lubricants for electric vehicles and hybrids

Rajesh Uppal November 27, 2021 Energy & Propulsion, Material Comments Off on lubricants for electric vehicles and hybrids 287 Views

Transport is responsible for around 23% of energy-related carbon dioxide emissions globally. This is expected to double by 2050. Motor vehicles also put a burden on society, especially in urban environments where they are chiefly responsible for noise and air pollution. Electric vehicles (EVs) have been considered as critical technology for addressing the concerns about energy cost, energy dependence and environmental damage. Electric cars bring nice driving experience in form of instant acceleration, quieter and smoother driving experience, cheaper to run as well as cheaper to maintain, and also has environmental benefits.

A number of large manufacturers are developing FHEVs because they see the technology as a good bridge until more advanced diesel and electric vehicles are fully developed. Full hybrid electric vehicles (FHEV) with additional features such as regenerative braking or stopping the engine during idle, can significantly improve fuel economy and lower CO2 emissions. It makes them increasingly popular with OEMs as a way to reach European emission legislation.

Depending upon the make, EVs can be of different types: battery, hybrid, plug-in hybrid, fuel cell battery, and solar electric vehicles . HEVs come in several designs: (1) classification based on electrical and mechanical power flows: series, parallel, series-parallel, or complex hybrids; (2) classification based on power levels and operation: full, micro, and mild hybrids. In the HEV, one unique feature is that it can shut off the IC engine when the vehicle has sufficient power to run from only the electric motors. This leads to more cooling of the IC engine and frequent starts and stops

As the automobile industry is making progress towards the new age where EVs will become the next big thing as they will overshadow internal combustion engine (ICE) vehicles and presumably dominate the world’s roads, a new set of lubricants and oils must be made specifically to tailor to the performance requirements of the electrical and gear components in the EVs. Environmental protection, resource utilization, and customer satisfaction are the key drivers for innovation in EV/HEV lubrication.

In aspects of mechanical performance, the EV/HEV technology presents several tribological challenges. The failure of bearings that may account for almost 40% of failures in motors in EV/HEV can be a major concern due to complex voltages in shafts and bearing currents. Premature bearing failures are accompanied by undesirable noise, vibration, and instability. A generalized solution to EV/HEV lubrication can be challenging because of a highly diverse bearing current range and design (He et al., 2020). Conventional solutions to friction and wear may also not be feasible in EV/HEV. For instance, the use of present-day friction modifiers like molybdenum dialkyl dithiocarbamates results in loss in effectiveness over miles accumulation. Hence, new strategies and solutions are needed to improve tribology performance.

Lubrication Challenges for Electric vehicles

EVs require lubricants in vital electrical components such as coolants for the car battery, gear oils for differentials, chassis, gear reducer, and wheels, brake fluids, and grease for other components of the EV.

Creating lubricants for EVs is a difficult endeavor as original equipment manufacturers have their own unique electric motor design, thus requiring a specific lubricant for their electric motors to fit their needs for great performance. EV lubricants have greater technical requirements compared to that of ICEs. The lubricants must target important specifications such as antiwear performance, friction reduction, efficiency, electrical compatibility and insulation, and electric motor and battery pack cooling. The current and future developments of EV lubricants have a common goal to minimize friction loss, enhance durability, bolster efficiency, and strengthen other performance aspects. Satisfying those salient goals for EVs would pave the way towards a greener future.

Efficiency is an important area of focus and correlates directly to the torque properties of EVs and translate into range. Since electric motors are designed to be compact to save weight and space, higher rotational speeds of up to 18,000 rpm must be dealt with. When there is high stress occurring on the motor, air can infiltrate the components and cause fluids to foam which causes damage to the components’ surfaces. Therefore, reducing torque through the development of low viscosity lubricants will help solve those issues and push EVs to dominate the automobile industry.

The distinguished difference between electric/hybrid vehicles from conventional ones is the intermittent on-and-off control of the internal combustion engine, whereas electric vehicles operate on electrical motors. This difference creates new requirements in lubricants with performance characteristics that are otherwise uncritical: electrical properties (such as electrical conductivity and breakdown voltage) and thermal properties (thermal conductivity, specific heat, among others), in addition to fluidic performance that we have studied for decades.

There is no combustion engine in the EV. The core jobs of lubricants remain the same. With the advancements of e-mobility technologies, those lubricants must play a major role in electrical compatibility, thermal management, and material adaptability. It is also expected in the near future that the EV transmissions and axles will accommodate an electric motor in the unit housing.

In this case, the presence of electric motor windings in the transmission would add up more copper in contact with the lubricants, and therefore more copper corrosion problems will arise . The large amount of heat produced at the motor windings will test the heat transfer ability of the lubricant as well as its thermal stability. To sustain these high temperatures, the need for use of new alloys and polymers for manufacturing vehicle components is expected, which may give rise to new compatibility concerns.

Lubrication challenges of hybrid vehicles

For enhancing the performance and efficiency of the vehicle, it is necessary that all the components involved in the power generation process should be optimized. Therefore, the study of lubricants is of fundamental importance. In a conventional IC engine vehicle, the lubricants used are engine oil, transmission fluids, and grease. The engine oil provides hydrodynamic lubrication to the engine, wear protection in metal-to-metal contact, cooling for internal engine parts along with many other performance enhancement and protective functions.

Be it automatic stepped transmission (AT), continuously variable transmission (CVT), or the dual-clutch transmission (DCT), the transmission fluid has the same broad purpose, that is, to create hydraulic pressure, dissipate heat, and protect the metal gears and other parts from wear. The main role of grease in automotive is to reduce frictional losses by lubricating bearings, i.e., most of the moving parts of the assembly. But along with the developments in automobile industries, the lubricants need to perform in harsh conditions and provide various performance and compatibility aspects.

The HEV has an electric motor alongside the combustion engine. Its combustion engine is smaller in size compared with the ICE vehicles of corresponding sizes. The size of the ICE engine becomes smaller and that of the electric batteries grows as the vehicle approaches the EV.

The DCT mechanism has the most efficient transmission technology in terms of mechanical efficiency. Therefore, most HEVs in the market have DCT modular transmission. In these types of vehicles, the e-motor is directly integrated with the DCT box and is cooled by transmission lubricant. Because the lubricating fluid is in contact with the electrical components, it is highly important that it has superior electric properties like electrical conductivity, dielectric constant, and dielectric strength.

During speeding up, a hybrid vehicle switches from one to the other engine at the proper time, yielding a win-win in terms of energy efficiency. This is translated into lower fuel consumption and reduced CO2-emission.

However, it also brings some specific challenges:

When the combustion engine kicks in when the hybrid car is already travelling at speed, it introduces a heavy load on cold start, increasing the risk of engine wear.
Because the operation time of the combustion engine is short it is more prone to moister and acid formation.
Under-usage of the combustion engine can lead to fretting wear.
Short journeys during which only the electric motor is used, increase the risk of water accumulation in the combustion engine.

These special engine operating conditions result in low oil temperatures for which the additive chemistry has to be compatible. In this harsh environment, water ingress and sludge formation can cause serious oil degradation over time. In combination with the use of bio-fuel, this can further accelerate the oil degradation process in HEV applications. It means only the highest quality engine oils are suitable for hybrid vehicles.

Lubricants for Electric and Hybrid vehicles

Recent progress in lubrication has been reported in areas such as bio-lubricants, mineral oil-based lubricants, nanoparticle additives, and carbon nanotube-based lubricants, among others.

The research efforts on lubricants have been of obtaining higher resistance to copper corrosion and compatibility with polymers employed in the electronic components of EV/HEV. This involves the design of new standard test methods to measure properties in EVs. Obtaining low viscosity and improvement of electric and thermal properties are other key areas of focus. The most successful approaches have been the use of nanotechnology-based anti-wear and friction lubricants, vapor phase lubrication, ionic liquids, and low-viscosity oils.

In EV, the importance of grease cannot be overlooked. In grease use, nanotechnology, synthetic base oils, and thickeners have demonstrated enhanced lubricity, higher service life, and low friction torque. Lithium grease has shown to impart the advantages of high adherence, non-corrosiveness, and moisture resistance making them compatible with several applications. Aluminum and urea greases perform well too; however, their production is associated with hazardous processing and constraints on process balance. There have been attempts to find green solutions to the lubrication problem.

There have been attempts to find green solutions to the lubrication problem. With low volatile organic compounds (VOCs), low compressibility, high dielectric strength, and good emulsifiability, bio-based lubricants have shown promise as alternatives to conventional oils. With chemical modifications (for high thermal stability and oxidative stability) and the use of suitable additives for load-bearing and friction properties, they can perform better than conventional lubricants. Bio-inspired designs are also used to improve efficiency. Nakanishi et al. proposed a bio-inspired oil seal that mimicked articular cartilage and had a comparatively lower frictional torque compared with traditional oil seals

The transmission fluid in HEV contains dispersants and needs to have insulating properties (low electrical conductivity) to avoid short-circuiting of motor parts. Tang et al. treated the dispersants in transmission fluid with phosphorus (P) and boron (B) to improve anti-wear and anti-friction properties.

Wheel bearings in EVs are important targets for improving efficiency. The high torque in the wheel bearings of an EV need to be well controlled. The grease used need to perform consistently at elevated temperature fluctuations.

Electric Vehicle Fluids and Lubricants Market

The global electric vehicle fluids and lubricants market size reached USD 1,178.8 Million in 2020 and is expected to register a CAGR of 18.7% during the forecast period, according to latest analysis by Emergen Research. Factors such as favorable government policies towards adoption of more environment-friendly and renewable energy resources and vehicles and increasing competition in the electric vehicle market to leverage opportunities emerging as a result of the steady shift towards EVs are some key factors driving market growth. In addition, rising focus on developing, manufacturing, and introducing more efficient and affordable EVs in the global market are other key factors expected to drive market growth going ahead.

Rising use of coolant fluid for thermal management systems in electric vehicles is driving growth of the heat transfer fluids segment, which is expected to register significant CAGR over the forecast period.

In terms of market share, hybrid electric vehicles (HEVs)/plug-in hybrid electric vehicles (PHEVs) segment is expected to register significant growth during the forecast period due to rising adoption of HEVs in developed and developing countries to reduce carbon emissions.
In terms of revenue, Asia Pacific is expected to register a significantly rapid CAGR during the forecast period due to increasing demand for electric vehicles in countries in the region.

Key players in the Electric Vehicle Fluids and Lubricants market include 3M, Castrol, Royal Dutch Shell, Total Lubricants, Valvoline Inc., Motul, Fuchs Petrolub SE, Engineered Fluids Inc, Exxon Mobil Corporation, and Lubrizol Corporation.