title: Numerical investigation of the in-cylinder flow-spray-wall interactions in direct injection engines
Creators: name: Pati, Andrea
Publisher: name: UNSPECIFIED
Year: 2022

description: Internal combustion engines (ICEs) are an essential and common power source for personal mobility and the transport of goods. Despite their improvement over the last decades, they are still responsible for a relevant fraction of greenhouse gas and pollutant emissions. It is thus crucial to optimize them to enhance their efficiency and minimize emissions. This task is far from trivial because different physical phenomena, from a strongly unsteady turbulent multiphase flow to mixture ignition and combustion, interact in a cause-effect chain inside an ICE. Hence, it is challenging to study a single phenomenon individually, and engine studies must combine several research areas into a single multidisciplinary approach. In this context, numerical investigation methodologies can flank the experimental investigations as they make it possible to analyze many aspects of the engine that can be prohibitive to measure. This thesis develops a numerical method for engine simulation, striving toward accuracy, flexibility, stability and computational efficiency. All of these are necessary for proficiency in the multidisciplinary numerical study of the engine, in both academia and industry. This framework is used to investigate the phenomena preceding the combustion phase in the Darmstadt research engine, a Direct Injection Spark Ignition (DISI) optically accessible engine. In this context, new mesh motion methodologies are developed that are suitable for DISI engines, characterized by complex geometries. Such methodologies are designed to achieve maximum accuracy for RANS and LES simulations with minimum effort. Moreover, methodologies for multi-cycle simulations are developed, including pre-processing and post-processing methods. The strategy is fully validated by simulating a full cycle of the Darmstadt engine running in motored condition at different operating conditions, using a RANS approach. Furthermore, this work investigates spray dynamics, including the spray impingement and wall film evolution, which are relevant in predicting the fuel/air mixture distribution. A full cycle of the Darmstadt engine with ECN Spray G direct injection is simulated under different operating conditions to study the fuel/air mixture distribution right before the ignition. It is found that a reduced engine load reduces the drag and the breakup of the spray, increasing the mass of the wall film, while a higher engine speed reduces mixture stratification. A new breakup model is proposed, specific for predicting the spray evolution during the intake phase (early injection). Moreover, the newly developed model reduces the need for parameter adjustment, increasing the simulation reliability. A novel spray post-processing methodology is applied to directly compare the simulation with Mie scattering measurements. A satisfactory agreement in terms of the penetration length, asymmetry of the spray and velocity field is found inside the engine. Additionally, the interaction between the fuel spray and flow field during the injection is studied on different planes, to develop a set of phenomenological models for the in-cylinder flow-spray interaction. In order to characterize the physical properties of biofuels for spray simulation, particular importance is placed on the formulation of fuel surrogates. Hence, a Genetical Algorithm (GA) strategy is formulated to define surrogates of biofuels to predict properties such as the distillation and the vapor pressure. Furthermore, ten consecutive motored cycles of the Darmstadt engine are simulated using an LES approach. Good agreement is found with experimental data. The LES results are then used to analyze the turbulence production in the engine and its correlation with Cycle-Cycle Variability (CCV). Last, due to the importance of the boundary layer in engine efficiency, a set of near-wall flow variables have been extracted and analyzed.