Interest on the fuel injection technology is increasing recently as its importance in the development of gasoline direct injection engines has become more evident. The main focus of this study is on the spray and flow characteristics of high pressure swirl injectors (HPSI) applicable to these engines. Prototype injectors were designed and fabricated. Comparison of the injector performance with an air-assisted fuel injector (AAFI) was also attempted. In order to achieve these goals, the computational fluid dynamics as well as the experimental measurement was utilized.
In Chapter 3, basic characteristics of HPSI nozzles are described. Prototype swirl-generating nozzle was fabricated by modifying an MPI injector nozzle. Experimental parameters included injection pressure, injection duration and ambient pressure. Spray tip penetration and atomization performance were also examined as a function of these parameters. The D_{32} of the swirl nozzle ranged from 20μm to 28μm at an injection pressure of 5MPa, ,which was about 30% smaller than that of the base MPI nozzles. The atomization performance was enhanced with lower ambient pressure and higher injection pressure. Drop size distribution was found to fit to the Rosin-Rammler distribution and to be less dispersed than that of the air-assisted fuel injector and less uniform than those of the diesel injector.
In Chapter 4, CFD evaluation of the internal flow of HPSI nozzles was discussed as a preliminary optimization process of the nozzle design. Measurements of the static flow rate were compared with computations, and the agreements were satisfactory in values and tendencies with respect to the needle lift. Initial cone angle was calculated, and compared with measurements by the direct photographic imaging method. It could be predicted satisfactorily by introducing an adequate correction factor. Considering these cross-checking for a validation of the computational model, parametric study was carried out to predict the flow rate and the initial cone angle with respect to the selected design parameters. The most significant parameter on both the static flow rate and the initial cone angle was found to be the orifice diameter. Effects of other parameters were also examined and discussed.
Transient flow inside the nozzle was also simulated with respect to the needle motion using the moving-grid utility of the CFD code. Dynamic flow rate was obtained and found to be rather precise compared with that by measurement. The influence of upper needle-seat angle and needle opening duration on initial transient flow was also surveyed.
In Chapter 5, performance and characteristics of the AAFI and the HPSI were compared in terms of flow rate, spray pattern, penetration, atomization quality and drop size distribution. ’The characterization processes of both injector sprays were arranged in parallel and were intended to survey nozzle performance. In order to define the spray pattern, a newly defined spray shape factor was introduced. Equations for D_32 prediction obtained in the previous studies were utilized to survey the variation of mean drop diameter of both injectors.
Main purpose of the present study is to help the design optimization of combustion chambers and injection nozzles for real engine application. Using characterization data of these sprays, operating factors such as injection timing, common rail pressure, and so on, can be evaluated and adjusted to obtain an optimum combustion quality. Also, analysis of internal flow can make it faster and easier to optimize the nozzle design, and eventually, the spray characteristics.
