Homogeneous lean combustion is expected to be a key technology to further improve the combustion and reduce emissions of spark-ignition direct-injection engines. The application of lean combustion is facing many challenges such as slow flame propagation and combustion fluctuations. Under severe operating conditions such as low-speed lean-burn conditions, the weak in-cylinder airflow worsens the fuel and air mixing yielding difficulties in stable flame kernel initiation and consequently deteriorating flame propagation. In this study, the effect of flash boiling spray on flame kernel generation, flame propagation, engine performance, and exhaust emissions of the spark ignition direct injection (SIDI) engine under homogenous lean-burn conditions are investigated. A single-cylinder four-stroke optical SIDI engine was used in this study. The in-cylinder flash boiling and subcooled sprays during engine operation were compared using the Mie scattering technique. The in-cylinder flame propagation was recorded using a high-speed camera and analyzed using a digital image processing model. The results reveal that both subcooled and flash boiling sprays have collapsed, but the liquid phase particle concentration intensity of the subcooled spray was higher compared with flash boiling spray, indicating a higher vaporization rate of the fuel droplets in the flash boiling case. Consequently, the air-fuel mixing is improved, and the wall-wetting and fuel-rich regions are reduced. The flame images and combustion analysis demonstrate that flash boiling spray can effectively improve the flame kernel initiation and accelerate flame propagation. Additionally, flash boiling lean-burn combustion showed higher in-cylinder pressure, more heat release, improved IMEP with advanced combustion phasing, and lower cyclic variation compared to subcooled case. The color model showed that the premixed flame was enhanced under the flash boiling spray. This study shows that flash boiling spray can be an effective method to improve combustion efficiency in homogenous lean-burn mode under low engine speed. |
