Research Progress of Influencing Characteristics of Lubricating Oil in Transcritical CO₂ Automotive Air Conditioner

Lubricating oil is a key factor affecting the performance and reliability of transcritical CO₂ heat pump air conditioners dealing with complicated application environments and the requirements of new energy vehicles. The type of oil products and the amount of charged oil play an important role in maintaining high performance and ensuring the safe and stable operation of the system. This study discusses the function and influence of lubricating oil in transcritical CO₂ automotive heat-pump air conditioners. Several common lubricating oils used in CO₂ automobile heat-pump air conditioners are summarized, and the performance of each lubricating oil is compared. The effects of adding lubricating oil on the performance of each component and the overall performance of the transcritical CO₂ heat-pump air-conditioning system are analyzed. Lubricating oil reduces the abrasion and working noise of moving parts and prevents refrigerant leakage and excessive discharge temperature in the compressor. However, excessive oil accumulation in the compression chamber leads to reduced volumetric efficiency and increases power consumption. In the heat exchanger and pipeline, the oil slick that forms on the heat exchanger wall increases the flow resistance and thermal resistance. Simultaneously, oil has an effect of restraining nuclear boiling and bubble disturbance in the evaporator, which deteriorates two-phase heat transfer. Finally, the existing research results, conclusions, and key research directions in related fields are discussed.