Lean burn mode of direct injection spark ignition engines (DISI) can improve the engine's thermal efficiency. However, it showed slow flame propagation and combustion instabilities which are strongly related to mixture distribution. Multiple injections can improve control over in-cylinder charge distribution and consequently on burn mode. Additionally, multiple injections can reduce spray impingement and enhance the turbulence in the compression stroke, particularly with late injection, but the time is not enough for spray evaporation. Fuel injection under flash boiling conditions boosts fuel vaporization and spray-airflow interaction of the late injections. This study investigates the effect of flash boiling multiple injections on the combustion of lean gasoline/air mixture in an optical DISI engine. This work uses Mie scattering, CFD, high-speed imaging, and flame images postprocessing model to investigate spray characteristics, spray-airflow interaction, flame propagation, and CH* digital intensity. The results reveal that flash boiling multiple injections has significantly mitigated the liquid spray intensity indicates the elevated rate of vaporization with reduced spray impingement and better interaction with the tumble flow compared to the subcooled single injection. The optimal engine performance and combustion were achieved when the second injection and highest tumble ratio point were timely synchronized, for example, 260°bTDC at 800 rpm and 280°bTDC at 1500 rpm. Thus, flash boiling multiple injections can effectively enhance the thermal efficiency by 54.65% and 11%, respectivly. The CH* digital intensity and heat release showed that flash boiling multiple injections could effectively improve the lean-burn operation of the DISI engine. |
