Optical Study on Spray and Two-Stage Ignition Characteristics for Diesel Spray Under Low Ambient Temperature and Density Conditions

Poor atomization and ignition difficulty due to the deterioration of environmental conditions restrict the cold-start performance of diesel engine. To investigate how the ambient and injection parameters affect diesel ignition characteristics at low temperature and density, liquid spray development was measured by back-illumination method; vapor spray and ignition process were visualized using high-speed shadowgraph method in constant volume combustion chamber. The results showed that liquid-vapor penetration and two-stage ignition delay have different sensitivities to variables: the variation of ambient density greatly affects the spray development while ambient temperature is the most significant parameter affecting ignition delay. Additionally, the change of injection pressure cannot cause significant change of both liquid penetration length and low temperature ignition, but increasing injection pressure promoted the vapor penetration length downstream development. Based on the data obtained, the empirical formulation in the form of power function was fitted for the stable stage of liquid penetration length, which proposed a reference for comparing the liquid phase development characteristics of diesel spray. Similarly, revise the Arrhenius-type ignition delay prediction formula and the correction coefficient K (K_LTI and K_HTI) was optimized quantitatively instead of fixed values, provides a preliminary theoretical basis for subsequent diesel spray model.