Kastengren, A., Duke, D., Swantek, A., Sevik, J. et al., "Time-Resolved X-Ray Radiography of Spark Ignition Plasma," SAE Int. J. Engines 9(2):693-703, 2016, doi:10.4271/2016-01-0640.
Understanding the short-lived structure of the plasma that forms between the electrodes of a spark plug is crucial to the development of improved ignition models for SI engines. However, measuring the amount of energy deposited in the gas directly and non-intrusively is difficult, due to the short time scales and small length scales involved. The breakdown of the spark gap occurs at nanosecond time scales, followed by an arc phase lasting a few microseconds. Finally, a glow discharge phase occurs over several milliseconds. It is during the arc and glow discharge phases that most of the heat transfer from the plasma to the electrodes and combustion gases occurs. Light emission can be used to measure an average temperature, but micron spatial resolution is required to make localized measurements. In this paper, we present the results of a proof of concept experiment that demonstrates the use of time-resolved x-ray radiography to measure the density of the plasma in the spark gap during the glow discharge phase of a conventional transistorized coil ignition system. A focused monochromatic x-ray beam from the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory was used to make time-resolved point measurements of the projected density, which decreases as the plasma temperature increases. Multiple measurement positions were combined to produce a two-dimensional map of the projected density both inside and surrounding the spark gap. A temporal resolution of 153 ns and spatial resolution of 5 μm were achieved. A range of ambient gases, densities (1 - 3 bar) and ignition coil charging durations (1 - 2.5 ms) were considered. At least 30 repeated measurements were made for each sample point and condition. All these parameters were found to substantially alter the structure of the plasma as measured by x-ray radiography. From the projected density measurements, changes in thermal energy can be calculated. These provide a quantitative measurement of the energy deposited into the gas; this is only a small fraction of the total spark energy. The measurements demonstrate the feasibility of time-resolved x-ray radiography for the study of plasma structure and development for SI applications.