## 2 Methodology
### Key Parameters for Elastohydrodynamic Lubrication Algorithm in External Shock Condition  
The external shock is equivalent to the interference on the elastohydrodynamic lubrication calculation. Whether the elastohydrodynamic lubrication algorithm can complete the calculation under interference is the key to the study. The calculation step size and the iterations number have an important influence on the completion of the calculation. Both EXCITE software and elastohydrodynamic lubrication algorithms are designed for calculations under normal operating conditions of diesel engines. Under external shock conditions, the simulation step size and the maximum iterations number need to be adjusted.  
In order to ensure that the iterative calculation can converge, a very small simulation step size and a high number of iterations are required. The smaller the calculation step size, the easier the calculation will converge, but the overall calculation time becomes longer. The initial simulation step size is reduced from 0.0625\({}^{\circ}\) CA (crankshaft angle) in normal conditions to 0.0078125\({}^{\circ}\) CA in shock conditions, the minimum step size is reduced from 0.0078125\({}^{\circ}\) CA in normal conditions to 1e-5\({}^{\circ}\) CA in shock conditions.  
EXCITE is an iterative calculation software. The meaning of the maximum iterations number is that when the iterative calculation reaches this value, if the accuracy requirement is not met, the result of the last calculation is retained, the iteration is stopped, and the calculation of the next time step size is performed. Under external shocks, the calculation of elastohydrodynamic lubrication is not easy to converge, so the maximum iterations number of 800 used in normal operating conditions is not enough for external shock conditions. The maximum number of iterations is increased from 800 in normal conditions to 10,000 in shock conditions.  
Decreasing the simulation step size and increasing the number of iterations will significantly increase the calculation time. When the main bearing adopts the spring damping model, the calculation time in the impact environment is 5-16 h. When the main bearing adopts the elastohydrodynamic lubrication model, the calculation time in the impact environment is 40 to 90 h.  
Joint parameters that can work normally under normal conditions may fail under external impact conditions. In this study, a calculation error report was encountered, the distance was too large at a main degree of freedom node of the thrust bearing. The thrust bearing works normally under normal working conditions. After analysis, it is found that the stiffness of the thrust bearing is selected according to the normal working environment and the stiffness value is small. In shock conditions, the external shock will excite the three-dimensional vibration of the diesel engine, the stiffness value of the thrust bearing is not sufficient to restrain the longitudinal vibration of the crankshaft, resulting in the excessive displacement at a main degrees of freedom node of the thrust bearing. After increasing the stiffness of the thrust bearing from 10,000 N/mm to 12,000 N/mm, the calculation of elastohydrodynamic lubrication in the impact environment can be successfully completed.  
By reducing the calculation step size, increasing the iterations number, and increasing the stiffness of the thrust bearing, the elastohydrodynamic lubrication calculation under the impact environment can be successfully completed.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Stiffness (kN/mm)** & **Damping (Ns/mm)** \\ \hline vertical shock absorber & 15 & 4 \\ horizontal shock absorber & 4000 & 4 \\ longitudinal shock absorber & 4000 & 4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Specifications of the shock absorbers.