Currently, the automotive industry is transitioning from traditional fossil fuel vehicles to electric vehicles (EVs) . One issue connected to this change is the relatively low range of these vehicles (especially at cold temperatures) . The most widely studied solution to this problem involves using the vapor injection system , , , . Scroll compressor, as the core component of a vapor injection system, is commonly used in automotive heat pump systems because of their compactness, capability of high-speed operation, liquid resistance, and other good characteristics .
As noted, vapor injection systems have been investigated in detail. Sun et al.  proposed and developed a new relationship to calculate various performance parameters of vapor injection compressors, which is two orders of magnitude faster than existing models. Tello-Oquendo et al.  compared the working mechanisms of vapor injection scroll compressors with those of two-stage scroll compressors at high-pressure ratios. Wang et al.  demonstrated the impact of injected entropy between vapor injection cycles with flash tanks and intermediate heat exchanger heat pumps. Tello-Oquendo et al.  designed a novel dual-source vapor injection heat pump, and the waste heat recovery can reach 100% by using a vapor injection compressor. Zeng et al.  established a thermodynamic model for simulating a two-phase vapor injection cycle and showed that the cycle has higher COP than others. Kim et al.  examined the effects of the injection hole shape of asymmetric scroll compressors under different climates and proposed an optimized design. Jung et al.  studied and optimized the effects of the injection-port angle for the injection compressor and the length of the internal heat exchanger. Chung et al.  studied the performance comparison of CO2 refrigerant for injection compressors under extreme heating and cooling conditions. Oquendo et al.  compared vapor injection scroll compressors with two-stage reciprocating compressors under extreme weather conditions, indicating that the former provided better efficiency and a superior COP at low-pressure ratios. Jang et al.  studied a new type of heat exchanger with a micro-tube structure for a vapor injection system. Moon et al.  assessed that the performance characteristics of an economizer would be affected by a vapor injection refrigeration system under various experimental conditions. Zhang et al.  performed comparative experiments to analyze the volume and compressor efficiency at different injection pressures in conjunction with low temperatures. Kang et al.  developed a scroll compressor with a double-vapor injection system and examined the heating capacities of this system, as well as those of a single vapor injection system and a system with no vapor injection.
With a scroll compressor’s relatively high rotational speed, the rotor and stator plates have minimal clearance when high pressure is generated in each chamber. The pressure is generally balanced by a back one so that the end of the moving plate will be separated from the bottom of the stator plate , . However, under-compression can occur under some heat pump operational conditions, affecting the back pressure balance. The change will, in turn, cause the moving disk to detach from the bottom of the static disk, leading to leakage and damage to the compressor . Thus, lubricating oil in the scroll compressor is essential. The oil acts as a sealant and also lowers the discharge temperature during the heat pump operation so that the scroll compressor’s service life can be increased . In the case of an actual automobile heat pump system, the lubricating oil is included in the automobile when produced, and the owner cannot adjust the type or quality of the oil. In contrast, the oil circulation rate (OCR) of air conditioning or refrigerant flow will be changed under different working conditions.
The effect of the OCR on compressor performance has been scrutinized in detail in previous works. Zhu et al.  collected oil samples in two locations and studied the factors affecting the oil circulation rate in ejector cooling cycles. Ossorio et al.  studied the oil circulation at different compressor speeds and evaporation temperatures, while Navarro et al.  investigated the OCR for different compressors and determined the effects under different working conditions. Dhayanandh et al.  examined the influence of the oil injection parameters on the performance of screw air compressors, and the lubricating oil circuit was designed according to the optimal oil filling parameters. Wu et al.  analyzed the oil injection flow rate of a semi-hermetic twin-screw refrigeration compressor and evaluated the effect of the oil injection position on compressor efficiency.
In the present work, a vapor injection scroll compressor with a low-pressure ratio was used to study the effects of injection pressure and OCR on the compressor performance of EVAC under two conditions, and the optimal combination of OCR and injection pressure were tested. This work can provide valuable reference for ensuring the high efficiency of vapor injection compressors under operating conditions. Finally, an empirical relationship between heating capacity with the vapor injection coefficient based on experimental data was provided.