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. Moreover, optical emission spectroscopy (OES) was used for evaluating vibrational temperatures based on the line-shift approach. Statistical analysis of arc morphological parameters, along with electric measurements on the primary and secondary coil circuit, were used for defining boundary conditions for the numerical simulations.
The CFD simulations are setup to match the experimental vibrational temperature profile during the discharge process. This key aspect, along with the secondary circuit electric characterization, allows to quantify the amount of thermal energy that is deposited in the gas and, at the same time, the heat dispersed at the spark plug electrodes. Furthermore, the model is able to predict the arc stretching for all investigated cases in terms of spatial and temporal development, providing detailed information for a better understanding of the interaction between the discharge and the flow field.