ignition systems and Turbulent Jet Ignition (TJI). In the present experimental work, an optically accessible setup was used to investigate the ignition occurrence and early flame development in the pre-chamber and subsequent main chamber ignition of methane/air mixtures at density conditions relevant to engine application. Two schlieren setups coupled with fast recording cameras allow the visualization of both the pre-chamber and main chamber. NRPD ignition with different pulse patterns in terms of pulse number and repetition frequency is applied in the pre-chamber. Conventional inductive ignition and NRPD-assisted ignition are compared for the first time in terms of flame development in the pre-chamber and hot jet ignition in main chamber. The effect of different air to fuel ratios is assessed in both laminar and turbulent pre-chamber conditions. Results show that while NRPD is always advantageous when compared to standard inductive ignition, the higher amount of energy added by increasing the number of pulses only affects the ignition success in very lean conditions, thus showing that increased energy is not the main governing mechanism for successful ignition. Irrespectively of the initial conditions (laminar or turbulent) in the pre-chamber, it is shown that an optimum jet velocity exists that ensures increased ignition probability in the main chamber, thus suggesting that mixing plays a major role in main chamber ignition. The combination of both techniques, NRPD and TJI, allowed assessing the optimum conditions in terms of number and frequency of NRPD pulses for a reliable ignition in lean conditions.ISSN:0196-8904ISSN:1879-222

One-dimensional diesel spray modeling of multicomponent fuels

[EN] The present work reports a one-dimensional model to predict the liquid length and spray penetration of diesel sprays when using blends of single-component fuels. A high-pressure liquid-vapour equilibrium has been implemented by means of fugacity coefficients, together with the hypothesis of a real-gas mixture to calculate the partial enthalpy of each component. The model has been validated in a first step by means of an experimental study using binary blends of n-decane and n-hexadecane, where the temporal evolution of the liquid length and vapor spray penetration have been measured. Results show that the model predicts adequately the spray penetration for the different fuels at various conditions. A six-component fuel has also been investigated. Results indicate that this fuel has a very similar evaporative behavior to n-hexadecane, which is confirmed by both experiments and model predictions.Part of this work has been developed within the frame of project B03T02 Modelling of Emission Formation and Exhaust Gas Aftertreatment, Multicomponent diesel Combustion Modelling and Validation with financial support of the "COMET K2-Competence Centres for Excellent Technologies Programme" of the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT), the Austrian Federal Ministry of Economy, Family and Youth (BMWFJ), the Austrian Research Promotion Agency (FFG), the Province of Styria, and the Styrian Business Promotion Agency (SFG). Thanks are also given to the supporting industrial and scientific project partners, namely Kompetenzzentrum-Das Virtuelle Fahrzeug Forschungsgesellschaft mbH (ViF), AVL List GmbH, OMV Refining and Marketing GmbH and Institute for Internal Combustion Engines and Thermodynamics (IVT) Graz University of Technology. Support for this research was partially provided by the Generalitat Valenciana inside the program Ajudes per a la realitzacio de projectes d'I+D per a grups de investigacio emergent (reference GV/2013/041), which is gratefully acknowledged. Also, the authors would like to thank the Pontificia Universidad Catolica del Peru for financing the first year of studies of W. Vera-Tudela and making it possible for him to start his program of PhD at the Universitat Politecnica de Valencia.Pastor Soriano, JV.; García Oliver, JM.; Pastor Enguídanos, JM.; Vera-Tudela Fajardo, WM. (2015). One-dimensional diesel spray modeling of multicomponent fuels. Atomization and Sprays. 25(6):485-517. https://doi.org/10.1615/AtomizSpr.2014010370S48551725