DCTF meets hybrid challenge
07 December 2020
27 October 2020
How the quest for efficiency is shaping the future of transmission fluids
Change in vehicle transmissions hardware is being driven by ever tightening emissions legislation, which compels the automotive industry to find every possible means to improve fuel efficiency. Leandro Benvenutti, Infineum Specialties Market Manager & OEM Advisor, talks to Insight about the advanced and specialised fluids that are being developed to support the needs of this continually evolving market.
To contribute towards fuel efficiency improvements, automotive OEMs are offering more transmission options in their vehicle ranges including automatics with an increasing number of gears as well as dual clutch (DCT) and continuously variable (CVT) transmission systems. However, as Infineum’s Leandro Benvenutti explains, the significant differences between these technologies mean that fluids designed for one application may not be suitable for use in the others.
“If we look at CVT and automatic systems for example, the fluids here are not interchangeable,” he confirms. “Although a CVT has a fairly simple operation, comprising two-pulleys with variable diameters in a continuous gear ratio, some technical challenges have been experienced with its lubrication. In this system, all of the engine force is transmitted by the contact of a belt or chain with the pulley, which creates metal-to-metal contact. It is essential that the lubricating oil ensures there is no slippage between these surfaces to prevent wear while also maintaining the correct friction factor - a big challenge for the CVT fluid. This is very different from a traditional automatic transmission fluid, which does not need to prevent slipping, but is required to ensure smooth gear shifts and enhanced gear protection. While there are some similar requirements for the fluids, such as wear protection and materials compatibility, a different suite of additives is needed to deliver the required performance to these two very different hardware technologies, which is why the fluids are not interchangeable.”
DCTs are different again as Leandro continues. “There are two types of dual clutch transmission on the market: dry clutch and wet clutch, what differs between them is where the clutch is installed in the vehicle. If the clutch is outside of the gearbox, as for example in the Ford with Powershift™, then a traditional manual transmission fluid is typically used. When the clutch is inside the transmission box, for example in most of the VW DSG (Direct Shift Gearboxes), it is known as a wet DCT and a specific fluid that can deliver the necessary friction requirements is needed. Again, this fluid is not compatible with traditional automatic transmission or CVT systems.”
Unlike the API and ACEA industry standards that cover passenger car and heavy-duty engine oils, there is nothing similar that applies to transmission fluids. Although, as Leandro explains, the JASO standard is required by some OEMs, however, it is only a baseline level of performance from which vehicle manufacturers build their own requirements.
“When a manufacturer starts a new transmission development, they select a fluid supplier who, in turn, selects an additive company to partner with. The three work closely for the duration of the project and design the fluid as a component of the transmission system. The hardware and fluid are developed concurrently, because when the transmission is ready the fluid must be available for first factory fill. Once the new transmission has been launched and the fluid specification released, products can be licensed for service filling – although demand for the new fluids is limited until the volume of installed transmissions increases. However, despite this lack of initial demand, the investment in the development and testing of these new fluids is not only long and expensive but also different for each OEM. What this means for consumers is that they can be confident that a fluid certified for use by the specific OEM will offer the right balance of properties including friction performance, lubrication of moving parts, corrosion protection and viscosity control to protect the transmission system over the life of the fluid.”
However, what Leandro feels can be confusing for end users is the term ‘multivehicle-ATF’ that has been applied to some transmission fluids.
“The multivehicle-ATF is designed to meet the lubrication needs of transmissions with similar technologies that also have similar requirements in terms of viscometrics and performance. This requires a robust additive system that is capable of delivering proven and reliable performance across many transmissions. The investment is high for additive companies since they must generate the technical support for the use of these multivehicle transmission fluids, which may include bench, laboratory, dyno, and/or field tests. In the end, the market gains a transmission fluid with broad application coverage for a given transmission type.”
One of the biggest drivers for change in the transmission market in recent years has been the improvement of vehicle fuel efficiency. Here, Leandro observes, one trend that has been emerging is the use of ultra low viscosity fluids.
“I see this ultra low viscosity (ULV) nomenclature as a further evolution of OEM product generations, for example, Ford and GM have MERCON® and DEXRON® ULV products respectively. While the older versions would not mention viscosity, in a newer generation of products, the fluid viscosity was decreased from the range of 7.0 to 7.5 cSt KV@100oC to a range of 6.0 to 6.5 cSt, which Ford called MERCON®LV (where LV refers to Low Viscosity) and is in the same range of DEXRON® VI. Then, another generation of OEM products was released with a viscosity in the range of 4 to 4.5 cSt, which the manufacturers have called ULV.”
However, as Leandro continues, this most recent drop in viscosity makes it more difficult to achieve a functional result, making it increasingly important for products to be designed to meet all the other automatic transmission performance parameters.
“In an automatic transmission the torque converter is coupled with a clutch so that energy can be saved. Originally torque converter clutches operated during steady state vehicle conditions, such as highway driving. But, with the advance of embedded electronics, the clutch can now be effectively controlled during low speed, transient manoeuvres such as driving around in the city. This means a specific converter clutch slip speed can be targeted to improve efficiency while minimising noise vibration and harshness or NVH. This requires a very delicate friction balance and, if the fluid is not correctly formulated, a vibration or shudder can arise. The driver may feel these occurrences as the fluid ages and the anti-shudder additives are consumed. It is important that when OEMs make the transition from a low viscosity to an ULV fluid the anti-shudder additive continues to perform as expected. In addition, wear must also be managed and, as the use of ULV fluids grows, we expect the use of higher quality base oils to continue.”
While the move to ULV fluids is one lever for fuel economy improvement, OEMs are looking for fuel efficiency gains across their fleet in order to meet upcoming regulations. Most are offering hybrid vehicle options, a trend that impacts both the transmission system and fluid design. As Leandro explains, the level of impact really is aligned to the level of hybridisation.
“The stop-start is the lowest level of hybridisation, which has no impact on the transmission system or fluid. The second level is mild hybridisation, where the electric motor assists a combustion engine, increasing the engine’s torque and reducing fuel consumption. However, the vehicle is not able to operate in electric-only mode and again there is no impact on the transmission system or fluid. When we get to the third level, full hybridisation, a larger more powerful electric motor, powerful enough to propel the vehicle itself, is either directly coupled to the transmission, or integrated inside the transmission case. The e-motor is in direct contact with the fluid and we begin to see some additional aspects of performance that must be considered regarding the transmission fluid.”
As Leandro continues, there are three main challenges that the fluid needs to address in full hybridisation: heat transfer, materials compatibility and electrical properties.
“Heat is generated and hot spots that did not exist previously have been observed. Although a cooling system with refrigerant fluid can be used, it makes the system more complex and is not an efficient way to remove the heat – a better option some OEMs are exploring is to cool the system with the transmission fluid. This means that the lubricant must be able to withstand higher temperatures and have a higher oxidation resistance. Another challenge in full hybrids is that new materials are being used, which can create compatibility issues. For example, chemical attacks on the plastics of the connectors, corrosion of the copper windings or resin degradation can all generate electric current leakages or even a short circuit in the transmission. There is also a question around the electrical properties of the fluid that requires attention. Because the electric motor works at high voltages, the fluid needs to ensure its isolation and I expect the volumetric resistivity of future fluids to be an important consideration. If the resistivity is too low, there could be a power outage and/or loss and consequently an engine malfunction. Conversely, if it is too high it can generate electrostatic charges that can result in an electric arc, which can damage or even destroy the transmission.”
However, as Leandro concludes, although the full hybrid hardware is very different and new challenges have been created, there are, as yet, no unique specifications for the transmissions of hybrid vehicles.
“We have seen no additional demands from OEMs for these new hybrid vehicle systems, which generally means that today conventional automatic transmission fluids are being used. So, what do I think will be the next generation after ULV transmission fluids? In my view it will be dedicated transmission fluid specifications for hybrid and electric vehicles because the ever increasingly higher voltages within these new systems require specialised fluids. Watch this space because I don't think that it will be too long until we will see new hybrid transmission fluid specifications from the OEMs.”