## 2 Modeling and Experimental Studies of the Geometry of Injector Nozzles
### Hitherto Existing Methods of Optimizing the Geometry of Injector Nozzles  
The static flow rate of high-pressure injector nozzles was characterized through measurements and computations as an optimization process of nozzle design [25]. Prototype injectors were fabricated by modifying multi-zone injection nozzles. Measurements of flow rate were compared with the computations with respect to the nozzle geometry and the needle lift. The initial cone angle was calculated and compared with measurements by the direct photographic imaging method. A parametric study was carried out to terms the flow rate and the initial cone angle for selected design parameters. Experimental study and optimization were carried out in the layouts and the structure of the high-pressure common rail fuel injection system for a marine diesel engine [26]. A novel optimization solution to improve the steady-state performance of the common rail fuel injection system designed for a ship engine retrofitting was proposed. The tests concentrate on optimizing the hydraulic layouts and the structure parameters to manage and use the model. In tests, the modified multi-objective genetic algorithm was employed to the reduction of rail pressure fluctuation. The nozzle needle design concept which was considered in the research project was focused on maximizing the contact area between the needle and its cartridge to reduce needle wear [27]. The injector feeding part was realized using two series of holes. The design was replenished by 3D numerical simulations which indicated the optimal feeding-hole number and geometry to obtain a maximum mass flow rate. The study's results indicated that the hole number substantially influences the flow losses along the internal flow path and the global mass flow rate. Cavitation inside the injector nozzle was observed as a critical phenomenon, which induces a decrease in the flow capacity of the nozzle, but also an increase in the spray outlet effective velocity. Some authors [14] have also performed visualization tests on transparent nozzles, but most of these studies are performed in scaled-up or simplified injector nozzles due to difficulties in production and optical measurements in manufactured geometries. An experimental research investigation was carried out to analyze the influence of conical and cylindrical orifice geometries on a common rail fuel injection subsystem [1]. This behavior of the injection rate in the different nozzles was characterized by using the non-dimensional parameters of cavitation number, discharge coefficient and Reynolds number. The results evidenced essential differences in the permeability of both nozzle geometries and resistance of the conical nozzle to cavitation. In the paper, a nozzle flow model was used to design an injector nozzle and obtain initial spray conditions for the common rail injection system [28]. The injector for dimethyl ether needs nozzle holes with larger diameters and a higher sack volume for the same injection duration. Additionally, the needle lift and needle seat diameter should be increased to achieve a minimum flow area ratio. In the conducted experimental investigation, an effort has been made to enhance the performance of a dual fuel combustion engine utilizing different nozzle offices [29]. In work, the injector nozzle with 3, 4 and 5 holes, each having 0.2, 0.25 and 0.3 mm hole diameter, respectively, and injection pressure in the injector subsystem (varied from 210 to 240 bar in steps of 10 bar) were optimized. The configurations of re-entrant type combustion chamber and 230 bar pressure in injection subsystem, 4 hole and 0.25 mm nozzle have contributed to maximum performance. Increasing the injection pressure course in injector space band downsizing the spray holes' orifice diameter has been the principal measures for self-ignition engine to influenced fuel--ambient gas mixture formation and combustion processes [30]. Additionally, the combination significantly accelerates the mixing of fuel and ambient gas and greatly decrease the soot formation.