Structure and Measurement of Atmospheric and High-Pressure Ignition Plasma
Authors: Shaffer James , Zare Saeid , Askari Omid 
Published: January 2022

Abstract
Experiments are conducted to understand atmospheric spark ignition process in more detail. The research done relates the electrical energy dissipated across the spark gap to the measured schlieren ignition volume. The result is the supplied electrical thermal energy. The study provides insight into the structure of plasma and the mechanisms which convert electrical power into heat. The research is done to support laminar burning speed calculations to increase accuracy and extend diagnostic techniques to conditions otherwise immeasurable. Typically, plasma measurements are taken via a Langmuir probe. However, for the automotive ignition plasma, this measurement technique is challenging because of the transient nature, high pressure, and temperatures involved. Therefore, several alternative techniques will be used in order to find the potential distribution of the plasma and unveil the structure of the plasma more specifically the cathode fall. Three different voltage measurements are taken in order to capture the cathode fall of the plasma. One method simply measures the potential using a high voltage probe. This method may be inaccurate because of the presence of charged ions, however, these results are compared to non-intrusive measurements where voltage data is extrapolated over various gaps sizes to zero length. It is generally agreed that the desired measurement for this work, the cathode fall, remain constant and depends on the composition of the gas and the electrode. Therefore, changing the system input power and the gap will only change the voltage drop across the bulk plasma. The linear change in voltage potential through variation of testing parameters like gap length can then be extrapolated to zero length of the bulk plasma or minimum energy value which should be equal the value of cathode fall and bulk plasma potential respectively. It was found that after excluding systemic losses such as electrical resistance and ignition coil inefficiencies, the primary loss within the plasma gap is the potential drop across the cathode sheath. Excluding the loss in the cathode fall results in a measured electrical data that is responsible for thermal discharge. In order to highlight the findings, electrical discharge energy is compared to the volume of the heated gas kernel in atmospheric air. Removal of the cathode fall data will show that the energy is proportional to the volume of heated gas whereas, before the change in energy dissipation between glow and arc plasmas prevented this relationship from being visible. The data and methods discussed in this research provides the means to determine the thermal energy of ignitions and sparks even when the spark is inaccessible or obscured. Further work will be done utilizing the power measurement found in this work in a model to predict the affected thermal spark volume. It is also proposed that further validation of the proposed measured electrical thermal energy should be compared to the energy measured with a calorimeter to determine any other inefficiencies in the plasma discharge process. Additionally, the experimentation done observes the cathode fall of only glow plasma, Additional work should be done to find the cathode fall of arc plasma.