## 3 The Research Results and Discussion
As can be seen in Figure 8, the spray cone angle increases, but both the fuel spray tip velocity and the penetration depth decrease simultaneously with the start of combustion (SoC), taking place earlier in the cycle owing to the higher air-fuel vapor mixing rate at the high boost pressure of 0.45 MPa and high temperature inside the cylinder. The \(D_{b}\)/\(T\) value affects the oscillation results of the spray penetration and the spray tip velocity. When increasing the combustion chamber diameter, their maximum value also grows, while their values decrease when the boost pressure decreases (Figure 9). In fact, the angle between the axis of the spray hole and the axis of the fuel injection nozzle (\(\varphi\)) changes as the \(D_{b}\)/\(T\) ratio varies. This angle should be in accordance with the diameter of the combustion chamber to ensure that fuel is injected into the space in the combustion chamber. The amount of fuel injected should not be too close to the cylinder head or too much on the surface of the combustion chamber. To achieve that, it is necessary to optimize this angle with the maximum value of the indicated power. In the study, this angle was also optimized for each case of the diameter of the combustion chamber. It can be seen that the \(\varphi\) angle changes according to the change in the diameter of the combustion chamber. Furthermore, the length of the nozzle holes increases too, while the wall thickness of the nozzle tip does not change (Figure 11). This causes disturbances in the fuel flow and the spray parameters. The maximum value of the spray tip velocity in all cases (Figures 9 and 10) exceeds the speed of sound. The excess air ratio does not depend on the \(D_{b}/T\) ratio, but it has a weak effect on \(p_{\rm max}\) (Figure 12) at the \(\varepsilon\) value unchanged condition. When the value of the fuel injection in a cycle is unchanged (\(Q=60\) mg), the excess air ratio \(\alpha\) and the maximum pressure in the cylinder \(p_{\rm max}\) increase due to the significant rise in the boost pressure (Figure 12b). The excess air ratio \(\alpha\) rises because the mass of the air entering the cylinder increases. Those variations are determined by the increase in the in-cylinder pressure at the end of the compression process. The \(\alpha\) and \(p_{\rm max}\) values grow by 2.84 and 2.1 times, respectively, when the boost pressure increases from 0.15 to 0.45 MPa (Figure 12a). The increase in the boost pressure has a positive effect on the mixing rate (the maximum value of the spray penetration decreased but the cone angles of the spray increased, Figure 9); therefore, the maximum increment rates in the in-cylinder pressure (\(dp/d\varphi\))\({}_{\rm max}\) and \(T_{\rm max}\) decrease as well (Figure 13). Thus, at the value of \(D_{b}/T=10\), the boost pressure variation from 0.15 to 0.45 MPa leads to a decrease of 38% in \((dp/d\varphi)_{\rm max}\) and of 9% in \(T_{\rm max}\). The decrease in the \((dp/d\varphi)_{\rm max}\) value has a positive effect on the production of NO\({}_{\rm x}\) emissions and noise in the operating process of a diesel engine. The growth in the boost pressure has a higher effect on the \((dp/d\varphi)_{\rm max}\) than that on the \(T_{\rm max}\) value. As the boost pressure increases, especially above 0.35 MPa, its role in reducing the values of both \((dp/d\varphi)_{\rm max}\) and \(T_{\rm max}\) decreases, especially significant in \(T_{\rm max}\). The highest values of \((dp/d\varphi)_{\rm max}\) and \(T_{\rm max}\) were achieved at the smallest diameter of the combustion chamber (Figure 13) and the biggest depth value of the combustion chamber (\(D_{b}/T=3.4\)), which is associated with more fuel striking against the combustion chamber wall until the autoignition occurs; however, the only exception for \(T_{\rm max}\) is at \(p_{in}=1.5\) bar. Figure 12: Dependence of the air–fuel ratio \(\alpha\) and \(p_{\rm max}\) on the \(D_{b}/T\) ratio (\(p_{ij}=300\) MPa): (**a**) dependence of the air–fuel ratio \(\alpha\); (**b**) dependence of \(p_{\rm max}\).