## 4 Results and Discussions
### Analysis of Train Energy Consumption
Under this condition, fresh air load was the smallest and the ACS energy consumption was the lowest at 38.47 kW. When the supply air temperature was changed without changing the FAV, the ACS energy consumption exhibited a trend coincident with the supply air temperature. Under the premise of the standard [12], maximising the supply air temperature can achieve the lowest ACS energy consumption. 1.2 No-Oxygen Supply Areas Where the Enthalpy of Ambient Air Is Lower than That of Indoor Enthalpy (Xining Station)  
Figure 9 shows the effect of different FAVs on the CO\({}_{2}\) PP and ACS energy consumption. Contrary to Changsha station, the enthalpy of the outdoor air at Xining station was lower than that of the indoor enthalpy; that is, the enthalpy of the mixed air containing fresh air and returned air was lower than that of the indoor enthalpy. As shown in Figure 10, the enthalpy difference in the air treatment process (C-L) decreased as the FAV increased. However, the residual heat and humidity in the train were both positive. Therefore, increasing the FAV helped reduce the cold load of the ACS and the CO\({}_{2}\) PP in the carriage simultaneously. The variation trend of ACS energy consumption with air supply temperature was opposite to that for the Changsha station. Increasing the supply air temperature increased the indoor temperature and simultaneously increased the enthalpy difference of C-L and L-S in the air treatment process. Therefore, using a lower supply air temperature and larger FAV can effectively reduce the energy consumption of the ACS. When a supply air temperature of 292 K and FAV of 0.544 m\({}^{3}\)/s were used, the temperature in the carriage was within the comfortable range of 24-28 \({}^{\circ}\)C, and it could be ensured that the CO\({}_{2}\) PP did not exceed the upper limit. Figure 8: Effect of fresh air volume on CO\({}_{2}\) partial pressure and the energy consumption of ACS (Changsha). 1.3 Oxygen Supply Areas Where the Enthalpy of Ambient Air Is Lower than That of Indoor Enthalpy (Wudaoliang Station)  
Table 5 lists the calculation results for Wudaoliang station, which has the lowest ambient temperature in the oxygen supply areas. The air supply conditions at Wudaoliang station are also listed in Table 5.  
Figure 10: Air-conditioning process (Xining). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Calculation** & **Basic** & **1** & **2** & **3** & **4** & **5** \\
**Conditions** & **Condition** & & & & & \\ \hline Temperature/K & 291 & 291 & 291 & 292 & 293 & 294 \\ Fresh air volume/m\({}^{3}\).s\({}^{-1}\) & 0.544 & 0.400 & 0.300 & 0.300 & 0.300 & 0.300 \\ Oxygen supply volume/m\({}^{3}\).s\({}^{-1}\) & 0.176 & 0.154 & 0.138 & 0.138 & 0.124 & 0.102 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Calculation conditions of Wudaoliang railway station. Figure 9: Effect of fresh air volume on partial pressure of CO\({}_{2}\) and the energy consumption of ACS (Xining). Figure 11 shows the effect of different FAV on the CO\({}_{2}\) PP and ACS energy consumption. The results indicate that when the FAV was 0.3 m\({}^{3}\)/s, the PP of CO\({}_{2}\) in the passenger breathing zone reached the maximum allowable value. At the same supply air temperature, energy consumption of the ACS reached a minimum value when the FAV was 0.4 m\({}^{3}\)/s. When the supply air temperature was 294 K, it could meet the minimum requirement of 24 \({}^{\circ}\)C in the carriage and maintain low ACS energy consumption. Figure 12 shows the O\({}_{2}\) PP in the train and the total energy consumption of the ACS and OSS under different calculation conditions (Table 5). The effect of changing the FAV on the energy consumption of ACS and OSS revealed significant differences in the calculation results. Increasing the FAV simultaneously diluted the concentrations of CO\({}_{2}\) and O\({}_{2}\) in the train. This explains the restrictive relationship between O\({}_{2}\) PP, CO\({}_{2}\) PP, and FAV.