The requirements placed on driveline fluids are similar for full hybrids and PHEVs and reflect the position of the e-motor or motors relative to the ICE. These relative positions are referred to herein as P0, P1, P2, P3 and P4. However, there are no industry-agreed references, so other terminology is in use.
P0 and P1 refer to the e-motor sitting before or after the ICE, respectively, and physically separated from the transmission. In both cases, the e-motor drives the existing transmission, boosting the ICE. Little change is anticipated for the transmission fluid. E-motors in these positions do not interact with the transmission fluids.
P2 and P3 place the e-motor immediately before or after the transmission. In both cases, the e-motor could be inside the transmission housing. If so, it is in contact with the transmission fluid, so fluid formulators must consider electrical compatibility. The motor configuration of the Toyota Prius approximates P2 and P3, although an alternative interpretation is that the P2 motor is part of the transmission.
The configuration in the Prius has led to the term “power split” (as denoted by PS in the illustration above). Toyota has also referred to this arrangement as an e-CVT, or electronic continuously variable transmission, that provides continuous and variable output from the transmission by use of a planetary gear set, which allows the ICE and both e-motors to work in concert.
P4 has the e-motor driving the rear axle, but P4 could also describe a motor driving the front wheels. The key feature is that the e-motor is physically separated from the transmission and ICE. A motor at P4 can either drive the axle via a drive shaft and a conventional axle or be integrated within the axle assembly. Some BEVs have the motor integrated with the axle box, so the characteristics of some BEV motors are like those at P4 in a hybrid. This arrangement is often called an e-axle.
With so many arrangements of hardware, there are many potential and different requirements on the lubricant. Current lubricants are not optimum, but with the small volumes required, OEMs are likely to use an off-the-shelf fluid. However, optimization is ongoing as EV volumes rise.
A vehicle with e-motors at the P0 and P1 positions would have little requirement for a novel transmission fluid.
If motors at P2 or P3 are integrated with the transmission, the transmission fluid will be in contact with the motor. Some additives used to modify the frictional properties of the transmission fluid are said to have electrical compatibility issues, which could mean re-formulation when the fluid is also required to cool the e-motor.
An e-motor at P2 could be integrated with a dual-clutch transmission or the transmission itself. A dual-clutch transmission requires traction from the fluid, as the plates and discs of the clutch packs engage. This is a significantly different frictional situation compared with a conventional ATF, which has no requirement for traction. Therefore, different fluids are required, depending on the mechanical hardware deployed.
An e-motor at P4 in a hybrid will have many of the same requirements placed on it as an EM driving the rear axle in a BEV. As noted above, this is often referred to as an e-axle and could describe an electric motor driving the front axle.
The Chevrolet Bolt features an integrated e-motor driving the front axle, for example.
Transmission fluids are a complex mix of base oils and additives. They are required by all vehicles, electrified or not, and share numerous performance requirements. Many automatic transmission fluids claim to be fill-for-life or for a significant part of the vehicle’s life before maintenance is required under normal operation. This means they must provide virtually unfailing strong wear and oxidation protection, optimum friction characteristics, compatibility with elastomer seals and outstanding low-temperature performance.
What is new in transmission fluids in the EV realm is the need for optimum electrical conductivity and resistivity, as well as better thermal conductivity for improved cooling.
As far as hybrid transmission fluids are concerned, the lubricants industry is still in the early stages of investigating and developing improved solutions for these complex vehicles, where oils are exposed to significant electrical currents. Today’s hybrid transmissions use existing fluids, and it is unlikely that OEMs will develop a dedicated hybrid fluid due to costs until sales reach a commercially viable tipping point.2
Unlike hybrids, BEVs have simpler transmissions, and most BEV transmissions are single speed. There is talk of two-speed transmissions, but that would still be simpler than automatic transmissions in ICE vehicles, which can have six speeds, not to mention duel clutch or continuously variable transmissions in hybrids. While fluids for these transmissions will need to combat corrosion, they also have to tolerate electrical conductivity, unlike a manual clutch transmission.
The efficient running speed of e-motors ranges between 3,000 and 10,000 rpm, so the simplest transmission is a step-down gear set, often called a reduction gear or e-axle, with a fixed ratio to convert the most efficient motor revolutions into just over 1,000 rpm at the wheel hub, given current sizes of wheels and tires. In the longer term, e-motors may run at more than 20,000 rpm.
The advantage of simplicity is offset by the fact that the e-motor operates for a great deal of its time outside its most efficient range, leading to reduced vehicle range, the most significant source of customer concern.
There are also issues relating to non-optimization of fluids with the simplest transmissions. The problems Tesla has had with its earlier models (reduction gear ratio of 9.73:1) could be related to wear due to high torque. Due to the high torque, Tesla employs a limiter to constrain wheel spin. Some believe it is intended to protect the bearings and gearbox from wear.
Other OEMs have incorporated multi-gear transmissions into EVs to address range anxiety by allowing the motor to operate at maximum efficiency over a wider range of road speeds. One of the end-user benefits of this is that a small motor can still exert torque at higher road speeds, allowing acceleration at around 100 km per hour. The drawbacks are additional weight and cost (another obstacle), plus perception of a less smooth ride relative to an EV with a single reduction gear.
This leaves the picture regarding transmissions for EVs somewhat unclear: simplicity dictates a single-speed reduction gear, but range anxiety and/or performance requirements could push OEMs towards multi-speed gearboxes.
Jaguar Land Rover, Audi, VW and Porsche launched EVs based on multiple e-motors, each with a single reduction gear between the e-motor and the wheels. The larger e-motors deployed in these vehicles are capable of vehicle speeds above 200 km/h without reaching maximum engine revolutions. An economical city EV will likely compete with current ICE-powered city cars on power and with a smaller e-motor may require a multi-speed gearbox to achieve an attractive combination of acceleration, range and top speed.
