Electrical Properties of Electric Vehicle Gear Oils

Abstract

This study compared the oxidation resistance of three commercial oils used in electric car transmissions. The tests were carried out on a stand equipped with a gear train in accordance with ASTM D5704. The changes in physicochemical and dielectric parameters as well as the degree of degradation were assessed by means of the FTIR spectral analysis method. Significant changes in physicochemical parameters were noticeable, including an increase in the acid number as well as an increase in kinematic viscosity at 40 °C and a decrease at 100 °C. The test results show that the oil dedicated to hybrid vehicles degraded the least, while the other oils, dedicated to electric vehicles, lost their lubricating properties to a significant extent. In addition, attention was paid to the abrasion generated during the operation of the gearbox, which has a fairly considerable impact on the change in the dielectric properties of the oils tested. In the future, more detailed research should be carried out on the effects of varying temperatures and of an electromagnetic field on the degradation of gear oils dedicated to EVs and to determine how their dielectric properties change.

1. Introduction

The purpose of gear lubricating oils is to form an oil film on the contact surface of metal components and to dissipate heat from friction nodes; in addition, they are designed to reduce the risk of damage to the lateral surfaces of gear teeth. Oils undergo ageing while in use. In conventional vehicles, the degradation process of these oils is triggered by thermal and mechanical factors, whereas in electric vehicles, there is an additional element, namely the electromagnetic field present within the motor and transmission space [1]. The authors of [2,3,4,5,6,7] indicate that for gear oils used in EVs, the priority features should include the following:

(1)

The prevention of corrosion of copper components (conductors, contacts);

(2)

Compatibility with polymers (sensor and insulation components) and elastomers (sealing materials) [5,8,9];

(3)

Low viscosity to dissipate heat quickly [6];

(4)

Good resistance to oxidation [4];

(5)

Balancing the values of electrical properties, such as electrical conductivity, dielectric constant, or gear oil breakdown voltage [2].

Both the viscosity and thermal conductivity of gear oils used in electric vehicles are very important parameters, with both of them being determined by the molecular structure and the interactions occurring within the material or liquid. The low viscosity of the liquid allows the molecules to move more freely, which can translate into a faster transfer of energy in the form of heat, while also leading to higher thermal conductivity. In general, lighter liquids, such as water, often have low viscosity and relatively high thermal conductivity. This is because the lower flow resistance in these liquids enables a faster transfer of thermal energy. In contrast to conventional internal-combustion-engine vehicles (in which gear oil is mainly used to lubricate the gearbox), EV drive systems require the use of oils that effectively dissipate heat. Due to their compact design, they are often exposed to high gear oil temperatures, generated under high loads and speeds, especially during rapid acceleration or regenerative braking [10]. An important function of gear oil is to ensure adequate cooling. Thanks to their rapid heat dissipation, these oils prevent large temperature changes that can accelerate oil degradation and consequently degrade the lubricating properties of the oil. This aspect of thermal management is crucial, as even relatively minor overheating can lead to the initiation of oxidation processes, thus reducing the oil’s ability to lubricate effectively, while accelerating component wear [11].

In electric vehicles, gear oils may come into contact with electric motor components, and therefore higher demands are placed on their electrical properties, including their electrical conductivity, dielectric constant, and breakdown voltage, compared to vehicles with internal combustion engines.

1.1. Electric Properties in EV

The value of the electrical conductivity must be carefully balanced with the other electrical properties. If it is too high, a short circuit or dangerous current leakage can occur. In contrast, if the conductivity is too low, electrostatic charges can build up in the system, which can consequently cause the oil to degrade through oxidation resulting from the formation of an electric arc. Gear oils used in EVs should have a very low level of electrical conductivity to provide effective insulation, prevent the generation of an electric arc, and protect sensitive drive system components [12].

The dielectric constant is a measure of a material’s ability to store electrical energy in an electric field. It describes how much electrical charge a material can store as compared to a vacuum. In electric vehicle systems, the dielectric constant is one of the important electrical parameters, especially for materials and lubricants used in the vicinity of electrical components. The dielectric constant affects the insulating capacity as well as the interaction of materials with electrical fields [13]. The higher its value, the better the material acts as an insulator, which translates into preventing current conduction.

The dielectric loss factor (tgδ) is defined as the ratio of the current associated with losses in the dielectric to the current associated with charge build-up on the capacitor plates. A common method for measuring dielectric losses in oils is to measure the capacitance of an oil sample in a known electric field. A known voltage is applied, and the current flowing through the oil is measured. The loss factor is related to the phase shift between the voltage and current, which is proportional to the dielectric loss. Energy losses during the polarisation of dielectrics are determined by the current leakage, the rotation of dipoles in dipole polarisation, and the movement of bound charge during macroscopic polarisation [14,15]. In the presence of a dielectric material, the electric field within that material is reduced. This is due to the polarisation of the material, where the charges in the dielectric material shift to counteract the field applied, thus reducing the electric field strength and improving its insulating capabilities [16].

If a dielectric is placed in an electric field, a portion of the energy of the field will be used to heat it. Dielectric loss refers to the power that is dissipated by the electric field in the form of heat in the dielectric. The ability of dielectrics to dissipate the energy of an electric field is estimated using the dielectric loss tangent. Changes in oil properties, even with a small number of impurities, can be seen from the measured values of the dielectric loss tangent. In addition, in order to assess the overall properties of dielectrics, a parameter of permittivity is used. It can be expressed as an absolute or relative value. Relative permeability characterises the properties of a dielectric and shows by how much the strength of the interaction between two electric charges in a dielectric medium is lower than that in a vacuum [17]. The lower the dielectric loss, the more effectively a substance acts as a dielectric material.

The dielectric loss factor values have not yet been determined for gear oils for electrically powered vehicles [18], whereas for transformer oils [19], its percentage change, at which the oil can still be used in a transformer, should not exceed the following values:

• 10% (at 70 °C) and 15% (at 90 °C) for transformers with a voltage of 110–150 kV;
• 7% (at 70 °C) and 10% (at 90 °C) for transformers with a voltage of 220–500 kV;
• 3% (at 70 °C) and 5% (at 90 °C) for transformers with a voltage of 750 kV.

The values of the percentage change in the dielectric loss factor during operation are defined at the national level, and those quoted above apply to transformer oils available on the Polish market [18].

The literature refers to the dielectric loss factor in the context of assessing the quality of transformer oils contaminated with copper or other metals [20]. Dissolved and suspended metals can significantly increase the ionic content of mineral oils, causing a significant, up to threefold, increase in the tgδ value. An additional factor is the catalytic effect of certain metals, especially copper, on oil oxidation/degradation processes. Experience shows that losses in the conductive and dielectric properties of oil can be reduced by removing cations and suspended metal particles from the oil. On the other hand, even if the high tgδ value is directly related to the copper content of the oil, it is not the only parameter responsible for the dielectric degradation of the transformer oil, with other polar compounds also playing a role in contributing to the increased electrical loss of the oil. However, it is a fact that the removal of copper from transformer oils enables a significant improvement in the residual oxidation stability of the oil. It is also the case that the metal content of transformer oils can contribute to the dielectric loss of these oils, as is confirmed by [21]. Copper, as one of the most important materials present in a transformer, is, statistically, the main (but not the only) component responsible for the increase in dielectric losses in insulating mineral oils. However, the dispersion of copper in oil, both in a dissolved and suspended form, remains to be investigated in terms of the mechanism of oxidation processes of both base oils and additives catalysed by copper and its complex compounds.

The composition of the oils must be optimised for resistance to the effects of an electromagnetic field generated by the electric vehicle motor [22]. The presence of additives, such as antioxidants or antiwear agents, can increase or decrease the dielectric constant, depending on their structure and chemical composition. Certain additives may introduce polar groups that increase the oil’s resistance to electrical breakdown, whereas others may introduce conductivity-enhancing materials that reduce dielectric strength [23]. Temperature also affects the dielectric constant, as its higher values can increase the mobility of ions in the oil, thus reducing its dielectric strength [24]. During operation, the oil ages, and its chemical structure changes, which can potentially affect the value of the dielectric constant and the insulating capacity of the oil. Gear oils in electric vehicles need to be particularly resistant to the oxidation process, as it leads to the formation of impurities that can lead to an increase in dielectric properties [22].

Breakdown voltage is the value that determines the maximum electric field strength or voltage that an insulating material can withstand before it loses its insulating properties and allows electric current to flow freely. Once a certain breakdown voltage value is exceeded, the structure of the insulating material is locally destroyed, resulting in a sudden increase in conductivity [25]. Electrical breakdown involves the formation of an electrical spark, which, in an extreme situation, can lead to the formation of an electric arc. In EVs, gear oils serve the function of insulators, preventing unintentional current flow in high-voltage systems. Electric vehicle systems operate at much higher voltages than those in conventional vehicles, typically within the range of 400 to 800 V, with some systems even reaching over 1000 V. A high breakdown voltage value is therefore essential for gear oils to ensure safe operation and prevent undesirable interactions between electrical and mechanical components in an electric vehicle drive system [12].

The value of breakdown voltage in electric vehicle gear oils is determined by the oil composition, the impurity content, the temperature changes occurring in the system, the oxidation process, and the viscosity of the oil. Some additives, especially polar particles, can lower the breakdown voltage, thus causing the conduction of an electrical current. Impurities, such as water and metal particles, drastically reduce the insulating strength. During thermo-oxidative degradation, by-products are formed, and high temperatures increase ion mobility, which lowers the breakdown voltage. Finally, low viscosity can reduce the breakdown voltage, thus allowing ions to move more easily [26]. A high breakdown voltage is necessary for oils in EVs to withstand strong electric fields without the consequence of electrodegradation, i.e., a process accelerated by the supply of thermal energy and often catalysed by impurities such as moisture or particles. Specialised lubricating oils in EVs should therefore be formulated with special additives in order to maintain this parameter at an appropriate level, balancing high dielectric strength with thermal stability to ensure that adequate quality is maintained at the elevated operating temperatures typical of EVs’ drive systems.

1.2. Electromagnetic Field

Another problem associated with the use of lubricating oils in electric vehicles is the degradation of the oil due to the presence of the electric field associated with high voltages and high current intensities in the electric motor windings. During the thermo-oxidative degradation process, reduction and oxidation (redox) reactions occur, leading to the formation of free radicals in the oil. Strong electromagnetic interactions in the vicinity of an electric motor can also affect various additives present in the oil, causing them to agglomerate in specific areas. The effect of high temperatures in such an ion-containing environment can lead to microbubble formation and further destabilisation and degradation of the oil, as oils containing microbubbles are more susceptible to degradation due to electrical breakdown [12]. Possible electrochemical processes on the surface can initiate further reactions of oil oxidation and degradation through both radical and ionic processes.

Due to the lack of standardised methods for testing dielectric properties dedicated to gear lubricating oils for EVs, there are no specifications defined for gear oils for electric cars that indicate the dielectric property values to ensure that these oils have an adequate lifetime. Standardised test methods only apply to the measurement of these properties for other products, for example:

• ‘ASTM D1816 Dielectric breakdown voltage of insulating liquids’,
• ‘ASTM D2624 Electrical conductivity of aviation and distillate fuels’,
• ‘ASTM D4308 Electrical conductivity of liquid hydrocarbons’.

2. Tests

Thermo-oxidative stability tests were carried out for gear oil samples in accordance with ASTM D5704 [27].

The tests were carried out using a single-speed gearbox with a gear ratio of 1:2. A test oil sample of 120 mL was introduced into the gearbox through the vent hole. Once the conditions in the gearbox had stabilised, i.e., the oil had been heated to the test temperature (163 °C), the drive motor was started and aeration was switched on (1 dm³/h). Each test was carried out for 50 h. The degree of oil degradation was measured according to the change in oil viscosity, the change in acid number, and the amount of pentane- and toluene-insoluble precipitates formed in the oil. This method assessed the thermal oxidative stability during the operation of the oil in frictional associations where oxidation occurs, particularly facilitated during lubrication when the oil is in a thin film, at elevated temperatures, with catalytic oxidation assisted by a metallic substrate.

The degree of oil degradation was assessed based on changes in the following parameters:

➢

Physicochemical parameters:

• Kinematic viscosity, according to ISO 3104 [28]: The determination of kinematic viscosity is carried out using a glass capillary viscometer. The measurement involves recording the time required for a specified volume of liquid to flow through the capillary at a controlled temperature (typically 40 °C or 100 °C). The kinematic viscosity is calculated as the result of the flow time and the viscometer’s calibration constant, with the result expressed in mm²/s.
• Viscosity index, according to ISO 2909 [29]: The viscosity index is determined based on the kinematic viscosity values measured at two temperatures: 40 °C and 100 °C. The calculation follows the formula provided in the standard, comparing the tested sample with reference oils of known viscosity behaviour. The viscosity index is a dimensionless number that indicates the degree to which a lubricant’s viscosity changes with temperature.
• Acid number, according to ISO 6618 [30]: The acid number is determined by titrating the sample dissolved in a solvent mixture (ethanol and toluene) with a potassium hydroxide solution in ethanol, using phenolphthalein as an indicator. The acid number is expressed as the number of milligrams of KOH required to neutralise the acids in 1 g of the sample (mg KOH/g).
• Water content, according to DIN 51777 [31]: The water content is determined by means of the distillation method using toluene. The sample is heated in the presence of toluene, and the separated water is collected and measured volumetrically in a properly calibrated receiving tube. The result is expressed as a mass percentage % (m/m).
• Weld load is the lowest applied load at which, under the conditions established in EN-ISO 20623 [32], welding of a rotating test ball with three stationary test balls will occur.

➢

Electric properties:

• The dielectric loss factor, tgδ, is a measure of the loss of energy in the dielectric and is defined as the ratio of the current associated with losses in the dielectric to the current associated with charge buildup on the capacitor cladding. Energy losses during dielectric polarisation depend on the leakage current, the rotation of the dipoles in dipole polarisation, and the movement of the bound charge during macroscopic polarisation. Analysis of the measurement involves matching the measured dielectric response with the ageing time of the individual oil samples tested.
• The power factor, PF, according to EN 60247 [33], in transformer oil is a measure that determines how much electricity is actually used for operation (active power) and how much is lost as reactive power. Power factor values for transformer oil are usually very low, typically less than 0.5% when corrected to 20 °C.
• Resistivity, according to EN 60247 [33], was measured at an ambient temperature using Megger’s IDAX-300 insulation analyser (Megger, Dover, UK).
• Conductivity was measured at an ambient temperature using Megger’s IDAX-300 insulation analyser, according to EN 60247 [33].

➢

These measurements were performed under the following conditions: AC voltage of 50 Hz, and electric field strength of 1 kV/mm, using an automatic high-voltage measuring bridge, type 2820, from Tettex (Haefely, Basel, Switzerland).

FTIR analysis, applying the recommendations of the DIN 51451 and ASTM E2412-23a standards [34,35]: Spectra were recorded using a Thermo Scientific Nicolet iS5 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a cuvette with an optical path length of 0.1 mm, with ZnSe windows. Spectra were recorded in the range of 600–4000 cm⁻¹.

3. Samples

On the market, there are few gear oils used in electric passenger cars. Three samples of commercially available oils were tested, as follows:

Oil A—A synthetic gear oil designed for gearboxes that are exposed to high pressures and for heavily loaded transmissions. According to the manufacturer, this oil is easy to apply even at low temperatures, while ensuring viscosity stability over a wide range of operating temperatures. Its advanced formula provides optimal protection for components, protects gearboxes from wear and corrosion, and ensures smooth gear changes.

Oil B—A synthetic oil for dry electric motor transmissions designed for electric and hybrid vehicles. According to the manufacturer, using this oil extends the life of the drive system and increases the vehicle’s range on a single charge. This is a new generation of oil that meets the requirements for increasing torque and maximum performance.

Oil C—A synthetic oil providing full protection for the transmission mechanism, while maintaining the performance of hybrid automatic transmissions. It has increased thermal conductivity to protect the drive system against the additional heat that is common in hybrid and electric vehicles. The increased oxidation resistance and oil film stability at high temperatures ensure optimal operating conditions for automatic transmissions.

Discussion of the Results

An assessment of the physicochemical parameters and dielectric properties before and after the degradation test was carried out for the gear oil samples. The results are provided in

Table 1. Summary of physicochemical and dielectric properties of tested oils before and after oxidation.

Properties	Unit	Oil A				Oil B				Oil C
		Fresh	Aged	Difference	Difference %	Fresh	Aged	Difference	Difference %	Fresh	Aged	Difference	Difference %
PQ index	-	*	11	–	–	*	368	–	–	*	12	–	–
Kinematic viscosity at 40 °C	mm/s2	32.06	36.68	4.62	4.14	32.39	43.89	11.5	35.5	36.43	36.73	0.3	0.8
Kinematic viscosity at 100 °C	mm/s2	7.625	7.300	−0.325	−4.2	6.402	4.872	−1.530	−23.9	7.618	7.703	0.085	1.1
Acid number	mg KOH/g	1.763	2.719	0.956	54.2	2.064	2.150	0.086	4.2	1.372	1.079	−0.293	−21.3
Water content	% (m/m)	0.204	0.012	−0.192	−94.1	0.309	0.17	−0.139	−44.9	0.104	0.013	−0.091	−87.5
Weld load (10 s)	N	2600	2500	100	−3.85	2100	2000	100	−4.76	2200	2100	100	−4.55
Dielectric loss factor, tgδ	-
at 20 °C		21.08	3.10	−17.98	−85.3	21.13	2.54	−18.59	−87.9	1.12	0.83	−0.29	−25.9
at 50 °C		64.20	10.05	−54.15	−84.3	63.95	11.17	−52.78	−82.5	4.51	3.33	−1.18	−26.2
at 70 °C		145.27	24.79	−120.48	−82.9	133.39	25.25	−108.14	−81.1	8.87	6.91	−1.96	−22.1
at 90 °C		201.42	39.55	−161.87	−80.4	176.24	38.99	−137.25	−77.9	13.63	11.35	−2.28	−16.7
Power factor (PF)	-
at 20 °C		0.9989	0.9517	−0.0472	−4.7	0.9889	0.9344	−0.0545	−5.5	0.7552	0.6432	−0.112	−14.8
at 50 °C		0.9999	0.9951	−0.0048	−0.5	0.9999	0.996	−0.0039	−0.4	0.9767	0.9582	−0.0185	−1.9
at 70 °C		1	0.9992	−0.0008	−0.08	1	0.9992	−0.0008	−0.08	0.9937	0.9899	−0.0038	−0.4
at 90 °C		1	0.9997	−0.0003	−0.03	1	0.9997	−0.0003	−0.03	0.9971	0.9961	−0.001	−0.1
Resistivity ρ in 20 °C	[Ωm]	1.38 × 107	1.14 × 108		726.09	1.78 × 107	1.08 × 107		−39.33	3.02 × 108	4.33 × 108		35.31
Conductivity in 20 °C	[S/m]	7.25 × 10−8	8.78 × 10−9		−87.89	5.61 × 10−8	9.24 × 10−8		64.71	3.31 × 10−9	2.31 × 10−9		−30.21

* not applicable.

.

It should be noted that the dielectric property determinations and their methodology are dedicated to the testing of transformer oil parameters, and, to date, there are no known literature references to the performance of these tests on samples of gear oils dedicated to EV vehicles/motors.

The graphs below (Figure 1, Figure 2, Figure 3 and Figure 4) show changes in the content of the physicochemical parameters and dielectric properties of the oils tested.

When analysing the results obtained, it should be noted that oils A and B behave similarly under the conditions of the ageing test applied. The viscosity at 40 °C increases markedly (from 14 to more than 35% of the initial value) with a simultaneous decrease in viscosity at 100 °C by approximately 4% to 24% of the initial value. A change at 100 °C may indicate a probable loss of viscosity modifiers. However, the oil’s behaviour at 40 °C is probably caused by the degradation processes of the oil base, leading to the emergence of a certain number of structures that are poorly soluble in the base oil, e.g., paraffin-like oligomers that thicken the oil at low temperatures. Such structures can be completely dissolved during heating and do not affect the oil’s viscosity at higher temperatures (100 °C). Oil C behaves completely differently, with an increase in viscosity observed at both temperatures; however, the increase is small and amounts to approximately 1% of the initial value. During testing, the water content decreases in all oils and the acid number increases. It should be noted that the greatest increase in acidity, of approximately 1 mg KOH/g, is observed for oil A, which is indicative of its strongest degradation. For oils B and C, the increase is approximately 0.1 mg KOH/g. Under the conditions of the tests carried out, oils A and B undergo similar degradation as a result of ageing, which deteriorates their dielectric properties. The dielectric loss factor (tgδ) decreases, depending on the temperature, from approximately 18 to 140–160 units (Table 1, Figure 3). As regards oil C, the dielectric loss coefficient (tgδ) of the initial oil is low, and amounts to approximately 1–14, depending on temperature, while the observed decreases range from 0.3 to 2.3 units. The power factors for oils A and B are typical of good dielectrics, whereas oil C has a relatively low power factor, especially at 20 °C, which is related to the water content of the oils tested. In addition, the high PQ index value in oil B after the test should be noted. This indicates poorer protection of the friction surfaces under the test conditions, and the generation of abrasion, or corrosion deposits, from the test transmission. It is the suspension of the resulting impurities and abrasion, rather than the ions, that is responsible for the change in conductivity.

Unfortunately, despite literature recommendations that such oils should be dielectric in nature [22], none of the commercially available oils tested are typical dielectrics. In addition, the performed tests show a clear deterioration in electrical properties (dielectric loss factor, electrical permeability, resistivity and conductivity).

The spectra of fresh oils are shown in Figure 1. The procedure was analogous to that used in research on the comparison of methods for determining the thermo-oxidative stability of base oils and other lubricants conducted in recent years at the INiG-PIB [36,37]. Figure 5 shows the FTIR fresh oils. Differential spectra of each oil sample pair before and after the test were analysed—see Figure 6.

When analysing the FTIR differential spectra of in-service oils, it is possible to quantify their degradation, as shown in

Table 2. Infrared (FTIR) spectral analysis of gear oils.

Oil	Phenolic Antioxidant Degradation		Change in the Structures of Associated Hydrogen Bonds		Changes in Carboxyl Structures/ Degree of Oxidation/		Changes in C-O Structures		Degradation of EP Additives (1st Band)		Degradation of EP Additives (2nd Band)
	cm−1	abs/0.1 mm	cm−1	abs/0.1 mm	cm−1	abs/0.1 mm	cm−1	abs/0.1 mm	cm−1	abs/0.1 mm	cm−1	abs/0.1 mm
A	3650	−0.0195	3300–3600	−0.0159	1728	0.0843	1191	0.0598	983	−0.2547	654	−0.1138
B	3647	−0.0103	3300–3600	−0.0230	1703	0.0218	1152	0.0573	981	−0.0485	675	−0.0676
C	3642	−0.0023	3300–3600	−0.0195	1716	0.0250	1150	−0.0147	941	−0.0207	–	–
	–	–	–	–	1734 *	−0.0353	–	–	–	–	–	–

* Ester additive depletion.

.

In the case of oil A, the bands associated with oil degradation processes are clearly visible, including a distinct band of approximately 1728 cm⁻¹ originating from the carbonyl/carboxyl groups present in the oxidation products (0.0843 abs/0.1 mm). This indicates the most far-reaching oxidation processes, being almost four times more intense than those in the case of oils B and C. This is confirmed by the change in the acid number, as described earlier. Within this range, an overlapping negative band, most likely related to the decomposition/decarboxylation of simple aliphatic carboxylic acids, of 1706 cm⁻¹, is also observed. In the second region associated with the initial oxidation processes (1100–1200 cm⁻¹), a band of approximately 1191 cm⁻¹, with a strong absorbance (0.0598/0.1 mm), similar to that observed in the oil B spectrum, is observed. In addition, a band related to antioxidant degradation (3650 cm⁻¹; −0.0195 abs/0.1 mm), a negative broad band in the 3300–3600 range, most likely related to water evaporation, is observed. Also present in the differential spectrum are negative bands associated with the degradation of the EP phosphoric additives (983 cm⁻¹; −0.2547 abs/0.1 mm), being five times more intense than those for B and ten times more intense than those for C.

In the case of oil B, the bands associated with oil degradation processes are clearly visible, including a distinct band of approximately 1703 cm⁻¹ originating from the acidic carboxyl groups present in the oxidation products (0.0218 abs/0.1 mm). This indicates less intense oxidation processes than those in the case of A, similar to those for oil C. In the second region associated with the initial oxidation processes (1100–1200 cm⁻¹), a band of approximately 1152 cm⁻¹, with a strong absorbance (0.0573/0.1 mm) similar to that observed in the oil B spectrum, is observed. In addition, a band related to antioxidant degradation (3647 cm⁻¹; −0.0103 abs/0.1 mm), a negative broad band in the 3300–3600 range, most likely related to water evaporation, is observed. Also present in the differential spectrum are negative bands associated with the degradation of the EP phosphoric additives (981 cm⁻¹; −0.0485 abs/0.1 mm), being significantly less intense than those for A and more than twice as high as those for C.

In the case of oil C, the bands associated with oil degradation processes indicate a medium degree of degradation, including a distinct band of approx. 1716 cm⁻¹ originating from the carbonyl/carboxyl groups present in the oxidation products (0.0250 abs/0.1 mm). This indicates less intense oxidation processes than those in the case of A, similar to those for oil B. In the second region associated with the initial oxidation processes (1100–1200 cm⁻¹), no significant bands are observed, and the absorbance at 1150 cm⁻¹, amounting to −0.0147 abs/0.1 mm), indicates a practical absence of oxidation products with C-O bonds. In addition, a weak band associated with antioxidant degradation is observed (3642 cm⁻¹; −0.0023 abs/0.1 mm). However, it should be noted that the phenolic antioxidant band is virtually invisible in the fresh oil, which indicates its low level in oil B. In the spectrum, as in the others, a negative broad band in the 3300–3600 range, most likely related to water evaporation, is observed. In the differential spectrum, only one negative band related to the degradation of the EP phosphoric additives is visible (941 cm⁻¹; −0.0207 abs/0.1 mm), being much weaker than that for the other oils tested.

4. Conclusions

This study was conducted on three commercially available gear oils intended for use in EVs. The selection criteria for the oils were their market availability and application. This study was experimental in nature, as there are currently no standardised methods for testing the dielectric properties of EV oils. Comprehensive analyses of physicochemical and dielectric properties were carried out, with an emphasis on the latter, since the physicochemical parameters of EV and conventional oils are similar.

Based on the analysis of changes in physicochemical properties and FTIR spectral assessment of oils during ageing tests performed according to ASTM D5704 (50 h at 163 °C in the presence of a copper plate catalyst), it can be concluded that the oils degraded and, in addition, their dielectric properties deteriorated. Also, the weld load changed within the error range.

In general, oils A and B have similar properties and behave similarly under test conditions, whereas oil C, dedicated to hybrid cars, has poorer dielectric properties but appears to be more stable under test conditions. Oils can be ranked, according to the degree of degradation, in the following order: A, B, C. Attention should be paid to the different courses of oxidation processes.

The tested oils A and B lost their dielectric properties with increasing temperature, as evidenced by the values of permittivity and the large values of the loss factor (tgδ) and the power factor (PF). During the measurements, there was interference due to excessive current, at the limit of the range of instruments designed for testing dielectric materials. No effect of changing the water content on dielectric properties of the oils tested was noted. However, mechanical impurities, including the abrasion generated during gearbox operation, have a significant impact on increasing the conductivity and dielectric properties of oils.

Although the literature clearly suggests that such oils should have dielectric characteristics, none of the tested commercially available oils demonstrated properties consistent with conventional dielectric materials. The experimental data revealed a noticeable degradation of dielectric performance, as indicated by increased dielectric loss factors, reduced resistivity, higher electrical conductivity, and lower relative permittivity. These outcomes highlight the limited applicability of the examined oils in systems requiring stable and reliable insulating behaviour.

In further research, it is planned to evaluate the chemical processes accompanying the degradation of gear oils dedicated to EVs under the influence of an electromagnetic field, with particular attention paid to the catalytic effect of metallic abrasion generated during transmission operation.