In-cylinder flow–flame interactions in a production engine via combined endoscopic PIV and high-speed chemiluminescence imaging

The relationship between in-cylinder flow structures and subsequent turbulent flame growth in a production spark-ignition engine was investigated using combined phase-locked particle image velocimetry (PIV) and high-speed flame chemiluminescence (CL) imaging via endoscopic access. Measurements were taken in a 4-cylinder production engine operating at 2000 rpm and 75 Nm torque with a stoichiometric fuel–air mixture. For two-dimensional flow field, a double-frame sCMOS camera was used to acquire 143 cycles of particle image pairs at 50°CA bTDC (15.5°CA before ignition timing). Simultaneously, time-resolved flame propagation images were captured every second combustion cycle using a high-speed CMOS camera (Vision Research Phantom v7.3) at a frame rate of 11 kHz. Broadband chemiluminescence imaging began at 50°CA bTDC and continued until 6°CA bTDC. The acquired images were analyzed in conjunction with pressure-derived heat release rates and mass fraction burned (MFB) to elucidate the relationship between engine performance and the physical characteristics of flame propagation. Analysis of instantaneous velocity fields from individual cycles unveiled substantial cyclic variations in flow structure. Combined flow–flame imaging demonstrated that the direction and magnitude of the flow near the spark region significantly influenced spark plasma orientation and early flame kernel development. Correlation map analysis indicated a strong positive correlation between local velocity magnitude and average flame speed, particularly in the vicinity of the spark plug. The high-correlation region is associated with the largest differences in the mean flow fields between slow and fast cycles.