Investigation of a novel leaner fuel spray formation for reducing soot in diffusive diesel combustion- Homogenizing equivalence ratio distribution in the lift-off region -

摘要

Automotive powertrain is diversifying. It is needed to adopt optimal powertrain considering CO_2 reduction and convenience for the users. Diesel engines are beneficial from the points of view of thermal efficiency and reliability, but further emission reduction is needed. To reduce soot formation and late combustion, lowering equivalence ratio at the lift-off position, or fuel-rich spray core is thought to be effective. One approach to realize this is by enhancing air introduction from entrainment section (“(1)Increasing entrainment amount”), diluting the entire spray core. In addition to the conventional approach, this study discusses an alternative approach in which entrained air is selectively supplied to over-rich region to homogenize equivalence ratio at lift-off point, (“(2)Homogenization”). The investigations were performed by varying not only injection method like injection pressure or nozzle hole specification, but also fuel properties. Oxygenated fuel investigation revealed that homogenizing equivalence ratio of spray core leads to fast combustion. Furthermore, lowering surface tension investigation revealed that it can shorten break-up length and promote air introduction from entrainment section without higher injection pressure, and it leads to reduce equivalence ratio of spray core. In addition, it was found that Jet number which is a dimensionless number for break-up can organize the smoke emission for different fuel or injection pressure. Ducted Fuel Injection (DFI) was investigated by both measurement and calculation, and it was found that the duct makes equivalence ratio distribution inside the spray to be more homogeneous, or promotes entrainment of air into spray core.