Because of increasingly stringent engine emissions and fuel economy standards, there is an urgent need for developing future diesel engines with higher efficiency and lower emissions. Therefore, low temperature combustion is currently being pursued to develop new types of advanced diesel engines. Since low temperature combustion is more sensitive to chemical kinetics, the understanding of the autoignition characteristics of diesel fuels under low-to-intermediate temperatures becomes important. In order to achieve the goal of higher efficiency and lower emissions diesel engines, both experimental and computational investigations of diesel fuels at low-to-intermediate temperatures need to be conducted, as the experimental autoignition results help develop a comprehensive understanding of diesel ignition and provide a validation database for model development, and a comprehensive chemical kinetic model of diesel is also imperative for accurate prediction of ignition and emissions characteristics of diesel engines. Because diesel fuels contain hundreds, even thousands of species, and the composition of diesel is too complex to model, it is also necessary to develop surrogate fuels, which are simpler mixtures that include fuel components representative of hydrocarbon classes found in diesel fuels, and can capture the essential chemical/physical properties and performance characteristics of the target diesel fuel to sufficient accuracy. Therefore, the work presented in the current dissertation aims to gain better understandings and fill in gaps in fundamental combustion data of diesel-surrogate components and surrogate fuel mixtures relevant to diesel fuels. Autoignition of trans-decalin at low-to-intermediate temperatures has been investigated first to get a better understanding of its autoignition characteristics, and the development of a detailed chemical kinetic model of diesel surrogates has been benefited from the results of trans-decalin. The agreements of the developed diesel surrogate model have been tested by comparing with the current autoignition results of diesel surrogates, and possible sources of discrepancies between experimental and simulated results have also been investigated. Based on that, binary blends of iso-cetane and tetralin are further chosen for autoignition investigation to help find out possible reasons causing those discrepancies and to further benefit the refinement and development of comprehensive diesel surrogate models.

The design process for development of engines could be made faster and less expensive with the help of computations which help understanding the processes prevalent in internal combustion engines. Running engine simulations are challenging as they need to accurately capture the fluid dynamic and chemical kinetic processes that occur in an engine. A major challenge in simulating chemical kinetic processes is the complexity of the fuel chemistry: real fuels are complex mixtures whose composition determines their physical properties and reactivity. The behavior of these real fuels can be conveniently represented using simpler mixtures often called â€surrogates mixtures†that match the key properties of the real fuels. Successful modeling of the ignition of real fuel first requires the formulation of an appropriate surrogate mixture whose compositions are carefully chosen in order to best emulate the combustion properties of the targeted real fuel. Then a comprehensive chemical kinetic model developed based on the surrogate fuel is used to simulate the combustion process of the real fuel. The work presented in the current dissertation intends to systematically study the surrogate modeling of diesel fuels. The study has been conducted to understand the ignition of surrogate fuel constituents and fully blended diesel fuels. Autoignition of tetralin, 1-methylnaphthalene, iso-cetane, and n-dodecane, the constituents of diesel surrogates, are investigated in the current dissertation. Besides, ignition of binary blends of the surrogate constituents has also been studied to investigate the effects of blending on ignition when neat components are blended to formulate a surrogate fuel. Furthermore, the ignition of two fully blended research grade diesel fuels has also been conducted inorder to provide quality ignition delay data for development and validation of chemical kinetic models of kinetic fuels.

A study was performed to elucidate the chemical-kinetic mechanism of combustion of toluene. The research was performed in collaboration Dr. Charles Westbrook and Dr. William Pitz at Lawrence Livermore National Laboratory (LLNL). A detailed chemical-kinetic mechanism for toluene developed at LLNL was employed. Numerical calculations were performed using this mechanism and the results were compared with experimental data obtained from premixed and nonpremixed systems. Under premixed conditions, predicted ignition delay times were compared with new experimental data obtained by I. Da Costa, R. Fournet, F. Billaud, F. Battin-Leclerc at Departement de Chime Physique des Reactions, CNRS-ENSIC, BP. 451, 1, rue Grandville, 51001 Nancy, France. Also, calculated species concentration histories were compared to experimental flow reactor data from the literature. Under nonpremixed conditions, critical conditions of extinction and autoignition were measured in strained laminar flows in the counterflow configuration. Numerical calculations were performed using the chemical-kinetic mechanism at conditions corresponding to those in the experiments. Critical conditions of extinction and autoignition are predicted and compared with the experimental data. Comparisons between the model predictions and experimental results of ignition delay times in shock tube, and extinction and autoignition in nonpremixed systems show that the chemical-kinetic mechanism predicts that toluene/air is overall less reactive than observed in the experiments. The principal objective of this research is to obtain a fundamental understanding of the physical and chemical mechanisms of autoignition and combustion of Diesel in nonpremixed systems. The major components of Diesel are straight-chain paraffins, branched-chain paraffins, cycloparaffins, and aromatics. The results of this research on toluene are expected to be useful in understanding the role of aromatics in combustion of Diesel.

This report includes the results of an investigation on the autoignition and combustion processes in diesel engines at low ambient temperatures. Experiments were conducted on three different single-cylinder direct-injection, four-stroke engines, using fuels of different cetane numbers and physical properties. Tests covered ambient temperatures ranging from 250C to -250C. The engines were soaked at least eight hours before a cold start test. The analysis indicated that the difficulty in starting diesel engines is caused by combustion instability at low temperatures. Combustion instability will cause the engine to misfire once before it fires again. This is referred to as 8-stroke-cycle operation. If it misfires twice, it is referred to as l2-stroke-cycle operation, and so on. This pattern was found to be reproducable. The engine may start on a l2-stroke-cycle operation at low temperatures, shift to an 8-stroke-cycle, and finally shifts to the regular 4-stroke-cycle. This pattern has been found not to be engine or fuel specific. A detailed thermodynamic and combustion analysis of the experimental data indicated that the cause for combustion instability is a combination of dynamic, physical and chemical kinetics factors. Recommendations are made to reduce combustion instability by using the electronic controls already available on engines.

There has been much recent progress in the area of surrogate fuels for diesel. In the last few years, experiments and modeling have been performed on higher molecular weight components of relevance to diesel fuel such as n-hexadecane (n-cetane) and 2,2,4,4,6,8,8-heptamethylnonane (iso-cetane). Chemical kinetic models have been developed for all the n-alkanes up to 16 carbon atoms. Also, there has been much experimental and modeling work on lower molecular weight surrogate components such as n-decane and do-decane which are most relevant to jet fuel surrogates, but are also relevant to diesel surrogates where simulation of the full boiling point range is desired. For the cycloalkanes, experimental work on decalin and tetralin recently has been published. For multi-component surrogate fuel mixtures, recent work on modeling of these mixtures and comparisons to real diesel fuel is reviewed. Detailed chemical kinetic models for surrogate fuels are very large in size. Significant progress also has been made in improving the mechanism reduction tools that are needed to make these large models practicable in multidimensional reacting flow simulations of diesel combustion. Nevertheless, major research gaps remain. In the case of iso-alkanes, there are experiments and modeling work on only one of relevance to diesel: iso-cetane. Also, the iso-alkanes in diesel are lightly branched and no detailed chemical kinetic models or experimental investigations are available for such compounds. More components are needed to fill out the iso-alkane boiling point range. For the aromatic class of compounds, there has been no new work for compounds in the boiling point range of diesel. Most of the new work has been on alkyl aromatics that are of the range C7 to C8, below the C10 to C20 range that is needed. For the chemical class of cycloalkanes, experiments and modeling on higher molecular weight components are warranted. Finally for multi-component surrogates needed to treat real diesel, the inclusion of higher molecular weight components is needed in models and experimental investigations.

The objective of the research was to obtain a fundamental understanding of the physical and chemical mechanisms of autoignition and combustion of diesel and JP-8 in non-premixed systems. Diesel and JP-8 are comprised of hundreds of aliphatic and aromatic hydrocarbon compounds. The major components of these fuels are straight chain paraffins, branched chain paraffins, cycloparaffins, aromatics, and alkenes. Detailed chemical kinetic mechanisms that describe combustion of many of the components in diesel and JP-8 are not available and are unlikely to be developed in the near future. As a consequence, it is necessary to develop surrogate fuels. The research was focused on developing the necessary scientific knowledge for developing these surrogate fuels. The experimental part of the research was performed employing the counterflow configuration. The fuels tested were n-heptane, n-decane, n-dodecane, n-hexadecane, cyclohexane, methylcyclohexane, toluene, and o-xylene because they represent the types of fuels in diesel and JP-8. Critical conditions of autoignition and extinction were measured. Flame structures were measured for non-premixed n-heptane flames and n-decane flames. For n-heptane and n-decane flames, numerical calculations were performed using detailed chemistry and the results were compared with experiments.