Effects of Injection and Intake Timing on Atomization and Combustion in a Hybrid Gasoline Direct Injection Optical Engine under the Low-Speed Low-Load Condition

With the growing trend of hybridization in modern engines, hybrid gasoline direct injection (GDI) engines are typically designed for high-load, mid-to-high-speed operation to achieve optimal fuel economy. However, these engines inevitably operate under low-speed, low-load conditions where insufficient intake air often leads to poor air-fuel mixing and weak turbulence, resulting in suboptimal combustion. Adjusting intake and injection timing presents a simple and effective approach to optimizing the combustion process in hybrid GDI engines. In this study, an optical engine with a combustion system geometry identical to that of an advanced hybrid GDI engine is used. The engine features a compression ratio of 15.0:1 and is equipped with a variable timing camshaft for intake timing control and an electronically controlled system for injection timing. High-speed color imaging, using transparent pistons and cylinder liners, captures the in-cylinder spray development and combustion processes. The interaction between the airflow, spray, and piston is analyzed to better understand its effects on combustion. The results show that advancing the intake timing, relative to the baseline intake and injection settings, enhances the interaction between the airflow and the fuel spray, improving flame kernel development and reducing pool fires caused by fuel films on the piston. In contrast, delaying the injection timing reduces pool fires by improving spray impingement on the piston; however, the delay prevents proper evaporation of the fuel droplets, causing the formation of a sooty yellow flame on the intake side due to the accumulation of fuel in the tumble region. This study demonstrates that careful tuning of intake and injection timings can optimize in-cylinder combustion, improve fuel-air mixing, and reduce emissions in hybrid GDI engines operating under low-speed, low-load conditions.