Rozprawy doktorskie na temat „Homogeneous combustion”

The work presented in this thesis is an investigation of the dynamics of unconfined hydrogen-air flames in the presence of buoyant effects and the determination of an ignition criterion for flame propagation between adjacent pockets of reactive gas separated by air. The experimental work was conducted using the soap bubble technique and visualized with high speed schlieren or large scale shadowgraph systems. A study was first conducted to determine the most suitable soap solution additive among glycerol, guar and polyethylene oxide for conducting the experiments, isolating guar as the best candidate. The soap solution was then used to study the dynamics of flames in single or multiple soap bubbles filled with reactive mixtures of different compositions. The soap bubble method was also further improved by designing a soap dispenser that can maintain a bubble indefinitely and a method to burst the soap solution prior to an experiment using timed heated wires. In the experiments with single bubbles, it was found that for sufficiently lean hydrogen-air mixtures, buoyancy effects become important at small scales. The critical radius of hemispherical flames that will rise due to buoyancy was measured and estimated using a model comparing the characteristic burning speed and the rise speed of the flame kernel. Excellent agreement was found between the model predictions and the measured critical flame radii. The experiments with multiple bubbles provided the scaling rules for flame transition between neighboring pockets of hemispherical or spherical shape separated by an inert gas. The test results demonstrated that the separation distance between the bubbles is mainly determined by the expansion ratio when the buoyancy effects are negligible, corresponding to near stoichiometric mixtures. For leaner mixtures with stronger buoyant effects, the critical separation distance was no longer governed by the expansion ratio alone, as buoyancy forces render the flame propagation across the inert gas more difficult. Visualization of the ignition dynamics confirmed that buoyancy forces tend to accelerate the first kernel up before ignition of the second kernel can be achieved.

CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Com o intuito de reduzir as emissões e melhorar a combustão em uma maior faixa de rotação e carga de um motor, foi proposto o estudo da combustão por compressão de misturas homogêneas (HCCI), este processo apresenta altas eficiências e baixas emissões, principalmente de NOx e fuligem. Assim, o objetivo do presente trabalho é a determinação das faixas de operação estável em um motor diesel, de alta taxa de compressão (20:1). O combustível utilizado foi gasolina tipo A, tendo em vista a sua grande produção, além das características de auto-ignição. Para atingir o objetivo proposto foram controladas a temperatura de entrada do ar e a quantidade de combustível da mistura, o que foi implementado sem modificação estrutural do motor. Os ensaios foram realizados com uma temperatura de alimentação entre 75 e 95 ºC, com rotação entre 1200 e 2200 RPM. Os valores para o fator lambda variaram, em função de um processo de combustão estável, entre 2 e 4. São apresentados os resultados experimentais obtidos em um dinamômetro de bancada, sobre os quais se fez uma análise do rendimento, para a faixa de melhor estabilidade da combustão. Para a mesma faixa foi realizada uma análise das curvas de pressão x tempo, caracterizando a auto-ignição como função da temperatura do ar e da riqueza da mistura. Os melhores rendimentos encontrados situam-se ao redor de 36,5 %, para uma temperatura de ingresso de 85 °C, para as maiores rotações pesquisadas.
The present study of homogeneous mixture compression ignition (HCCI) was proposed in order to reduce emissions and improve combustion at a higher speed range and load, this process has high efficiency and low emissions mainly NOx and soot. Therefore, the aim of this study was to determine the ranges of stable operation in a diesel engine of high compression ratio (20:1), operating in HCCI. The fuel used was gasoline type A, given its large production, besides the good characteristics of auto-ignition. To achieve this purpose were controlled inlet air temperature and the amount of fuel in the mixture, these were implemented without structural modification of the engine. The tests were conducted with a feed temperature between 75 and 95 ° C, with rotation between 1200 and 2200 RPM. The values for the lambda factor varied between 2 and 4, as a function of a stable combustion process. The experimental results here reported were obtained on a dynamometer bench, on which, it was made a performance analysis for the better stability combustion range. Additionally for this range, an analysis of the curves of pressure vs. time was performed, characterizing the auto-ignition as a function of air temperature and the richness of the mixture. The best results found are located around 36.5% at an intake temperature of 85 ° C for the highest speed studied.

The work presented in this thesis is intended to investigate the effects of fuel properties, injection strategy and timing on autoignition and combustion characteristics of a Homogeneous Charge Compression Ignition (HCCI) engine with a negative valve overlap (NVO) strategy. Conventional (pressure-transducer based) measurements and passive optical research have contributed to understanding of the chemical-physical sites of HCCI autoignition and combustion. This experimental work was undertaken on matching thermal and optical single cylinder research engines in configurations derived from a production Jaguar V8 engine. A thermal engine study using a range of fuels including conventional gasoline and primary reference fuels has been performed to gain insight into autoignition and combustion characteristics of various chemically dissimilar blends or components. This was done at different operating conditions by varying the engine speed and the proportion of residuals trapped. These measurements have shown that the autoignition and combustion characteristic of an HCCI operated engine are highly dependent on fuel blend composition and are also affected by engine operating conditions. It was found that the autoignition process type which the mixture undergoes, whether it is one- or two-step, depends very strongly both on fuel blend composition and on engine operating conditions. More specifically the presence and also proportion of particular chemical compounds in a blend could significantly contribute to the alteration of the process type. Similar experiments using the chosen engine operating points were repeated on the optical engine using passive optical diagnostics such as imaging and spectroscopy. Thereby it was possible to gain insight into the chemistry of one-step and two- step ignition processes. The image analysis of the port fuel injected (PFI) HCCI operation have been carried out for stoichiometric and lean conditions. A crank-angle resolved high-speed imaging technique was employed a piston crown window for optical access to the combustion chamber. The spatial repeatability nature of autoignition occurrence and the directions of combustion progress were evaluated using especially developed image processing technique. The insight into the expansion rates of burned areas and of the spreading velocities of reacting structures fronts was also gained by introducing two new image processing techniques. Various direct injection strategies (single and split injection) and timings, including fuel injection prior to and during the negative valve overlap period were optically investigated. The comprehensive study included the application of three diagnostic instruments: the Complementary Metal-Oxide Semiconductor (CMOS) high-speed colour imager, the intensified Charge Couple Device (CCD) and the imaging spectrograph. Among the other observations the applied passive techniques, the imaging and the spectroscopy in conjunction with adequate image processing techniques have shown that the combustion behaviour and also the colour of the burning mixture are dependent on the fuel injection scheme. With the investigated split (double) injection, when some of fuel is injected prior to TDC NVO the combustion behaviour is significantly different than when it is injected during even at TDC (NVO). There is a strong indication that a form of incandescence occurs during the NVO, which probably comes from the glowing soot. This is further supported by a quantification of the emitted luminescence and spectroscopic measurements during this phase.

Dans le contexte de la maîtrise des émissions de gaz à effet de serre et donc de la consommation des carburants, le moteur Diesel occupe une place privilégiée grâce à son meilleur rendement de combustion. Toutefois, les émissions de NOx et de particules sont un de ses handicaps. Pour limiter les émissions de ces polluants à l'échappement, les combustions basses températures comme la combustion HCCI (Homogeneous Charge Compression Ignition) représentent des alternatives prometteuses. La combustion HCCI est basée sur l'auto inflammation d'un mélange pauvre air/carburant dilué par de l'EGR. Les émissions de NOx et de particules diminuent d'un facteur de 10 à 100. Cependant, la zone de fonctionnement du moteur restreinte, le contrôle délicat de la combustion, le niveau de bruit et les émissions d'HC et de CO doivent être améliorés pour permettre un développement important de ces nouveaux modes de combustion. Une formulation adaptée du carburant fait partie des axes de progrès. L'objectif de cette thèse est justement de comprendre l'impact de la chimie du carburant pour améliorer le contrôle et l'initiation de la combustion HCCI. Trois dispositifs expérimentaux sont utilisés : un réacteur auto agité, une machine à compression rapide et un banc moteur. L'impact de différentes familles chimiques telles que des oléfines, des acétals et des naphtènes a été étudié. Cette thèse a mis en évidence l'influence de la chimie du carburant sur la combustion HCCI. Ainsi, il a été démontré que les doubles liaisons des oléfines ou bien encore les tensions cycliques des naphtènes constituaient des critères chimiques clés pour améliorer l'initiation et la propagation initiale de la combustion.

HCCI engines are a class of engines which use high compression ratio to ignite a charge of air-fuel mixture, essentially eliminating the need for spark plugs. This contrasts with diesel engines (although HCCI can be used for diesel engines) where the fuel is injected near the top dead center of the compression stroke regime. Gasoline HCCI engines are of significance because, it attempts to improve the characteristics of the engine for example the thermal efficiency. High compression ratio comes with higher thermal efficiency, yet the peak temperature remains low enough to have low production rates of harmful oxides of nitrogen and formation of soot. However, there are certain challenges associated with such type of engine, one of which and perhaps the most important of all is how to control the combustion rate. Flow dynamics and chemical-kinetics analysis, is essential to predict combustion timing, duration, and rate. The objective of this study is to analyze a HCCI engine using, simulation analysis models including a three-dimensional CFD simulation model. Simulation analysis is carried out using a generic HCCI engine, initially with simplified chemical kinetics, and then using detailed chemical kinetics and using RANS turbulence CFD model. A sensitivity analysis of the effect of RPM on the combustion time, burn duration, heat release, efficiency and emission concentration are carried out.

A one-dimensional, steady-state numerical model of the combustion of homogeneous solid propellant has been developed. The combustion processes is modeled in three regions: solid, two-phase (liquid and gas) and gas. Conservation of energy and mass equations are solved in the two-phase and gas regions and the eigenvalue of the system (the mass burning rate) is converged by matching the heat flux at the interface of these two regions. The chemical reactions of the system are modeled using a global kinetic mechanism in the two-phase region and an elementary kinetic mechanism in the gas region. The model has been applied to RDX, HMX and GAP. There is very reasonable agreement between experimental data and model predictions for burning rate, temperature sensitivity, surface temperature, adiabatic flame temperature, species concentration profiles and melt-layer thickness. Many of the similarities and differences in the combustion of RDX and HMX are explained from sensitivity analysis results. The combustion characteristics of RDX and HMX are similar because of their similar chemistry. Differences in combustion characteristics arise due to differences in melting temperature, vapor pressure and initial decomposition steps. A reduced mechanism consisting of 18 species and 39 reactions was developed from the Melius-Yetter RDX mechanism (45 species, 232 reactions). This reduced mechanism reproduces most of the predictions of the full mechanism but is 7.5 times faster. Because of lack of concrete thermophysical property data for GAP, the modeling results are preliminary but indicate what type of experimental data is necessary before GAP can be modeled with more certainty.

Actuellement, les principaux thèmes pour le secteur de transport sont le réchauffement global et la crise énergétique, ce qui encourage les chercheurs à développer des technologies alternatives et efficaces. Le concept ‘HCCI’ (combustion d’une charge homogène, allumée par compression) est l’une des solutions pour le moteur de véhicules. Ce mode de combustion, indépendant d’une notion de propagation de flamme, permet de réduire fortement les émissions critiques de NOX et de suies dans les gaz d'échappement. Cette combustion de type HCCI du carburant diesel se caractérise par une combustion à deux étapes. Parallèlement, l’apparition de nouveaux carburants, comme le bio-alcool, est une autre voie de recherche. Les bio-alcools ont un nombre d’indice d'octane élevé qui peut se mélanger avec du carburant diesel pour optimiser la combustion de HCCI des carburants diesel. L’objectif de cette thèse est donc de caractériser les deux étapes de la combustion HCCI en étudiant l’influence de l’impact de l’ajout d’une fraction d’alcools dans diesel. La comparaison avec un mélange d’iso-octane, hydrocarbure à indice d'octane élevé de paraffine et des mélanges dilués via les gaz d’échappement est aussi analysée en tant que verrous potentiels pour améliorer la combustion de type HCCI. Dans cette thèse, le n-heptane est choisi comme composé principal représentatif du diesel, l'éthanol et 1-butanol sont choisis comme bio-alcools. L’analyse présentée ici se repose sur trois approches différentes : l’analyse expérimentale de la pression cylindre, l'analyse d'images de chimiluminescence spontanée de certaines espèces et les résultats issus de la modélisation cinétique de la combustion
Currently, the major issues for the transportation sector are the global warming and energy crisis which encourage researchers to develop an alternative green efficient technology. The homogeneous charge compression ignition (HCCI) can be one of solutions for the automotive engine. This combustion concept is independent on the high temperature flame propagation which releases lowest critical emissions (NOX and PM) in the exhaust gas. HCCI combustion of diesel fuel presents specific characteristic of two-stage ignition that over-advances the main heat release. As the importance of bio-alcohol fuels increases, it is interesting to evaluate the potential of the fuels, to optimize the HCCI combustion of diesel fuels. This is the objective of this phD thesis. The two-stage ignition characteristic of the diesel hydrocarbon is described and the influence of alcohol fuel fraction in diesel blends is investigated in comparison with high octane paraffin hydrocarbon diesel blends and EGR addition. All potentials are concluded to the potential for HCCI combustion improvement. In this thesis, n-heptane was selected as the major diesel representative component and ethanol and 1-butanol as the considered alcohol fuels. Three approaches were used based on experimental cylinder pressure analysis, the chemiluminescence emissions image analysis and the chemical kinetic analysis results from the engine modeling. A detailed chemical kinetic scheme was specifically developed from sub-scheme of all considered fuel

Une des solutions très prometteuses pour la réduction des émissions polluantes et de la consommation des moteurs à combustion interne est la mise en place de combustions alternatives pour une optimisation à la source. En particulier, de nombreuses études sont menées autour du développement d’une combustion en charge homogène par auto inflammation appelée « Homogeneous Charged Compression Ignition ». Cependant, le contrôle de ce mode de combustion nécessite une recherche avancée incluant une approche interdisciplinaire associant la chimie, la mécanique des fluides et les contraintes d’un fonctionnement moteur. Cette thèse a pour objet l’étude de l’auto inflammation en mode homogène traité d’un point de vue numérique et expérimental. L'étude numérique s’intéresse au développement de mécanismes réduits de carburants de référence. Ces mécanismes réduits sont introduits dans le cadre de modèles plus généraux simulant un fonctionnement de cycle moteur, se focalisant sur trois carburants de référence ; le n-heptane, l'isooctane et le toluène. Trois mécanismes et un mécanisme « mélange » sont ainsi proposés et font l’objet tout d’abord d’une validation numérique et ensuite d’une validation expérimentale. Le mécanisme final « mélange » est composé de 62 réactions et de 49 espèces. En outre, l'interaction des oxydes d’azote NO avec les hydrocarbures est incluse par un appendice cinétique. La validation expérimentale est effectuée dans une gamme de paramètres respectant les conditions de fonctionnement d’un mode HCCI. Les gammes de température d'admission sont comprise entre 35 et 70 °C, avec une richesse entre 0,28 et 0,64 et des taux de compression entre 7 et 14, et ceci pour différentes teneurs des 3 composées de base formant le carburant. En particulier, les carburants testés présentent une gamme complète de proportion de n-heptane/iso-octane. En ce qui concerne le cas des mélanges de n-heptane/toluène, la valeur maximum étudiée du pourcentage volumique de toluène est de 40 %. Des analyses comparatives entre l'essence, le gasoil et leurs substituts de référence sont proposées. En outre, l'effet de NO sur le processus d'auto-inflammation est étudié jusqu'à une addition de 170 ppm. Une étude complète des effets de l’introduction d’EGR est présentée avec en particulier les effets de la dilution par N2 et CO2, les effets du niveau thermiques sur une gamme de 30 à 120 °C, et les effets chimiques de certaines espèces comme le CO, le CH2O et de CH3CHO. L'analyse expérimentale a été soutenue par le mécanisme validé. De cette analyse aboutit un ensemble de conclusions en ce qui concerne le contrôle du processus d'auto-inflammation en mode HCCI.

This thesis is about modelling of the combustion and emissions of dual fuel HCCI engines for design of “engine combustion system”. For modelling the combustion first the laminar flamelet model and a hybrid Lagrangian / Eulerian method are developed and implemented to provide a framework for incorporating detailed chemical kinetics. This model can be applied to an engine for the validation of the chemical kinetic mechanism. The chemical kinetics, reaction rates and their equations lead to a certain formula for which the coefficients can be obtained from different sources, such as NASA polynomials [1]. This is followed by study of the simulation results and significant findings. Finally, for investigation of the knock phenomenon some characteristics such as compression ratio, fuel equivalence ratio, spark timing and their effects on the performance of an engine are examined and discussed. The OH radical concentration (which is the main factor for production of knock) is evaluated with regard to adjustment of the above mentioned characteristic parameters. In the second part of this work the specification of the sample engine is given and the results obtained from simulation are compared with experimental results for this sample engine, in order to validate the method applied in AVL Fire software. This method is used to investigate and optimize the effects of parameters such as inlet temperature, fuels ratio, diesel fuel injection timing, engine RPM and EGR on combustion in a dual fuel HCCI engine. For modelling the dual fuel HCCI engine AVL FIRE software is applied to simulate the combustion and study the optimization of a combustion chamber design. The findings for the dual fuel HCCI engine show that the mixture of methane and diesel fuel has a great influence on an engine's power and emissions. Inlet air temperature has also a significant role in the start of combustion so that inlet temperature is a factor in auto-ignition. With an increase of methane fuel, the burning process will be more rapid and oxidation becomes more complete. As a result, the amounts of CO and HC emissions decrease remarkably. With an increase of premixed ratio beyond a certain amount, NOX emissions decrease. With pressure increases markedly and at high RPM, knock phenomenon is observed in HCCI combustion.

Depuis plusieurs années, l'une des problématiques sociétales est de diminuer les émissions de polluants et de gaz à effet de serre dans l'atmosphère. Le secteur du transport terrestre est directement concerné par ces considérations. Le moteur Diesel semble promis à un bel avenir grâce à son rendement supérieur à celui du moteur à allumage commandé, conduisant à de plus faibles rejets de CO2. Cependant, sa combustion génère des émissions d'oxyde d'azote (NOx) et de particules dans l'atmosphère. Les normes anti-pollution étant de plus en plus sévères et les incitations à diminuer les consommations de carburant de plus en plus fortes, le moteur Diesel est confronté à une problématique NOx/particules/consommation toujours plus difficile à résoudre. Une des voies envisagées consiste à modifier le mode de combustion afin de limiter les émissions polluantes à la source tout en conservant de faibles consommations. La voie la plus prometteuse est la combustion HCCI (Homogeneous Charge Compression Ignition) obtenue par injections directes précoces. Plusieurs limitations critiques doivent cependant être revues et améliorées : le mouillage des parois par le carburant liquide et le contrôle de la combustion à forte charge. Le but de cette thèse est ainsi de mieux comprendre les phénomènes mis en jeu lors de la combustion HCCI à forte charge obtenue par des multi-injections directes précoces. Une méthodologie a été mise au point afin de détecter le mouillage des parois du cylindre, ce qui a permis de comprendre l'effet du phasage et de la pression d'injection sur cette problématique. Une stratégie optimale de multi-injections permettant d'atteindre une charge élevée sans mouiller les parois a ainsi été développée et choisie. Nous avons ensuite pu mettre en évidence le potentiel de la stratification par la dilution en tant que moyen de contrôle de la combustion en admettant le diluant dans un seul des 2 conduits d'admission. Des mesures réalisées en complémentarité sur le même moteur mais en version 'optique', ont permis, à partir de la technique de Fluorescence Induite par Laser, de montrer que concentrer le diluant dans les zones réactives où se situe le carburant permet un meilleur contrôle de la combustion, ce qui permet d'amener le taux de dilution a des niveaux faisables technologiquement.

Dans un contexte de recherche de nouveaux modes de combustion propres, la combustionhomogène à allumage par compression HCCI s’inscrit comme une stratégie prometteuse.Cependant, cette combustion est limitée par un niveau élevé de bruit. La recherche descarburants permettant de relaxer cette contrainte constitue l’objectif global de cette étude.Particulièrement, on s’intéresse ici à l’influence de l’Indice de Cétane, de la volatilité et de lacomposition chimique des carburants sur les Délais d’Auto-Inflammation et sur les vitesses decombustion globales évaluées par les taux maximaux d’accroissement de la pression et dudégagement d’énergie apparente. L’étude se base dans un premier temps sur l’analyse d’essaissur banc moteur dans lesquels on a testé plusieurs carburants de synthèse à l’état pur et enmélange avec un Gazole conventionnel. Dans un deuxième temps des essais ont été préparés etréalisés sur Machine à Compression Rapide avec deux configurations en injection directe et enmélange homogène. Les essais Moteur ont permis d’orienter les paramètres expérimentauxciblés sur ce dispositif. D’autre part, pour étudier les régimes de combustion, des mesures dechamps de température locale ont été réalisées en mélange inerte (N2, CO2, Ar) par FluorescenceInduite par Laser avec un traceur Toluène. L’étude montre les limites des paramètres habituelspour caractériser l’adéquation carburant combustion HCCI et propose un nouveau critère basésur la dépendance des délais d’auto-inflammation à la température et à la richesse
Advanced combustion strategies such as Homogeneous Charge Compression Ignition (HCCI)usually enable cleaner combustion with less NOx and Particulate Matter emissions comparedto conventional Diesel combustion. However, these strategies are difficult to implement due todifficulties related to combustion timing and burn rate control. Lately various studies have beenfocusing on extending advanced combustion functioning with new technologies and withsearching fuels properties to enable such combustion modes. This study is focused on theimpact of fuel Cetane Number, volatility and chemical composition on Ignition Delay, HeatRelease Rate and Pressure Rise Rate. The study is based on three complementary experiments.First, several synthetic fuel was tested on a research engine and analysis was focused on theHeat Release Rate. Secondly, experiments on a Rapid Compression Machine were performedto study the auto-ignition phenomena at homogeneous conditions with surrogate fuels (blendsof n-Heptane and Methyl-Cyclohexane). Analysis of the combustion regimes was supported bya study of the temperature field based on a Toluene Laser Induced Fluorescence experiment ininert (N2, CO2, Ar) mixture. Finally, the RCM was adapted to allow direct injection of fuel tostudy the auto-ignition at less homogeneous conditions. Results showed the limits of theconventional fuels properties to describe an adequate fuel formulation for the HCCI combustionmode. A new criterion based on the dependency of ignition delays to temperature and air fuelratio variations is proposed

La propagation de flammes turbulentes dans des milieux réactifs inhomogènes concerne un grand nombre d’applications pratiques, y compris celles qui reposent sur des cycles de combustion à volume constant. Les hétérogénéités de composition (richesse, température,dilution par des gaz brûlés, etc.) sont issues de plusieurs facteurs distincts tels que la dispersion du spray de gouttelettes de combustible et son évaporation, la topologie de l’écoulement ainsi que la présence éventuelle de gaz brûlés résiduels issus du cycle précédent. La structure des flammes partiellement prémélangées qui en résultent est significativement plus complexe que celles des flammes plus classiques de diffusion ou de prémélange. L’objectif de ce travail de thèse est donc de contribuer à l’amélioration de leur connaissance, en s’appuyant sur la génération et l’analyse de base de données de simulations numériques directes ou DNS (Direct Numerical Simulation). Celles-ci sont conduites avec le code de calcul Asphodele qui est basé sur l’approximation de faible nombre de Mach. Le combustible de référence retenu est l’iso-octane.La base de données est structurée suivant cinq paramètres qui permettent de caractériser l’écoulement turbulent ainsi que l’hétérogénéité de composition du milieu réactif. Dans un premier temps, des configurations bidimensionnelles ont été considérées en raison du coût élevé induit par la description détaillée de la cinétique chimique. L’étude des ces différents cas de calcul a permis de mettre en lumière plusieurs mécanismes fondamentaux de propagation dans les milieux hétérogènes en composition. Une réduction significative des coûts de calcula pu ensuite être obtenue grâce au développement d’un modèle chimique simplifié optimisé.Son utilisation a permis d’étendre les analyses à de
The propagation of turbulent flames in non-homogeneous reactive mixtures of reactants concerns a large number of practical applications, including those based on constant volume combustion cycles. The composition heterogeneities (equivalence ratio, temperature, dilution by burnt gases, etc.) result from several distinct factors such as the dispersion of the spray of fuel droplets and its evaporation, the flow field topology as well as the possible presence of residual burnt gases issued from the previous cycle. The resulting partially premixed flames structure is significantly more complex than the one of more conventional diffusion or premixed flames.The aim of this thesis work is therefore to contribute to the improvement of their understanding, by proceeding to the generation and analysis of a new set of direct numerical simulations (DNS) databases. The present computations are performed with the low-Mach number DNS solver Asphodele. The database is structured according to five parameters that characterize the turbulent flow as well as the composition heterogeneity of the reactive mixture. First, because of the high numerical costs induced by the detailed description of chemical kinetics, two-dimensional configurations were considered. The study of these various simulations highlights several fundamental mechanisms of flame propagation in heterogeneous mixtures. Then, a significant computational cost saving has been achieved through the development of an optimized simplified chemistry model. The use of the latter allowed to overcome the major bottleneck of high CPU costs related to chemical kinetics description and thus to extend the analysis to three-dimensional configurations. Some of the conclusions obtained previously were reinforced

Les reactions chimiques preponderantes dans differentes zones sont determinees et conduisent a la formation d'un schema global. La stoechiometrie et les parametres cinetiques sont determines et introduits dans la resolution des equations des quantites transportables dans la zone reactive. Ils conduisent a la determination numerique de la vitesse de combustion normale de ce type de propergol. Evaluation numerique de l'action des additifs de plomb sur la combustion du propergol, a differentes pressions

[ES] El trabajo de investigación presentado en esta tesis es el resultado de varios años dedicados al desarrollo, la implementación y la optimización de dos tecnologías combinadas: un concepto de combustión innovador y una arquitectura de motor de nuevo diseño. Esta investigacion se ha realizado en el marco de una colaboración con Renault SA, como continuación de las actividades realizadas en el proyecto europeo POWERFUL (POWERtrain for FUture Light-duty vehicles) por un lado,y en el marco del proyecto europeo REWARD (Real World Advanced technologies foR Diesel engines), devenido como continuación del proyecto POWERFUL en el marco del programa de investigación Horizonte 2020, por otro lado. Los principales objetivos de estos estudios eran evaluar el potencial del concepto de combustión parcialmente premezclada (PPC) operando con gasolina como combustible en un innovador motor de 2 tiempos de válvulas en culata, y luego diseñar una nueva geometría de motor de 2 tiempos utilizando la arquitectura Uniflujo para superar los principales problemas y limitaciones observados durante la primera etapa, que se pueden resumir principalmente en el rendimiento de barrido (especialmente trabajando en cargas elevadas). La metodología diseñada para este trabajo de investigación sigue un enfoque teórico-experimental. La evaluación del concepto de combustión PPC operando con gasolina se llevó a cabo principalmente con un enfoque experimental con el apoyo del análisis en línea directamente en el banco de ensayo, seguido de un exhaustivo tratamiento posterior de los datos y de un análisis detallado del proceso de combustión utilizando herramientas de diagnóstico. Por el contrario, el desarrollo del nuevo motor Uniflujo de 2 tiempos consistió principalmente en iteraciones sobre modelado 3D-CFD, si bien las actividades experimentales fueron fundamentales para validar las diferentes soluciones propuestas y evaluar su sensibilidad ante diferentes parámetros de interés utilizando una metodología de Diseño de Experimentos (DoE). La primera parte del trabajo se ha dedicado a la comprensión de los procesos termodinámicos involucrados en la combustión operando con el concepto PPC en un motor de 2 tiempos de válvulas en culata utilizando gasolina como combustible, y a evaluar su potencial en términos de emisiones contaminantes, consumo de combustible y ruido. Por último, se ha realizado un trabajo de exploración para ampliar en la medida de lo posible el rango de funcionamiento de este concepto de combustión en esta configuración específica del motor, investigando especialmente el rendimiento en cargas bajas en todo el rango de regímenes de giro del motor, y estableciendo también las principales limitaciones para la operación en cargas altas. La segunda parte de la tesis se ha centrado en el desarrollo y optimización teórica de un motor Uniflujo de 2 tiempos de nuevo diseño, incluyendo su fabricación y validación experimental. El objetivo principal era optimizar, utilizando principalmente simulaciones 3D-CFD, el rendimiento de barrido de esta arquitectura de 2 tiempos mediante el diseño de nuevas geometrías de puertos de admisión, permitiendo un gran control sobre el flujo de aire hacia y a través del cilindro para barrer al máximo los gases quemados y minimizar el cortocircuito de aire fresco hacia el escape. Las soluciones óptimas se evaluaron experimentalmente siguiendo la metodología DoE, antes de comparar finalmente los resultados de rendimiento de barrido con la anterior arquitectura de motor de 2 tiempos con válvulas en culata.
[CA] El treball de recerca presentat en aquesta tesi és el resultat de diversos anys dedicats al desenvolupament, la implementació i l'optimització de dues tecnologies combinades: un concepte de combustió innovador i una arquitectura de motor de nou disseny. Aquesta recerca s'ha realitzat en el marc d'una col·laboració amb Renault SA, com a continuació de les activitats del projecte europeu *POWERFUL (*POWERtrain *for *FUture Light-*duty *vehicles) d'una banda, i en el marc del projecte europeu *REWARD (Real *World *Advanced *technologies *foR Dièsel *engines), es devingut com a continuació del projecte *POWERFUL en el marc del programa d'investigació Horitzó 2020, d'altra banda. Els principals objectius d'aquests estudis eren avaluar el potencial del concepte de combustió parcialment premesclada (PPC) operant amb gasolina com a combustible en un innovador motor de 2 temps de vàlvules en culata, i després dissenyar una nova geometria de motor de 2 temps utilitzant l'arquitectura Uniflux per a superar els principals problemes i limitacions observats durant la primera etapa, que es poden resumir principalment en el rendiment d'escombratge (especialment treballant en càrregues elevades). La metodologia dissenyada per a realitzar aquests treballs de recerca segueix un enfocament tant experimental com teòric. L'avaluació del concepte de combustió PPC operant amb gasolina es va dur a terme principalment amb un enfocament experimental, però sempre amb el suport de l'anàlisi en línia directament en el banc d'assaig, seguit d'un exhaustiu tractament posterior de les dades combinat amb una anàlisi detallada del procés de combustió utilitzant eines de diagnòstic. Per contra, el desenvolupament i el disseny del nou motor Uniflux de 2 temps va consistir principalment en iteracions sobre modelatge 3D-CFD, si bé les activitats experimentals van ser fonamentals per a validar les diferents solucions proposades i avaluar la seua sensibilitat davant una sèrie de paràmetres d'interés utilitzant una metodologia de Disseny d'Experiments (DoE). La primera part del treball s'ha dedicat a la comprensió dels processos termodinàmics involucrats en la combustió operant amb el concepte de combustió PPC en un motor de 2 temps de vàlvules en culata utilitzant gasolina com a combustible, i a avaluar el seu potencial en termes d'emissions contaminants, consum de combustible i també de soroll. Finalment, s'ha fet un treball d'exploració per a ampliar en la mesura que siga possible el rang de funcionament d'aquest concepte de combustió utilitzant eixa configuració específica del motor, investigant especialment el rendiment en càrregues baixes en tot el rang de règims de gir del motor, i establint també les principals limitacions per a l'operació en càrregues altes. La segona part de la tesi s'ha centrat en el desenvolupament i optimització teòrica d'un motor Uniflux de 2 temps de nou disseny, incloent la seua fabricació i validació experimental. L'objectiu principal era optimitzar, utilitzant principalment simulacions 3D-CFD, el rendiment d'escombratge d'aquesta arquitectura de 2 temps mitjançant el disseny de noves geometries de ports d'admissió, permetent un gran control sobre el flux d'aire cap a i a través del cilindre per a escombrar al màxim els gasos cremats i minimitzar el curtcircuit d'aire fresc cap a l'escapament. Les solucions òptimes es van fabricar i van avaluar experimentalment seguint la metodologia DoE, abans de comparar finalment els resultats de rendiment d'escombratge amb l'anterior arquitectura de motor de 2 temps amb vàlvules en culata.
[EN] The research work presented in this thesis is the result of several years dedicated to the development, implementation and optimization of two combined technologies: an innovative combustion concept and a newly designed engine architecture. These investigations have been performed in the framework of a research collaboration with Renault SA following up the activities performed along the European POWERFUL project (POWERtrain for FUture Light-duty vehicles) on the one hand, and in the framework of the European REWARD project (REal World Advanced technologies foR Diesel engines), brought as a continuation of the POWERFUL project in the frame of the Horizon 2020 research program, on the other hand. The main objectives of these studies were to evaluate the potential of the Partially Premixed Combustion (PPC) concept operating with gasoline fuel in an innovative 2-Stroke poppet-valve engine, and then to design a new 2-Stroke engine geometry using the Uniflow architecture to overcome the main problems and limitations observed during the first stage, which can be mainly summarized to the scavenging performance (especially at high loads). The methodology designed for performing these investigation is based on both experimental and theoretical approaches. The evaluation of the gasoline PPC concept was carried out mainly experimentally, but always supported by online analysis directly on the test-bench and followed by a thorough post-processing of the data combined with a detailed analysis of the combustion using combustion diagnostic tools. On the contrary, the development and design of the new 2-Stroke Uniflow engine consisted mainly of 3D-CFD iterations, but experimental testing was crucial to validate the different solutions proposed and evaluate their sensitivity to a set of parameters of interest using a Design of Experiments (DoE) methodology. The first part of the work has been dedicated to the understanding of the thermodynamical processes involved in the combustion in a poppet-valve 2-Stroke engine operating with the gasoline PPC concept, and to evaluate its potential in terms of pollutant emissions, fuel consumption and also noise. Finally, a wide exploration has been performed to extend as much as possible the operating range of this combustion concept using that specific engine configuration, especially investigating the low loads performance throughout the full range of engine speeds, and also laying out the main limitations for high-to-full load operations. The second part of the thesis has been focused on the development and theoretical optimization of a newly designed 2-Stroke Uniflow engine, leading to manufacture and experimental validation. The main objective was to optimize, using mainly 3D-CFD modeling simulations, the scavenging performance of this 2-Stroke architecture by designing new intake ports geometries and to enable a great control over the air flow into and through the cylinder in order to scavenge the burnt gases as much as possible while minimizing the fresh air short-circuit to the exhaust. The optimum solutions were then manufactured and experimentally tested following a DoE methodology, before finally comparing the results of the scavenging performance to the previous 2-Stroke poppet-valve engine architecture.
Thein, KJL. (2021). Evaluation of combustion concepts and scavenging configurations in a 2-Stroke compression-ignition engine for future automotive powerplants [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/164044
TESIS

In this dissertation, the evolution of a pressure wave driven by a harmonic signal on the boundary during gas combustion is studied. The problem is modeled by a nonlinear, hyperbolic partial differential equation. Steady-state behavior is investigated using the perturbation method to ensure that enough time has passed for any transient effects to have dissipated. The zeroth, first and second-order perturbation solutions are obtained and their moduli are plotted against frequency. It is seen that the first and second-order corrections have unique maxima that shift to the right as the frequency decreases and to the left as the frequency increases. Dispersion relations are determined and their limiting behavior investigated in the low and high frequency regimes. It is seen that for low frequencies, the medium assumes a diffusive-like nature. However, for high frequencies the medium behaves similarly to one exhibiting relaxation. The phase speed is determined and its limiting behavior examined. For low frequencies, the phase speed is approximately equal to sqrt[ω/(n+1)] and for high frequencies, it behaves as 1/(n+1), where n is the mode number. Additionally, a maximum allowable value of the perturbation parameter, ε = 0.8, is determined that ensures boundedness of the solution. The location of the peak of the first-order correction, xmax, as a function of frequency is determined and is seen to approach the limiting value of 0.828/sqrt(ω) as the frequency tends to zero and the constant value of 2 ln 2 as the frequency tends to infinity. Analytic expressions are obtained for the approximate general perturbation solution in the low and high-frequency regimes and are plotted together with the perturbation solution in the corresponding frequency regimes, where the agreement is seen to be excellent. Finally, the solution obtained from the perturbation method is compared with the long-time solution obtained by the finite-difference scheme; again, ensuring that the transient effects have dissipated. Since the finite-difference scheme requires a right boundary, its location is chosen so that the wave dissipates in amplitude enough so that any reflections from the boundary will be negligible. The perturbation solution and the finite-difference solution are found to be in excellent agreement. Thus, the validity of the perturbation method is established.

La combinaison d'une technologie alternative comme l'allumage par compression de la charge (HCCI) avec une énergie renouvelable comme la biomasse est un bon compromis pour réduire les émissions polluantes à la source. L'un des problèmes majeurs de cette combustion est le contrôle du moment de l’allumage ; l'injection directe de particules solides en suspension dans le mélange réactif permet de contrôler le début de la combustion. Dans la première partie de ce travail, la combustion de la biomasse gazeuse a été étudiée expérimentalement sur une machine à compression rapide dans des conditions HCCI. En raison de la complexité de ce mode de combustion, une approche simplifiée du processus a été simulée numériquement sur Chemkin-Pro pour étudier les effets des conditions initiales et de la composition du combustible sur les aspects physico-chimiques de la combustion ainsi que sur les émissions produites. Après avoir mis en évidence les avantages de la combustion pauvre, et d’un combustible type (biogaz/gaz de synthèse) dans la réduction les émissions polluantes, une base de données a été constituée pour sélectionner une gamme optimisée de conditions pour des émissions minimales et une réactivité spécifique. La deuxième partie de ce travail a été consacrée au contrôle du moment de l'allumage. Un injecteur de particules a été conçu pour injecter directement la poudre de biomasse dans le mélange réactif. Ce système a été combiné avec la machine à compression rapide, permettant l'étude de la combustion de biomasse dans différentes conditions initiales avant son injection dans le gaz réactif. Les résultats de cette étude ont mis en lumière les effets prometteurs de l'injection de poudre sur le contrôle de la combustion homogène pauvre
Combining an alternative technology like charge compression ignition (HCCI) with renewable energy such as biomass is a good compromise to reduce pollutant emissions at the source. One major problem of this combustion is the control of ignition timing; the direct injection of solid particles in suspension into the reactive mixture may control the start of ignition. Hence, the work was divided into two parts. In the first part, gaseous biomass combustion was studied experimentally on a rapid compression machine under HCCI conditions. Because of the complexity of this combustion mode, a simplified approach of the process was simulated numerically on Chemkin-Pro to study the effects of initial conditions and fuel composition on the physico-chemical aspects of the combustion as well as the produced pollutant emissions. After highlighting the advantages of lean combustion, and a fuel type biogas/syngas in reducing pollutant emissions, a database was built to select an optimized range of conditions for minimal emissions and a specific reactivity. The second part of this work was dedicated to ignition control. A particle injector was conceived to inject biomass powder into the reactive mixture directly. This system was combined with the rapid compression machine, allowing the study of biomass combustion with different initial conditions prior to its injection into reactive gas. The results of this study shed light on the promising effects of powder injection in the control of lean homogeneous combustion

The principle of controlled auto-ignition (CAI) is to mix fuel and air homogeneously before compressing the mixture to the point of auto-ignition. As ignition occurs simultaneously, CAI engines operate with lean mixtures preventing high cylinder pressures. CAI engines produce small amounts of nitrogen oxides (NOx) due to low combustion temperatures while maintaining high compression ratios and engine efficiencies. Due to simultaneous combustion and lean mixtures, CAI engines are restricted between low and mid load operations. Various strategies have been studied to improve the load limit of CAI engines. The scope of the project is to investigate the consequences of varying valve timing, as a method to control the mixture temperature within the combustion chamber and therefore, controlling the mixture auto-ignition point. This study presents computational fluid dynamics (CFD) modelling results of transient flow, inside a 0.45 litre Lotus single cylinder engine. After a validation process, a chemical kinetics model is combined with the CFD code, in order to study in-cylinder temperatures, the mixture distribution during compression and to predict the auto-ignition timing. The first part of the study focuses on validating the calculated in-cylinder velocities. A mesh sensitivity study is performed as well as a comparison of different turbulence models. A method to reduce computational time of the calculations is presented. The effects of engine speed on charge delay and charge amount inside the cylinder, the development of the in-cylinder flow field and the variation of turbulence parameters during the intake and compression stroke, are studied. The second part of the study focuses on the gasoline mixture and the variation of the valve timing, to retain different ratios of residual gases within the cylinder. After validation of the model, a final set of CFD calculations is performed, to investigate the effects of valve timing on flow and the engine parameters. The results are then compared to a fully homogeneous mixture model to study the benefits of varying valve duration. New key findings and contributions to CAI knowledge were found in this investigation. Reducing the intake and exhaust valve durations created a mixture temperature stratification and a fuel concentration distribution, prior to auto-ignition. It resulted in extending the heat release rate duration, improving combustion. However, shorter valve timing durations also showed an increase in heat transfer, pumping work and friction power, with a decrease of cylinder indicated efficiency. Valve timing, as a method to control auto-ignition, should only be used when the load limit of CAI engines, is to be improved.

Commercial fuels are mixtures with large numbers of components. Continuous thermodynamics is a technique for modelling fuel mixtures using a probability density function rather than dealing with each discreet component. The mean and standard deviation of the distribution are then used to model the chemical reactions of the mixture. This thesis develops the necessary theory to apply the technique of continuous thermodynamics to the oxidation reactions of hydrocarbon fuels. The theory is applied to three simplified models of hydrocarbon oxidation: a global one-step reaction, a two-step reaction with CO as the intermediate product, and the four-step reaction of Müller et al. (1992), which contains a high- and a low-temperature branch. These are all greatly simplified models of the complex reaction kinetics of hydrocarbons, and in this thesis they are applied specifically to n-paraffin hydrocarbons in the range from n-heptane to n-hexadecane. The model is tested numerically using a simple constant pressure homogeneous ignition problem using Cantera and compared to simplified and detailed mechanisms for n-heptane. The continuous thermodynamics models are able not only to predict ignition delay times and the development of temperature and species concentrations with time, but also changes in the mixture composition as reaction proceeds as represented by the mean and standard deviation of the distribution function. Continuous thermodynamics is therefore shown to be a useful tool for reactions of multicomponent mixtures, and an alternative to the "surrogate fuel" approach often used at present.

Im Rahmen dieser Arbeit werden die Potenziale zur Steigerung des Gesamtwirkungsgrads von stöchiometrisch homogen betriebenen Ottomotoren in der Teillast untersucht. Im Gegensatz zur konventionellen Laststeuerung über die Drosselklappe bezeichnet die betrachtete, drosselfreie Laststeuerung die Quantitätsregelung einzig über den Hubverlauf der Gaswechselventile. Nach einer Zusammenfassung bisheriger Untersuchungen zur drosselfreien Laststeuerung werden konkurrierende Bilanzierungsverfahren von Ladungswechsel- und Hochdruckteil von 4-Takt Verbrennungsverfahren vorgestellt. Anhand theoretischer Betrachtungen folgt für eine belastbare Bewertung der Prozessgüte allein die Bilanzierung in den Grenzen der Unteren Totpunkte (UT-UT). Im ersten Teil der motorischen Untersuchungen am Vollmotor wird das effektive Potenzial mechanisch variabler Ventiltriebe ermittelt. Dabei bleibt die Verbrauchsverbesserung gegenüber einem gedrosselten Referenzmotor aufgrund sinkender Restgasverträglichkeit als Folge einer nachteiligen Abnahme der Ladungsbewegung hinter den Erwartungen zurück. Im Widerspruch zu mechanisch gekoppelten Systemen wird zur bedarfsgerechten Anpassung der Ladungsbewegung die Forderung nach maximaler Flexibilität der Ventilhubgestaltung abgeleitet. Im zweiten Teil der motorischen Untersuchungen am Einzylinder-Forschungsmotor werden die maximalen Freiheitsgrade eines nockenwellenlosen Ventiltriebs basierend auf dem Prinzip eines elektromotorischen Linearantriebs systematisch eingesetzt. Neben konstruktiven Maßnahmen zur Beeinflussung des Einströmvorgangs in den Brennraum wird die Reduzierung der Drosselverluste durch Hubverlaufsformung sowie gezielte Restgasverdünnung im Vergleich von interner zu externer Rückführung betrachtet. Der Einfluss der Gemischbildung wird über die konkurrierende Darstellung von innerer und äußerer Kraftsteinspritzung aufgezeigt. Neben den maximalen Potenzialen werden ebenso die Grenzen der Entdrosselung dargestellt. Im Gegensatz zu mechanischen Systemen gelingt zwar die Realisierung einer bedarfsgerechten Ladungsbewegung mit Hilfe vollvariabler Ventilhübe, jedoch wird eine fortgesetzte Verbrauchsverbesserung durch die Gewährleistung einer sicheren Entflammung limitiert.

Le moteur downsizé à allumage commandé constitue l'une des voies principales explorées par les constructeurs automobiles pour améliorer le rendement et réduire les émissions de dioxyde de carbone des motorisations essence. Il s'agit de combiner une réduction de la cylindrée avec une forte suralimentation afin d'améliorer le rendement du moteur, en particulier à faibles et moyennes charges. Leur mise au point est limitée par l'augmentation des combustions anormales, dont le contrôle par forte dilution peut également entraîner l'apparition de variabilités cycliques importantes. Actuellement, la compréhension des nombreux paramètres intervenant dans l'apparition de ces phénomènes et de leurs interactions, reste encore imparfaite. Dans ce contexte, l'objectif de ce travail est de contribuer à la compréhension des mécanismes impliqués dans les processus de propagation des flammes turbulentes. Cette étude est réalisée dans une enceinte de combustion sphérique haute pression haute température, équipée de ventilateurs générant une turbulence homogène et isotrope. La première partie de ce travail est consacrée à l'étude de la combustion prémélangée laminaire isooctane/air. Dans un deuxième temps, l'aérodynamique de l'écoulement dans l'enceinte est finement caractérisée par Vélocimétrie Laser Doppler et Vélocimétrie par Images de Particules. Enfin, la propagation des flammes turbulentes est étudiée en termes de vitesse à partir de visualisations par ombroscopie. Une loi unifiée, permettant de décrire la propagation des flammes turbulentes indépendamment des conditions thermodynamiques initiales, de l'intensité de la turbulence et de la nature du mélange réactif est notamment proposée.

碩士
國立臺灣科技大學
機械工程系
105
With the development of science and technology and the rise of environmental awareness, people request more and more on the engine performance and pollution emissions. Many scholars pointed out that the use of “gasoline direct injection technology" (Gasoline Direct Injection, GDI) will meet the demands of less fuel consumption, improve the thermal efficiency and reduce the pollution and so on. Therefore, this paper aims to analyze the fluid motion inside GDI engine by means of numerical simulation, making it possible to find out the way to improve the GDI engine. GDI technology can reduce the fuel consumption because it can form a thin layer of gasoline in the combustion chamber, the key to the design of thin layered combustion system is the use of different fuel injection timing, so that mixed oil and air distribution layer rather than homogeneous Distribution, if the oil is concentrated in the vicinity of the plug, and other areas of chamber is relatively thin, that can be in the case of a very high overall air-fuel ratio, resulting in sufficient torque, in order to obtain lower fuel consumption. However, the chamber of the internal combustion engine is very complex and difficult to observe. Therefore, this paper uses ANSYS FLUENT to simulate the in-cylinder flow field. Continuing and improving the results of the literature[1], we first use ANSYS GEOMETRY modify the engine geometry given by the vendor, then use ANSYS MESH to divide the completed geometric model into grid, and finally enter ANSYS FLEUNT to set the numerical model and initial and boundary conditions, Then start the simulation. The simulation will be applied on cold flow field to check the energy conservation and continues law, then check the simulation results if it is in accordance to the experimental results. According to the operating conditions given by the experiment, we simulate homogeneous combustion mode of the GDI engine. Finally, we try different injection time to design the best stratified combustion system.

碩士
國立成功大學
系統及船舶機電工程學系碩博士班
94
The homogeneous charge compression ignition (HCCI) engine can enhance the thermal efficiency of DI engine and reduce the formulation of pollutant. Because the homogeneous charge compression ignition needs to adopt the formation of homogeneous mixture gas in the intake process, it is very important to analyzing the combustion performance of a homogeneous charge compression ignition engine with different intake premixed gas and its contents. This article is aimed at the KUBOTA RK-125 which is a single cylinder direct injection diesel engine, and carries out the numerical simulation. KIVA3V-RELEASE2 is used as the subject of the program, according to modifying the different intake gas contents in program adopting different proportion of the gasoline, ethanol and methanol under different engine speeds. Also the intake port and in-cylinder distribution of equivalence ratio, heatrelease rate of engine, flow and temperature field are investigated for different intake premixed gas and its contents, and the results between the simulation and the experiment will be compared . The research has already entered the gasoline, ethanol and methanol successfully in the intake process, and succeed in initialing the chemical reaction mechanism of KIVA3V-RELEASE2 using the RNG model of turbulence mixing combustion. The simulation result show that following proportion being the higher, heat of vaporization supreme methanol occur burn in lower temperature and its pressure peak value will rise because of increasing in-cylinder absorption of heat . The simulation combustion process in this research are expected to be a reference value when the close cycle diesel engine system installs with HCCI engine in the future.

碩士
國立臺灣科技大學
機械工程系
104
For the purpose of the optimal control development for a homogeneous charge compression ignition (HCCI) engine, sensitivity of the combustion timings is analyzed based on experiment data and a crank-angle-based model. The 500 c.c. single cylinder engine is equipped with an intake air heating system and an exhaust throttle. The physics-based model is first validated against the experiment data at various intake temperatures, air-to-fuel ratios (AFR) and residual gas fractions. The results show that the combustion timings are more sensitive to the in-cylinder temperature at valve closing (Tivc) in the operating condition when the intake temperature and AFR are lower. The impact from the in-cylinder gas composition, on the other hand, is negligible compared to the effect of the temperature.

碩士
國立雲林科技大學
機械工程系碩士班
91
This research is done against the YAMAHA ME200F three cylinders direct injection diesel engine. The engine exhaust emissions, the cylinder pressure, and the heat releases characteristics are measured and analyzed to study the effects of gasoline injection in the intake manifold on the engine combustion generosity. The best fuel injection angle and the optimized 92 CPC gasoline injected rate are studied under balanced and unbalanced total fuel energy consumed in the diesel engine in order to understand this relatively homogeneous combustion phenomena. The first part of this research the gasoline fuel is injected into the intake manifold. Since the fuel energy is increased, the engine speed and torque output are increased as expected. The second part of this research the injected diesel fuel amount is reduced (under a fixed engine speed) according the fuel heating value contained in the gasoline fuel. Thus, the total fuel energy (including the diesel and the gasoline fuel) injected into the cylinder are remained the same. The measurements indicate the best angle of fuel injection in the intake manifold is 64°BTDC. The NOx and Smoke emissions are obviously improved in this fuel injected into the manifold study. The NOx is reduced because the relatively homogeneous temperature distribution in the diesel engine combustion chamber. The smoke is reduced because the non-homogeneous environment has been improved. However, the CO and HC emissions are deteriorated because the HC vapor is surrounding the cylinder liner boundary layer area. The engine performance under low speed and low load is suffered because the failure of supplementary fuel ignition and worse fuel-air mixing. If the engine speed and load are increased, this disadvantage of gasoline injection into intake manifold is adjusted.

Thesis (M.S.)--University of Wisconsin--Madison, 2001.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 208-211).

博士
國立臺北科技大學
機電學院機電科技博士班
107
The efficiency of an internal combustion engine is essential for automobiles and motorcycles. Several studies have demonstrated that homogeneous charge compression ignition is a promising technology for realizing high engine efficiency and low emissions. This study investigated the combustion characteristics of HCCI using a motorcycle engine with various fuels including kerosene, n-heptane, and dimethyl ether (DME) which has lower autoignition temperature and higher cetane number. The engine performance, combustion characteristics, and thermal efficiency were analyzed from experimental data. The dual fuel and exhaust gas recirculation were incorporated to expand the engine operating range. From the heat release analysis, two-stage combustion is clearly observed. And the test results revealed that better combustion pattern can be achieved by adjusting air-fuel ratio, the rates of dual fuel, and exhaust gas recirculation. The HCCI engine performs with lower brake specific fuel consumption and lower CO and NO emissions compared with the original SI engine. Energy balance analysis revealed that lower heat loss due to low cylinder gas temperature of lean combustion contributed to higher efficiency. Air cycle simulation with MFB of double-Wiebe function executed indicates that a better combustion pattern led to higher thermal efficiency. The HCCI engine was also installed in a range-extended electric motorcycle and displayed 132% fuel consumption improvement compared with the original motorcycle of spark ignition engine.

Vapor generation is central to the flow dynamics within fuel injector nozzles. Because the degree of atomization affects engine emissions and spray characteristics, quantification of phase change within diesel fuel injectors is a topic of design interest. Within the nozzle, the large pressure gradient between the upstream and downstream plena induce large velocities, creating separation and further pressure drop at the inlet corner. When local pressure in the throat drops below the fluid vapor pressure, phase change can occur with sufficient time. At the elevated temperatures present in diesel engines, this process can be dependent upon the degree of superheat, motivating the modeling of heat transfer from the wall. By modeling cavitation and flash boiling phenomena as a departure from equilibrium conditions, the HRMFoam model accurately reproduces canonical adiabatic flows. An experimentally determined relaxation time controls the rate at which vapor is generated, and includes model constants tuned for water and a diesel fuel surrogate. The model is shown to perform well for several benchmark experimental cases, including the work of Reitz, Lichtarowicz, and Nurick. With the implementation of the Farve-averaged energy equation, the present work examines and validates the transport of enthalpy through the fixed heat flux and fixed wall temperature boundary conditions. The pipe heat transfer experiments of Boelter and Allen are replicated using the kEpsilon, Realizable kEpsilon, and Spalart-Allmaras models. With proper turbulence model selection, Allen's heat transfer coefficient data is reproduced within 2.9%. Best-case bulk temperature rise prediction is within 0.05%. Boelter's bulk temperature rise is reproduced within 16.7%. Turbulent diffusivity is shown to determine radial enthalpy distribution.

Homogeneous Charge Compression Ignition (HCCI) combustion is an alternative combustion mode in which the fuel is homogeneously mixed with air and is auto-ignited by compression. Due to charge homogeneity, this mode is characterized by low equivalence ratios and temperatures giving simultaneously low nitric oxide (NOx) and soot in diesel engines. The conventional problem of NOx-soot trade-off is avoided in this mode due to absence of diffusion combustion. This mode can be employed at part load conditions while maintaining conventional combustion at high load thus minimizing regulatory cycle emissions and reducing cost of after-treatment systems. The present study focuses on achieving this mode in a turbocharged, common rail, direct injection, four-cylinder, heavy duty diesel engine. Specifically, the work involves a combination of three-dimensional CFD simulations and experiments on this engine to assess both traditional and novel strategies related to fuel injection. The first phase of the work involved a quasi-dimensional simulation of the engine to assess potential of achieving HCCI. This was done using a zero-dimensional, single-zone HCCI combustion model with n-heptane skeletal chemistry along with a one-dimensional model of intake and exhaust systems. The feasibility of operation with realistic knock values with high EGR rate of 60% was observed. The second aspect of the work involved three-dimensional CFD simulations of the in-cylinder process with wall film prediction to evaluate injection strategies associated with Early Direct Injection (EDI). The extended Coherent Flame Model-3Zone (ECFM-3Z) was employed for combustion simulation of conventional CI and EDI, and was validated with experimental in-cylinder pressure data from the engine. A new Uniformity Index (UI) parameter was defined to assess charge homogeneity. Results showed significant in-homogeneity and presence of wall film for EDI. Simulations were conducted to assess improvement of charge homogeneity by several strategies; narrow spray cone angle, injection timing, multiple injections, intake air heating, Port Fuel Injection (PFI) as well as combination of PFI and EDI. The maximum UI achieved by EDI was 0.78. The PFI strategy could achieve UI of 0.95; however, up to 50% of fuel remained trapped in the port after valve closure. This indicated that except EDI, none of the above-mentioned strategies could help achieve the benefits of the HCCI mode. The third part of the work involved engine experimentation to assess the EDI strategy. This strategy produced lower soot than that of conventional CI combustion with very short combustion duration, but led to high knock and NOx which is attributed to pool fire burning phenomenon of the wall film, as confirmed by CFD. An Optimized EDI (OptimEDI) strategy was then developed based on results of CFD and Design of Experiments. The Optim EDI consisted of triple injections with split ratio of 41%-45%-14% and advancing the first injection. This strategy gave 20% NOx and soot reduction over the conventional CI mode. Although this strategy gave encouraging results, there was a need for more substantial reduction in emissions without sacrificing efficiency. Hence, a novel concept of utilizing air-assisted Injection (AAI) into the EGR stream was employed, as this implied injecting very small droplets of fuel into the intake which would have sufficient residence time to evaporate before reaching the cylinder, thereby enabling HCCI. The fourth and final part of the work involved engine experimentation with AAI, and combination of OptimEDI with AAI. Results with 20% EGR showed that 5 to 10% of AAI gave further reduction in NOx but not in soot. With experiments involving 48% EGR rate, there was soot reduction of 75% due to combined AAI-EDI. NOx was negligible due to the high EGR rate. Thus, the significant contribution of this work is in proving that combining AAI with EDI as a novel injection strategy leads to substantial NOx and soot reduction.