## 5 Discussion
### Optimization Design of Fuel Injection Speed Under Different Gear Structure Parameters
#### 5.2.2 Design of Fuel Injection Speed for Each Cone Angle at Different Speeds  
This section establishes five injection speeds (12.5, 15, 17.5, 20, and 22.5 m/s) based on the pressure and velocity distribution at different positions below the nozzle under various operating conditions. Through simulation calculations for the oil volume fraction under each injection speed in the nine operating conditions of Section 4, recommendations for optimized design of injection speeds are provided for different structural parameters of gears.  
Figure 19: Distribution of volume fraction of gear lubricating oil under different cone angles and rotational speeds.  
Figure 20: Curve graph of the variation in oil volume fraction with gear speed under different cone angle conditions.  
Figure 21 shows the variation in gear oil volume fraction with injection speed at different pitch angles and at gear speeds of 6000, 8000, and 10,000 rpm. Through a detailed analysis of these curves, the following conclusions can be drawn: Firstly, under high-speed conditions of 8000 rpm and 10,000 rpm, as the injection speed increases, the volume fraction of gear oil usually shows an upward trend. This phenomenon is mainly due to the higher injection speed effectively overcoming the resistance generated by high-speed airflow, thereby increasing the coverage area of lubricating oil on the gear surface. High-speed airflow generates strong airflow shear forces in the gear meshing area, hindering the adhesion and distribution of lubricating oil. However, by increasing the injection speed, lubricating oil can penetrate the airflow more effectively and be evenly distributed on the gear surface, thereby increasing the oil volume fraction. In contrast, at a lower speed of 6000 rpm, the volume fraction of gear oil shows a trend of first increasing and then decreasing with the increase in injection speed. This characteristic can be attributed to the relatively low air pressure in the gear meshing area at 6000 rpm, which only requires a relatively small injection speed to achieve the best fuel injection effect. At lower speeds, the airflow shear force in the gear meshing area is smaller, making it easier for lubricating oil to adhere to the gear surface. However, once the optimal injection speed is reached, any further increase in injection speed may lead to lubricant accumulation or splashing, resulting in a decrease in lubricant volume fraction. Excessive spraying speed may lead to excessive accumulation of lubricating oil on the gear surface and even cause the oil splash out of the meshing area, thereby reducing the effectiveness of the oil volume fraction.  
Figure 21: The relationship between oil volume fraction and injection speed at different gear speeds and cone angles.  
In addition, under specific speed conditions, the total oil volume fraction increases with the increase in gear pitch cone angle. This phenomenon can be attributed to the increase in pitch cone angle, which causes the gear surface to tilt more, thereby expanding the contact area during gear meshing. A larger contact area requires more lubricating oil to cover these surfaces, effectively reducing friction and suppressing wear. The increase in cone angle results in longer contact lines in the gear meshing area, requiring lubricating oil to cover a larger area, thus increasing the oil volume fraction. In summary, the analysis results in Figure 21 not only reveal the complex effects of gear speed, injection speed, and pitch cone angle on oil volume fraction but also provide an important theoretical basis for optimizing gear lubrication systems. By adjusting the injection speed and pitch angle reasonably, the coverage efficiency of lubricating oil can be effectively improved, and the friction and wear in the gear meshing area can be reduced, thereby extending the service life of the gear and improving its operating efficiency. This research has important engineering application value for the lubrication design and optimization of high-speed and high-load gear systems.