Chemical Kinetic Characterization Of Autoignition And Combustion Of Diesel And Jp 8

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.

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.

Abstracts are given for 6.1 basic research in chemical propulsion sponsored by the Army Research Office and the Air Force Office of Scientific Research.

MIL-T-83133C grade JP-8 is of interest to the U.S. Army as the single fuel for the battlefield. The conversion to JP-8 occurred primarily to improve the safety of aircraft, although the single fuel for the battlefield concept (and the similarity of jet fuel to diesel fuel) is centered on the use of aviation kerosene in all Air Force and Army aircraft and ground vehicles. Detailed chemical kinetic mechanisms that describe combustion of many of the components in JP-8 are not available and are unlikely to be developed in the near future. Hence there is a need to study the characteristics of JP-8 experimentally. The following is the final progress report on our developments and accomplishments through May 2001- May 2004. The detail of the experimental facilities including two combustion chambers, spherical and cylindrical, optical set up, a high temperature oven and also our thermodynamic model used to calculate burning speed were discussed in the last reports and will be briefly discussed here. Measurements have been done in these facilities for gaseous and liquid fuels over the wide range of temperature and pressure. In the last year we developed a new heating system for fuel injection line in cylindrical vessel. The liquid fuel line in the spherical vessel was redesigned. Burning speeds of premixed JP-8 air have been measured for a range of temperature and pressure. Pictures of JP-8 flame have been taken using the high speed CCD camera in the cylindrical chamber. The results are presented in this report.

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.

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.

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.

We have developed a model of the diesel fuel injection process for application to analysis of low temperature non-sooting combustion. The model uses a simplified mixing correlation and detailed chemical kinetics, and analyzes a parcel of fuel as it moves along the fuel jet, from injection into evaporation and ignition. The model predicts chemical composition and soot precursors, and is applied at conditions that result in low temperature non-sooting combustion. Production of soot precursors is the first step toward production of soot, and modeling precursor production is expected to give insight into the overall evolution of soot inside the engine. The results of the analysis show that the model has been successful in describing many of the observed characteristics of low temperature combustion. The model predicts results that are qualitatively similar to those obtained for soot formation experiments at conditions in which the EGR rate is increased from zero to very high values as the fueling rate is kept constant. The model also describes the two paths to achieve non-sooting combustion. The first is smokeless rich combustion and the second is modulated kinetics (MK). The importance of the temperature after ignition and the equivalence ratio at the time of ignition is demonstrated, as these parameters can be used to collapse onto a single line all the results for soot precursors for multiple fueling rates. A parametric analysis indicates that precursor formation increases considerably as the gas temperature in the combustion chamber and the characteristic mixing time are increased. The model provides a chemical kinetic description of low temperature diesel combustion that improves the understanding of this clean and efficient regime of operation.