Simultaneous Measurements of Flame Propagation Velocity and Unburned Mixture Velocity in a Spark-Ignition Engine

The purpose of this study is to evaluate the behavior of unburned mixture compressed by the propagating flame front in a spark-ignition engine. This study employs a compression-expansion machine (CEM) to achieve simultaneous visualization of in-cylinder unburned mixture flow and the propagating flame using a bottom-view approach, aiming to understand the structure of turbulent premixed flames and combustion phenomena. By using high-speed color camera, CW YAG laser and flame-resistant solid tracer particles, both in-cylinder unburned mixture flow and the propagating flames were visualized simultaneously. By utilizing the characteristic scattering light of laser-traced particles and the propagation of blue flames, green channel (G-channel) was extracted to detect unburned mixture flow within the cylinder, while blue Channel (B-channel) were extracted to detect propagating flames. Methods for detecting flames from the visualization images and evaluating techniques for simultaneously measuring flame propagation velocity and unburned mixture flow velocity were investigated. A comparison of the unburned mixture velocity and the flame propagation velocity revealed that the unburned mixture is compressed by the propagating flame. Regarding the vicinity of the flame front, the vorticity was calculated from the velocity distribution of the unburned mixture obtained through experiments and compared with the flame front shape. It was confirmed that the vortices in the unburned mixture were intensified in a manner that follows the shape of the elongated and protruding flame.