Diesel air-fuel mixing and combustion have been investigated in a Rapid Compression Machine (RCM). The measurements were performed at high injection pressures up to 260 MPa and under reacting and non-reacting conditions. The spray was injected through solenoid-controlled multi-hole injectors. Two nozzles were applied with orifice diameters of 175 μm (D175) and 150 μm (D150), respectively. The visualization of the penetration of the liquid and the gaseous phase as well as the spray cone angle under evaporative, non-reacting conditions was carried out by the shadowgraph imaging technique in combination with a high speed camera. For combustion studies the flame luminosity of the flame as well as the chemiluminescence signals emitted by the OH radicals in the UV range were detected.
Investigations revealed different behavior of the macroscopic spray characteristics with the two applied nozzles when increasing the injection pressure from 200 MPa to 260 MPa. With the larger nozzle diameter (D175) the spray penetration and the spray propagation velocity increase as the injection pressure is increased. On the contrary to that, with the smaller nozzle diameter (D150) an increase of the injection pressure had no effect on the spray velocity. With 260 MPa a higher spray penetration was only observed at the beginning of the injection due to the faster opening of the needle. The further propagation of the tip of the spray was similar with 200 MPa and 260 MPa. With both applied nozzles the injection pressure has little effect on the penetration length of the liquid phase. At an applied injection pressure of 200 MPa the near-nozzle spray angle is wider with D175, whereas similar spray angles were observed at 260 MPa. From the measurements in reacting atmosphere an earlier ignition of the fuel and a faster combustion could be shown with nozzle D150. In addition, a higher combustion pressure was measured. This can be attributed to better air-fuel mixing and a higher premixed portion, which was confirmed by the analysis of the spray angles in the far-nozzle region obtained from the shadowgraph images at non-reacting conditions.