The shortening of lift-off length associated with jet-wall and jet-jet interaction in a small-bore optical diesel engine

Jet-wall and jet-jet interactions are important diesel combustion phenomena that impact fuel-air mixing, flame lift-off and pollutant formation. Previous studies to visualise a wall-interacting jet in heavy-duty diesel engines suggested that the shortening of lift-off length could occur due to the recirculated hot combustion products that are entrained back into incoming diesel jet. The significance of this effect, known as re-entrainment, can be higher in small-bore engines due to shorter nozzle-to-wall distance and increased wall curvature. In this study, we performed hydroxyl chemiluminescence imaging using an intensified CCD camera and high-speed imaging of natural soot luminosity using a CMOS camera in an automotive-size optical diesel engine. To provide detailed understanding of the reacting jet under the influence of re-entrainment as well as jet-jet interaction, various jet trajectories were investigated using one and two-hole injectors coupled with a modified piston that allows (1) the identification of the shortening of lift-off length, (2) the measurements of lift-off lengths for varying degrees of jet-wall interactions and (3) the clarification on inter-jet spacing effects on the lift-off lengths. Findings from the measurements support the re-entrainment theory because the shortening of lift-off length occurs only before the end of injection when the strong jet momentum induces the ambient gas entrainment. The shortening also shows good correspondence with the penetration of the recirculated jet head back towards the centre of the combustion chamber. Simultaneous imaging of the bowl-wall-interacting jet and cylinder-liner-wall-interacting jet depicts a shorter lift-off length for the bowl-wall-interacting jet, which further supports the importance of re-entrainment on diesel combustion. Inter-jet spacing effects on the lift-off length are also studied utilising two-hole injectors with two inter-jet spacing angles (51.4° and 102.8°). A narrower spacing between the jets results in the shorter lift-off length of the primary jet due to the higher jet-jet interaction even if short injection duration is used to suppress the re-entrainment. For tested conditions of this study, the shortening of lift-off length appears to be more sensitive to jet-jet interaction than to the re-entrainment caused by the jet-wall interaction.