## 1 Introduction  
Electric vehicles have been proved to be one of the most effective solutions to reduce greenhouse gas and air pollutant emissions in the transport sector, besides promoting low noise [1, 2]. However, a considerable contribution to the environmental impact of cars is given by high global warming potential (GWP) refrigerants [3, 4]. Therefore, ever-stricter regulations aiming at phasing out these working fluids are being implemented all over the world. Looking for new environmentally friendlier refrigerants for mobile air conditioning units has become one of the hottest research topics in the field of EVs in recent years [5]. Thanks to its negligible GWP, zero ozone depletion potential (ODP), non-toxicity, non-flammability and favorable thermo-physical properties [6, 7], CO\({}_{2}\) (R744) has become one of the most preferred working fluids for cooling systems of vehicles [8, 9]. Since the introduction of the transcritical running mode [10], there have been countless studies on the performance of CO\({}_{2}\) refrigeration systems. Compared with high-GWP working fluids, the performance of transcritical CO\({}_{2}\) refrigeration units is lower at high cooling medium temperatures [11, 12]. Therefore, in order to improve the performance of transcritical CO\({}_{2}\) air conditioning systems, many methods have been studied [13, 14]. The expedients to improve the performance of transcritical CO\({}_{2}\) systems are based mainly on two approaches: optimizing system components and enhancing system layout. Regarding the optimization of system components, this method mainly includes the performance enhancement of the gas cooler, evaporator and compressor. [PERSON] et al. (2015) developed a simulation model of an automobile gas cooler and proposed a new heat exchanger optimization design method. [PERSON] et al. (2016) proposed an integrated fin and micro-channel gas cooler and developed a segment-by-segment model to analyze the design scheme maximizing the heat exchange performance. [PERSON] et al. (2017) proposed a novel CO\({}_{2}\) heat pump system with series gas cooler (SGC) configuration and compared its performance with that of the conventional CO\({}_{2}\) system. The introduction of SGC was found to lead to an average increase of 33.7% in the system heating capacity and of 35.0% in the system coefficient of performance (COP). [PERSON] et al. (2018) implemented a fast and accurate genetic algorithm to predict the performance of CO\({}_{2}\) microchannel gas coolers in automobile air conditioning systems. In addition, the performance of microchannel evaporators and its improved structure design were studied in [PERSON] et al. (2019); [PERSON] et al. (2020); [PERSON] et al. (2020). [PERSON] et al. (2020) explored the internal flow characteristics of a scroll compressor to enhance the performance of R744 mobile heat pump air conditioning systems. The above studies have resulted in significant advances in the investigation of maximizing the overall performance of the system by optimizing system components. Besides, regarding enhancing the system cycle to improve the transcritical CO\({}_{2}\) cooling performance, various solutions have been implemented, such as overfed evaporators, two-phase ejectors and parallel compression ([PERSON] et al., 2020). A literature review revealed that the influence of the gas cooler air flow rate on the transcritical CO\({}_{2}\) electric vehicle air conditioning system performance has not been investigated comprehensively. On the one hand, the efficiency of transcritical CO\({}_{2}\) systems is significantly sensitive to the refrigerant temperature upstream of the expansion valve, which is affected by the air flow rate within the gas cooler substantially. On the other hand, the gas cooler air flow rate varies considerably in vehicles due to the running state and the stationary state.