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7 Conclusions

The internal combustion engine is by far the most important power train for all kind of vehicles today. Up to now there is no alternative to this kind of engine, and it is for sure that it will keep its leading position for at least the next three to five decades. However, it has to be continuously improved, and great efforts have to be made in order to increase efficiency and to fulfill future emission legislation. In this context, the reduction of engine-out raw emissions by applying improved or new mixing formation and combustion concepts will be one of the key measures to keep the internal combustion engine up to date.

A systematic and precise control of mixture formation with modern highpressure injection systems, including fully variable rate shaping, variable nozzle geometry, pressure-modulated injection etc., will become crucial for realizing future combustion concepts. However, due to of the growing number of free parameters, the prediction of spray and mixture formation is becoming increasingly complex. For this reason, the optimization of the in-cylinder processes using 3D computational fluid dynamics (CFD) is becoming increasingly important. In this book, the state of the art in modeling in-cylinder spray and mixture formation processes in internal combustion engines has been presented and discussed. This includes the description of the mathematical treatment of twophase in-cylinder flows consisting of a continuous and a dispersed phase, the derivation of the relevant conservation equations, and the detailed description and discussion of all CFD models needed to describe the relevant sub-processes during spray and mixture formation.

The research work that is necessary to develop and to continuously improve these models is of great importance for several reasons. Simulation models, which have been carefully adjusted to a specific range of boundary conditions, can be used to perform extensive parametric studies, which are usually faster and cheaper than experiments. Much more important is the fact that despite the higher uncertainty compared to experiments, numerical simulation can give much more extensive information about the complex in-cylinder processes than experiments could ever provide. Using numerical simulations, it is possible to calculate the temporal behavior of every variable of interest at any place inside the computational domain. This allows getting a detailed knowledge of the relevant processes, and is a prerequisite in order to improve them. Furthermore, the numerical simulation can be used to investigate processes that take place at time and length scales or at places that are not accessible and thus cannot be investigated experimentally. Last but not least, the research work that is necessary to increase our knowledge about the relevant processes and to improve the CFD models, reveals new and unknown

velocity and thermal boundary layers cannot be resolved by the gas phase grid and have to be described by so-called wall functions. Usually the grid cells at the wall are too coarse, and the range of applicability of the wall-functions is exceeded. A possible approach is to refine the mesh near the walls until the desired grid spacing is obtained. However, due to the strongly changing flow quantities, the boundary layer thickness is highly transient and the sizes of all wall cells must be adapted dynamically to the required values. This still causes considerable problems today. For this reason, a new and completely gridindependent approach would be highly desirable.

ξ As discussed in Chap. 6, future mixture formation and combustion concepts for diesel as well as gasoline engines will utilize almost exclusively high-pressure direct injection. The higher the injection pressure, the more energy for spray and mixture formation must be provided by the injection system itself, and the more important the influence of internal nozzle flow on primary and secondary spray break-up, Chap. 2. Especially the primary break-up, which is responsible for the starting conditions of the liquid droplets that penetrate into the cylinder volume, is significantly influenced by the three-dimensional turbulent and cavitating flow that emerges from the nozzle holes. Primary break-up models, which describe the relevant effects during this first disintegration of the liquid jet in the vicinity of the nozzle, are highly desirable in order to eliminate uncertainties regarding the starting conditions of the spray in CFD calculations. Today, there are already some promising approaches to solve this problem in the case of diesel injection, Chap. 4. However, further research work is necessary to extend them for the use in the case of high-pressure gasoline injection. Furthermore, effects like needle seat throttling, flash-boiling, pressure-modulated injection, variable nozzle geometry, highly transient operation etc. have to be included. The most important problem regarding the modeling of primary break-up however is the validation of new models. Due to the very dense primary spray and the small dimensions, this region is hardly accessible by optical measurement techniques. Thus, it is difficult to understand the relevant processes and to verify CFD models.

ξ The basic challenge in describing the evaporation process of fuel droplets is the choice of an appropriate reference fuel that represents the relevant behavior of the fuel. Especially in the case of new tailored synthetic fuels, e.g. for HCCI applications, a single reference fuel can often not predict the relevant subprocesses during evaporation, ignition, and combustion with adequate accuracy, and multi-component fuel models become increasingly desirable. A possible approach to describe such a multi-component fuel has been presented in Chap. 4. Compared to the conventional auto-ignition combustion concepts, the ignition delays of HCCI concepts are significantly longer, and in this case detailed ignition chemistry models that can predict the two-stage ignition of realistic multi-component fuels with sufficient accuracy are also needed.

ξ Another challenge is the improvement of turbulence modeling. Turbulence greatly influences the spray and mixture formation process as well as the sub-