Correlation between fuel injection characteristics and in-cylinder flow development in an optical GDI engine under catalyst heating conditions

Abstract

This study aimed to investigate the effects of various fuel injection strategies on in-cylinder flow and flame propagation characteristics using an optical GDI engine operated under catalyst heating conditions. Under single injection conditions, the in-cylinder flow and turbulence characteristics varied significantly depending on the injection timing. Specifically, late injections including and after bTDC 240° enhanced in-cylinder flow and increased turbulent kinetic energy, leading to improved flame propagation speed and mixture homogeneity. In contrast, early injections at bTDC 320° and 280° disrupted or weakened the in-cylinder flow due to spray momentum, thereby deteriorating mixture formation and combustion characteristics. Therefore, from the perspectives of combustion stability and particulate matter reduction, relatively late injection timings were more favorable. In the multiple injection experiments, injection timing influenced flow asymmetry and kinetic energy distribution; however, the average flow velocity and tumble ratio were similar to those of the single injection case at bTDC 240°. Consequently, when the final injection occurred earlier than bTDC 90°, the flame propagation speed was comparable to that of the single injection at bTDC 240°. However, when the final injection timing was later than bTDC 90°, flame propagation characteristics deteriorated, making such conditions unsuitable for catalyst heating operation. Lastly, when a small amount of fuel was injected just before ignition, sufficient injection quantity led to enhanced turbulence and improved flame propagation, while also increasing the diffusion flame area and the potential for particulate emissions. This study provides experimental insights into the interaction between fuel injection and in-cylinder flow, serving as a foundation for establishing optimal injection strategies under catalyst heating conditions.