A novel control method for the automotive CO2 heat pumps under inappropriate refrigerant charge conditions

CO₂ heat pumps are increasingly widely used in automotive air conditioning, and the demand for refrigerant charge varies greatly under different modes and operation conditions. However, due to the limited space in vehicles, the accumulator cannot fully meet the refrigerant balance requirements for the variable operating conditions, and states of inappropriate refrigerant charge were frequently observed. To enhance system performance and safety under inappropriate charge conditions, an experimental setup and a mathematical model with a maximum error of 6 % were established to investigate the operating characteristics under inappropriate charge conditions. An investigation into the interplay among discharge pressure, the appropriate refrigerant charge range, and refrigerant distribution was conducted. It elucidated how refrigerant charging states (undercharged, adequately charged, or overcharged) manifest in terms of refrigerant distribution and optimal discharge pressure. A novel control method based on refrigerant distribution regulation has been proposed. The proposed control logic could increase the system IPLV by 36.8 % when the refrigerant charge deviates from the appropriate range by 12.12 %, and the greater the dysregulation in the refrigerant charge, the greater the improvement. This control logic would be more enhanced if dysregulation was considered.