Flame Image Velocimetry Analysis of Flame Front Turbulence and Growth Rate in an Optical Direct-Injection Spark-Ignition Engine

High-speed flame imaging has been widely used to investigate flame propagation in optically accessible direct-injection spark-ignition (DISI) engines. Previous studies utilized a high-speed movie to measure the overall growth rate of the flame and to analyze the flame shape and its correspondence with engine performance and efficiency. This study proposes the flame image velocimetry (FIV), a new diagnostic method enabling time-resolved, two-dimensional flame front vector extraction and turbulence intensity calculation. The high-speed camera is used to record the propagating petrol flame, and contrast variations are tracked to derive flow vectors. The PIVlab, a Matlab-based open-source code, is used for this flame front FIV analysis, and the systematic optimization of processing parameters is performed. The raw flame images are preprocessed using the contrast-limited adaptive histogram equalization (CLAHE) filter before a four-step fast Fourier transform (FFT) is applied. The interrogation window size for each step is optimized to achieve the highest flow vectors, which, for the studied cases, return 84-84-24-24 pixels with a half overlap. A total of 100 combustion cycles are FIV processed for each test condition to tackle the inherent cyclic variations. The Reynolds decomposition is applied to individual cycles to derive high-frequency component magnitude, which is interpreted as turbulence intensity. A spatial filtering method is used for the decomposition with optimized cut-off lengths for minimal cyclic variations of the measured turbulence intensity. The new FIV method is proven useful in a case study using two injectors with different nozzle structures. The results show that a smaller hole diameter and counterbore hole shape leads to a higher flame front vector magnitude, overall higher turbulence intensity, and more uniform distribution of turbulence than the injector with a larger hole diameter and cylindrical hole shape. This FIV result explains the higher engine power output and lower cyclic variations measured for the smaller hole injector.