Electric Vehicle Heat Pump Efficiency Boosted by Vapor Injection Research
As electric vehicles (EVs) continue to dominate the automotive landscape, manufacturers and researchers alike are intensifying efforts to improve energy efficiency, especially in auxiliary systems. Among these, the air conditioning and heating system stands out as a major contributor to energy consumption. Unlike internal combustion engine vehicles, which can utilize waste heat from the engine for cabin warmth, EVs must rely on alternative methods during cold weather. Traditionally, this has meant using electric resistance heaters such as Positive Temperature Coefficient (PTC) heaters. While effective, these systems significantly drain battery power, reducing driving range—often by 30% or more in winter conditions.
To address this challenge, automakers and academic institutions have been exploring heat pump technology as a more energy-efficient alternative. Heat pumps transfer thermal energy from the outside air into the cabin, offering higher efficiency than direct electric heating. However, their performance drops sharply in low-temperature environments, leading to reduced coefficient of performance (COP) and elevated compressor discharge temperatures—issues that undermine their viability in colder climates.
A breakthrough in this domain comes from a team of researchers at the University of Shanghai for Science and Technology. In a recent study published in a leading thermal engineering journal, Xia Yang, Jiang Ziqi, Zhang Bin, Tian Yafen, and Li Kang present a comprehensive numerical investigation into the vapor injection characteristics of a compact scroll compressor designed specifically for electric vehicle heat pump systems. Their work offers new insights into optimizing compressor performance under real-world operating conditions, particularly through enhanced vapor injection (EVI) techniques.
The research focuses on a 38 cm³/r displacement scroll compressor with a short scroll profile—specifically, a two-pair working chamber (N=2) design. This configuration is notable for its compact size, lightweight construction, and robustness at high rotational speeds, making it ideal for integration into space-constrained EV platforms. Unlike conventional scroll compressors with longer profiles (N=3), short-profile units are more prone to under-compression issues, especially in heating mode. Under-compression occurs when the built-in volume ratio of the compressor does not match the system pressure ratio, resulting in energy losses and higher discharge temperatures.
To mitigate these challenges, the team implemented a vapor injection strategy, where additional refrigerant is introduced midway through the compression process. This technique, also known as flash-gas bypass or economizer injection, has been widely studied in stationary HVAC systems but remains less explored in automotive applications—particularly for compact, short-profile compressors used in EVs.
The researchers developed a three-dimensional, transient numerical model of the compressor using advanced computational fluid dynamics (CFD) software. The model was built based on precise geometric parameters and simulated the entire compression cycle, including suction, compression, vapor injection, and discharge phases. Refrigerant R134a was selected as the working fluid due to its widespread use in automotive air conditioning systems and favorable thermodynamic properties.
One of the critical aspects of the simulation was the turbulence modeling approach. The team employed the RNG k-ε turbulence model, which provides improved accuracy in flows with high strain rates and strong curvature—conditions commonly found within the narrow scroll chambers. This choice allowed for a more realistic representation of internal flow patterns, pressure distribution, and heat transfer phenomena.
Boundary conditions were carefully defined to reflect actual operating scenarios. Suction pressure, suction temperature, injection pressure, injection temperature, and discharge pressure were all set according to typical EV heat pump operating ranges. The simulation assumed adiabatic wall conditions, neglecting heat exchange with the external environment, which is a reasonable approximation given the short cycle duration and minimal time for significant heat loss.
To ensure computational accuracy without excessive resource consumption, the team conducted a grid independence study. Several mesh configurations ranging from 97,000 to over 1.1 million cells were tested. Results showed that beyond 735,000 cells, further refinement yielded diminishing returns in terms of solution accuracy while drastically increasing computation time. Therefore, the final simulations were performed using a mesh of 735,000 elements, striking an optimal balance between precision and efficiency.
Before analyzing the effects of vapor injection, the researchers validated their numerical model against experimental data. Performance metrics such as discharge temperature, total mass flow rate, compressor power, and overall efficiency were compared across multiple operating points. The results showed strong agreement, with discrepancies remaining within acceptable limits—maximum errors of 5.8% for discharge temperature, 6.4% for mass flow rate, 7.2% for power consumption, and 9.2% for efficiency. These deviations were attributed primarily to the absence of lubricating oil in the simulation, which in real-world operation helps cool internal components and reduces leakage.
With the model validated, the team proceeded to investigate how varying injection pressure affects compressor performance. Simulations were conducted at two key rotational speeds: 5,000 rpm and 6,000 rpm—representative of typical mid-to-high load conditions in EV applications.
As injection pressure increased from 0.30 MPa to 0.55 MPa, several trends emerged. First, suction mass flow rate decreased. This counterintuitive result stems from the proximity of the injection port to the suction chamber. At certain crank angles, the injection passage directly connects with the suction zone. Since injection pressure exceeds suction pressure, some refrigerant flows backward into the suction path, reducing net inflow. However, the injected mass flow increases with higher injection pressure, and its growth outpaces the decline in suction flow. Consequently, total refrigerant mass flow rises—by up to 19.9% at 5,000 rpm.
This increase in total mass flow directly impacts heating capacity. More refrigerant circulating through the system means greater heat rejection in the condenser, translating into higher cabin heating output. At both 5,000 and 6,000 rpm, heating capacity rose steadily with injection pressure, achieving maximum improvements of 20.5% and 17.1%, respectively, compared to non-injection operation.
However, the benefits come at a cost: increased power consumption. As more refrigerant is compressed and the injection process adds complexity to the thermodynamic cycle, the compressor requires more mechanical work. Power draw rose in parallel with mass flow, peaking at the same levels as heating capacity. This leads to a crucial trade-off: while both heating output and power input increase, the rate of improvement in heating capacity slows down at higher injection pressures.
This dynamic is reflected in the system’s coefficient of performance (COP), defined as the ratio of heating output to electrical input. COP initially improves with injection pressure, reaching optimal values of 3.21 at 5,000 rpm (0.50 MPa injection) and 3.37 at 6,000 rpm (0.40 MPa injection). Beyond these points, COP begins to decline, indicating diminishing returns. The existence of an optimal injection pressure suggests that real-world control strategies should dynamically adjust injection based on operating conditions rather than using a fixed setting.
Discharge temperature behavior followed a similar non-linear trend. Initially, injection reduced discharge temperature—down to 83.6°C at 5,000 rpm and 79.7°C at 6,000 rpm—because the injected vapor acted as a coolant, mitigating under-compression losses. However, beyond a certain injection pressure (around 0.40–0.50 MPa), the injected refrigerant’s temperature (maintained at 5°C superheat) became relatively high compared to the in-chamber gas, negating the cooling effect. As a result, discharge temperature began to rise again, eventually surpassing baseline levels.
Compressor efficiency—the ratio of isentropic work to actual work—also exhibited a peak. It improved at lower injection pressures due to reduced throttling losses and better matching of internal pressure to system requirements. But as injection pressure climbed, interference with suction flow and rising power demands caused efficiency to drop. Maximum efficiency was observed at 0.35 MPa injection pressure, highlighting the importance of moderation in injection strategy.
Volumetric efficiency, on the other hand, consistently declined with increasing injection pressure. This metric reflects the ratio of actual suction volume to theoretical displacement. Because injection disrupts the suction process and reduces net inflow, volumetric efficiency suffers—a known limitation of mid-cycle injection in compact compressors.
Having established the role of injection pressure, the team turned to injection temperature. Holding injection pressure constant at 0.40 MPa—the optimal point for COP at 6,000 rpm—they varied injection temperature from 14°C to 26°C.
The results revealed a much weaker influence. As injection temperature increased, refrigerant density decreased slightly, leading to a small reduction in injected mass flow (3.38%) and total mass flow (1.27%). Discharge temperature rose marginally—by up to 3.1°C—due to reduced cooling effect and slightly higher under-compression losses. Power consumption increased slightly, while heating capacity remained nearly unchanged. Consequently, COP showed only a minor downward trend.
Both compressor efficiency and volumetric efficiency decreased slightly with higher injection temperature, but the changes were minimal. This indicates that, within the tested range, temperature has a secondary effect compared to pressure. The primary driver of performance variation is the thermodynamic state and pressure level of the injected vapor, not its temperature.
These findings have important implications for EV thermal management system design. They suggest that precise control of injection pressure is critical for maximizing heat pump efficiency, especially in cold climates where every watt-hour counts. In contrast, injection temperature can be managed with less stringency, allowing for simpler system configurations.
Moreover, the study underscores the advantages of short-profile scroll compressors in automotive applications. Despite their susceptibility to under-compression, the integration of vapor injection effectively compensates for this limitation, enabling high heating capacity and improved COP without requiring larger, heavier compressors. The compact design also facilitates integration into tight engine bays or underfloor modules, preserving vehicle packaging flexibility.
The research also points to future directions. While this study focused on steady-state performance, real-world driving involves frequent transients—accelerations, decelerations, and changing ambient conditions. Adaptive control algorithms that modulate injection pressure in real time could further enhance system efficiency. Additionally, optimizing the location, size, and geometry of the injection port may yield additional gains. Previous studies have shown that angled or multi-port injection can improve mixing and reduce flow disturbances.
Another area for exploration is the use of alternative refrigerants. While R134a remains common, its global warming potential (GWP) has prompted a shift toward lower-GWP options such as R1234yf or natural refrigerants like CO₂. The behavior of vapor injection may differ significantly with these fluids due to variations in thermophysical properties, necessitating tailored design approaches.
From a manufacturing perspective, the findings support the development of variable-injection compressors—units capable of enabling or disabling injection based on demand. Such systems could operate in high-efficiency injection mode during cold weather and switch to standard mode in milder conditions, extending component life and reducing complexity.
For automakers, the implications are clear: vapor injection is not just a theoretical concept but a practical tool for enhancing EV range and comfort. As battery costs remain high and consumer expectations for winter performance grow, technologies that improve energy utilization will become increasingly valuable. This study provides a solid foundation for integrating advanced compression technologies into next-generation EVs.
In conclusion, the work by Xia Yang and colleagues represents a significant step forward in understanding and optimizing vapor injection in compact scroll compressors for electric vehicles. By combining rigorous numerical modeling with experimental validation, they have delivered actionable insights into how injection pressure and temperature influence key performance metrics. Their results confirm that strategic use of vapor injection can substantially improve heating efficiency, reduce discharge temperatures, and enhance overall system COP—critical factors in the quest for longer-range, more comfortable electric vehicles.
The study demonstrates that even small design refinements can yield meaningful gains in real-world performance. As the automotive industry continues its transition to electrification, research like this will play a vital role in overcoming technical barriers and delivering sustainable, high-performance transportation solutions.
Xia Yang, Ziqi Jiang, Bin Zhang, Yafen Tian, Kang Li, University of Shanghai for Science and Technology, Journal of Thermal Science and Engineering Applications, DOI: 10.1115/1.4056789