Driving Force: Axles are at the heart of an electric revolution
Posted: July 12, 2018 by Jim Park
The low profile of the motor on Dana’s eS5700r axle used on Class 3 vehicles allows the van body to be mounted closer to the chassis, improving cargo capacity.
TORONTO, Ont. — We’re at the dawn of a new era in the propulsion of heavy trucks. Electric powertrains are opening new packaging and integration possibilities, including driven axles. Traditional transmissions, driveshafts, power dividers or differentials are no longer required.
Everything changes once you get over the idea that we need an internal combustion engine at the front of the truck. Cooling requirements determine the placement of the engine in current configurations. Ditch the diesel and you dispense with the need for a driveshaft to get the rotational forces back to the drive wheels. Placing electric motors in, on or near the drive axles also frees up tons of frame space for batteries and the associated electronics — about four tons worth of space and weight to be more precise.
A battery-electric Class 8 tractor requires several tons of batteries, and the safest and most convenient place for them is under the cab between the frame rails. You don’t want to share that space with a driveshaft. It’s much easier to route wires around the batteries than to put a driveshaft through the middle of the battery pack.
“From a complexity and weight standpoint, taking out all those pieces makes a lot of sense to me if you’re doing a clean-sheet design,” says Rick Mihelic, director of future technologies studies at North American Council for Freight Efficiency and a former vehicle development engineer at Peterbilt. “The other side of that is the 100 years of experience we have in our current systems. We got where we are with them because they are necessary and they work.”
Which brings us to the drive axles. We need drive axles to drive the truck, obviously, and to support the load on the frame. We can’t radically change the purpose or the basic axle design overnight because of the need for compatibility with legacy components like brakes and suspensions. And that’s what differentiates startups like Nikola, Tesla and Thor from the established OEMS and Tier 1 suppliers.
“If you own a product line based around a diesel engine and a conventional drivetrain, and you want to introduce an electric motor to that, the most cost-effective way to do that is to replace one component at a time in the model rather than completely redesigning the whole model line,” Mihelic says. “If you have all that existing infrastructure and inertia in current products, it’s very difficult to turn to a clean sheet design. All your capital investment has to consider the entire product line and how long you expect it to continue producing revenue.”
The startups have none of that overhead or inertia. They can go directly to what the established providers will get to eventually, and they can take out all the redundant components. The startups are talking about going right to production because they need cash. They have no revenue source until they have a product they can sell, while the OEMs and the Tier 1s have to deal with the capital investment to support an entire family of products.
“They have to reallocate money very judiciously,” Mihelic says. “That’s especially challenging when you consider the potential market for this technology will be very small for the near future. They make money on volume. They have to be careful not to starve how they make their fortunes while wading into the electric side of it.”
That said, Tier 1 suppliers Dana and Meritor have a line of electric drive axles they are happy to talk about. Daimler Trucks just announced a foray into electric propulsion but told Today’s Trucking it was a bit early to be talking about specifics at this time.
Thomas Healy, founder and CEO of Hyliion, a startup that makes a hybrid-electric drive axle for use in 6×2 tractors, making them effectively 6×4 tractors, sees three pathways to drive axle electrification.
“We will see an evolution take place with electric axles,” he says. “Right now, the solution we’re using is an electric motor that mounts perpendicular to the drive axle. Power comes in from one direction and gets rotated 90 degrees by a pinion gear. The next evolution will be an inline motor mounted in the center of the axle. My personal feeling is the third evolution will be an in-wheel-motor setup. It won’t even be an axle as we know it today. You’ll have a motor located in the wheel hub.”
Wheel-hub, brake and wheel-end component supplier Conmet announced in March that it had partnered with Protean Holdings to develop an electric in-wheel drive system for the medium- and heavy-duty commercial vehicle markets. The deal pairs Protean’s automotive wheel-end-motor expertise with Conmet’s commercial vehicle wheel end experience. According to the Conmet press release, “The jointly developed electric wheel end system will provide vehicle packaging advantages, reduce complexity, and minimize drivetrain losses for truck, tractor and trailer applications.”
Using a traditional axle housing with a motor mounted where the carrier and driveshaft input would normally go is an easy and economical first step toward electrically powered drive axles.
Tier 1 options
Offerings from Dana and Meritor reflect Healy’s first two pathways. Meritor’s eAxle features an electric motor combined with a two-speed gear box mounted directly to the front of a traditional axle housing. The motor and gear assembly essentially takes the place of the forward carrier assembly and still uses the differential. From a mechanical perspective, it’s similar in shape and design to current conventional drive axles.
“You can put the motor in a lot of different places, so you have to go through the pros and cons of each different architecture,” says John Bennett, Meritor’s newly appointed vice-president and chief technology officer. “We use a single motor in-line with the current carrier. By integrating the motor that way, we can nest a lot of the gearing inside the motor. Our eAxle changes very little. You can literally plug the motor into the housing and the suspension mounts do not change.”
In keeping with Mihelic’s earlier notion of a managed transition to electric power, Meritor’s option offers great potential. Among the challenges is dealing with what’s called overhung mass, such as the weight of the motor and gear assembly bolted to the front of the axle housing, like the carrier is now. Bennett says special attention is required to make sure the joint between the housing and the mounted motor stays true and doesn’t develop leaks. To address that, Meritor is opting for a smaller, more efficient and lighter motor with a two-speed gearbox to more closely match motor speed to wheel speed.
This sort of an axle configuration would be suited to Class 6 and 7 trucks and school buses with a gross vehicle weight up to 33,000 lb. Class 7 and 8 trucks and 4×2 and 6×2 tractors would possibly require an extra set of reduction gears at the wheel end. Heavy Class 8 highway and vocational trucks would require a separate motor for each axle in a tandem setup along with wheel end gear reduction as well. Published literature suggests Meritor will use existing axles, such as the 12X, 14X and 17X with an “e” designation, as in 14Xe.
Meanwhile, Dana, is going in a slightly different route. Following a joint venture announced in June with Quebec-based TM4, which designs and manufactures motors, power inverters, and control systems for electric vehicles, Dana will develop a line of e-axles with the motors build into or alongside the axle housing in what it calls a parallel-axis configuration.
TM4, incidentally, is a subsidiary of Hydro-Québec, the province’s electrical utility company. According to the June 22 announcement, “The transaction establishes Dana as the only supplier with full e-Drive design, engineering, and manufacturing capabilities – offering electro-mechanical propulsion solutions to each of its end markets.”
Dana is currently using two different axles for Class 3 vehicles and Class 4-6. The sS5700r features an inline motor and gearing built right into the axle. In fact, the motor housing is part of the axle. The slightly heavier eS9000r has an externally mounted motor that is still inline with the axle housing. It’s geared directly to the axle shaft inside a more traditional-looking axle housing. Both are designed as drop-in replacements for traditional drive axles.
“Parallel-axis gears typically run a little bit more efficiently than you’d expect in a hypoid gear set,” says Ryan Laskey, vice-president of commercial vehicle engineering at Dana. “With hypoid gear sets you can traditionally lose a couple of percentage points in efficiency during that torque transition in the 90-degree phase. In a parallel-axis arrangement, you’re losing probably half of that. It’s small, but in a battery powered system we want to do everything we can do to reduce energy consumption. Anything you can do in the bearings and the gears to reduce frictional losses will provide additional range, which will be critical in the timeframe of the early adoption phase with the cost of batteries as high as they are.”
We won’t see many of these axles in revenue service in the Class 8 space for some time, although Dana’s sS5700r is installed in a number of Class 3 trucks built by the Workhorse Group and currently being evaluated by United Parcel Service in the U.S.
Mihelic sees a 15- to 20-year timeframe before traditional drive axles become scarce. It may be 30 years or more before electric and hybrid takes the place of diesel power.
“A strong argument can be made that diesel will be around for a long time yet because it’s very well suited to the longhaul environment and the infrastructure is already in place,” he says. “That said, the people who buy trucks understand the fundamental differences and inherent advantages to each type of powertrain. They will have one truck for city work and another for the highway, it depends on how much they want to optimize for particular duty cycles. There’s no single solution in everyone’s future.