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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.

UV-chemiluminescence from the excited hydroxyl-radical (OH*) has been used as a marker for the high-temperature reacting zone in spark-ignited engines for quite some time. In research engines with large optical access, sensitive camera systems make it possible to obtain images of the flame that can be used for, e.g., determining the flame-front's propagation speed [Aleiferis et al., Combust. Flame 136 (2004) 283-302]. However, on one hand such optical engines are limited in their speed and load range, on the other, typical UV endoscopes make wide-field imaging at low light levels challenging. Here, a large-aperture UV endoscope is used to capture sequences of OH* chemiluminescence during early flame propagation in a nearly unmodified production engine. We compare three imaging systems: phase-locked single-shot imaging, phase-locked double-frame imaging, and “high-speed” cinematography at kHz repetition rates.