E-Axles Speed Electrification

In the quest for ever-lower CO2 emissions, and more immediate needs to retain vehicle access in some of the world’s most-populous cities, the e-axle has found its calling. With most dedicated, fully electrified platforms still gestating in OEM product pipelines, the e-axle seems an automotive component born to electrify current architectures.

Often described as eAWD, the e-axle is an electro-mechanical propulsion system contained in an axle structure housing an electric motor, the power electronics and some form of gearing/differential. Used in conjunction with a conventional ICE or hybrid powertrain, an e-axle can add electric propulsion to an unpowered vehicle axle, hence the eAWD moniker.

Consolidation of electronics, motor and differential makes the e-axle an ideal “bridge” component in terms of packaging flexibility, electrification and performance gains across vehicle segments. As nearly every transport segment will require some form of electrification to remain viable within the CO2 emissions glidepath, the e-axle is an ingenious and in-demand solution.

Zero-emission vehicle modes are becoming crucial as more municipalities decree electric-only zones. GKN supplies the rear e-axle on the Volvo XC90 T8 Plug-in Hybrid eAWD, that combined with its 10.4 kwh battery, earns the SUV an all-electric EPA-rated range of 19 miles. (GKN; Volvo)

As one Tier 1 noted dryly: “We're very busy in the space.”

Engineers interviewed for this article note that the e-axle business is booming, thanks to the technology’s adaptability. However, many Tier-1 suppliers are discovering that “modular” does not equal “plug-in.” The same interest that’s creating such demand is also fueling iteration complexity, along with new integration challenges for suppliers.

What’s the draw?

Even with certain market regions in flux due to political shifts, ever-stricter reductions in global CO2 emissions continue. More recently, zero-emission zones in major cities are creating demand for products that can run electric-only. The e-axle is a single component that can provide multiple solutions.

Michael Lembke, senior director of electrification and hybridization at Dana, says that CO2 regulations are still the biggest draw but, “they vary significantly by country.” China’s role is large, as it’s mandating electric vehicle percentages. “They're not just mandating fleet CO2; they're telling the OEMs eight percent of your fleet have to be battery electric,” Lembke noted.

Magna Powertrain’s integrated eDrive systems contain the gearbox, e-motor and inverter. The Mid system pictured here provides up to 3,800 N·m at the wheels, and can be applied to both hybrid and battery electric vehicles. (Magna)

Though the latest Chinese regulations will boost e-axle applications, other markets are seeing increased electrified need.

“It's not just China, where we have the biggest growth rate over the next couple of years,” explained Theo Gassmann, VP of advanced engineering for GKN Driveline. “All the premium OEMs in Europe and in the U.S. are investigating and investing in electric vehicles and plug-in hybrids. The CO2 agenda, local pollution issues, zero emissions zones—all these are pushing hard on electrification.”

According to Graham Evans, a principal analyst in automotive technology at IHS Markit, the e-axle segment will see significant growth. “For our light-vehicle forecast from 2017 to 2023, we see compounded annual growth of 35% for front e-axles and 46% for rear e-axles,” Evans noted.

The IHS light-vehicle forecast includes front–e-axle applications growing from 750,000 units in 2017 to more than 4.5 million by 2023. During the same time frame, rear/AWD e-axle applications are expected to increase from 320,000/110,000 to more than 3 million/2.5 million, respectively. With major cities and other localities continuing to adopt zero-emission zones, commercial applications should only add to those projections.

“If you think about the commercial segment, the way people do work in a city and deliver goods, those vehicles would need to be fully electric,” said Seth Metzger, VP light-vehicle engineering at Dana. “Vehicles that move into the city and move out, they will have to go into a full-electric operating mode for some period of time.”

Rapid iteration, cost challenges

Fortunately for vehicle manufacturers, e-axles are a solution available now—and can be adapted to current architectures. “Many OEMs need to take an existing vehicle architecture, a vehicle that has been designed for an engine, transmission, prop-shaft, exhaust system and the fuel tank,” Dana’s Lembke explained. “And within that given architecture—because time is of the essence—fit an electro-mechanical propulsion system into that same vehicle architecture.”

ZF's new fully-integrated electric axle drive system will enter volume production before the end of 2018 with a European manufacturer. (ZF)

Such adaptability is key to e-axle adoption. “Companies have legacy platforms, legacy architectures to be considered,” explained Walter Sackl, global director of product management, driveline systems at Magna Powertrain, “and the e-axle is one of the units which can transform an architecture into a hybrid system. The nice thing is that an e-axle usually fits into the packaging space of a standard vehicle.”

E-axle technology is also evolving swiftly, making the latest applications, even within legacy platforms, more easily packaged from even a few years ago. “Every new e-axle application has new features, more integration, and it’s more compact, more sophisticated than the generation before,” GKN’s Gassmann said. “When we started, it was an electric motor bolted to a transmission, an inverter somewhere in the trunk, with a thick cable connecting everything. Now we are talking about integrated systems with an inverter, motor and gearbox pretty much bolted together and it’s one turn-key unit.”

Any new system faces engineering challenges and the e-axle has its own set of hurdles. One of the largest is OEM customers trying to gain share in a segment that’s being driven more by regulation than consumer demand.

“Cost is extremely challenging,” admitted Dana’s Metzger. “A lot of the OEs have been upfront that they’re not making money on battery electrics. As we get more volume it could help [market] share and reduce the costs of batteries, motors and inverters. Ultimately, we have to drive those costs down, because the OEMs have to be able to make money.”

GKN’s new eTwinsterX e-axle features a space-saving coaxial format, an output of 120 kW (161 hp) and 210 N·m (155 lb·ft) of torque, a two-speed transmission, as well as full torque-vectoring capability. (GKN)

Volume will be key, as well as reducing pricing pressure on some of the raw materials going into e-axles. “It's the second most expensive component within an EV, and all this competition means that the cost per unit is going to come down fairly quickly,” said Evans from IHS Markit.

He said the primary concern is in the rare-earth metals within the motor. “There’s a lot of interest in developing motor applications that lean away from the use of rare-earth metals,” Evans explained, “so this supply-chain constraint won’t exist and won’t hamper growth.”

Beyond cost challenges, all the suppliers who spoke with AE mentioned NVH, which is an entirely new aspect in electrified platforms.

“Because electric cars are nearly silent inside and out, you tend to hear every noise from the gears, electronics and e-motor,” noted Michael Wetzel, head of product/project management axle drive systems, e-mobility division, ZF Group. “And as compact as the components are, you’ve a high power-to-weight ratio, and that makes it complicated to get all the NVH measures implemented.”

To address NVH on a system level, advanced simulation tools are needed. “From the very first approach, you have to keep in mind the individual components and their interactions,” said GKN’s Gassmann. “Then you have to have the experience to get the basic ingredients right so it’s quiet and efficient. It's about oil, it's about bearings, it's about advanced lubrication concepts to minimize churning losses.”

There are also the e-axle’s inherent high-speed inputs. While peak speeds in a high-performance ICE may be up to 8000 rpm, peak input speeds in the next-generation electric-propulsion system will approach 20,000 rpm—making them "a totally different animal and very challenging for NVH,” Gassmann asserted.

Another common challenge in the expanding e-axle business is managing complexity. “We have a history of around six powertrain arrangements in the traditional ICE world – front transverse-engine with FWD, front longitudinal-engine with RWD and so on,” said Magna’s Sackl. “When we transfer into those future architectures, we need to handle a large number of different architectures—maybe 42 and beyond.”

Closer OEM integration

For suppliers used to solving mechanical issues, this can mean working more closely with their OEM customers and with greater integration challenges, particularly on the software front.

“The OEMs are now pushing the whole system integration effort through the first-tier suppliers, which is a great opportunity but a big challenge as well,” acknowledged Gassmann. “It's a huge shift, because we used to do the mechanical integration and the gearbox stuff, but now we’re actually doing complete system integration.”

Dana’s new eS5700r e-Drive axle is designed as a “drop-in” solution for medium-duty truck and bus suspensions. It recently entered service on a new Dongfeng bus application in China. Using a liquid-cooled motor and inverter, and including disc brakes, the unit weighs only 673 lb (305 kg). (Dana)

“In the past, a mechanical solution decided the function,” explained Magna’s Sackl. “Now, we’re transferring to electronics—this includes electronically controlled clutches for example—which makes things more complex because the software controls need to be embedded in the structure.

“The portion of the Tier-1’s scope is by far larger than it was,” he continued. “Now we deliver a large portion of the software function associated with it. So for us, a large portion of our engineering staff—almost 50 percent—are dedicated just to software function.”

48-volt vs. high-voltage

Where does 48-volt fit into the e-axle equation? Most of the suppliers we spoke to see continued growth of 48-volt to power a host of new comfort and chassis technologies, but the future of electrified propulsion will require higher voltages. “The main goal for 48-volt is to generate power for other systems,” Magna’s Sackl explained, “which is completely different from the high-voltage approach, where the primary aim is to propel the car.”

“We’re currently working mainly on high-voltage systems,” ZF’s Wetzel echoed. “In the high-voltage systems, you have power outputs of 50 kilowatts or more, and this gives the possibility to drive fully electric in the inner cities.”

“We don’t see a huge market for an e-axle with 48-volt, because the issue is with the torque it can deliver,” GKN’s Gassmann said. “You either have something that can provide some EV-drive capability downtown, with enough torque to get over the curb or up the gradient in the carpark, but then the system will only be available at lower speed.

“If you go for CO2 [reduction], you need the system to operate at higher speeds as well, otherwise you’re losing all the CO2 recuperation and boosting potential,” Gassmann continued. “To accomplish both, you have to add complexity to the transmission with a 2-speed, which is then spoiling the whole approach, because 48-volt is considered low-cost hybridization.”

Vehicle dynamics + CO2

One of the key roles as suppliers, is helping create products that consumers want to buy, and not just via efficiency. With existing vehicle platforms, engineers can continue to reduce CO2 and the size of the ICE with e-axle boosting. The real draw, however, will come from added capability and performance.

“Electric axles will probably eat some of the all-wheel-drive mechanical market, that's for sure,” GKN’s Gassmann noted. “And it’s not lost business; it’s actually gaining business, because an e-axle adds significantly more value than mechanical all-wheel-drive.”

“Where we've received RFQ’s for performance cars, the majority of those have had torque vectoring as a request, where you're not just purchasing it for fuel economy,” said Dana’s Metzger. “Of course, that’s a benefit—but it's also really for performance and the joy of driving.”

“Vehicle dynamics, plus CO2, is our main goal. For us it’s not either or,” said Magna’s Sackl. “At the end of the day, we are positioning ourselves very much in the space of ‘fun to drive,' with vehicle dynamics to make consumers happy.”

Development accelerates

Whatever the draw, be it regulatory or consumer driven, there’s enormous activity in the e-axle field. The key players AE spoke with all noted the near-seismic shift in the industry.

“If you look at the pace of change, what has happened in electrification in the last five years, it’s equivalent to what happened with combustion engines and transmissions in the last 25 years,” opined Dana’s Lembke. “There's not a month that goes by where an OEM isn’t announcing, ‘by the year X, we’re going to have so many hybrids or so many battery electrics in production. We seem to see that almost every week, particularly out of China.”

Noted Steven LaChance, VP Customer & Program Engineering, GKN Driveline: “The all-wheel-drive side is exciting because we get to solve a lot of unique problems with integration. But the e-technology is really exciting. It's an exciting time to be an engineer.”