In this study

a CO2 heat pump system test bench for electric vehicles with an oil circulation rate testing device was built to investigate the influence of the oil circulation rate on the performance of the compressor and the heat pump under typical vehicle operating conditions. Under refrigeration conditions

the increase in the oil circulation rate from 1.17% to 5.26% increased the volumetric and isentropic efficiencies of the compressor by 13.6% and 5.6%

respectively

Under heating conditions

when increasing the oil circulation rate from 1.51% to 6.68%

the volumetric and isentropic efficiencies of the compressor increased by 7.19% and 15.7%

respectively

while the discharge temperature of the compressor decreased from 155.5 °C to 146 °C. Unlike the flow resistance and heat exchange capacity of the air cooler in the high-temperature section

the oil circulation rate exhibited a significant effect on the evaporation process in the low-temperature section. The analysis of the influence of the oil circulation rate on the overall performance of the heat pump system demonstrated that the heat pump system performance improved when the oil circulation rate is approximately 4%.