STUDY OF WATER INJECTION STRATEGIES TO ENHANCE EVAPORATION IN SI-ENGINES THROUGH 3D-CFD SIMULATIONS

Due to the increasingly severe restrictions in terms of CO2 emissions to be fulfilled within 2020, and the further reductions set for the following years, the Automotive field is now facing a period of intensive technological development and transformation. In the present dissertation a deep analysis of the water injection concept in SI-engines (spark ignition engines) to enhance higher efficiency, already adopted in the past mainly on aircrafts and race cars and now revisited to be applied on large-scale vehicle production, has been done. Since there is an important difference between gasoline and water properties, all the already assimilated considerations concerning both gasoline injection events and post-injection phenomena (atomization, evaporation, mixing) are no longer suitable when injecting water. As first step a solid knowledge of water main characteristics has been obtained, highlighting the main differences respect to gasoline. After that, thanks to the 3D-CFD tool QuickSim developed by Dr.-Ing. Marco Chiodi during his PhD at Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS)/ Institut für Verbrennungsmotoren und Kraftfahrwesen (IVK) of University of Stuttgart, the analysis of the most relevant parameters affecting water evaporation has been performed. The reason behind this great interest in water evaporation depends on the ability of this particular component to highly reduce the mixture temperature (water-air/water-air-fuel) thanks to its greater HOV (heat of vaporisation) compared to gasoline. This characteristic will be later explained as one of the major advantages of water injection strategy. Another important consideration to be considered when dealing with this liquid is that water is injected into an airstream which composition consists mainly of air but with a smaller quantity of water vapour. This little amount does not affect in a large-scale injection events involving gasoline. However, this is no longer true for water, which injected quantity is highly dependent on the relative humidity of the air. An analytical study of this behaviour has made possible not only to understand how much the initial relative humidity of the external air affects the maximum amount of water to be injected, but also to implement this knowledge in the 3D-CFD tool, so that to obtain a more reliable and realistic evaluation of this concept. The final task of this work involved the possibility to implement in QuickSim a model to evaluate water wall impingement. Again, because of the difference between water and gasoline properties, a more detailed analysis must be performed on this aspect. Thanks to the 3D-CFD commercial software STAR-CD a first evaluation of this phenomenon has been possible. Later, a series of different sub-models have been considered to choose the main settings that would have been applied on QuickSim to finally obtain all the possible instruments to correctly describe and examine water injection strategy.