Em motores de combustão interna, o entendimento do processo de combustão é de fundamental importância para o desenvolvimento de modelos de queima empregados em simuladores de ciclos termodinâmicos, sendo a curva de pressão medida na câmara uma das principais fontes de informações a respeito deste processo. Além disto, este entendimento fornece subsídios valiosos para o projeto e desenvolvimento de novas câmaras de combustão, de componentes e do próprio motor. O objetivo deste trabalho é, a partir da medição da pressão na câmara de combustão de motores de ignição por centelha, obter informações a respeito da evolução da combustão, tais como taxa de liberação de calor e fração de massa queimada em função da posição angular do virabrequim. Para tanto, supõe-se a câmara de combustão dividida em três zonas, a saber: a de gases não queimados, composta pela mistura fresca admitida pelo motor e uma fração de gases residuais; a de gases queimados resultantes da combustão e a de gases não queimados contidos em frestas. Uma frente de chama adiabática, com formato esférico e interagindo com as paredes da câmara é admitida como interface entre as zonas queimada e não queimada. São também considerados os efeitos da dissociação química dos produtos da combustão, de troca de calor com as paredes e os de vazamento de gases da câmara para o cárter (blow-by). O modelo construído a partir destas hipóteses foi traduzido em um código computacional e aplicado a curvas de pressão geradas por um simulador e obtidas experimentalmente em um motor específico. / Understanding the combustion process in IC engines is very important to develop combustion models used in thermodynamic cycle simulators, and the in-cylinder pressure history is the basic source of information about this process. Besides, this understanding provides valuable knowledge to design and develop new combustion chambers, components and the engine itself. The purpose of the present work is to obtain information about combustion development in SI engines from in-cylinder pressure measurements, in terms of variables such as heat release and mass fraction burned as a function of crank angle. The combustion chamber is divided into three zones, comprising burned gases, mixture of fresh charge and residual gases, and gases in crevices. A spherical and adiabatic flame front that interacts with the chamber walls is assumed to separate the burned and unburned zones. The proposed model considers chemical dissociation at high temperatures, heat transfer and blow-by as well. The model built from the assumptions above was implemented as a numerical code and applied to a specific engine, using both cylinder gas pressure data from a cycle simulator and experimentally obtained.

<p>Cultivating cross-disciplinary understanding across science and engineering instruction will be essential to preparing the next generation of scientists to prosper in an increasingly interdisciplinary STEM workforce. This study reports on the culmination of a project that has been aimed at addressing this challenge by investigating how science and engineering students use the first law of thermodynamics, a guiding principle of the crosscutting concept of energy and matter, to transfer across disciplinary boundaries. A qualitative interview study was undertaken in which chemistry-, engineering-, and physics-major students were recruited and tasked with addressing familiar and unfamiliar first law problems that incorporated the systems, language, and notation of each respective discipline. Guided by the dynamic transfer framework, data were analyzed via a general inductive approach to categorize the contextual resources, or target tools, students leveraged to address the provided problems. This analysis revealed three unique guiding epistemologies whose frequency and character of emergence signify field-specific approaches to transferring into an unfamiliar disciplinary context. Connections are drawn to highlight the capacity of the derived findings and developed methodology to support cross-disciplinary understanding in the classroom and in future transfer of learning research.</p>

Thesis (Ph.D.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

Many industrial processes have low-temperature waste heat sources that cannot be efficiently recovered. Low grade waste heat has generally been discarded by industry and has become an environmental concern because of thermal pollution. This has led to the lookout for technologies which not only reduce the burden on the non-renewable sources of energy but also take steps toward a cleaner environment. One approach which is found to be highly effective in addressing the above mentioned issues is the Organic Rankine Cycle (ORC), which can make use of low- temperature waste heat to generate electric power. Similar in principle to the conventional cycle, ORC is found to be superior performance-wise because of the organic working fluids used in the cycle. The focus of this study is to examine the ORC using different types of organic fluids and cycle configurations. These organic working fluids were selected to evaluate the effect of the fluid boiling point temperature and the fluid classification on the performance of ORCs. The results are compared with those of water under similar conditions. In order to improve the cycle performance, modified ORCs are also investigated. Regenerative ORCs are analyzed and compared with the basic ORC in order to determine the configuration that presents the best thermal efficiency with minimum irreversibility. The evaluation for both configurations is performed using a combined first and second law analysis by varying certain system operating parameters at various reference temperatures and pressures. A unique approach known as topological method is also used to analyze the system from the exergy point of view. Effects of various components are studied using the exergy-wheel diagram. The results show that ORCs using R113 as working fluid have the best thermal efficiency, while those using Propane demonstrate the worse efficiency. In addition, results from these analyses demonstrate that regenerative ORCs produce higher efficiencies compared to the basic ORC. Furthermore, the regenerative ORC requires less waste heat to produce the same electric power with a lower irreversibility.