Lubricants in Electric vehicles

Introduction

The tribological requirements for Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs) differ from those of Internal Combustion Engine (ICE) vehicles. In EVs, key concerns include a lubricant’s thermal and electrical properties, copper corrosion, and compatibility with the elastomers/polymers used in their components. Maintaining proper lubrication at speeds exceeding 25,000 rpm is crucial for protecting seals, bearings, and gears from friction and wear. Additionally, the utilization of advanced materials in batteries and motors necessitates the formulation of new lubricants compatible with these materials as they come into contact with these crucial components. Ensuring compatibility is essential to prevent hazards caused from the explosive electrolytes of batteries and motor parts. Furthermore, the adoption of low-viscosity lubricants is driven by the need for efficient heat transfer within EV/HEV systems.

Properties of EVs lubricants

Electrical properties

Lubricants utilized in EVs and HEVs encounter electrical currents flowing through the lubricated bearings while safeguarding the contacting surfaces. This current flow occurs on surfaces electrically connected to the electric motor. Lubricants with inadequate electrical properties may lead to Electric Discharge (ED) damage. To ensure effective protection, it is imperative to select lubricants with appropriate electric resistance and lubricant dielectric strength throughout their lifespan. Achieving suitable electric impedance and dielectric strength can be accomplished by altering the Base Oil (BO) or by employing additives.

Preventing electric damages

To prevent electric damages it is important to maintain low electric resistance than possessing a high dielectric constant. While neat non-polar BOs like PAO and mineral oil boast dielectric breakdown voltages in the range of 10 kV. It is significantly higher than typical motor bearing voltages and the presence of impurities or additives can drastically reduce this breakdown voltage. Studies indicates that ED damage can occur at bearing voltages as low as 100 V, and the dielectric breakdown voltage of non-conductive grease may decrease to just a few volts over prolonged testing periods. Therefore, completely relying on high dielectric strength is unrealistic for preventing ED damage.

Thermal properties

The molecular structure of BOs plays a crucial role in determining the thermal capacity and thermal conductivity of a lubricant. A higher number of “quantum states” in the molecular structure corresponds to increased thermal capacity, requiring more energy input to raise the temperature. The thermal conductivity of a BO is linked to its molecular diffusivity, with easier molecular movement resulting in higher thermal conductivity. This relationship between BO properties and lubricant viscosity can constrain lubricant selection when considering both tribological conditions and thermal management. Nanoparticles added to lubricants can significantly enhance thermal conductivity and capacity by increasing carriers of thermal energy. For instance, adding 0.8 vol.% of silica nanoparticles can double thermal conductivity. However, nanoparticle additives may decrease the specific heat of the lubricant. Despite lacking experimental evidence in EV/HEV lubricants, nanoparticle additives show promise in optimizing thermal properties and improving tribological performance, presenting potential benefits for powertrain cooling design.

Thermal management

Effective thermal management is crucial for EVs and HEVs. Optimized performance of electric motors relies on maintaining thermally controlled operating conditions. Achieving this requires a thermal path with high thermal conductivity between the source of energy loss and the thermal sink. Lubricated contacts serve as one of the most important thermal paths in EVs/HEVs. Furthermore, circulating lubricant can provide additional cooling for electric motors. Inadequate thermal management can increase the resistance of copper wires in electric motors, leading to decreased efficiency. Additionally, high temperatures within the electric motor can demagnetize permanent magnets and reduce motor lifespan. Therefore, ensuring effective thermal management is essential for maximizing the performance and longevity of EV and HEV systems.

Reference

[1] Chen, Y., Jha, S., Raut, A., Zhang, W. and Liang, H., 2020. Performance characteristics of lubricants in electric and hybrid vehicles: a review of current and future needs. Frontiers in Mechanical Engineering, 6, p.571464.

[2] https://www.tribonet.org/industry-news/electric-vehicle-lubricants/

[3] https://www.mosil.com/blog/contact-lubricants-for-electric-vehicles/

[4] https://addinol.de/en/products/lubricants-for-the-automotive-sector/oil-electric-cars/