| Optimization and upgrade of fuel supply system is key towards meeting the increasingly stringent demands of emission and fuel consumption regulations. Due to its superiority as compare to other, electronically controlled high-pressure common rail fuel injection system is widely used nowadays. Fuel injection rate measurement from each nozzle hole of a multi-hole injector significantly contributes to the performance optimization of fuel supply system and its relative systems (i.e. air intake system and combustion system). Flow characteristic differences among nozzle holes are one of the main causes of fuel injection differences during spray and combustion.Therefore to determine the influence of this phenomenon on subsequent processes, the flow characteristics and fuel injection process of each hole of a multi-hole diesel injector was investigated in this study.The technique and equipment capable for measuring fuel injection rate under high injection pressure simultaneously from each nozzle hole of a multi-hole injector was designed and constructed. With this, the technique for measuring and testing fuel injection rate from each hole of a multi-hole diesel injector of a high-pressure common rail system was proposed and established. The accuracy of the measuring system was checked with an injection flow rate meter. After validating the measuring system, fuel injection characteristics of an evenly arranged six-hole injector under different operation conditions (40 MPa?160 MPa injection pressure) were then measured and analyzed. Moreover, numerical nozzle model of the evenly arranged six-hole injector was established and used to investigate in-nozzle hole flow and injection characteristics differences among the nozzle holes under the following conditions: needle motion perfectly centered (symmetric motion), eccentric needle motions from either elastic deformation or not (asymmetric motion), injection pressure variation, different nozzle types (Blind Hole Nozzle-SAC nozzle and Valve Covered Orifice-VCO nozzle). After investigating the effect of in-nozzle inner surface characteristics on fuel flow and injection properties, the following conclusions were reached:Under various working conditions (range from high injection pressure and fuel mass to low injection pressure and fuel mass), hole-to-hole fuel injection rate of a modern diesel engine can be accurately measured by the appropriate selection of a piezoelectric force sensor and charge amplifier set-up with adjustable magnification and sensitivity. With the same nominal dimensions, differences in fuel injection characteristics among nozzle holes are obtained. However with an increase in either the pulse width or the injection pressure, the injection uniformity among nozzle holes is improved effectively. Under the same conditions, cycle-to-cycle variation of total injection quantity from the nozzle is always lower than the cycle-to-cycle variation of the injection quantity from each nozzle hole. This is because of the mutual dependence of each nozzle hole injection with other hole in the nozzle. With an increase in either the pulse width or the injection pressure, the cycle-to-cycle injection homogeneity is improved effectively.Under the same operation conditions and with a perfectly centered needle motion,VCO nozzle has early cavitation inception, faster cavitation development,stronger cavitation intensity, relatively lower fuel injection rate and cycle fuel injection quantity and higher non-uniform coefficient of cycle fuel injection quantity among nozzle holes as compared to SAC nozzle.Irrespective of the nozzle type (SAC or VCO nozzle), the nozzle hole relatively further from the needle displacement during inelastic eccentric needle movement,develops the earliest cavitation formation in its upper surface and inner part,stronger cavitation intensity and higher fuel flow velocity. This phenomenon is much more highlighted with increasing needle eccentric displacement and fuel injection pressure. Also, the relatively closest nozzle hole to the eccentric needle motion, develops cavitation at its lower surface while the farthest hole, develops no cavitation formation during injection process. More fuel flows through the relatively large opening created by the displaced needle and enters into the hole closer to the needle displacement via the pressure chamber during the eccentric needle movement. High velocity fuel flow areas are located at the upper parts of the holes close to the needle displacement whereas in the holes relatively further,high velocity flow areas are located at the lower part of the hole. The holes relatively closer to the needle during the eccentric motion, have higher fuel injection rate and cycle fuel injection quantities comparatively. The effect of eccentric needle motion on flow is stronger in VCO than in SAC nozzle. Also the non-uniform coefficient of cycle fuel injection quantity among VCO nozzle holes is higher than that of SAC.Under elastic eccentric needle displacement, the flow area around the closest hole to the needle gets blocked considerably by the needle while the flow area around the hole farthest from the needle gets relatively bigger. During the initial stages of fuel injection, hole-to-hole fuel injection rate and fuel flow velocity variations corresponds with the variations in flow area. That is the hole with higher flow area has higher fuel flow velocity and fuel injection rate comparatively. The flow characteristics and injection characteristics of the fuel inside the hole closest to the needle is more responsive to the needle's eccentric motions. That is, the fuel injection quantity of the hole at the initial stages of injection is high together with the cycle fuel injection quantity, however at a more advance stage of injection,the hole's cycle fuel injection quantities become relatively low. The non-uniform coefficient of cycle fuel injection quantity among nozzle holes under eccentric needle motion are higher than those from perfectly centered needle motion, also hole-to-hole variations in cycle fuel injection quantities are relatively higher under increasing degree of needle eccentricity.The development of cavitation inside the holes is suppressed by the holes inner surface roughness. That is with the ratio of inner surface roughness height and hole diameter Ra/d increment, the cavitation intensity inside that hole reduces,and its fuel flow velocity and the gas volume fraction at the outlet gradually reduces. Also the hole's turbulent kinetic energy increases. However, under different inner hole surface roughness or different inner nozzle surface roughness's (hole, needle valve and needle seat surfaces), the mass flow rate of the hole is not significantly influenced. This is as a result of the comprehensive influence between cavitation and fuel flow velocity. |
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