Vapor/Liquid Behaviors in Split-Injection D.I. Diesel Sprays in a 2-D Model Combustion Chamber

Some experimental investigations have shown that the trade-off curve of NOx vs. particulate of a D.I. diesel engine with split-injection strategies can be shifted closer to the origin than those with a single-pulse injection, thus reducing both particulate and NOx emissions significantly. It is clear that the injection mass ratios and the dwell(s) between injection pulses have significant effects on the combustion and emissions formation processes in the D.I. diesel engine. However, how and why these parameters significantly affect the engine performances remains unexplained. The effects of both injection mass ratios and dwell between injections on vapor/liquid distributions in the split-injection diesel sprays impinging on a flat wall have been examined in our previous work. In this paper, the behaviors of the split-injection diesel sprays in a 2-dimensional model combustion chamber, which was installed in a high-temperature and high-pressure constant volume vessel filled with nitrogen, was observed by use of the ultraviolet-visible laser absorption-scattering (LAS) imaging technique. The effect of the injection mass ratios and the effect of the dwell(s) between injections on the distributions of fuel vapor and droplets were clarified through qualitative imaging of the optical thickness of vapor and the optical thickness of droplets, respectively. The findings give an implication to the potential relation between the vapor/liquid behaviors in the split-injection sprays and the reduction mechanism of NOx and particulate emissions of the D.I. diesel engine.