Academic literature on the topic '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<br>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.<br>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.

<p> 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.</p><p>