## 4 Results and Discussions
### Analysis of Train Energy Consumption  
The FAV mainly depends on the CO\({}_{2}\) content inside and outside the carriage and that produced by the passengers. Preliminary studies have indicated that the minimum FAV required by passengers increases with an increase in altitude, leading to dilution of the O\({}_{2}\) concentration, and increases the energy consumption of oxygen production [7]. Therefore, to design a reasonable installed capacity of the ACS and OSS, this study used the meteorological parameters of train z264 passing through each station as the calculation condition and calculated the lowest energy consumption of the ACS and OSS along the Qinghai-Tibet line. The entire railway line can be divided into three sections based on the effect of FAV on train energy consumption with changes in outdoor climatic conditions: (1) non-oxygen supply areas where the enthalpy of ambient air is higher than the indoor enthalpy; (2) non-oxygen supply areas where the enthalpy of ambient air is lower than the indoor enthalpy; and (3) oxygen supply areas where the enthalpy of ambient air is lower than the indoor enthalpy. We analyse and discuss with respect to representative stations in the three sections. The net heat input of each wall and heat dissipation of the heat source at the three representative stations are listed in Table 4. A negative value indicates that heat was transferred from the interior to the exterior of the train. 1.1 No-Oxygen Supply Areas Where the Enthalpy of Ambient Air Is Higher than That of Indoor Enthalpy (Changsha Station)  
Figure 8 shows the effect of FAV on the ACS energy consumption and CO\({}_{2}\) PP in the train under different supply air temperatures. The ACS energy consumption included two parts: cooling process after mixing fresh and returned air, and heating process before supplying air. The recommended value of 20 m\({}^{3}\)/(h\(\cdot\)person) for FAV was chosen as the basic parameter for calculation [21]. The results indicate that the CO\({}_{2}\) average PP of the passenger breathing zone was only 136 Pa, which is lower than the upper limit of the CO\({}_{2}\) PP. The air quality in the carriage was good under basic conditions. A larger FAV corresponds to a larger fresh air load of the ACS, which leads to an increase in the ACS \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Station**} & \multicolumn{6}{c}{**Enclosure Structure Heat Transfer Quantity/W**} & \multicolumn{6}{c}{**Solar Radiation/W**} & \multicolumn{1}{c}{**Total**} \\ \cline{2-11}  & **Left Wall** & **Right** & **Rool** & **Floor** & **Left Window** & **Right** & **Left Window** & **Right** & **Passenger/W** & **Heat** \\ \hline Changsha & 440.1 & 421.1 & 648.4 & 309.0 & 228.5 & 244.5 & 0 & 0 & 11368.0 & 13659.6 \\ Xining & 274.9 & 254.6 & 480.8 & 273.1 & 10.7 & 79.8 & 0 & 0 & 11368.0 & 12742.1 \\ Wudaololang & \(-\)293.1 & \(-\)287.6 & \(-\)673.8 & \(-\)52.1 & \(-\)190.5 & \(-\)285.0 & 670 & 43 & 11368.0 & 10298.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The heat gains of the train. Figure 7: Schematic diagram of the air circulation in the train. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & **Velocity Speed** & **Temperature** & **Fresh Air** & **O\({}_{2}\) Concentration (Nachitai-Lhasa)** \\ \hline value & \(1\,\mathrm{m/s}\) & 18 \({}^{\circ}\)C & 20 m\({}^{3}\)/(h\(\cdot\)person) & 25\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Basic calculation conditions. energy consumption. For FAVs of 0.4 m\({}^{3}\)/s, 0.3 m\({}^{3}\)/s, and 0.2 m\({}^{3}\)/s, we obtained 138, 143, and 154 Pa, respectively, as the average PP of CO\({}_{2}\) in the passenger breathing zone. As the FAV decreased to 0.2 m\({}^{3}\)/s, the average CO\({}_{2}\) pressure in the breathing zone exceeded the upper limit, and the air quality could not meet the hygiene requirement. The critical FAV that met the CO\({}_{2}\) PP requirement of the passenger breathing zone was 0.23 m\({}^{3}\)/s.