title: Modeling and scale-resolving simulations of knocking combustion initiation in a spark ignition engine
Creators: name: Kircher, Magnus
Publisher: name: UNSPECIFIED
Year: 2024

description: Climate change has made the reduction of greenhouse gas emissions one of today's top priorities. Currently, individual mobility contributes significantly to global emissions. To reduce these emissions, there is a trend towards electric vehicles. However, combustion engines will remain relevant and the combustion efficiency must be further improved to reduce the carbon dioxide emissions associated with the combustion of fossil fuels. In this regard, downsizing technology is promising but poses an increased risk of engine damage due to knocking combustion. Here, the stochastic occurrence of knocking combustion requires a knock control that retards the spark timing from the knock limit leading to non-optimal combustion. Therefore, the knock limit is a major limiting factor for engine efficiency. In this context, the knock resistance is an important parameter that depends on the exact fuel composition. For gasoline fuels, however, the fuel composition is not standardized. Therefore, surrogate fuels are needed for numerical investigations. These surrogate fuels are chosen such that the combustion and auto-ignition properties of gasoline fuels are captured. A particular auto-ignition characteristic is the negative temperature coefficient (NTC) behavior, which might change the knocking combustion initiation. However, there is still a deficiency of comprehensive research on this subject. In this thesis, a systematic investigation of the knocking combustion initiation, as well as its cycle-to-cycle variations, at the knock limit for surrogate fuels with NTC behavior is performed. First, a measurement campaign investigating the differences in the knocking behavior of three surrogate fuels and gasoline is analyzed. A model-based analysis shows that the NTC behavior of the fuels is relevant for the operating conditions and can potentially change the knocking combustion initiation mechanism. Given these insights, an existing precursor auto-ignition model is extended to capture the non-linear auto-ignition process of fuels with NTC behavior. The model is validated in 0-D configurations before being applied in two 3-D multi-cycle LES studies. Here, the model is capable of reproducing experimental trends of local and global knock quantities. An influence of the NTC behavior on the local auto-ignition process is found taking into account temperature stratification and turbulent flame propagation. Based on the single cycles of the multi-cycle LES, the cycle-to-cycle variations of knocking combustion initiation are analyzed. Correlations are found that show an influence of large-scale flow structures on the combustion process. The associated cycle-to-cycle variations in the local flame propagation in turn lead to local and thus global differences in the subsequent auto-ignition process. In summary, the findings of this thesis significantly extend the understanding of the cause-and-effect chain of knocking combustion initiation of surrogate fuels with NTC behavior for operating conditions at the knock limit.