| With increasing environmental protection awareness globally,pollutants reduction and fuel consumption have become one of the important challenges in diesel engines.The optimization of injection processes in terms of fuel injection rates and spray characteristics,are crucial in improving the performance and emission characteristics of diesel engines.Due to hole-to-hole geometric differences within symmetric multi-hole diesel injectors,there are diesel fuel flow characteristics differences among the holes during fuel injection.However the equipment and methods available currently,can only be used to determine the total injection rate from a multi-hole diesel injector.They cannot measure the possible hole-to-hole differences in injection rates among the nozzle holes.In this regard,the development of a hole-to-hole injection rate measuring system,will be immensely significant since it could serve as a guide for optimizing multi-hole fuel injector at the design and quality control stages during manufacturing.With regards to the mass and momentum conservation laws,a transient measuring method based on spray momentum flux was used to study the behavior and stability of injection from each nozzle hole of a multi-hole injector.Considering research background,research status and the challenges associated with the traditional measurement technique,a spray momentum flux experimental test rig that ensured the simultaneous and continuous data acquisition from all the holes of a multi-hole injector was constructed.The test rig was connected to a high pressure common rail test bench.In this research,the following works have been done:Before the commencement of experiment,injection rate measuring system for each nozzle hole(consisting of hardware and software interface system)was designed and developed based on the spray momentum flux measurement principle.Afterwards,a spray momentum flux experimental test rig was constructed from the design.In this research,details of the data acquisition system are presented together with the types of sensor systems.The two types of sensors systems(an ‘amplifier integrated sensor with a fixed measuring range' and an ‘amplifier-sensor setup with adjustable measuring range')are also analyzed and compared with one another.Furthermore,the accuracy of the customized spray momentum flux experimental test rig was investigated through series of experiments.With the aid of a common-rail pump test bench,the influence of sensor target distances and sensor target-axis angles on the measured result were analyzed.It was observed that,sensor target distance of less than 15 mm and sensor target-axis angle within the range of 90°± 5° were the optimum settings required to ensure reliable measurements.Validation of cycle fuel injection quantities from each nozzle hole of the eight hole nozzle was performed with the aid of an EFS injection flow meter.It was observed that the relative errors between experimentally determined and EFS measured cycle fuel injection quantities was less than 5%.After validating the measuring system,a two-layered 8-hole,four valve diesel engine injector was tested with the customized spray momentum flux experimental setup.The influence of injection pulse and injection pressure on fuel injection characteristics,in terms of injection rate,cycle fuel injection quantities of each nozzle hole,the relative average deviation of each nozzle hole fuel quantities,were then analyzed under different operation conditions.With the increment of injection pressure or injection pulse width,the variation coefficient of the cycle injection quantities of each nozzle hole and the cycle fuel injection quantities difference between the lower and the upper layered nozzle holes declines gradually.Hence the uniformity of the fuel injection quantity was improved.A three-dimensional(3D)gas-liquid two-phase flow model of an 8-hole two-layered diesel injector nozzle,was established and validated to ascertain its accuracy by comparing injection rates results obtained from it to those obtained experimentally(with a spray momentum flux experimental setup).This showed that,the established model can be adequately used to predict transient injection rates and other flow characteristics within and from the nozzle holes at various operating conditions.At the same time,the effects of nozzle hole location on the characteristics of the two-phase flow,were analyzed numerically(from simulation results).This explains why the average flow velocities of fuel flow through the lower layered holes are larger than those through the upper layered nozzle holes.From the cavitation and the flow velocity distributions of the liquid phase in the double-layered nozzle,it is clear that the larger cavitation formation in the upper layered holes resulted in a smaller effective flow area,which ensured that the injection rates from the upper layered nozzle holes were lower than the injection rates from the lower layered hole. |
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