## 3 The Research Results and Discussion  
This study was carried out at an engine speed of 1400 rpm, fuel injection pressure of 300 MPa, and with the boost pressure varying from 0.15 to 0.45 MPa with a change step of 0.1 MPa. The simulation was performed when the \(D_{b}\)/\(T\) ratio varied from 3.4 to 10 at \(\varepsilon=15.4\), which corresponded to a change in \(D_{b}\) from 67 to 100 mm. During the simulation, the start of injection (SoI) with each speed mode needed to be optimized to achieve the lowest indicated fuel consumption (\(g_{i}\)) and the biggest indicated power (\(N_{i}\)). Figure 8 illustrates the result of the SoI optimal with a fuel injection pressure of 300 MPa and engine speed of 1400 rpm. From the results, the SoI optimal for this case was chosen as 5 degrees of crank angle BTDC. At this point, the \(N_{i}\) obtained was the largest, and the \(g_{i}\) was the smallest. Figure 7: Checking the accuracy of the 3D model in the AVL Fire with experimental results: (**a**) 150 MPa of injection pressure and 0.1 MPa of the inlet pressure; (**b**) 300 MPa of injection pressure, \(Q=45\) mg and 0.07 MPa of the inlet pressure; (**c**) 300 MPa of injection pressure, \(Q=60\) mg and 0.1 MPa of the inlet pressure; (**d**) 300 MPa of injection pressure, \(Q=120\) mg and 0.15 MPa of the inlet pressure. Influence of Geometric Parameters of the Combustion Chamber and Boost Pressure on the Characteristics of the Injection Fuel Spray  
Figures 9 and 10 demonstrate the characteristics of the development of the fuel spray as a function of time. Figure 8: The optimal Sol at engine speed 1400 rpm and fuel injection pressure of 300 MPa. Figure 9: Effect of the boost pressure on the spray characteristics and the mean temperature at an engine speed of 1400 rpm, \(p_{ij}=300\) MPa, \(Q=60\) mg and \(D_{b}=100\) mm: (**a**) vapor penetration; (**b**) liquid penetration; (**c**) penetration; (**d**) spray tip velocity; (**e**) spray cones angle; (**f**) mean temperature. At the initial stage of the injection (up to 0.05 ms), the boost pressure and the \(D_{\text{b}}\)/\(T\) ratio had a weak effect on the injection process (Figures 9 and 10). The oscillation of both the liquid and the vapor phases' penetration and the spray tip velocity started after 0.1 (ms) time from the Sol. The length of penetration was limited by fuel evaporation and its movement in the gaseous state. As the boost pressure increased, the maximum values of the penetration length and the spray tip velocity decreased due to greater air resistance (Figure 9). The boost pressure defines the pressure in the engine cylinder; therefore, the higher the boost pressure, the higher the spray deceleration. In this case, the number of fuel droplets at the periphery and on the shell of the spray increased. As a result, the width of the tip edge increased--in other words, the spray cone angle increased. Figure 10: Effect of the combustion chamber geometry on the spray characteristics and the mean temperature at an engine speed of 1400 rpm, \(p_{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{ \  
The values of the spray penetration and the spray cone angle changed significantly when the boost pressure increased from 0.15 to 0.25 MPa, while those values changed slightly with an increase in the boost pressure from 0.25 MPa to 0.35 MPa. With an increase in \(D_{b}\) (\(D_{b}\)/\(T\) ratio), the spray parameters (liquid penetration) varied a little (Figure 10). This behavior can be explained by the fact that the pressure in the combustion chamber and the fuel injection pressure do not depend on \(D_{b}\)/\(T\) (because the values of all the fuel injection pressure, the boost pressure, and the compression ratio \(\epsilon\) are constant).