3.5.1. Energy Efficiency

While EVs are extremely efficient in energy consumption, reports have shown that as much as 57% of the energy used by the car is for overcoming frictional losses [29]. The main components considered to make up the large frictional loss include motor, tires, steering system, wheel bearings, joints, suspensions, and a few others [29]. Friction losses observed in electrical motors (EMs) are attributed to the heat produced by rotors, vibration, wear, and going against wind resistance. In rolling bearings, the main sources of friction loss are from churning, sliding, which is the source of the greatest friction loss of bearings, rolling, and seal sliding friction [30]. For these, a good lubricant or grease is expected to properly offset the frictions. To improve the energy efficiency of grease in these bearing applications, synthetics, primarily polyalphaolefins (PAOs), are used. Given the mix of sliding and rolling contact in the load zone of point contact bearings (ball and spherical roller bearings), PAO-formulated greases have improved traction properties compared to mineral oil-formulated greases. Greases are mainly used in bearings and gears due to them being more of a solid than oil lubricants, as they will not leak out of components and cause damage. Since the usage of greases is much easier with bearings and gears, about 85% of all bearings are lubricated with grease. As increasing energy efficiency is a big focus in the EV industry, all opportunities to reduce friction will help achieve the goal of reaching a range of up to 400 miles [31].

Energy efficiency can be correlated to lubricating film thickness. Thinner lubricants reduce viscous friction, allowing more energy to be conserved. However, greases will reduce lubricating film thickness at high temperatures and bring their own challenges on wear protection of sliding surfaces. A thinner lubricating film will be closer to the mixed and boundary lubrication regimes where wear is a concern. Striking a balance between the capability to remain in a full film lubrication regime and the wear protection towards thinner lubricating films is a crucial turning point that enhances energy efficiency using low-viscosity lubricant or grease. Several papers have emphasized that a new approach using either surface coatings or lower-roughness surfaces will ensure a thin lubricating film to separate surfaces and mitigate wear.

One of the most significant challenges for EVs is to extend the driving range besides energy efficiency. This challenge also imposes the infrastructure of wide-spread recharging stations. Without recharging infrastructure, all passengers must be prepared to give up the 300- to 400-mile ranges they are accustomed to in a conventional vehicle. Automotive engineers are looking for innovation approaches to improve energy efficiency by making vehicles lighter and by reducing torque in all components.

## 3.5.2. Electrification Components

The projected growth of EVs and HVs in the automotive industries has made evolutional changes in propulsion system electrification components. Powertrain and driveline components will be enhanced by electric motors and E-drive configuration. Electrification components including electric motors or regenerative brake components will be integrated in this advanced configuration with thermal managemen<sup>t</sup> systems for cooling applications. Electric motors and power electronics will be in contact with the cooling fluids. Coolants are required to cool motors and power electronics by removing heat for thermal management. In addition, automotive lubricants for electrified propulsion systems must function as effective coolants and reduce the corrosion of copper windings, on top of needing composites and rare earth magnetic materials while upholding wear and oxidation protection and energy efficiency.

Dr. Kuldeep K. Mistry [28] at The Timken Co. in Ohio also indicated that the development of electric vehicles will influence the selection and development of gear oils, coolants, and greases, because the lubricating oils or greases will be in contact with electric modules, sensors, and circuits. It will be affected by electrical current and electromagnetic fields. Soon, the number of electrical connections is expected to quadruple. In this context, one of the key life-limiting considerations will be corrosion.

Although grease oils will prevent moisture from reaching the surfaces, thus preventing rust, the presence of electromagnetic fields will create another challenge. Electromagnetic fields can propagate without the need for a solid medium. The amount of energy transferred is dependent on the intensity of the field, the frequency of its oscillation, and the dielectric properties of the material. The more energy the object absorbs, the more quickly the object will be heated. The object will gain heat as the frequency of the electromagnetic field increases. However, if the temperature diffusion rate is slower than the rate at which the electromagnetic field releases its energy, the temperature will increase rapidly [31]. As electric motors produce electromagnetic fields, there is a chance for the premature degradation of greases. The electrical discharge and free radicals will react with the oxygen in greases, creating hydrogen peroxide and continuing the chain of free radical reactions. This leads to the oxidation of base oils and thickeners, which causes a loss in lubricity. In addition, components within the grease will start to separate and the thermal effects from the electrical discharge will cause certain parts to evaporate and eventually cause grease failure [18,31]. To this degree, experiments have shown that a conductive lubricant will help with its corrosion prevention. This is because non-conductive greases will trap the energy from electrical currents and electromagnetic fields, leading to a sudden release and causing grea<sup>t</sup> damage [18,31]. However, if the material is conductive, then while the electrical currents will pass through, it will be in a much smaller amount and cause less damage.
