As hybrid electric vehicles continue to gain market share and hardware technology develops, there are three main concerns around the role and nature of the lubricating fluids used in the drivetrain, especially as the technology becomes more complex and drivetrain engineers look for the next level of efficiency.

The first issue concerns the electrical properties of the fluids and their compatibility with an increasing number of electrical components, wires and sensors.

The second has to do with viscosity and the race to maximize efficiency while maintaining protection for hardware, gears and bearings. Current transmission fluids are between 6 centistokes and 4 cSt, as measured by ASTM D445 at 100 degrees Celsius. But the desire to go thinner exists, and lubricant additives must be properly balanced to protect hardware.

The third area of concern is thermal transfer properties of driveline fluids, as they must be able to cool components such as electric motors, which produce localized areas of extreme high temperature.

Not only has the industry not yet developed lubricants that address these needs, it still needs to develop tests and methods to measure them. All these factors are raising questions that cannot be answered by current lube testing and evaluation methodologies, observed Monica Beyer, head of commercial HEV drivetrain strategy at Lubrizol. Its a whole new area of investigation that requires more sophisticated ways of looking at things.

Because they combine two different powertrains, hybrids require lubricants that accommodate both. Developing these fluids will be no simple task, especially as vehicle designs continue to evolve.

The different aspects of lubricant function and protection are increasing in numbers and broadening compared with the existing fluids in the marketplace, Beyer explained. In addition to the traditional hardware, such as gears and bearings, fluids need to work with and protect an increasing number of sensors, the polymer systems and resins used to coat or hold in place wiring, and in some cases exposed copper wiring and contacts.

Lubrizol has been working to understand the potential impacts of this increased exposure of lubricating fluids to electrical components and is developing new testing protocols that are more productive and relevant for the industry.

One area of development is an improvement in the test for copper corrosion. Today, with copper and a variety of copper alloys being the metals of choice for many electronic components in electrified vehicles, corrosion of these parts can lead to new issues, such as sudden interruptions of current, false sensor readings, malfunctions of control systems and loss of drivability.

The right lubricant additive technology is essential for preventing corrosion. The most common way to evaluate an additives effect on a copper surface is the ASTM D130 standard test method, which compares copper test coupons that have been immersed in a lubricant for several hours. While corrosion is a highly complex and chaotic multicomponent scientific problem, this decades-old test is simply a soak method done at stable temperature and provides no detail about the chemical kinetics (reaction rate and the factors that impact this rate), or the time dependence of the corrosion process.

Additionally, there is an inherent assumption that additives that are shown with this test to protect copper will also be suitable to protect copper alloys. Shortcomings like these are not surprising, however, given the fact that ASTM D130 was originally designed to evaluate the corrosiveness of crude oils alone, not fully formulated lubricants containing complex additive packages.

Providing only a visual comparison based on what color the copper strip turns, this test may indicate passing performance of a product that would ultimately fail under real-world conditions. A better measure of a fluids performance would provide insights into the kinetics and mechanisms of copper corrosion in the presence of lubricant additives over a range of operating temperatures.

A procedure developed by Lubrizol under the direction of Senior Research Chemist Gregory Hunt is shedding new light on the kinetics and mechanisms of metal corrosion, specifically that of copper and copper alloys. Using a new wire resistance test, were able to monitor corrosion in real time, said Hunt. The new test allows for easy assessment of the corrosion mechanism and challenges industry-held beliefs around lubricant additive corrosion processes, especially at elevated temperatures.

Corrosion processes are proven to be highly dependent upon temperature. In the interest of condensing testing time, ASTM D130 tests are accelerated by increasing temperatures, which presumes the dominant kinetic mechanism remains constant between real-world and accelerated conditions.

The new wire corrosion test demonstrates that certain lubricants do not maintain the same mechanism between real world and typical test temperatures, according to Hunt. It provides a detailed understanding of a lubricants copper corrosion behavior with temperature and time parameters based on real operating environments. Further, the test can be completed within three days, enabling economical testing at multiple temperatures.

Though its not an industry requirement, Lubrizol has been using this new wire corrosion test when evaluating the performance of its additives. If it were just about performing well in a test, ASTM D130 would suffice, Hunt stated. But we need to know how our products are going to actually perform under real conditions. We need to be able to offer our customers that level of confidence.

Other work is aimed at enabling a better understanding of oil conditions under different temperatures more reflective of actual operating conditions. Simulating the real-world conditions as accurately as possible is vital, as controlling the integrity of a fluid throughout its life remains a key factor in preventing potential problems-especially as viscosity decreases and operating environments vary more and become more extreme.

Lubrizol is also evaluating potential test methods to assess thermal transfer properties of fluids. Under conditions demanding high power output from the motor, temperatures will spike well above normal operating ranges. Assuming that the motor is in contact with a lubricant, fluids that can efficiently transfer heat away from the transmission will help original equipment manufacturers design more efficient and lighter-weight systems.

Ultimately, this all indicates that the industry may be at a turning point in terms of its lubrication fluid requirements. There is an emerging raft of necessary performance attributes, such as wire coating compatibility and electrical conductivity, which are not inherent, nor tested for, in current transmission fluids.

Are current transmission fluids still suitable, and are they the best fit for these new environments? It depends.

Currently, existing fluids are being used in hybridized versions of both manual and automatic transmissions, but as can be seen, these fluids may not always be the best solution. In the same way that different OEMs and suppliers are using different hardware solutions for HEVs, they are also using different lubricating fluids, with the default option being whatever is readily available today. This has the potential to be a limiting factor, as potential hardware development could be held back using traditional fluids in the thinking and development process.

To help evaluate the suitability of todays fluids, enable development of potential new hardware and define and develop tomorrows fluids, collaboration within the industry will be key. The lubricant base oil and various additives all play a role in determining what these new fluids will look like and how they can enable HEVs to perform at their best. To further reinforce this need for collaboration, any adjustments in lubricant technology may also have unintended effects elsewhere.

Looking to the near future, it is possible to imagine that throughout the entire drivetrain, a whole new range of lubricating fluids may be used. Specialized fluids will be able to provide performance in electrical compatibility, at extremely low viscosities, at speeds higher than 20,000 revolutions per minute and in an operating environment of sustained excursions at high temperature.

Beyer concluded, As HEVs become more mainstream, their performance and reliability will gain greater exposure and become more apparent to a greater number of consumers. The drivetrain plays a critical role in the driver experience, as it is central to how a car feels when driven. Working together, industry stakeholders can get ahead of any emerging issues in lubricant-related performance to deliver reliable and durable new drivetrain technology to the market.

Keith Corkwell has more than 27 years of experience in the lubricant industry. He is general manager of driveline additives, viscosity modifiers and custom solutions at the Lubrizol Corp. He has also served as global business manager of the companys heavy-duty engine oil additive and fuel additive businesses.