| As emission regulations become stricter nowadays,improving injection process in internal combustion engines(ICE),is one of the ways of enhancing combustion characteristics and reducing pollutants emissions.Combustion characteristics are known to be influenced by the quality of air-fuel mixtures,which in turn are affected by the quality of injection characteristics.But since injection characteristics are also influenced by internal nozzle geometry,such as needle displacement,the characterization of the dynamic effect of the needle displacement on injection will also influence mixture formation,combustion and emission characteristics from direct injection IC engines.Spray injection from multi-orifice nozzles is generally non-uniform due to the eccentric movements associated with their needle dynamics during needle lift.Therefore,determining the trajectory of the needle eccentricity and characterizing the effect of the eccentricity on injection will provide other possibilities of enhancing combustion characteristic and reducing pollutant emissions.Hence in this study,the injection characteristics differences from the orifices of a symmetric VCO nozzle used in direct injection ICE,were numerically studied and analyzed under on-axis and off-axis needle displacements.The novelty of the study is the characterization of spray jets from a multi-orifice nozzle experiencing off-axis needle displacement and the estimation of the magnitude of the off-axis displacement,the needle experienced.To validate the model of the VCO nozzle,the needle's effect on internal nozzle fuel flow and injection characteristics from each orifice of the model(obtained from on-axis needle displacement CFD simulation),was analyzed and compared with experiment results obtained from the VCO injector with the aid of a customized spray momentum flux test rig.The validation experiment was conducted under the principle of spray momentum flux.Ensuring the accurate application of the principle,piezoelectric force sensors were placed perpendicular to the axial directions of the spray jets from each orifice of the nozzle.The sensors were placed at distances of not more than 10 mm from each orifice exit and at an angle that eliminated the effect of inertia forces on the result.The CFD simulation and experiment were compared at two different injection pressures(160 and 40MPa).After the injection rate validation,a subsequent validation was conducted by comparing the spray jet penetration from each nozzle orifice(in a 1 MPa environment)with experimental results.Coupled internal nozzle fuel flow and spray simulation were performed for comparison.Using back-light photography technique,the spray jets developments and penetrations from each orifice of the nozzle in a 1 MPa constant volume chamber,were captured and measured with a spray analyzer software,experimentally.Near nozzle spray penetrations,were compared since they are directly influenced by the internal nozzle dynamics.From the compared analysis,it was evident that the internal needle dynamics of the multi-orifice VCO injector was not axis symmetric during the experiment,but off-axis.After the validation,numerical investigation of the characteristic effect of a two-stage off-axis needle displacement on internal nozzle flow and injection from the orifices of the nozzle was carried out.Two-stage off-axis needle displacement was formulated to replicate the recovering process of the needle after experiencing elastic deformation.At close position,the needle under high pressure experiences elastic deformation.As the initial stages of the lift,the needle starts to recover from the deformation by displacing radial along the internal cone of the nozzle.The radial displacement continuous(as the lifting progresses)till the needle assumes its natural state.From that point onwards,the needle only experiences vertical displacement as it moves towards the maximum lift stage.The initial stage of the needle lift,where the needle continuously glides along the internal cone of the nozzle body before it completely departed the body,was considered as the first stage,while the second stage was considered to begin from the moment at which the needle completely departed the nozzle body.To ensure that the investigation was three dimensional(3D),three different radial planes of the two-stage off-axis needle displacement were simulated and their effect on internal nozzle flow and spray characteristics was determined and compared to the flow and spray characteristics obtained under the on-axis needle displacement.The first plane,involved the displacement of the needle along a reference orifice plane(Orifice displacement),while the second plane involved the displacement of the needle along a direction that made 15~o from the reference orifice plane(15~o displacement).The third plane involved the displacement of the needle along a direction that made 30~o from the reference orifice plane(30~o displacement).The result showed that,under the off-axis needle displacements,cavitation formations and fuel flow velocities within the nozzle orifices were highly dependent on the trajectory of the needle eccentricity,especially at the early stages of injection.Also,off-axis displacement at the immediate areas of the orifices(within the nozzle),highly increased the penetrations of the spray jets from the orifices.Even though fuel flow through orifices closer to the radially displaced needle had higher injection rates,the spray jets they developed had wider spread and shorter penetrations as compared to spray jets from orifices that were relative further from the radially displaced needle.This is because,spray jet from the orifices closer to the needle displacement emerged with high velocities from the edges of the orifices due to the formation of string cavitation.On the quantities of fuel injected per cycle(Cycle fuel injection quantities),off-axis needle displacement influenced the orifice-to-orifice quantities.The orifices that were closed to the displaced needle injected fewer fuel quantities than orifices that were further from the displaced needle.Due to the displacement at the initial stages of the lift,the needle partially blocks the orifices situated along its displaced direction,hence decreasing the quantity of fuel flowing through them.On the other hand,the orifices situated at the opposite direction of the needle experiences no blockage and therefore injects according to their maximum potential.In this regard,the eccentric needle movement was considered as one of the main reasons causing spray jet differences among the nozzle orifices.The magnitude of the off-axis displacement within the multi-orifice VCO nozzle was numerically estimated by comparing near nozzle spray penetrations obtained from simulation(under three different degrees of needle eccentricity,i.e.0.02,0.04 and 0.06 mm)to the penetrations obtained experimentally.The radial displacement of the needle was estimated to be around 0.06 mm since the spray jet penetration from the 0.06 mm eccentricity,compared best with experiment result than the spray jet penetration from 0.02 and 0.04 mm eccentricities. |
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