High-Speed Imaging of Early Flame Growth in Spark-Ignited Engines Using Different Imaging Systems via Endoscopic and Full Optical Access

This work investigates the image quality achievable with a large-aperture endoscope system and high-speed cameras in terms of detecting the premixed flame boundary in spark-ignited engines by chemiluminescence imaging. The study is an extension of our previous work on endoscopic flame imaging [SAE 2014-01-1178]. In the present work, two different high-speed camera systems were used together with the endoscope system in two production engines to quantify the time-resolved flame propagation. The systems were cinematography with a CMOS-camera, both with and without an intensifier, the latter variation being used in a four-cylinder automotive engine as well as in a single-cylinder motorcycle engine. An algorithm with automatic dynamic thresholding was developed to detect the line-of-sight projected flame boundary despite artifacts caused by the spark and the large dynamic range in image brightness across each time series. A fundamental problem in evaluating the systems’ efficacy is that it is not clear what constitutes “correct” detection of flame boundary. To help clarify this question, the endoscopic results are compared among each other and to a “best-case scenario”, which was unintensified high-speed imaging with a large-aperture commercial camera lens in an engine with full optical access. We find that for the part-load operation examined here, broadband unintensified imaging through the endoscope yields useful and physically plausible results during all of the early flame propagation. Imaging is most challenging during inflammation, when the spark ignites the first small volume of mixture. For the conditions examined here, the chemiluminescence of the flame then is just at the level of the camera’s background-corrected read-out noise, while the spark in the immediate vicinity can still be very bright.