## 4 Results and Discussions  
Previous studies have investigated the influence of ASP on the temperature [13; 15], velocity [15; 22], and pollutant concentration fields [26] in Qinghai-Tibet railway trains when they passed through some stations. The results showed that adjusting the air supply temperature based on outdoor climatic conditions at different stations was beneficial for passenger comfort. For train z264, the interior temperature, CO\({}_{2}\), and O\({}_{2}\) contents in the train were numerically calculated when it crossed 18 stations in the plains and plateau. The control target was the requirements of temperature and oxygen PP in AC trains specified in the \"TB/T 1932-2014\" standard of the railway transportation industry in China [12]. The lumped-parameter method was used to establish an energy-balance model for the train. \[\rho V\frac{dh}{d\tau}=Q_{R}+Q_{XF}+R+\alpha_{n}(T_{i}-T_{n})A-Q_{PF} \tag{2}\]  
where \(h\) is the air enthalpy, kJ/kg; \(Q_{\rm R}\) is the passenger heat dissipation, \(W\); \(Q_{\rm XF}\) is the fresh air load, \(W\); \(R\) is the solar radiation through train windows, \(W\); \(\alpha_{n}\) is the convective heat transfer coefficient of interior wall, \(W\)/(m\({}^{2}\cdot\)K); \(T_{i}\) is the interior wall temperature, K; \(T_{n}\) is the interior air temperature, K; \(A\) is the interior wall area, m\({}^{2}\); \(Q_{\rm PF}\) is the heat removed by exhaust air, W.  
The CO\({}_{2}\) concentration (<0.15%) specified in the standard [12] applies to plains; however, there is no standard for plateaus. The optimal value of CO\({}_{2}\) PP in the aerospace field is 500 Pa, which significantly surpasses the limit value in the standard [12]. In human respiratory physiology, pulmonary gas exchange is caused by the pressure difference between the alveolar gas and blood around the alveoli [28]. Therefore, referring to the upper limit of 0.15% CO\({}_{2}\) concentration in the plains [12], it was more reasonable to use a PP of 152 Pa (equivalent to 0.15% CO\({}_{2}\) concentration in the plains) as the CO\({}_{2}\) limit in the plateau. The equations for calculating the CO\({}_{2}\) and O\({}_{2}\) PP are as follows:  
\[P_{\rm CO_{2}}=P_{A}\cdot y_{\rm CO_{2}} \tag{3}\]  
\[P_{\rm O_{2}}=P_{A}\cdot y_{\rm O_{2}} \tag{4}\]  
Figure 5: Grid independence validation. where \(y_{\text{CO}_{2}}\) and \(y_{\text{O}_{2}}\) are the CO\({}_{2}\) and O\({}_{2}\) volume concentrations, respectively, and \(P_{\text{A}}\) is the atmospheric pressure. The above results indicate that the distribution of CO\({}_{2}\) in the carriage is nonuniform, and the average CO\({}_{2}\) concentration in the carriage does not represent the real level in the breathing zone of passengers. A comparison between the average CO\({}_{2}\) PP in the carriage and the average CO\({}_{2}\) PP in the breathing zone under the same FAV (20 m\({}^{3}\)/h\(\cdot\)person) is shown in Figure 6. It can be observed that the average CO\({}_{2}\) PP in the carriage is lower than that in the breathing zone when the train is running. Basically, if the upper limit of the average CO\({}_{2}\) PP in the carriage is used as the reference parameter for adjusting the FAV, the CO\({}_{2}\) released by passengers would exceed the allowable upper limit. Therefore, the average CO\({}_{2}\) concentration in the carriage does not represent the real level in the breathing zone of passengers, and the lumped parameter method is no longer applicable for the calculation of the CO\({}_{2}\) PP. In summary, the ASP was adjusted based on the limits of the above standard [12], in which the CO\({}_{2}\) PP and O\({}_{2}\) PP calculations were based on the passenger breathing zone as the target area. The energy consumption of the ACS and OSS under each condition was calculated to obtain the working conditions for minimum energy consumption that satisfied both comfort and hygiene conditions. Figure 7 shows a schematic of air circulation in the train. The energy consumption of the ACS is related to the altitude and ambient temperature outside the train.