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A simplified model for multicomponent droplet vaporization is developed and implemented in a multidimensional model for flows, sprays and combustion in engines. The model is applied to study the vaporization characteristics of a multicomponent droplet under Diesel conditions, the distribution of the vapor components in a Diesel spray and the distribution of the components in a Diesel engine. It is shown that for typical warm Diesel engine operating conditions, the droplets vaporize sufficiently rapidly that the stratification of the different components in the spray is not significant. However, under engine starting conditions and, in particular, cold starting conditions, there is a significant stratification of the different components of the fuel. When the species are stratified, the heavier and slower vaporizing components are predicted to be on the periphery of the spray envelope. However, these components also take longer to reach there.

Stoichiometric operation is one possible approach for reducing in-cylinder pollutant formation in diesel engines. High levels of exhaust gas recirculation (EGR) combined with stoichiometric operation may be employed to decrease soot and NO emissions from the engine. In this work, in-cylinder conditions are estimated for a diesel engine near top dead center, prior to the start of injection, for different levels of EGR. Two modes of engine operation are considered: the first is operation with excess air such that the overall equivalence ratio is 0.5, and the second is stoichiometric operation. These conditions are employed in separate studies to understand the influence of both EGR and mode of operation on pollutant formation and ignition. N-heptane is used as a representative fuel. Its oxidation chemistry is modeled using a reduced 159-species, 1540-step mechanism. A kinetics-based soot model and NO sub-mechanism are employed to investigate pollutant formation.

Swirl in Diesel engines is known to be an important parameter that affects the mixing of the fuel jets, heat release, emissions, and overall engine performance. The changes may be brought about through interactions of the swirling flow field with the spray and through modifications of the flow field. The purpose of this paper is to investigate the interaction of the swirl with sprays in a Diesel engine through a computational study. A multi-dimensional model for flows, sprays, and combustion in engines is employed. Results from computations are reported with varying levels of swirl and initial turbulence in two typical Diesel engine geometries. It is shown that there is an optimal level of swirl for each geometry that results from a balance between increased jet surface area and, hence, mixing rates and utilization of air in the chamber.

For practical, extensive 3-D computations for engine improvements, each physical submodel needs to be the simplest that is compatible with the accuracy of all other physical submodels and of the numerics. The addition of one progress variable controlled by one Arrhenius term is shown to be adequate to reproduce Diesel ignition delay in 2-D and 3-D computations. The rest of the model is that used for years by the authors to optimize combustion in reciprocating and rotary engines with premixed and non-premixed charges, including all of its model constants. This minimal Diesel autoignition submodel reproduces well trends and magnitudes of ignition delay versus chamber temperature and pressure. As in experiments, it is found that multiple ignition sources develop in rapid succession at various locations around the fuel spray after the first ignition event.

Results are presented of 3-D computations of direct injection of gaseous methane and of liquid tetradecane through a multi-hole injector into a Diesel engine. The study focusses on the distribution of fuel/air ratio within the resulting gas and spray jets under typical Diesel conditions prior to ignition. It is shown that for a significant time after start of injection, the fraction of the vapor fuel which is in richer-than-flammable mixtures is greater in gas jets than in sprays. For methane injection, it is also shown that changing some of the flow conditions in the engine or going to a poppet-type injector, does not result in improved mixing. An explanation of these results is provided also through an analysis of the self-similar gas jet and 2-D computations of gas and spray jets into constant pressure gas. A scaling for time and axial distance in the self-similar gas jet also clarifies the results.

Results of three-dimensional computations of non-vaporizing and vaporizing Diesel sprays in a very high pressure (up to 18.4 MPa without combustion) environment are presented. These pressures and corresponding density ratios of ambient gas to injected liquid are about a factor of two greater than those in current Diesel engines. The spray model incorporates a line source for drops, heat, mass and momentum exchange between the gas and liquid phases, turbulent dispersion of drops, collisions and coalescences, and drop breakup. The accuracy of the model is assessed by making comparisons of computed and measured spray penetrations. Reasonable agreement is obtained for a broad range of conditions. A scaling for time and axial distance clarifies these results.

In a previous study, the authors compared the fuel-air mixing characteristics of gas jets and sprays in Diesel engine environments in the absence of combustion. A three-dimensional model for flows and sprays was used. It was shown that mixing was slower in gas jets relative to fast-evaporating sprays. In this study, which is an extension of the previous one, the direct-injection of gasesous methane, gaseous tetradecane and liquid tetradecane are studied using the same three-dimensional model. This study concentrates on combustion. It is shown that the fuel-air mixing rate and hence the burning rate are initially slower with gas injection.

Results of comparisons of computed and measured soot and NO in a direct-injection Diesel engine are presented. The computations are carried out using a three-dimensional model for flows, sprays and combustion in Diesel engines. Autoignition of the Diesel spray is modeled using an equation for a progress variable which measures the local and instantaneous tendency of the fuel to autoignite. High temperature chemistry is modeled using a local chemical equilibrium model coupled to a combination of laminar kinetic and turbulent characteristic times. Soot formation is kinetically controlled and soot oxidation is represented by a model which has a combination of laminar kinetic and turbulent mixing times. Soot oxidation appears to be controlled near top-dead-center by mixing and by kinetics as the exhaust is approached. NO is modeled using the Zeldovich mechanism.

Recent experimental results have shown that the penetration length of the liquid phase in a Diesel spray under normal operating conditions is relatively short compared to the penetration length of the overall jet. In addition, the results indicate that, for a significant fraction of the injection duration, the mass and volume of the injected fuel that is in the liquid phase is relatively small compared to the total volume and mass of fuel injected. Based on these considerations, a Virtual Liquid Source (VLS) model for Diesel sprays has been developed which treats the liquid region of the spray as a source of mass, momentum and energy without directly computing the liquid phase. The penetration length of the liquid phase along the axis of injection is obtained from recent measurements.

Interactions between the jets in a multi-hole injector and between the jet and the wall may affect the fuel-air mixing processes in a direct-injection Diesel engine. These interactions are the subject of the investigation in this work. It is known that in the case of free jets, for a given total mass and momentum flow rate, increasing the number of holes would result in an increase in the mixing rate. In the case of a multi-hole injector in an engine, however, if the number of holes are increased beyond an optimum value, the interaction between the jets themselves may result in a reduced mixing. In the limit of increasing the number of holes, a hollow-cone jet would result. The fuel-air mixing in the hollow-cone jet is shown to be slower than in a multi-hole injector with an optimum number of holes.