An experimental investigation of oil circulation ratio influence on heating performance in an air condition heat pump system for electrical vehicles

•Influence of OCR in air conditioning heat pump system for electric vehicle was studied.•Variation of OCR showed obvious effect on COP and heating capacity.•Correlation of OCR vs. optimal performance in system was investigated. Oil, lubricating and cooling the sliding parts of the compressorcomponents exposed to friction, could prevent refrigerant gas leakage in compressor chamber of vapor compression air conditioning system. For air condition heat pump system in electric vehicles, no oil circulating routes and oil separator are used to drive oil from system to compressor, making oil retainedmore difficult to control. Oil accumulated in heat exchanger and expansion valve would enhance pressure drops and reduce heat transfer capacity, leading to decrease of entire heating performance in cold climate. In this paper, a newly designed air conditioning heat pump system with R134a and PAG46 oil is introduced firstly, and oil circulation experiments were conducted on a full-scale electric vehicle. Typical heat transfer characteristics and system performance parameter were recorded and analyzed subsequently under different compressor oil charge conditions, including excessive charge and insufficient charge. Oil circulation rates in system were also measured and exhibited an increasing trend with compressor oil charge mass. Furthermore, heating performances of the system under different oil circulation rate were compared by various heating capacity and pressure drops. Results showed that both excessive and insufficient charge might decrease heating performance, i.e., determination of suitable oil charge is quite important for air condition heat pump system.