An investigation of mixture formation and in-cylinder combustion processes in direct injection diesel engines using group-hole nozzles

Mixture properties of the transient fuel spray injected by group-hole nozzles were quantitatively studied in a constant volume chamber via the laser absorption scattering (LAS) technique, and compared with conventional single-hole nozzles. Specific areas investigated included non-evaporating and evaporating ambient conditions, and the conditions of free spray and spray impinging on a flat wall. The particular emphasis was on the effect of one of the key parameters of the group-hole nozzle structure, namely, the interval between orifices. Subsequently, in-cylinder ignition and combustion processes were investigated by direct flame visualization in a single-cylinder optical research engine using the group-hole nozzle and the standard multi-hole nozzle, together with heat release analysis. Results show that the group-hole nozzle can produce a smaller Sauter mean diameter (SMD) of droplets under the non-evaporating condition. Under the evaporating condition, for the free spray cases, there exists a trade-off between the fuel evaporation and the spray penetration in the application of the group-hole nozzle. With the increase in interval between orifices, the ratio of evaporation increases, but the spray tip penetration decreases. For the spray impinging on a flat wall, the group-hole nozzle can enhance the spray tip penetrations to some extent compared with the single-hole nozzles. Flame images taken in a direct injection optical engine show that the enhancement of the fuel atomization and evaporation by the group-hole nozzle may contribute to a decrease in overall flame luminosity level, suggesting reduced soot formation in diesel engines.