3.5.3. Noise

The absence of an ICE to mask vibrations, harshness, squeaks, and rattles will trigger more problems such as NVH and buzz, squeak, and rattle (BSR). Chad Chichester has reported, in a recent Society of Tribologists and Lubricant Engineers (STLE) publication [28,29], that NVH and BSR can affect sensors that are increasingly used in vehicle safety and guidance. The choice of greases used in EVs/HVs is different from those available on the market today and can reduce or eliminate noise which will help make vehicles safe.

#### *3.6. Future Development Trends for Grease Used in EVs/HVs*

As EVs are different from ICEs, it is important to know their different future requirements. Of the multitude of greases on the market, lithium greases are a top choice because of their high adherence, non-corrosiveness, and moisture resistance—all of which allow them to be compatible with and protect components [28,29]. Lithium grease has shown to impart the advantages of high adherence, non-corrosiveness, and moisture resistance, making them compatible with several OEM applications such as EVs/HVs. Aluminum and urea greases perform well too; however, their production is associated with hazardous processing and constraints on process balance.

It is predicted that the use of eco-friendly and bio-degradable greases will increase. Additionally, corrosion protection, low-temperature performance, and water resistance, among others, will rise in interest [29–31]. It is also important to consider that it is more favorable to produce a grease with low torque functionality through thickeners, base oils, and additives while ensuring that the properties of the electrical and surfaces remain unaltered. In the future, it is likely that instead of general greases for all types of EVs, custom-made ones will be favored due to the variabilities in EV designs and the factors they will bring into the formulation of greases [18,32]. While PU greases are currently not commonly used in EVs, they might be of interest in the future due to their lifelong sealing functions, long-life properties at high temperatures, and low noise characteristics. PU grease formulations have inherent oxidation inhibition chemistry as part of the thickener, leading to long life at elevated temperatures. When coupled with PAO stocks, these products can contribute to significant improvements in grease life at elevated bearing temperatures. Furthermore, improved filtration practices and thickener reaction control have contributed to lower noise properties by controlling particles' size and overall particle contamination. This leads to quieter noise levels in rotating bearings.

Desired lubricant properties in EVs/HVs [15,33,34] have been grouped into a table as shown in Table 2. All the required lubricant properties that need to test their performance characteristics are compared with the conventional lubricants used in the ICEs. This table indicates that these newly developed tests such as electrical conductivity, thermal conductivity, extreme speed, and bearing protection capability are crucial performance characteristics to meet HV or EV performance requirements.

**Table 2.** Desired lubricant properties in electric vehicles (EVs)/hybrid vehicles (HVs) compared with in internal combustion engine vehicles (ICEVs) (adapted from [15,33,34]).


1 In reference to specific lubrication types required for vehicles in Figure 3B. 2 The location numbers are in reference to the labels present in Figure 3B.

**Figure 3.** (**A**) Comparison of battery and ICE sizes and (**B**) an image of the main areas where lubrication is required in an EV (Tesla), hybrid electric vehicle (HEV) (Volkswagen NetCarShow), and ICEV (Subaru Forester showroom), respectively [15].

#### *3.7. ASTM Standard Test Development for Grease Used in EVs or HVs*

Various ASTM standard tests [34] have been developed to ensure the required performance specifications for grease operation in EVs or HVs. Among these ASTM standard tests, the most important test parameters are shown in Table 3. For example, oil viscosity is developed according to the load, speed, and operating temperature of the application [32]. While viscosity should be reduced to minimize friction loss, too low of a viscosity hinders durability and causes the lubricant to leak out of the bearings. This brings into play oxidation properties and dropping point when in extreme temperatures. Oxidation, enhanced by spark discharges, deteriorates the oil and increases the chances of sludge buildup, which hinders thermal control from the motor. Additives are added to modify these properties, but some may be counterintuitive and shorten the life span of the grease. Lastly, the lubricant must maintain electrical properties such as volume resistivity, dissipation factor, and dielectric strength to avoid electrical losses in the system [32]. Overall, the lubricant must be formulated to balance all these requirements. Table 4 depicts specific grease properties that must be tested today for operation in EVs or HVs.+

**Table 3.** Laboratory test specifications for electrical properties (adapted from [34]).


**Table 4.** ASTM laboratory test specifications for functional driveline fluids or greases (adapted from [34]).


In addition, ASTM Corrosion Tests [32] for grease have been developed such as the ASTM D4048 Copper Corrosion from Grease, D5969 Corrosion of Grease in Sea Water, and D1743—Corrosion Preventive Properties of Lubricating Greases. ASTM offers other basic lab test methods for specific properties related to electric fields and thermal cooling conditions, which have ye<sup>t</sup> to be reviewed and assessed for greases used in EVs or HVs. A future publication for further standard test methods will be addressed.
