The Impact of Oil Circulation Rate on the Cooling Capacity of Electric Vehicle Car Refrigeration Systems

The rapid advancement of new energy vehicles has significantly expanded the thermal management objectives from the traditional passenger cabin air conditioning to include critical systems such as battery cooling. For an electric vehicle car, the energy consumption of its refrigeration system has a profound impact on the overall driving range. Among various factors, the Oil Circulation Rate (OCR) within the refrigerant circuit is a critical parameter that substantially influences the performance of the entire refrigeration system. Unlike compressors in internal combustion engine vehicles, electric vehicle car compressors offer advantages such as a wide operating speed range and compact size. However, operating at high speeds necessitates an excellent lubrication environment to ensure the longevity of internal mechanical components. While lubricating oil is essential for compressor reliability, its circulation throughout the refrigeration system can negatively affect heat exchanger efficiency and overall cooling capacity. This study investigates the effect of OCR on the cooling capacity of a battery cooling system in an electric vehicle car, analyzing data from tests conducted under different oil charges and compressor speeds.

Optimizing the oil charge is a common practice to manage OCR. Studies have shown that reducing the OCR from 3% to 0.5% can be achieved by optimizing the oil charge, but insufficient oil (oil starvation) can compromise compressor durability. Research indicates that an OCR of around 4.5% might offer a balanced performance for the compressor itself. Performance testing under different OCR conditions has demonstrated that reducing the system’s OCR can increase automotive air conditioning cooling capacity by approximately 10%. The primary factors influencing OCR are compressor speed and evaporating temperature, with OCR increasing notably as compressor speed rises. A higher OCR can degrade evaporator heat transfer efficiency, leading to an attenuation of system cooling capacity. In automotive air conditioning systems, OCR can reach up to 10%, potentially causing a cooling capacity reduction of up to 10%. Therefore, it is crucial to explore the impact of OCR on the refrigeration system of an electric vehicle car to ensure adequate internal lubrication for the electric compressor’s service life while minimizing the detrimental effect on system cooling capacity.

Test Bench for the Electric Vehicle Car Compressor Battery Cooling System

Test Methodology and System Description

The experiments were conducted to evaluate the influence of different oil charges on the cooling capacity of a battery cooling system under an ambient temperature of 48°C. The test matrix encompassed various oil charges at two different compressor speeds, as detailed in the table below.

The system utilized a variable-speed horizontal compressor designed for electric vehicle car applications, with a displacement (V_d) of 8.0 × 10^{-5} m³. The refrigerant was R134a, and the lubricant was Polyol Ester (POE) oil HAF68. The schematic of the battery cooling system is illustrated in the principle diagram. Gaseous refrigerant discharged from the compressor enters the condenser for cooling. The condensed liquid refrigerant then passes through a refrigerant mass flow meter and an oil circulation rate analyzer. After being throttled by an electronic expansion valve (EXV), it flows through a plate heat exchanger, where it absorbs heat from the battery coolant. The vaporized refrigerant then returns to the compressor suction port. On the coolant side, the battery coolant is circulated by a pump, rejects heat in the plate heat exchanger, passes through a coolant mass flow meter, absorbs heat from an electric heater (simulating battery heat load), and returns to the pump.

The key instrumentation used for data acquisition is listed in the following table, ensuring precise measurement of temperatures, pressures, flow rates, and OCR.

Data Analysis Methodology

The system mass flow rate of the oil-refrigerant mixture, denoted as $\dot{m}_{mix}$, is measured directly by the mass flow meter. The system oil circulation rate, $\omega_o$, is measured by the dedicated analyzer. The mass flow rate of circulating oil is therefore calculated as:

$$
\dot{m}_o = \dot{m}_{mix} \times \omega_o
$$

Consequently, the mass flow rate of the circulating refrigerant (pure R134a) can be determined by:

$$
\dot{m}_r = \dot{m}_{mix} – \dot{m}_o
$$

The specific enthalpy of the pure refrigerant at the compressor suction, $h_{r,s}$, is calculated from the measured suction pressure ($P_s$) and suction temperature ($T_s$). Similarly, the specific enthalpy at the expansion valve inlet, $h_{r,vi}$, is calculated from the inlet pressure ($P_{vi}$) and temperature ($T_{vi}$). Ignoring the effect of oil, the cooling capacity provided by the pure refrigerant cycle is:

$$
Q_r = \dot{m}_r \times (h_{r,s} – h_{r,vi})
$$

To account for the presence of oil, the specific enthalpy of the oil-refrigerant mixture is evaluated considering its composition. For a mixture containing oil, liquid refrigerant, and vapor refrigerant, the specific enthalpy $h_{mix}$ is:

$$
h_{mix} = \frac{\dot{m}_o}{\dot{m}_{mix}} \times h_o + \frac{\dot{m}_{r,l}}{\dot{m}_{mix}} \times h_{r,l} + \frac{\dot{m}_{r,v}}{\dot{m}_{mix}} \times h_{r,v}
$$

where $h_o$, $h_{r,l}$, and $h_{r,v}$ are the specific enthalpies of oil, liquid refrigerant, and vapor refrigerant, respectively. As evaporation proceeds, the concentration of lubricating oil in the liquid phase increases. The local oil concentration in the liquid phase, $\omega_l$, is defined relative to the refrigerant vapor quality $x$:

$$
\omega_l = \frac{\omega_o}{\omega_o + (1 – \omega_o)(1 – x)}
$$

The temperature-pressure relationship for the R134a and HAF68 mixture is complex. An empirical correlation fitted using software (e.g., DateFit) can be used. A general form for such a relationship, relating pressure $P$ (in bar absolute), temperature $T$ (in Kelvin), and refrigerant mass fraction in the liquid phase $\omega$ (where $\omega_l + \omega = 1$) with coefficients $a_1$ to $a_9$, is:

$$
\log_{10}(P) = a_1 + \frac{a_2}{T} + \frac{a_3}{T^2} + \log_{10}(\omega) \times \left( a_4 + \frac{a_5}{T} + \frac{a_6}{T^2} \right) + \log_{10}^2(\omega) \times \left( a_7 + \frac{a_8}{T} + \frac{a_9}{T^2} \right)
$$

Neglecting the heat of mixing, the actual cooling capacity of the oil-refrigerant mixture circulating in the system is then:

$$
Q_{mix} = \dot{m}_{mix} \times (h_{mix,s} – h_{mix,vi})
$$

where $h_{mix,s}$ and $h_{mix,vi}$ are the specific enthalpies of the mixture at the compressor suction and expansion valve inlet, respectively.

Analysis of Experimental Results

Variation of Oil Circulation Rate

The measured Oil Circulation Rate (OCR) for different oil charges at compressor speeds of 3000 r/min and 6000 r/min is summarized in the table below. For the electric vehicle car system, a clear trend is observed: OCR increases with both the amount of oil charged into the system and the compressor speed.

At 3000 r/min, OCR rises from 1.62% to 5.86% as the oil charge increases from 0.15 kg to 0.35 kg. At 6000 r/min, OCR rises more sharply from 4.50% to 9.61% over the same oil charge range. For a fixed oil charge, increasing the compressor speed from 3000 r/min to 6000 r/min raises the OCR by approximately 2.75 to 5.06 percentage points. Notably, the rate of OCR increase accelerates when the oil charge exceeds 0.25 kg (250 mL). For instance, at 6000 r/min, increasing the charge from 0.15 kg to 0.25 kg raises OCR by 2.65 percentage points, whereas increasing from 0.25 kg to 0.35 kg raises it by 2.46 percentage points from a much higher base. This indicates a non-linear relationship where the system’s tendency to carry oil increases significantly once a certain oil inventory threshold is surpassed. This behavior is critical for the thermal management design of an electric vehicle car, which often requires the compressor to operate at high speeds.

System Pressure Characteristics

The suction and discharge pressures are fundamental parameters reflecting the operating state of the refrigeration cycle in the electric vehicle car. The following table presents the average values measured under different test conditions.

The data shows that higher compressor speed leads to an increase in discharge pressure and a decrease in suction pressure. This is primarily due to the increased mass flow rate and associated pressure drops across system components. The higher pressure ratio at 6000 r/min also affects compressor efficiency and the thermodynamic state of the refrigerant-oil mixture throughout the cycle.

Cooling Capacity Variation and Key Influencing Factors

The cooling capacity calculated for both the pure refrigerant ($Q_r$) and the oil-refrigerant mixture ($Q_{mix}$) is presented in the table below. The difference between these two values, $\Delta Q = Q_r – Q_{mix}$, represents the cooling capacity degradation directly attributable to the presence of circulating oil.

The data clearly shows that $Q_{mix}$ is consistently lower than $Q_r$, and the degradation $\Delta Q$ generally increases with higher OCR. At 3000 r/min, $\Delta Q$ remains relatively stable at 0.38 kW for oil charges up to 0.25 kg but then increases to 0.74 kW at 0.35 kg oil charge, corresponding to the notable jump in OCR from 2.54% to 5.86%. At 6000 r/min, $\Delta Q$ grows more significantly, from 1.01 kW at 4.50% OCR to 2.26 kW at 9.61% OCR. This confirms that a higher OCR has a more severe impact on cooling capacity, which is a critical performance metric for the climate control and battery thermal management of an electric vehicle car.

The reduction in cooling capacity stems from a decrease in the useful enthalpy change across the evaporator. The enthalpy difference for the mixture, $\Delta h_{mix} = h_{mix,s} – h_{mix,vi}$, is smaller than that for the pure refrigerant, $\Delta h_r = h_{r,s} – h_{r,vi}$. This is primarily because oil retains a portion of the liquid refrigerant that does not vaporize, and the sensible cooling of both the oil and this retained liquid consumes a part of the refrigeration effect that would otherwise be available for useful cooling. The proportion of enthalpy difference reduction and the corresponding cooling capacity reduction are compared in the following analysis.

The relationship is evident: as OCR increases, the reduction in the evaporator-side enthalpy difference becomes more pronounced, directly leading to a greater loss in cooling capacity. For the electric vehicle car compressor operating at 3000 r/min, when OCR rises from about 1.62% to 5.86%, the enthalpy difference reduction increases from approximately 5.2% to 12.93%, causing a cooling capacity reduction from 3.66% to 7.50%. At the higher speed of 6000 r/min, more relevant for peak cooling demands in an electric vehicle car, the effect is more severe. As OCR increases from 4.50% to 9.61%, the enthalpy difference reduction climbs from 11.66% to 22.89%, resulting in a significant cooling capacity penalty ranging from 7.50% to 14.69%.

Effect of Compressor Speed Increase

Comparing performance between 3000 r/min and 6000 r/min for the same oil charge provides insight into the scaling behavior of the electric vehicle car refrigeration system. The key parameter changes are summarized below.

When the compressor speed doubles from 3000 r/min to 6000 r/min, the volumetric flow rate theoretically doubles. However, due to a decrease in suction density caused by lower suction pressure, the actual mass flow rate of the oil-refrigerant mixture increases by approximately 70%. Simultaneously, the higher speed induces a higher OCR, which reduces the effective enthalpy difference $\Delta h_{mix}$ across the evaporator by about 20%. The net effect on cooling capacity, given by $Q_{mix} = \dot{m}_{mix} \times \Delta h_{mix}$, is an increase of roughly 30-40%, not 100%. This non-linear scaling is crucial for the thermal management strategy of an electric vehicle car. It demonstrates that simply raising compressor speed to meet higher cooling demands (e.g., fast charging or aggressive driving) comes with diminishing returns and significant efficiency penalties due to increased pressure ratios and oil circulation effects.

Conclusion

This investigation into the impact of oil circulation rate on the cooling capacity of an electric vehicle car refrigeration system yields several key conclusions. The performance of the thermal management system in an electric vehicle car is highly sensitive to the amount of lubricating oil circulating with the refrigerant.

1. OCR Dependence on Operating Conditions: The Oil Circulation Rate increases with both the amount of oil charged into the system and the compressor speed. For a given oil charge, increasing the compressor speed from 3000 r/min to 6000 r/min raised the OCR by 2.75 to 5.06 percentage points. The relationship is non-linear, with OCR increasing more rapidly once the system oil inventory exceeds a certain threshold.
2. Cooling Capacity Degradation Mechanism: The presence of circulating oil reduces the effective enthalpy difference across the evaporator. This is because oil retains dissolved liquid refrigerant that does not vaporize, and sensible cooling of the oil and this retained liquid consumes part of the refrigeration effect. The cooling capacity of the oil-refrigerant mixture ($Q_{mix}$) is therefore always lower than that calculated for the pure refrigerant ($Q_r$).
3. Quantitative Impact of OCR: The degradation escalates with increasing OCR.
   • At a compressor speed of 3000 r/min, as OCR increased from 1.62% to 5.86%, the reduction in evaporator enthalpy difference rose from 5.2% to 12.93%, leading to a cooling capacity reduction from 3.66% to 7.50%.
   • At the higher, more demanding speed of 6000 r/min for an electric vehicle car, as OCR increased from 4.50% to 9.61%, the enthalpy difference reduction was more severe, climbing from 11.66% to 22.89%, which resulted in a substantial cooling capacity penalty of 7.50% to 14.69%.
4. Non-Linear Scaling with Speed: Doubling the compressor speed from 3000 r/min to 6000 r/min increased the mixture mass flow rate by about 70% but decreased the evaporator enthalpy difference by about 20% due to higher OCR and system pressures. Consequently, the net increase in actual cooling capacity ($Q_{mix}$) was only approximately 30-40%, not the theoretical 100%. This highlights a key design constraint for electric vehicle car thermal management systems.

In summary, optimizing the lubrication system for an electric vehicle car compressor involves a critical trade-off. Sufficient oil charge is necessary to ensure reliable lubrication, especially at high speeds. However, excessive oil charge leads to a high OCR, which significantly impairs system cooling capacity and efficiency. Therefore, for the thermal management design of an electric vehicle car, it is imperative to determine a minimum but adequate oil charge that guarantees compressor durability while minimizing the negative impact of oil circulation on the overall refrigeration performance, thereby contributing to extended driving range and effective battery thermal management.