Dr. Gareth Fish
Different types of lubricating greases have long been used in numerous applications in passenger cars. But as electrification continues apace, certain grease formulations will need to account for some of the inherent, unique challenges in an electrified drivetrain.
From the development of improved drainage and sewer systems in large metropolises in the 19th century to the reduction of smog generated by vehicles, reducing air pollution has been a goal of humanity for hundreds of years. And as governments around the world continue to pursue aggressive emissions reduction targets in their efforts to mitigate the effects of climate change, we can collectively expect to see new technologies emerge that will make progress toward those goals.
In the automotive space today, one important technological change has been an increasing shift toward the electrification of the drivetrain. Electric cars have a history dating back to the latter part of the 19th century, but it wasn’t until Toyota launched the first modern hybrid electric vehicle (HEV) in 1997 that electrified cars began to gain traction on roads around the world. Though low gas prices initially kept demand low, it was nevertheless the start of the trend that has only grown in the ensuing decades.
Today, HEVs and fully electrified vehicles (EVs) are commonplace, and their adoption is expected to sharply rise as the technology continues to mature. Importantly, HEVs and EVs differ from traditional internal combustion engine (ICE) powered vehicles in a few fundamental ways, and have necessitated changes to the ways we think about common lubricants and greases that are essential to reliable operation.
As of 2019, roughly 483,000 metric tons of grease were sold into the automotive industry each year to provide a wide variety of functionality most drivers probably take for granted. On a given passenger car vehicle—including everything from small sedans to light-duty trucks—there are between 50-60 individual parts that require some form of grease to operate as intended, including wheel bearings, joints, window winders, seat rails, sunroof mechanisms, steering racks, door hinges, brake mechanisms, shock absorbers and many others.
Automotive greases vary in their formulation depending on their intended use, but many contain molybdenum disulfide and graphite as additives (typically with an additive treat rate between 4%-6%). Lithium, lithium complex and polyurea are also common thickeners depending on the geographic region of formulation. Across the vehicle, automotive greases are generally intended to provide one or several of the following four functions: Corrosion protection; lubrication for bearings or other componentry; water resistance; and anti-squeak performance.
Inside the vehicle, greases typically serve light lubrication and antisqueak duties. They must often be compatible with plastics and will not be replaced throughout the vehicle’s lifetime, making long-term reliability a key performance characteristic. Exterior greases generally are expected to provide antiwear, anticorrosion, and antioxidant capabilities. They must also be able to withstand on-road conditions (rain, snow, and more) without being washed off. Likewise, they are not typically replaced or reapplied throughout the vehicle’s useful life.
A typical passenger car angular contact (AC) wheel bearing provides a good example of an application that demands high levels of grease performance. The right formulation can help reduce churning losses, requiring optimized base oil viscosity and consistency. Synthetic fluids here can deliver lower losses at comparable viscosity levels; Diurea thickeners can additionally provide some improved efficiency.
Elsewhere, greases are found in a variety of electric motors that are included in a typical ICE vehicle, including cooling fans, fuel pumps, starter motors, power steering systems, braking systems and more. Here is where we can begin to identify some of the challenges greases must contend with in increasingly electrified drivetrains.
Greases in HEVs
By and large, the majority of greases on HEVs will likely remain the same as those used in today’s ICE vehicles, given that the functionality of things like seat rails, sunroofs, automatic windows, and other applications will not fundamentally change.
We are anticipating some changes, however. For instance, starter motor greases will no longer need to be formulated with high shock load resistance, because starter motors can now be permanently engaged for stop-start functionality in hybrids. Elsewhere, transmission electric motor bearings may be grease lubricated or oil lubricated when incorporated in the gearbox or differential.
Increasing battery capacities will have implications, as well. Typical hybrids utilize batteries that are capable of lasting up to 50 miles without a charge from the ICE engine, but larger batteries which increase overall vehicle weight can have some implications for wheel bearing greases. Supporting extra loads may involve some changes to the bearings and the greases required to lubricate them.
Greases for EVs
While HEVs are closely comparable to ICE technology in terms of their demands of on-vehicle grease applications, true battery EVs have other implications. 2017 saw global EV sales reach 1 million and 2019 saw sales reach 2 million. As of 2020, EVs make up 4.2% of all light vehicles made. Currently, Europe is the leading market for EVs. As these numbers grow, the automotive industry must be familiar with the changing demands of many common greases found throughout the vehicle.
Increased weight. As noted, increasing vehicle weight due to battery size has implications for wheel bearings. Take, for example, that a 60 kWh electric battery weighs around 950 pounds, contributing to a total midsize EV weight of about 3,500 pounds. This vehicle will have a range of about 250 miles. A similar sized hybrid with an ICE, electric battery and electric drive motor will typically weigh about the same. Meanwhile, an equivalent ICE-only vehicle will only weigh 2,500-2,800 pounds depending on the engine size. This increased weight, both in HEVs and EVs, has the potential to reduce bearing life by up to 30%. Light trucks may have to change the wheel bearing types to support the extra load.
New greases. Elsewhere, EVs will incorporate a few entirely new greases without comparable applications in ICE vehicles. First, transmission electric motor bearings may be grease lubricated when incorporated within the gearbox or differential. Second, electric motor bearing greases in these applications must deliver long life, low noise, conductivity or insulative performance, and energy efficiency.
Obsoleted greases. There are several traditional ICE grease applications that will simply become obsolete in most EVs. These include many applications through the driveshafts, like center bearings, high-speed constant velocity (CV) joints, universal joints, and sliding splines. Accessory drive bearings will also be obsoleted, along with the need for grease in water pumps, engine cooling fan bearings, alternators and belt tensioner pulley bearings. Starter motors and their associated greases are not required for EV operation.
Modified greases. Many greases required in today’s ICE vehicles will also be required in EV applications, with some notable changes.
One of the most important characteristics of electric bearing motor greases, for example, is their electrical conductivity. There are two primary concerns here: First, if a grease’s conductivity is too high, it could lead to current leakages and short-circuiting. There is no hard-and-fast limit on how high is too high; it will depend on the specific application. Second, if a grease’s conductivity is too low, this can lead to static electricity buildup and arcing, which can cause significant damage to the motor bearings. Again, there is not a set limit—the specific application will determine how low conductivity should be. OEMs and grease formulators will need to work closely together to determine the optimum levels of conductivity.
Certain greases in EVs also have a significant opportunity to contribute to greater vehicle efficiency, and therefore to improving the range of EVs. These applications include:
- Drivetrain joints and bearings
- Front end accessory drive bearings
- Wheel bearings
- Steering mechanisms
Within these four critical applications, however, delivering higher efficiency comes with some inherent challenges. For example, at low speeds, good lubrication films are not generated. There is a tendency for in-boundary lubrication, which can result in energy losses. While moving to a thicker base oil will increase lower speed film thickness, it will lead to higher speed churning losses. Meanwhile, at higher speeds, the lubrication films generated tend to be thicker. In these instances, lower-viscosity greases can deliver thinner films, which reduce churning losses. However, if the film is too thin, component durability may be compromised.
A balance must be struck, and advanced formulations will be required to deliver the necessary performance. The right base oil selection, thickeners, and additive packages can all contribute to a formulation that is ideal for an efficient EV grease.
As the number of electrified vehicles on roadways continues to increase around the world, it is critical for stakeholders to consider the ways in which grease formulations will be impacted. While there are many unknowns, it is clear that certain critical greases can help deliver increased energy efficiency and range, along with overall vehicle durability. Grease formulations that strike the balance between lower viscosities to reduce churning loss while maintaining the highest level of protective properties can deliver on those goals. As an industry, we must work together to develop the formulations that can help EVs meet their fullest potential for a cleaner future.
Dr. Gareth Fish
Dr. Gareth Fish is a technical fellow at The Lubrizol Corporation. He is an internationally recognized lubricants industry professional with over 37 years of experience. Dr. Fish earned a B.Sc. in Chemistry and a Ph.D. in Tribology from Imperial College of Science, Technology and Medicine, in London, England. He is a Chartered Scientist, has earned STLE CLS and NLGI CLGS certifications, authored more than 60 technical papers on grease and tribology and three book chapters and been awarded three U.S. patents and has taught more than 70 public classes on grease and tribology. In addition, Dr. Fish has won nine industry prizes for papers and publications on lubricating grease, including the NLGI Achievement Award for his more than 30-year contribution to the grease industry.