**4. Conclusions**

Greases have been used in manufacturing and automotive industries and have been rapidly evolving in recent decades to meet the demands of modern automobiles. While some greases have specific niche applications, the most common ones are those that are versatile and can be used in a wide variety of situations. Lithium-based greases, for example, are some of the most popular greases on the market due to their wide temperature tolerance and high dropping point. The reason for their popularity is due to the effective solution for a major cause of grease failure—high temperature. Greases operating at high temperatures and loads for ICEVs' components cannot be applied for EVs. EVs or HVs, while they also produce heat, prefer to operate in cooler conditions, requiring specific greases that can function at those temperatures as well.

Furthermore, one important aspect of EVs or HVs not present in ICEVs is the specific performance requirements of electrification components. These create electromagnetic fields and electrical currents, leading to potential issues as the field can transfer energy without the need for a physical medium. It became common for greases—especially thermally and electrically non-conductive ones—to absorb too much of the energy and eventually degrade due to oxidation and evaporation. The difference in operating conditions leads to a difference in requirement of lubricants in the two vehicles. The preferred cooler temperature of EVs leads to the pursuit of both a lower viscosity and density of lubricants while the opposite is true for ICEVs. While ICEVs are abundant today, EVs will undoubtedly become the future and their lubrication and grease components will be a focal point.

The major differences between EVs and conventional ICEVs can be grouped into the following technical areas: energy efficiency, noise, vibration, and harshness (NVH) issues and the presence of electrical current and electromagnetic fields from electric modules, sensors and circuits, and bearing lubrication. Additional considerations include thermal transfer, seals, corrosion protection, and materials' compatibility. The authors will review the future development trends of EVs/HVs on driveline lubrication and thermal management requirements. The future development of electric vehicles will globally influence the selection and development of gear oils, coolants, and greases, as they will be in contact with electric modules, sensors, and circuits and will be affected by electrical current and electromagnetic fields.

The increasing presence of electrical parts in EVs/HVs will demand the corrosion protection of bearings and other remaining mechanical components. Thus, it is imperative that specialized greases should be explored for specific applications in EVs/HVs to ensure maximum protection from friction, wear, and corrosion to guarantee the longevity of the operating automobile.

**Author Contributions:** Conceptualization, R.S., R.C., S.T. and R.M.; literature review and formal analysis, R.C. and S.T.; writing—original draft preparation, S.T. and R.C.; writing—review and editing, R.S., S.T. and R.M.; revisions and supervision, R.S., S.T. and R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data from reference [14] (Figures 1 and 2) and reference [15] (Table 2 and Figure 3) are publicly available from https://www.semanticscholar.org/paper/Decomposit ion-Kinetic-of-Greases-by-Thermal-Rhee/80d4d8f1ac0213173967ad1b6b4d15c3689f9026 (accessed on 2 March 2021) and https://doi.org/10.3389/fmech.2020.571464 (accessed on 2 March 2021), respectively. Data from Table 1 (reference [8]) and Tables 3 and 4 (reference [34]) have restricted access. Data from [8] is available at https://www.elsevier.com/books/the-tribology-handbook/neal e/978-0-7506-1198-5 (accessed on 2 March 2021) and [34] is available at https://www.astm.org/DIG ITAL\_LIBRARY/MNL/PAGES/MNL6220121208809.htm (accessed on 2 March 2021).

**Conflicts of Interest:** The authors declare no conflict of interest.
