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An experimental study was carried out to investigate the effects of ultra-high injection pressure and a microhole nozzle on the characteristics of evaporating diesel spray.The diesel fuel was supplied by an electronically controlled ultra-high pressure fuel injection system.Three injection pressures were applied: 80 MPa,160 MPa,and 250 MPa(i.e.,ultra-high injection pressure).Four types of injectors were used,each with the identical eight nozzle holes.The four injectors have nozzle hole diameters of 115 μm,105 μm,95 μm,and 85 μm(i.e.,micronozzle hole),respectively.After measuring the injection quantity and rate,Mie scattering and shadowgraph were performed to visualize the liquid and vapor phases of the diesel spray in a constant volume combustion chamber(CVCC).The ambient temperature and pressure in a diesel engine were simulated by using CVCC.The experimental results showed that the injection quantity and injection rate under ultra-high injection pressure were increased greater than 80%,compared with the injection pressure of 80 MPa,regardless of the nozzle hole diameter.With the same injection pressure,decreasing the nozzle hole diameter from 115μm to a micro-nozzle hole,decreases the injection quantity and injection rate by 45%.Ultra-high injection pressure was effective to compensate for the effect of using a micro-nozzle hole,and maintained the appropriate injection quantity and injection rate.Injection pressure had no significant effect on liquid penetration,even in ultra-high injection pressure.However,the rate of vapor penetration increase was much higher in ultra-high injection pressure.Decreasing the nozzle hole diameter,with the same injection pressure,led to a reduction in the liquid penetration and the rate of vapor penetration.The vapor spray angle was increased with the smaller nozzle hole diameters.The analysis of the experimental results showed that the time required to evaporate all of the liquid phases under ultra-high injection pressure was shortened by 40%,compared with the injection pressure of 80 MPa.The smaller nozzle hole diameter was effective to form the uniform air-fuel mixture after the end of injection.