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Nowadays internal combustion engines can operate under lean combustion conditions to maximize efficiency, as long as combustion stability is guaranteed. The robustness of combustion initiation is one of the main issues of actual spark-ignition engines, especially at high level of excess-air or dilution. The enhancement of the in-cylinder global motion and local turbulence is an effective way to increase the flame velocity. During the ignition process, the excessive charge motion can hinder the spark discharge and eventually cause a misfire. In this perspective, the interaction between the igniter and the flow field is a fundamental aspect which still needs to be explored in more detail to understand how the combustion originates and develops. In this work, a combined experimental and numerical study is carried out to investigate the flow field around the spark gap, and its effect on the spark discharge evolution.

Lean operation of spark ignition engines is a promising strategy for increasing thermal efficiency and minimize emissions. Variability on the other hand is one of the main shortcomings in these conditions. In this context, the present study looks at the interaction between the spark produced by a J-type plug and the surrounding fluid flow. A combined experimental and numerical approach was implemented so as to provide insight into the phenomena related to the ignition process. A sweep of cross-flow velocity of air was performed on a dedicated test rig that allowed accurate control of the volumetric flow and pressure. This last parameter was varied from ambient to 10 bar, so as to investigate conditions closer to real-world engine applications. Optical diagnostics were applied for better characterization of the arc in different operating conditions. The spatial and temporal evolution of the arc was visualized with high-speed camera to estimate the length, width and stretching.

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.