## 4 Orthogonal Experiment Analysis  
### Model Validation and Orthogonal Experiment Analysis  
A simulation analysis was carried out on the test rig. Table 3 lists the lubrication parameters of the original machine under the test conditions. Figure 7 illustrates the orbital paths of the experiment and the dynamics simulation. As shown in the figure, the load on the bushing of the connecting rod's small end is mainly concentrated in the lower part, and the piston pin orbital path is also located in the lower part. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Value** \\ \hline Bushing diameter & 30 mm \\ Bearing clearance & 20 \(\upmu\)m \\ Oil temperature & 75 \({}^{\circ}\)C \\ Roughness of journal & 0.4 \(\upmu\)m \\ Roughness of bushing & 0.4 \(\upmu\)m \\ \hline \end{tabular}
\end{table} 
Table 3: Lubrication parameters of the test rig. Figure 6: Piston pin dynamics analysis. The correlation coefficient is used to evaluate the similarity between the measured and calculated orbital paths and are calculated as follows:  
\[r(Z_{1},Z_{2})=\frac{Cov(Z_{1},Z_{2})}{\sqrt{Var(Z_{1})Var(Z_{2})}} \tag{11}\]  
\[Z_{1}=\sqrt{x_{1}^{2}+y_{1}^{2}} \tag{12}\]  
\[Z_{2}=\sqrt{x_{2}^{2}+y_{2}^{2}} \tag{13}\]  
where \(Cov(Z_{1}\), \(Z_{2})\) is the covariance of \(Z_{1}\) and \(Z_{2}\); \(Var(Z)\) is the variance of \(Z\); \(x\), \(y\) represent the horizontal and vertical coordinates of the two orbital paths, respectively; and the correlation between the measured and calculated orbital paths is 0.81. Because the orbital path mounting base is fixed to the small end of the connecting rod, there are evident macro-movements and impacts of the connecting rod during the operation. The micro-amplitude vibration of the base is inevitable, resulting in an error in the measurement of the orbital path. It should be noted that, in general, the two paths exhibit a strong correlation, which confirms the accuracy of the calculation model. Figure 8 illustrates the friction power from the small-end bearing and piston pin boss on both sides. As shown in the figure, the friction power generated by the small-end bearing is much greater than that generated by the piston pin boss. Figure 9 illustrates the distribution of the total oil film pressure in the three bearings at maximum pressure, where the oil film pressure from the two pin bosses is on the upper bushing near the small end of the connecting rod, and the oil film pressure of the small-end bushing on both sides is on the lower part of the bushing. Figure 8: Friction power of bushing and piston pin boss. Figure 7: Comparison of the experimental and calculated orbital paths. In accordance with the previous section, the tribological characteristics of the bushing have the greatest impact on the performance of the entire machine, and only the lubrication performance of the bushing is analyzed in later sections. Based on the analysis of the piston pin lubrication characteristics, the final evaluation indexes were MOFT, maximum friction power, and MOFP. Research findings indicate that the influencing factors are A (oil temperature), B (oil supply pressure), C (surface roughness of bushing), D (surface roughness of journal), E (bearing clearance), and F (engine speed). There is a great deal of importance in determining the test level because if the amplitude of the level is too narrow, the results may not represent the expected result, whereas if the amplitude is too wide, the results may not be representative of the criteria. In this study, the level values are selected uniformly within the level range of the internal quantitative factors based on relevant research findings combined with engineering reality. According to the design requirements of the orthogonal experiment, \(L_{m^{N}}\left(m^{k}\right)\), \(m=3\), \(k=6\), and \(N=3\), where \(L\) represents the orthogonal table, \(m\) indicates the number of factor levels, \(k\) is the experimental influence factor, and \(m^{N}\) indicates the number of times the test is conducted.