## 4 The Impact of Partial Oxygen Pressure and Temperature on Internal Pressure Fluctuations
### The Impact of Partial Oxygen Pressure Standards  
At an altitude of approximately 3500 m, the partial oxygen pressure in the air is around 13.7 kPa, which may cause discomfort due to insufficient oxygen. To prevent passengers from experiencing altitude sickness and to ensure that the oxygen supply inside the carriage meets the national standard [31], this study sets different oxygen supply standards: 13.7 kPa, 14.0 kPa, 14.3 kPa, and 14.6 kPa. These oxygen supply levels satisfy the body's requirements for partial oxygen pressure and ensure that the diffuse oxygen environment inside the train has an oxygen concentration greater than 22.3%, with a safe oxygen concentration below 25.5% [26]. Meanwhile, the internal temperature is set to 20 \({}^{\circ}\)C to analyze the impact of oxygen supply on internal pressure fluctuations. Figure 9 shows the effect of different oxygen supply standards on internal pressure fluctuations with valve openings set to 100%, 75%, 50%, and 25%, respectively. The figure demonstrates that under the influence of different internal oxygen partial pressure standards as a single factor, the impact on internal pressure fluctuations can reach up to approximately 1800 Pa, accounting for 18.11% of the total internal pressure fluctuation amplitude. When the valve opening is fixed, the higher the internal oxygen partial pressure standard, the greater the impact on internal pressure fluctuations. As the valve opening decreases, the system becomes more sensitive to variations in oxygen supply standards, leading to more pronounced internal pressure fluctuations. Compared to the 100% valve opening, the impact of oxygen supply standards on internal pressure fluctuations at 25% valve opening increases by approximately 200 Pa, representing an increase of about 12.23%, indicating a certain coupling effect between the oxygen supply system and the air ducts. \begin{table}
\begin{tabular}{c c c c c c} \hline Time & 1 & 3 & 10 & 30 & 60 \\ interval/s & 500 & 800 & 1000 & 1500 & 2000 \\ \hline \end{tabular}
\end{table} 
Table 2: Internal pressure comfort standards (composite). Figure 8: Comparison of internal pressure fluctuations with and without considering temperature and oxygen supply. Figure 10 shows the variation in internal pressure peak values under different oxygen supply standards as the train passes through the extreme tunnel. The figure indicates that as the oxygen supply standard increases, the maximum positive pressure value gradually rises, particularly when the valve opening is large and the oxygen supply standard exceeds 14.1 kPa, where the maximum positive pressure value becomes more pronounced with changes in the oxygen supply standard. Conversely, the maximum negative pressure value decreases as the oxygen supply standard increases. The peak-to-peak value gradually decreases with increasing oxygen supply standards, indicating that the oxygen supply system has a certain compensatory effect in mitigating overall internal pressure fluctuations under these extreme tunnel conditions. Figure 10: Internal pressure peaks under different internal oxygen partial pressure standards. (**a**) Maximum positive pressure; (**b**) Maximum negative pressure; (**c**) Peak-to-peak value. Figure 9: Impact of internal oxygen partial pressure standards on internal pressure fluctuations at different valve openings: (**a**) \(\alpha=100\%\); (**b**) \(\alpha=75\%\); (**c**) \(\alpha=50\%\); (**d**) \(\alpha=25\%\). Based on the analysis of Figure 10, it is evident that the internal pressure fluctuations are most pronounced when the valve is fully open at 100%. Therefore, the condition with 100% valve opening was selected to calculate the maximum internal pressure variations under different oxygen supply standards, as shown in Figure 11. As shown in the figure, within the 10 s interval of maximum internal pressure variation, except for the condition with an oxygen supply standard of 14.3 kPa, the internal pressure variations under all other conditions exceed the internal pressure comfort evaluation standard.