Optical Diagnostics of Inversed-Delta Rate Shaping Diesel Spray Flame towards Reduction of Late Combustion

In our previous work, diesel late combustion heat release is suspected to originate from rich fuel mixture cloud stagnating at the spray tip. Injection rate shaping is gaining attention as an attractive strategy to control diesel spray combustion characteristics where it could be an effective approach in reducing the late combustion. Progressive ramp-down injection rate as in "inversed-delta" shape is achieved by using a novel rate shaping injector called TAIZAC (TAndem Injectors Zapping Activation); rate shaping can be realized by controlling the actuation timing of two directly-connected commercially available injectors. To investigate the potential of inversed-delta rate shaping for reduction of diesel late combustion, simultaneous high-speed UV laser diffuse back illumination (DBI), UV emissions and soot luminosity imaging of inversed-delta and conventional rectangle-injected spray flames conducted in a constant volume combustion chamber are compared. In inversed-delta injection, faster combustion duration and higher initial heat release comparable to high pressure rectangle injection is observed from pressure-derived heat release rate analysis. Faster disappearance timing of spray tip rich mixture and UV emissions are noted in inversed-delta diesel spray indicating that late combustion heat release due to the rich mixture is reduced. Spray tip of the inversed-delta spray initially penetrates faster but it seems to be slowed down to which the rectangle spray tip penetration eventually surpasses it. It can be suggested that the inversed-delta rate shaping by using TAIZAC injector has a mixing and combustion characteristics as in high pressure injection, with the merits of low penetration as in low pressure injection.