Modeling Diesel engine combustion using pressure dependent Flamelet Generated Manifolds

Flamelet Generated Manifolds (FGMs) are constructed and applied to simulations of a conventional compression ignition engine cycle. To study the influence of pressure and temperature variations on the ignition process after the compression stroke, FGMs with several pressure levels are created. These pressure levels, and the corresponding average temperatures are obtained from experimental curves. Auto-ignition is taken into account by tabulating igniting laminar diffusion flames, with n-heptane as a surrogate for diesel. The chemistry is parameterized as a function of the mixture fraction and a reaction progress variable, and pressure variations are accounted for by using the pressure as an extra degree of freedom.General steady combustion phase characteristics are compared to experimental observations. They show a correct phenomenological spray flame picture with a flame lift-off, fuel rich inner region and a diffusion flame at the periphery. The resolution effect of pressure discretization in the database to ignition prediction is studied by performing simulations with 1, 3, 5 and 7 pressure levels in the FGM. The results show that not more than 5 pressure levels are needed to represent the chemistry evolution during an engine cycle. All tested reaction mechanisms for n-heptane give a short burn duration, especially the reduced ones. As a result, at the moment of ignition the pressure rise rate is over-predicted.