Zero and Multi-Dimensional Modeling of Internal Combustion Engines with respect to Ion Current and Soot Formation

Abstract: The work presented in this thesis can be divided into the three following fields of investigation: First of all the ionization model developed in this thesis is meant to simulate the thermal ionization process or the so-called second peak. Equilibrium assumptions have been adopted for the modeling of the thermal ionization, in which Saha’s equation was derived for singly ionized molecules. The investigation is focused on the thermal ionization and electron attachment of six chemical species, plus their positive or negative singly charged ions and the electrons. This ionization model was developed to run in post-process to a main combustion model that can handle the non-linear behavior promoted by the ionization process, using the temperature and pressure history from the two-zone combustion model. While the above ionization model had been developed, a homogenous multi-zone combustion model based on detailed chemical kinetics and capable of simulating a full engine cycle was being developed in parallel, starting from an existing two-zone model. The two-zone model consists of a burned and an unburned zone, separated by a thin adiabatic flame front. The multi-zone model differs in the handling of the burned gas. In the multi-zone case a number of burned zones are present. The number of zones is decided by the temperature difference between the flame front and the last generated burned zone. Since the main focus of this thesis was on problems where detailed chemical kinetic information is important, such as ignition of complex fuels and emission formation, a primary goal was to integrate detailed chemistry into engine simulation tools. Second, a multi-zone combustion model based on an existing two-zone model is developed and validated against the experimental results. The validation is done by using detailed chemical mechanism consisting of 141 species and about 1405 reactions. The model is a zero-dimensional model capable of simulating a full engine cycle. The existing two zone combustion model consists of a burned and an unburned zone, separated by a thin adiabatic flame front. The multi-zone model differs in the handling of the burned gas. In the multi-zone case a number of burned zones are present. The number of zones is decided by the temperature difference between the flame front and the last generated burned zone. The detailed chemical mechanism is taken into account in each zone, while the propagating flame front is calculated from the Wiebe function. Each zone is assumed to be a homogeneous mixture with a uniform temperature, mole and mass fractions of species. The spatial variation of the pressure is neglected, i.e., it is assumed to be the same in the whole combustion chamber at every instant of time. Auto-ignition is handled by the chemical kinetic model. As the unburned zone is assumed homogeneous, the effect of auto ignition is a single pressure peak. The model is not designed to predict the pressure oscillations seen in engine knock. The model is validated against experimental data for five different cases of fuel-to-air ratio, and the results were very good for comparing experiments. Third, in this last part the soot formation and oxidation under a real diesel engine combustion condition is investigated, by using the method of moment coupled with a CFD commercial code. The soot formation model (Method of Moment) used is based on a detailed description of the chemical and physical processes involved in the formation of soot. The soot particle size distribution function (PSDF) is described in terms of its statistical moment to reduce the numerical burden. The engine used for the simulation is a two stroke diesel engine. The simulation results for three different cases of injection pressures were found to have a very good agreement with experimental results.