Mechanisms of fuel injector tip wetting and tip drying based on experimental measurements of engine-out particulate emissions from gasoline direct-injection engines

Abstract

Gasoline fuel deposited on the fuel injector tip has been identified as a significant source of particulate emissions at some operating conditions of gasoline direct-injection engines. This work proposes simplified conceptual understanding for mechanisms controlling injector tip wetting and tip drying in gasoline direct-injection engines. The objective of the work was to identify which physical mechanisms of tip wetting and drying were most important for the operating conditions and hardware considered and to relate the mechanisms to measurements of particulate number emissions. Trends for each of the physical processes were evaluated as a function of engine operating conditions such as engine speed, start of injection timing, engine load, fuel rail pressure, and coolant temperature. The effects of fuel injector geometries on the tip wetting and drying mechanisms were also considered. Several mechanisms of injector tip wetting were represented with the conceptual understanding including wide plume wetting, vortex droplet wetting, fuel dribble wetting, and fuel condensation wetting. The main tip drying mechanism considered was single-phase evaporation. Using the conceptual understanding for tip wetting and drying mechanisms that were created in this work, the effects of engine operating conditions and fuel injector geometries on the mechanisms were compared with experimental results for particulate number. The results indicate that measured particulate number was increased by increasing injected fuel mass. Increasing injected fuel mass was suspected to increase tip wetting via wide plume wetting and vortex droplet wetting mechanisms. Particulate number was also observed to increase with hole length. Longer hole length was suspected to result in higher tip wetting via vortex droplet and fuel dribble wetting mechanisms. Longer timescale was found to decrease particulate number emissions. Lower speeds and early injection timings increased the timescale. Similarly, higher coolant temperature decreased particulate number. The coolant temperature influenced tip temperature resulting in higher tip drying.

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“…33 Another mechanism of PM formation, which can be possibly partially mitigated by higher injection pressure, is injector's tip wetting, leaving a liquid residue to slowly vaporise, lowering the mixture quality. 34,35 Interestingly, the bottom plot in Figure 12 also shows that the PN variation span due to changes in injection pressure is the smallest in the case of Reference fuel. Having the lowest vapour pressure, and the highest T10, T50, T90, and FBP (Table 3), the Reference fuel is the least volatile fuel among the three tested gasolines.…”

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Investigation of Fuel Injector Tip Wetting Using Laser-induced Fluorescence under Flash Boiling Conditions

“…There are many explanations about the mechanisms of tip wetting. For instance, it is held that tip wetting is mainly contributed by three factors: (1) Fuel impingement on the edge of the pre-hole, (2) vortex tip wetting, and (3) fuel dribble/dripping [1]. The first factor is mainly the results of overly expanding spray plumes impinging onto the wall of step holes.…”