## 2 Methodology
### Boundary Conditions  
The firing sequence and interval angle of the gas are set according to the actual conditions of the diesel engine. The gas explosion pressure uses the measured data and is made into a data format file, which contains the gas explosion pressure value of a complete working cycle. The data file contains two columns, the first column is time, the unit is crankshaft angle, the second column is gas explosion pressure, and the unit is MPa. EXCITE software reads this data file and assigns the gas explosion pressure to the main degree of freedom nodes of the pistons in the load data table. In the subsequent calculation process, the EXCITE software will automatically load the gas explosion pressure at the corresponding time on the corresponding main degree of freedom nodes of the pistons. Figure 4: The schematic diagram of the anti-shock calculation model of the 16-cylinder V-type diesel engine. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Parameters** \\ \hline  & 83 (No. 1 Bearing) \\ Main bearing width (mm) & 90 (No. 2–8 Bearing) \\  & 88 (No. 9 Bearing) \\ Main journal diameter (mm) & 220 \\ Main bearing radial clearance (mm) & 0.125 \\ Groove width on upper bearing bush (mm) & 22 \\ Diameter of bore on main journal (mm) & 20 \\ Oil type & 4 th generation locomotive lubricants \\ Kinematic oil viscosity at 100 \({}^{\circ}\)C (mm\({}^{2}\)/s) & 14.7 \\ Kinematic oil viscosity at 40 \({}^{\circ}\)C (mm\({}^{2}\)/s) & 146.7 \\ Oil density (Kg/m\({}^{3}\)) & 896.2 \\ Oil temperature (\({}^{\circ}\)C) & 88 \\ Lubrication oil supply pressure (MPa) & 0.49 \\ Surface roughness of main journal Rrms (\(\upmu\)m) & 0.5 \\ Surface roughness of main bearing bush Rrms (\(\upmu\)m) & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Specifications of the main bearings. The gas explosion pressure is transmitted to the block through the pistons, the connecting rods, and the crankshaft. The external shock load uses experiment data, as shown in Figure 5. The x-axis is time and the unit is seconds, and the y-axis is shock displacement and the unit is mm. The external shock load is the vertical shock displacement of diesel engine foundation and is saved in a dat format file. The dat file contains two columns of data. The first column is the time in seconds, and the second column is the impact displacement in mm. The dat file is imported into the EXCITE software. In EXCITE, the diesel engine foundation is specified to move according to the impact displacement in the dat file. The foundation of the diesel engine had no movement in other directions except for the vertical direction. There are no other constraints on the engine block except for the shock absorbers. The engine block is driven by the shock absorbers and vibrates with the vertical displacement movement of the foundation. In order to simplify the calculation model, this research assumes that the foundation is a rigid body. However, the length of this diesel engine is more than 3600 mm, and the installation location is in the naval engine room. Due to the long length of the diesel engine, the influence of the elasticity of the foundation should be taken into consideration, and the influence of this factor will be added in the future research. In this study, both the engine block and the crankshaft are assumed to be linear elastic bodies. The dynamic characteristics of the engine block and crankshaft are preserved in the reduction, the response of block and crankshaft to gas explosion pressure, inertial force, and external shock load is calculated. The vertical shock is the main part of underwater explosion load applied to the navy ship and the main part of shock damage of shipboard equipment [59]. This research mainly focuses on the vertical response of the diesel engine under external shock.