| Summary: | To mitigate the contemporary environmental challenges and to respect the regulations on the progressive ban of hydrofluorocarbons (HFC), natural fluid CO<sub>2</sub> was selected as an ideal refrigerant alternative in the transportation domain. In this study, the optimal CO<sub>2</sub> charging amount and the refrigerant distribution in a bus air conditioning/heat pump system were analyzed in detail. The results showed that there was a plateau (so named by the best charging amount) of the CO<sub>2</sub> charging amount in which the system performance reached an optimal value and maintained it nearly unchanged during this plateau while the performance declined on both sides of the plateau. In addition, the ambient temperature was found to have little effect on the determination of the refrigerant charging plateau, while the refrigerant distribution was affected by the ambient temperature to some extent. Due to the large thermal load and thermal inertia on a bus, the data and conclusions obtained are different from those of ordinary electric small passenger vehicles. This article aims to discover some quantitative parameters and lay a theoretical foundation in this field which is lacking relevant research. Through the research based on the GT-Suite simulation platform, we simulated the transcritical CO<sub>2</sub> cycle applied on a bus, and the performance changes of the bus system (COP 1.2–2.2, refrigerating capacity 9.5–18 kW) under different charging rates (3–8 kg) were obtained.
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