## 2 Modeling and Experimental Studies of the Geometry of Injector Nozzles
### Numerical Simulations Description  
In paper [15], simulated performance at different load conditions for a constant speed self-ignition engine was determined using GT-Power software. The critical parameters were optimized for achieving the desired fuel nozzle hole diameter. A very close validation with less than 1% difference is achieved at full load. In comparison, a less than 10% difference is achieved at part loads. The simulation model predicts the power, torque, brake specific fuel consumption, in-cylinder pressure, outlet temperature and NOx with very good accuracy at different loads. The optimized spray hole diameter of the testing engine was validated with the experimental results and compared with the baseline model. The work in [6] studied a new sub model of simulation model that correlated the discharge coefficient of the injector nozzle with the design variables of the nozzle and injection conditions. This solution enables the prediction of the effect of microvariations in the nozzle hole geometry as well as the variations in the main design parameters on self-ignition evolution and combustion processes. The working cycle simulation model has predicted changes in engine performance and toxic emissions due to injector nozzle design changes. The authors of the work in [31] showed that separate measurement of these single jet flow injection quantities is necessary to evaluate the injection holes. A measuring adapter for the determination of the injection quantities for individual spray holes was elaborated here. The influence of variation in shape and geometry of the individual nozzle components on singlejet flow injection quantities can be measured and simulated with a measuring adapter. It was demonstrated that the smaller the spray hole diameter, the larger the mass flow difference between the individual nozzle holes due to the increase in variation in the manufacturing process. The macro- and micro-geometrical state of the nozzle hole surface and the transient areas of the nozzle interior into the spray hole have a strong influence on the flow conditions within the range in the space of the nozzle needle seat and within the spray holes and thus on flow stream values and flow symmetry. Design features such as the angle in which the inclination-angle of individual nozzle hole [32], and manufacturing processdriven parameters, such as the shape of the grinding radius of spray hole inlet edge substantially, influence the quantity of fuel mass flux through the individual nozzle hole. Fuel pressurization up to 300 MPa, as required by modern common rail self-ignition engines, resulted in significant variation of the physical fuel properties (density, viscosity, heat capacity and thermal conductivity) relative to those at atmospheric pressure and room temperature conditions [33; 34]. The high acceleration of the fuel at velocities reaching 700 m/s as it is flowed through the nozzle hole orifices to induce cavitation was found. The simulations were assuming variable properties reveal two processes strongly affecting the fuel injection quantity and its temperature. The intense pressure and density gradients at the central part of the spray hole induce fuel temperatures even lower than that of the inlet fuel temperature. The local values can exceed the fuel boiling point and induce reverse heat transfer from the liquid to the nozzle's metal nozzle body. Local values of the thermal conductivity and heat capacity affect the transfer of heat produced at the nozzle surface to the flowing fuel. That creates large temperature gradients within the flowing fuel, which must be considered for accurate simulations of the flow through injector nozzles. Two different injector designs have been studied: one with sharp-inlet cylindrical holes and one with tapered holes with inlet rounding [33]. The findings indicated considerable changes in the flow development but also bulk flow values such as the volumetric efficiency of the injectors and the mean fuel injection temperature.