The demands on motor vehicles are steadily increasing. They have to be powerful and competitive but also environmentally friendly. This is not only due to the tight pollutant emission regulations, but also due to public interests. Although alternatives to internal combustion engines like hybrid or electro vehicles are under investigation and are already used for certain applications, it will take several decades until a wider use is possible. Thus conventional internal combustion engines have to be improved further. There is still potential like downsizing with supercharger, variable valve stroke or variable compression. But to be able to further improve the internal combustion engines, a deeper understanding of the combustion process is necessary. As experiments give only limited information, computational fluid dynamics (CFD) is a very powerful tool to gain more knowledge of the processes occurring within the engine and to improve the processes. In industry, the Eddy Dissipation Model is still a commonly used combustion model. As this is a very simple model, it is not successful to describe all the complex processes occurring in internal combustion engines. So this work focuses on the development of a suitable combustion model for direct injection engines in industrial applications. In such applications, combustion occurs under partially premixed conditions. The model presented is based on a flamelet concept with an additional transport equation for the reaction progress variable to account for the partially premixed regime. Different approaches are compared to close the source term of the reaction progress variable transport equation. The main effort has been spent for the correct description of the laminar flame speed, which changes with pressure, temperature and mixture composition, which strongly vary in internal combustion engines. As the laminar flame speeds are derived in the context of premixed combustion, the validity range of the correlations is not wide enough for diffusion flames. So they are extended to the flammability limits, for which correlations are derived within this work to account for the pressure and temperature dependence. To also account for the turbulence effects, an effective laminar flame speed is introduced, used in pre-integrated tables, which also depends on the mean mixture fraction and mixture fraction fluctuation. The developed model is validated on a hydrogen jet flame, on a piloted methane- and heptane-air flame, and on a methane-air swirl burner. Especially for the hydrogen jet flame, and the piloted methane flame, a detailed set of experimentally obtained variables is available, like species mass fractions and temperature distributions. In comparison to the Eddy Dissipation Model, which is based on the assumption of a single step reaction, it could be seen that the Eddy Dissipation Model is not reliable enough to account for the complex processes occurring in internal combustion engines, whereas the flamelet model together with the reaction progress variable and the effective laminar flame speed shows promising and reliable results.    «

The demands on motor vehicles are steadily increasing. They have to be powerful and competitive but also environmentally friendly. This is not only due to the tight pollutant emission regulations, but also due to public interests. Although alternatives to internal combustion engines like hybrid or electro vehicles are under investigation and are already used for certain applications, it will take several decades until a wider use is possible. Thus conventional internal combustion engines have to...    »