| The stringent emission regulations to diesel engines have resulted in higher requirements on the emission characteristics,economic characteristics and dynamic characteristics of diesel engines.Diesel engines would emerge different performance due to different injection rates.The injector is the core component of the supply system.Theoretically,the injection characteristics and the internal flow characteristics of each hole should be the same.However,the errors in machining,hydraulic grinding and the differences in the structure design lead to the eccentric needle movement during the needle movement.This causes non-uniformity of the spatial and temporal fuel distributions in the combustion chamber,leading to a compromised combustion and emission characteristics from the diesel engines.Therefore,it is essential to research the effect of the needle eccentric motion on the consistency of the injection of the highpressure common rail system,which would make the combustion and emission characteristics optimized.With regards to spray momentum flux,a measuring system including the control system,the high-pressure common rail test bench and the fuel injection test cabinet was designed,developed and used to measure the injection rates from the holes of a multi-hole injector.The supporting software was also compiled and improved.It could simultaneously test the injection rate of each hole of the multi-hole injector with the multiple acquisitions in multi-cycle.The cycle fuel injection quantity of each hole was tested to validate the accuracy of the established test bench.A three dimensional(3-D)model of the nozzle was used to analyze the injection characteristics of the nozzle after validating the model with experimental results.The validation was conducted by comparing experiment and simulation injection rate results,acquired simultaneously from all the holes of the injector(with a customized spray momentum flux experimental test rig)and the model.The comparison yielded only 5 % relative error between the results.Therefore,the model’s prediction ability was within acceptable limits.Further analysis shows that the fuel injection rates and the cycle fuel injection quantities of the lower layered holes are higher than those of the upper layered holes.The injection rate difference between the upper and lower layer of holes was analyzed.The needle eccentric motion model without elastic deformation was established for simulation.The injection rate,the cycle fuel injection quantity and the internal flow characteristics of each hole under different needle eccentricities,different needle eccentric directions and different operating conditions were analyzed.The difference of the injection characteristics and the cavitation characteristics are exacerbated with the increase of the needle eccentricity.The cycle fuel injection quantity of the relatively closest nozzle hole to the eccentric needle motion are higher and the cavitation in the same hole are weaker.The upper layered holes are more susceptible to needle eccentricity compared to the lower layered holes.The eccentric motion of the needle has a greater impact on the injection rate when the needle is not fully opened.The needle eccentric motion model with the elastic deformation was established for simulation for the comparison of the established model without the elastic deformation.The results indicate that the elastic deformation of the needle appear during the opening phase,the injection rate of each hole changes sharply.The injection rate of the relatively closest nozzle hole is lower at the initial stage.The cavitation first appears on the lower surface of the relatively closest nozzle hole,which gradually decreases as the needle rises on wall of the needle valve body.The cavitation gradually transfers to the upper surface of the injection hole with the vertical rise of the needle valve.The uniformity of the injection becomes worse as the needle eccentricity increases in two eccentric models. |
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