Numerical Investigation of Water Injection Effects on Flame Wrinkling and Combustion Development in a GDI Spark Ignition Optical Engine

The new real driving emission cycles and the growing adoption of turbocharged GDI engines are directing the automotive technology towards the use of innovative solutions aimed at reducing environmental impact and increasing engine efficiency. Water injection is a solution that has received particular attention in recent years, because it allows to achieve fuel savings while meeting the most stringent emissions regulations. Water is able to reduce the temperature of the gases inside the cylinder, coupled with the beneficial effect of preventing knock occurrences. Moreover, water dilutes combustion, and varies the specific heat ratio of the working fluid; this allows the use of higher compression ratios, with more advanced and optimal spark timing, as well as eliminating the need of fuel enrichment at high load. Computational fluid dynamics simulations are a powerful tool to provide more in-depth details on the thermo-fluid dynamics involved in engine operations with water injection. The main intent of this work is to explore the effectiveness of port water injection installation in an optical access GDI engine operated with fully open throttle and fixed spark timing. Commercial gasoline is used as a fuel, while water is delivered through 2 injectors in the intake manifold. The injected water mass is 30% of the fuel mass. Numerical combustions are validated against experimental data, taking into account the blow-by effect with a crevices model, not negligible in an optical access engine. The G-equation turbulent combustion model is used in a RANS framework. Multi-cycle simulations are performed with water injection, also focusing on the wall film dynamics and the spray evolution. This study highlights the positive impact of water injection on lowering charge temperatures before ignition, with a consequent reduction in the peak pressure. Predicted heat release rates match measured data for both the baseline and the water injection cases. In addition, the CFD model allows to have additional insight on the in-cylinder processes. Remarkably, the analysis of the predicted flame front details agrees with the experimental imaging results in detecting an increase of the flame wrinkling and a transition in the combustion regime in the presence of water, despite the concurrent reduction of the overall burning rate.