Influence of lubricating oil circulation characteristics on the performance of electric vehicle heat pump system under low temperature conditions

Electric vehicles frequently employ heat pump systems for winter heating. In winter low-temperature environments, the viscosity of lubricating oil increases, forming an oil film as the refrigerant enters the heat exchanger, consequently impacting the heat transfer efficiency and overall performance of the heat pump system. This paper presents the design and implementation of an experimental heat pump system tailored for electric vehicles. The experimental setup incorporates a specialized device for lubricating oil sampling and an assessment methodology for evaluating heat pump functionality. Under a range of low-temperature conditions, the system underwent comprehensive testing, enabling a detailed analysis and synthesis of the flow characteristics and the influence of lubricating oil within both the heat pump system and the heat exchanger. The results reveal that the oil circulation rate is directly affected by the compressor oil charge, and maintaining the oil circulation rate within the range of 3 %-4 % results in a peak COP of the system. When the ambient temperature is 0 degree celsius and the oil filling volume is 200 g, the oil circulation rate is 3.87 %, and the COP can reach 1.93. In addition, as the oil retention in the evaporator increases by 15.67 g, the COP value decreases by 21 %. Similarly, the increase in oil retention in the condenser from 2.43 g to 6.25 g resulted in a corresponding decrease in heat transfer capacity of 0.07 kW/K, leading to an overall deterioration of the system's performance.