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Combustion has played a central role in the development of our civilization which it maintains today as its predominant source of energy. The aim of this book is to provide an understanding of both fundamental and applied aspects of low-temperature combustion chemistry and autoignition. The topic is rooted in classical observational science and has grown, through an increasing understanding of the linkage of the phenomenology to coupled chemical reactions, to quite profound advances in the chemical kinetics of both complex and elementary reactions. The driving force has been both the intrinsic interest of an old and intriguing phenomenon and the centrality of its applications to our economic prosperity. The volume provides a coherent view of the subject while, at the same time, each chapter is self-contained.
This research book provides state-of-the-art advances in several areas of energy generation from, and environmental impact of, fuels and biofuels. It also presents novel developments in the areas of biofuels and products from various feedstock materials along with thermal management, emission control and environmental issues. Availability of clean and sustainable energy is of paramount importance in all applications of energy, power, mobility and propulsion. This book is written by internationally renowned experts from around the globe. They provide the latest innovations in cleaner energy utilization for a wide range of devices. The energy and environment sustainability requires a multipronged approach involving development and utilization of new and renewable fuels, design of fuel-flexible combustion systems and novel and environmentally friendly technologies for improved fuel use. This book serves as a good reference for practicing engineers, educators and research professionals.
Practical fuels are a complex mixture of thousands of hydrocarbon compounds, making it challenging and difficult to study their combustion behavior. It's generally agreed that in order to study these complex practical fuels a much simpler approach of studying simple fuel surrogates containing limited number of components is more feasible. Ethanol and n-heptane have been studied as primary reference fuels in the surrogate study of gasoline and diesel over the past few decades. The objective of the following thesis has been to study the autoignition characteristics of ethanol and n-heptane and validate chemical kinetic mechanisms. The validation of a chemical kinetic mechanism provides a deeper insight into the combustion behavior of the fuels which can be further used to study advanced combustion concepts. Experiments have been conducted on the rapid compression machine (RCM) and validated against mechanisms from literature study. Rapid compression machines have been primarily used to study chemical kinetics at low to intermediate temperatures and high pressures for their accuracy and reproducibility. For the following study experiments span over a range of temperature (650-1000 K), pressure (10, 15 and 20 bar) and equivalence ratio ([phi]=0.3, 0.5, 1). Experimental data based on the adiabatic volumetric expansion approach have been modeled numerically using the Sandia SENKIN code in conjunction with CHEMKIN. Experiments have been primarily focused on validating kinetic mechanisms at low to intermediate temperatures and elevated pressures. Ignition delay time data from experiments have been deduced based on the pressure and time histories. A brute sensitivity and flux analysis has been performed to reveal the key sensitive reactions and the dominant reaction pathways followed under the present experimental conditions. Improvements have been suggested and discrepancies noted in order to develop a valid chemical kinetic mechanism. Under the present experimental conditions for the study of ethanol, reactions involving hydroperoxyl radicals, namely C2H5OH+HȮ2 and CH3CHO+ HȮ2 as well as the formation of H2O2 from HȮ2 radical and its subsequent decomposition have been found to be sensitive. Based on the following, improvements and developements have been suggested to increase the accuracy and predictability of the mechanisms studied. Ignition delay data from experiments have been compared against those obtained from the mechanism used in the study for n-heptane. Discrepancies have been found in the low temperature region, with the mechanism under predicting the first ignition delay. The causes for the discrepancy have been noted to be due to the NTC behaviour exhibited during the two stage ignition of n-heptane. At low temperatures the reaction pathway proceeded by chain branching mainly due to the ketohydroperoxide species reaction pathway has been analysed. As the temperature of the reaction increases the reaction pathway is dominated by the ȮOH species propagation resulting in the formation of conjugate olefins and [Beta]-decomposition products, a further investigation of which can help improve the predictability of the mechanism.
This dissertation discusses the results from three different studies aimed at understanding the importance of fuel chemical structure during low temperature combustion (LTC) strategies, like homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) employed in internal combustion (IC) engines wherein the focus is on high octane fuels. Boosted intake air operation combined with exhaust gas recirculation, internal as well as external, has become a standard path for expanding the load limits of IC engines employing LTC strategies mentioned above as well as conventional diesel and spark ignition (SI) engines. However, the effects of fuel compositional variation have not been fully explored. The first study focusses on three different fuels, where each of them were evaluated using a single cylinder boosted HCCI engine using negative valve overlap. The three fuels investigated were: a regular grade gasoline (RON = 90.2), 30% ethanol-gasoline blend (E30, RON = 100.3), and 24% iso-butanol-gasoline blend (IB24, RON = 96.6). Detailed sweeps of intake manifold pressure (atmospheric to 250 kPaa), EGR (0 -- 25% EGR), and injection timing were conducted to identify fuel-specific effects. While significant fuel compositional differences existed, the results showed that all these fuels achieved comparable operation with minor changes in operational conditions. Further, it was shown that the available enthalpy from the exhaust would not be sufficient to satisfy the boost requirements at higher load operation by doing an analysis of the required turbocharger efficiency. While the first study concentrated on load expansion of HCCI, it is important to mention that controlling LTC strategies is difficult under low load or idle operating conditions. To ensure stable operation, fuel injection in the negative valve overlap (NVO) is used as one of method of achieving combustion control. However the combustion chemistry under high temperature and fuel rich conditions that exist during the NVO have not been previously explored. The second study focused on examining the products of fuel rich chemistry as a result of fuel injection in the NVO. In this study, a unique six stroke cycle was used to segregate the exhaust from the NVO and to study the chemistry of the range of fuels injected during NVO under low oxygen conditions. The fuels investigated were methanol, ethanol, iso-butanol, and iso-octane. It was observed that the products of reactions under NVO conditions were highly dependent on the injected fuel's structure with iso-octane producing less than 1.5% hydrogen and methanol producing more than 8%. However a weak dependence was observed on NVO duration and initial temperature, indicating that NVO reforming was kinetically limited. Finally, the experimental trends were compared with CHEMKIN (single zone, 0-D model) predictions using multiple kinetic mechanism that were readily available through literature. Due to the simplicity of the model and inadequate information on the fuel injection process, the experimental data was not modeled well with the mechanisms tested. Some of the shortcomings of the 0-D model were probably due to the model ignoring temperature and composition spatial inhomogeneities and evaporative cooling from fuel vaporization.Though the results from the NVO injection and boosted NVO-HCCI studies are enlightening, the fundamentals of the autoignition behavior of gasoline, alcohols, and their mixtures are not entirely understood despite the interest in high octane fuels in compression engines from a point of view of better thermal efficiency. The third study focused on higher octane blends consisting of binary and ternary mixtures of n-heptane and/or iso-octane, and a fuel of interest. These fuels of interest were toluene, ethanol, and iso-butanol. In this study, the autoignition of such blends is studied under lean conditions ([phi] = 0.25) with varying intake pressure (atmospheric to 3 bar, abs) and at a constant intake temperature of 155 °C. The blends consisted of varying percentages of fuels of interest and their research octane number (RON) approximately estimated at 100 and 80. For comparison, neat iso-octane was selected as RON 100 fuel and PRF 80 blend was selected as RON 80 fuel. It was observed that the blends with a higher percentage of n-heptane showed a stronger tendency to autoignite at lower intake pressures. However, as the intake pressure was increased, the non-reactive components, in this case, the higher octane blend components (toluene, ethanol, and iso-butanol), reduced this tendency subsequently delaying the critical compression ratio (CCR) of the blends. The heat release analysis revealed that the higher octane components in the blends reduced the low temperature reactivity of n-heptane and iso-octane. GC-MS and GC-FID analysis of the partially compressed fuel also indicated that the higher octane components did affect the conversion of the more reactive components, n-heptane and iso-octane, into their partially oxidized branched hydrocarbons in the binary/ternary blends, and reduced the overall reactivity which resulted in a delayed CCR at higher intake pressures.
Autoignition of combustible materials, particularly hydrocarbon fuels, play an important role in the fire hazard in the storage and handling of these materials. The minimum autoignition (or spontaneous ignition) temperature of a given fuel is an important flammability property of the material, but its experimentally determined value is markedly dependent on the method and apparatus employed for its determination. Some of the complex phenomena associated with autoignition, such as cool flames, zones of nonignition, multiple ignition, ignition delay, and hot-surface ignition, are defined and discussed. The numerous experimental and other factors which may influence autoignition temperatures are discussed, and their relations to autoignition phenomena are described. Some of the factors are chemical structure and composition, fuel-air ratio, concentration of oxygen, surface-volume ratio, geometry of the containing vessel, and pressure.