## 4 The Impact of Partial Oxygen Pressure and Temperature on Internal Pressure Fluctuations
### The Impact of Temperature Standards  
To withstand the harsh plateau environment, the train's air conditioning heating system and electric heating devices are used simultaneously to meet the thermal comfort needs of passengers. According to relevant domestic and international standards, UIC 553 [32] specifies that, under these conditions, the internal temperature of the train should be greater than 22 \({}^{\circ}\)C, while [33] requires that the internal temperature be above 20 \({}^{\circ}\)C. Additionally, the literature [23] mentions that when operating in the mountainous regions of western China, an internal temperature of at least 18 \({}^{\circ}\)C is sufficient. Therefore, to quantitatively analyze the impact of temperature on internal pressure fluctuations under extreme tunnel conditions, this study investigates the effects of internal temperatures set at 18 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, 22 \({}^{\circ}\)C, and 24 \({}^{\circ}\)C, with the oxygen supply standard set at 14.0 kPa, on internal pressure fluctuations. Figure 12 illustrates the impact of different heating temperature standards on internal pressure fluctuations with valve openings set to 100%, 75%, 50%, and 25%, respectively. The figure shows that the influence of temperature as a single factor on internal pressure fluctuations can reach up to approximately 600 Pa, accounting for 5.74% of the total internal pressure fluctuation amplitude. When the valve opening is fixed, the impact of internal temperature on pressure fluctuations under extreme tunnel conditions increases as the temperature rises. The influence of temperature on pressure fluctuations is more pronounced when the valve opening is larger. As the valve opening decreases, the overall impact of temperature diminishes; specifically, at a 25% valve opening compared to a 100% valve opening, the overall impact of temperature on internal pressure fluctuations is  
Figure 11: Maximum internal pressure variation under different oxygen supply standards. reduced by approximately 70 Pa, a decrease of about 13.01%, indicating a certain coupling effect between temperature and the air ducts. Figure 13 illustrates the variation in internal pressure peak values across different temperatures as the train traverses the extreme tunnel. The figure demonstrates that as the temperature increases, the maximum positive pressure gradually elevates, while the maximum negative pressure diminishes. When the valve opening is smaller, the peak-to-peak pressure value marginally decreases with the temperature rise, indicating that temperature may exert a moderating and attenuating impact on internal pressure fluctuations under these conditions. Conversely, when the valve opening is larger, the internal temperature exerts minimal influence on the peak-to-peak pressure value. Overall, higher temperatures may result in elevated positive pressure and reduced negative pressure, with only a negligible impact on the peak-to-peak pressure values. Based on the analysis of Figure 14, it is evident that internal pressure fluctuations are most pronounced when the valve is fully open at 100%. To assess the impact of internal temperature on pressure comfort, the condition with 100% valve opening was selected to determine the maximum internal pressure variations over different intervals. Figure 14 compares the maximum internal pressure variations at different temperatures. The figure shows that the overall pressure variation rate increases gradually with longer intervals, and there are certain differences in the maximum internal pressure variations under different temperatures. Over longer intervals (30 s, 60 s), the differences in pressure variation under different temperatures are minimal. However, over shorter intervals, the internal pressure variation tends to decrease as the internal temperature increases.