## 3 Results and Discussion
### Influence of Gas Cooler Air Volume Flow Rate on Key Temperatures and Pressures
Since the pressure difference between the discharge pressure and PBE was so small and constant, which was caused by the almost unchanged pressure drop in the  
Figure 2: Effect of the air volume flow rate in the gas cooler on the key temperatures of the investigated system. gas cooler and pipes, only the discharge pressure was taken as parameters to evaluate the optimum gas cooler air volume flow rate. It can be seen from Figure 4 that the isentropic (\(\eta_{\rm I}\)) and volumetric (\(\eta_{\rm V}\)) efficiencies of the compressor generally showed a slowly growing trend as the gas cooler air volume flow rate increased. Therefore, it could be concluded that the increase in air volume flow rate on the gas cooler side improves the isentropic efficiency and volumetric efficiency of the compressor. However, these efficiencies were not suitable as parameters for assessing whether the gas cooler air volume flow rate was optimal, since they were not significantly sensitive to changes in gas cooler air volume flow rate. Figure 4: Effect of the air volume flow rate in the gas cooler on the compressor isentropic (\(\eta_{\rm I}\)) and volumetric (\(\eta_{\rm V}\)) efficiencies. Figure 3: Effect of the air volume flow rate in the gas cooler on the pressures of the investigated system. Figure 5 presents the characteristics of the cooling capacity, compressor power input and COP with respect to the gas cooler air volume flow rate. As the gas cooler air volume flow rate increased, the compressor power input decreased rapidly at first and then gradually stabilized. By contrast, the cooling capacity was found to first increase, then gradually stabilize and finally, slightly decrease. As the gas cooler air volume flow rate increased, the heat exchange rate between the refrigerant and the air in the gas cooler was noteworthy. The CO\({}_{2}\) gas cooler outlet temperature gradually decreased, resulting in a progressive reduction in enthalpy of the refrigerant at the outlet of the gas cooler and thus in an increase in cooling capacity. By contrast, the optimal discharge pressure value gradually decreased as the gas cooler air volume flow rate increased, which had a negative impact on the increase in cooling capacity. In general, under the combined effect of the gas cooler outlet temperature and discharge pressure, the trend of the cooling capacity initially increased rapidly, then remained unchanged and finally decreased slightly. The COP of the air conditioning system first increased rapidly with the gas cooler air volume flow rate and then the rate of increase gradually reduced. Therefore, it could be concluded that there is an optimal air volume flow rate and the performance was no longer significantly improved as the air volume flow rate exceeded the optimal value. The COP and the cooling capacity are the conventional evaluation parameters for directly judging whether the air volume flow rate of the gas cooler is optimal. However, in the actual driving process, the cooling capacity and COP of mobile air conditioner units cannot be measured. Therefore, it is necessary to propose some other criteria, which can be quantified in real time and used to intuitively reflect whether the air volume is optimal. After the above analysis, the discharge temperature, the CO\({}_{2}\) gas cooler outlet temperature and the discharge pressure were adopted as evaluation parameters for assessing whether the gas cooler air volume flow rate was optimal. In order to further improve this evaluation, additional experimental exploration and data analysis were carried out. Figure 6 shows the evaluation parameters and the change rate of the cooling capacity and COP with gas cooler air volume flow rate. It was found that the change rate of the system evaluation parameters gradually decreased as the gas cooler air volume flow rate increased from 500 m\({}^{3}\cdot\)h\({}^{-1}\) to 2500 m\({}^{3}\cdot\)h\({}^{-1}\). Continuing to increase the gas cooler air volume flow rate, the change rate of the evaluation parameters considered in Figure 6 was less than 2%.