拡大VCOディーゼルノズルにおける針弁の偏心が噴孔内流れに及ぼす影響の数値解析

抄録

The VCO (Valve Covered Orifice) nozzle is known to reduce unburned hydrocarbon emission from direct-injection diesel engine. However, high-pressure fuel injection causes needle eccentricity which results in non-uniformity of sprays and deterioration of emission characteristics. Present study numerically investigates the needle eccentricity effects on the internal flow of the large-scaled VCO nozzle by using STAR-CCM+ (ver. 8.02.011), and obtained results are compared with those of a corresponding experiment. Swirling flow is formed in the nozzle hole due to the needle eccentricity. As increasing the swirl strength, air entrainment from the nozzle hole outlet is generated, and spray cone angle is decreased. However, in the range of large needle eccentricity, growth of the swirling flow decays, and the spray cone angle is saturated at certain value. These tendencies are also observed in the experiment. Thus, it is confirmed that the present numerical simulation reproduce the actual phenomena qualitatively. Furthermore, effects of edge curvature of the nozzle hole inlet is also numerically investigated as a key factor acting on the internal flow characteristics. As the edge curvature increases, the swirling flow in the nozzle hole considerably grows, and the air entrainment into the nozzle hole is greatly enlarged. Also, spray characteristics is extremely changed.