## 3 The Research Results and Discussion
Figure 13: Dependence of \((dp/d\varphi)_{\rm max}\) and \(T_{max}\) on the \(D_{b}/T\) ratio (\(p_{ij}=300\) MPa): (**a**) dependence of \((dp/d\varphi)_{\rm max}\); (**b**) dependence of \(T_{\rm max}\). As the diameter of the combustion chamber grows, the \((dp/d\varphi)_{\rm max}\) value reasonably or slightly decreases. At \(p_{in}=0.45\) MPa, there is a gradual decrease in \((dp/d\varphi)_{\rm max}\) with an increase in \(D_{b}\). Actually, the maximum pressure increment rate decreases by 13% when the \(D_{b}\)/\(T\) ratio rises from 3.4 to 10. The \((dp/d\varphi)_{\rm max}\) value also goes down significantly by 12.5% at \(p_{in}=0.25\) MPa and within the variation range of the \(D_{b}\)/\(T\) ratio from 3.4 to 5.6. To explain the results presented in Figure 13 in more detail, the characteristics of the RoHR (Figure 14) and the in-cylinder temperature distribution (Figures 15 and A1 in Appendix B) at different diameters of the combustion chamber are displayed. It is known that the fuel spray parameters are affected by the following factors. (1) The more that fuel injection pressure increases, the greater the fuel injection speed and the energy of the spray movement. In this study, the fuel injection pressure is kept constant; therefore, it does not affect the spray rate. (2) Increasing the boost pressure and compression ratio leads to deceleration of the spray (an increase in the width of the tip edge) and creates more fuel droplets; they are stalled on its periphery and the shell. The fuel injection pressure determines the energy of movement, while the boost pressure determines the energy of the spray deceleration. (3) Temperature of fuel flow in the cylinder and the boost pressure rise, which makes the fuel spray warm up and accelerates the ignition. (4) The bigger the diameter of the combustion chamber is, the longer the spray of the fuel, which requires more time for its warmup, ignition, and combustion in the volume [39,40]. Figure 13 shows that the spray tips move and quickly reach the wall of the combustion chamber. The smaller the diameter of the combustion chamber is, the greater the amount of the fuel portion which is injected spreads along the cylinder wall. This fuel warms up slowly (for the beginning of the heat release rate characteristics, see Figure 14), evaporates  
Figure 15: In-cylinder temperature distribution with \(D_{b}/T=3.4\) and \(D_{b}/T=10\). from the hot wall surface, ignites, and burns near the parietal zone. The heat release process is delayed, which can be seen at the end of the rate of heat release. When the boost pressure is 0.25 MPa, more fuel ignites. The fuel droplets warm up faster; therefore, they lose more energy when the spray moves. As a result, less fuel (compared to \(p_{in}\) = 0.15 MPa) falls on the cylinder wall, which results in more heat by the fuel spray from the burned fuel bring into the combustion chamber. Usually, the fuel spray absorbs more heat when it spends more time in the combustion chamber volume due to a higher total surface area of the spray fuel droplets directly exposed to the hot in-cylinder compressed air charge. However, the heat transfers from the gases to the fuel spray depend upon the injection pressure, combustion chamber design, pressure, temperature, and many variable parameters to be considered in each specific case. The fuel that falls on the cylinder wall evaporates and ignites quickly (a more rapid onset of RoHR, Figure 14). The combustion duration is still quite long, although it is less than with the value \(p_{in}\) = 0.15 MPa. With a further increase in the boost pressure from 0.35 to 0.45 MPa, the more fuel warms up and ignites in volume, and the little fuel drops on the wall of the combustion chamber (Figure 15). As a consequence, the combustion process passes faster. In the beginning, the RoHR is higher, and the combustion process is shorter (Figure 14).