## 4 Orthogonal Experiment Analysis
### Influence of Rotational Speed  
The rotational speed has a significant impact on the lubrication performance. Figure 17 shows the rotational speeds of the bushing and piston pin at different crankshaft speeds. As shown in the figure, the piston pin rotational speed increases with an increase in the crankshaft speed. As the piston moved upward, the bushing decelerated forward and reached the maximum speed when the crank pin was turned 90\({}^{\circ}\). The rotation of the piston pin is driven by the bushing, and its speed gradually accelerates from the still state at the bottom dead center (BDC) and reaches the maximum speed near the midpoint of the stroke. It then decelerates and rests at the top dead center (TDC). During the upward motion of the piston, the piston pin was driven by a bushing to reverse its rotation. The piston  
Figure 16: (**a**) Hydrodynamic lubrication friction power in different bushing roughnesses. (**b**) Asperity contact friction power in different bushing roughnesses.  
Figure 15: (**a**) Bushing roughness influences MOFT. (**b**) Bushing roughness influences minimum oil film thickness ratio.  
pin gradually accelerated and reached its maximum near the middle of the stroke, then decelerated and returned to the BDC. Compared with the rotation speed of the bushing, the piston pin rotation speed was much slower, and there was consistent relative sliding between the two movements.  
Figures 18 and 19 show the MOFT and asperity contact percentage, respectively, at different rotational speeds. When the piston approaches the TDC at each speed, the oil film thickness is the smallest, and the asperity contact percentage is the highest. As the rotational speed increased, the MOFT increased, and the asperity contact percentage decreased. Based on Figure 17, it can be seen that this is mainly due to the increase in rotational speed, which results in a greater difference in rotational speed between the piston pin and bushing, making it more favorable for the oil film to form between the two surfaces, resulting in more hydrodynamic lubrication and a reduced roughness contact rate.  
Figure 19: Asperity contact percentage.  
Figure 17: (**a**) Engine speed influences bushing rotation speed. (**b**) Engine speed influences piston pin rotation speed.  
Figure 18: MOFT in different rotation speed.  
Figure 20 shows the friction power generated in the asperity contact and hydrodynamic lubrication at different speeds. It can be observed from the figure that the friction power generated in the asperity contact and hydrodynamic lubrication increases with increasing speed. Compared with hydrodynamic lubrication, the friction power generated by the asperity surface contact increases significantly with an increase in speed.  
In combination with the MOFT change curve, with an increase in engine rotation speed, the oil film thickness also increases slightly; thus, the hydrodynamic lubrication and asperity contact friction coefficient are relatively unchanged. Because the asperity contact has a higher friction coefficient than hydrodynamic lubrication, the friction power of the asperity contact increases more significantly with increasing rotating speed.  
The oil film thickness increased at a higher rotation speed, which enhanced lubrication conditions. Additionally, the relative rotational speed of the piston pin and bushing increased, resulting in a significant increase in friction power. As a result, the piston pin and bushing rotational speeds should be balanced during the design process to maximize lubrication while minimizing friction power.