Near-nozzle microscopic characterization of diesel spray under cold start conditions with split injection strategy

•Ultra-high speed imaging and cooling system were developed.•Effects of low fuel temperature on spray were studied.•Impact of split-injection strategy on spray behavior was studied.•Effects of fuel properties on spray characteristics were studied.

Near-nozzle microscopic characteristics of diesel spray under room temperature (25 °C) and low temperature (−2 °C) were investigated by microscopic imaging technique. The primary breakup of winter diesel (WD) and rapeseed methyl ester (RME) sprays were investigated with single and split injection strategies. It was shown that increased viscosity and surface tension under low temperature lead to much poorer dispersion. Under low injection pressure with split injection strategy, the first split injection was unexpectedly severely affected by both temperature and dwell, with significant breakup characteristic differences when dwell varied. By contrast, the second split under low injection pressure tended to be affected only by temperature rather by dwell. High injection pressure considerably alleviated the breakup characteristic difference of the first split injection caused by temperature and dwell although the effects of fuel properties were still seen, leading to better fuel dispersion and more predictable spray characteristics. In addition, RME with higher viscosity and surface tension consistently presented much poorer dispersion quality compared with WD even under high injection pressure where the influence of fuel properties may be insignificant.