Evaluation of dynamic characteristics in the start-up process of transcritical CO₂ heat pumps for electric vehicles

CO₂ heat pumps in electric vehicles (EVs) exhibit excellent heating performance, however, the challenges of prolonged startup time and slow heating need to be addressed, particularly at low temperatures during start-up process. In this study, an experimental system was developed and a mathematical model was established and validated to investigate the dynamic performance of CO₂ heat pumps in EVs during start-up. The start-up process of CO₂ heat pumps was investigated under different temperatures, compressor speeds and refrigerant charges, and the dynamic characteristics of warm start-ups were compared with those of cold start-ups. Furthermore, a novel approach involving a compressor bypass was proposed to reduce start-up time at low temperatures in winter. The results demonstrated that an optimal shut-down time for the bypass was identified, achieving the fastest start-up and reducing start-up time by 82.3 % at −25 °C. To evaluate the start-up process, an evaluation index was established based on the intuitionistic fuzzy analytic hierarchy process, categorizing the start-up effect into five levels: excellent, good, ordinary, poor, and terrible. This study provides a reference for rapid heating, control logic optimization and dynamic performance evaluation of CO₂ heat pump for EVs under real-world driving scenarios in winter.