As the number of battery-electric heavy trucks continues to grow, the more we are taught about their motors and batteries. One thing that isn’t discussed in any meaningful detail, however, is how the transmissions work. So, we asked four major OEMs exactly what’s going on between the electric motors and wheels on their battery-powered offerings.
DUAL DAF OPTIONS
DAF has gone for two solutions to cover its full vehicle line-up, although this time only one of them actually has a gearbox at all. “On the XBs – and the LF before that – there’s just a direct drive motor with an output flange [shown below], that bolts to a prop shaft in the normal way,” says DAF’s marketing manager Phil Moon. A gearbox is seen as unnecessary in this instance, because the motor itself operates over a large rev range. “Matching the motor to the road speed right up to 90kph is eminently achievable.”
Achievable doesn’t always mean desirable, however. As you increase the speed of a motor, it’s then turning the armature at a higher speed as well, which takes slightly more energy. Gearing the drivetrain at higher road speeds to reduce the speed of the motor itself, therefore, produces a gain in efficiency. “Then, of course, at higher weights, you want some torque multiplication, in order to ensure your motor has enough tractive effort to start off and climb hills,” says Moon.
“On the XD and XF, which are operating at weights of up to 42 tonnes or more, it’s useful to have a gearbox for those two reasons,” he adds. As well as needing the multiplication factor at low speeds, these vehicles are likely to spend a much higher proportion of their working lives cruising on motorways. “We don’t actually need a lot of power at that point, but we do want maximum efficiency, so we can lower the motor speed.”
Needless to say, there’s a fine balance to be struck here, between the inefficiencies inherent in using any kind of gearing and those potentially appearing at speed within the motor itself. The more gears you have to go through, the less efficient that side of the balance becomes. “On our bigger vehicles, which actually have two motors, you’ve got a three-speed gearbox,” Moon says. “But it’s all one unit, built by ZF, the gears being integral to the motor itself.”
The unit itself is very compact, about the size of the gearbox in a traditional truck, and DAF has mounted it in the same place. From there, the output shaft can simply connect to the prop shaft in the normal way. “Generally it starts off in second gear, the middle of the three, then when you get up to cruising speed it changes up,” Moon explains. “But when you’re starting off heavily laden or on a slope, it uses the lower gear, effectively a form of crawler.” Determination of this is, however, entirely controlled by ECUs, just like in a full automatic. “There’s no manual intervention by the driver at all,” Moon confirms. “It’s just a two-pedal drive situation – press the throttle, the vehicle responds accordingly.”
VOLVO'S VENTURE
Volvo is another manufacturer using two different solutions to cover its full vehicle line-up. Firstly, its 4x2 tractor, which can run at 42 tonnes with a tri-axle trailer, uses the existing tried and tested, 12-speed I-Shift gearbox. Like all Volvo BEVs, this heaviest product boasts a prop shaft to transfer drive to the rear axle. Together, this use of existing technology has enabled Volvo to both be quick to market and also to offer a familiar gearbox PTO.
Perhaps the biggest difference between Volvo’s diesel and electric drivetrains is the lack of a clutch. “The heavy 4x2 FH and FM tractors have three electric motors, each connecting to the front housing of the gearbox, behind which are three reduction shafts,” says Mark Collins, e-mobility product manager. (FH/FM gearbox pictured below.) These motors rotate at between 5,000 and 8,000rpm, which is much faster than a diesel engine, hence the need to downspeed. “With those shafts, which are configured in a triangle, there’s a reduction gear, with a single shaft then connected to the gearbox.”
While all 12 forward gears and four reverse are available for use, the lower gears are used far less than with an ICE. “Even when the vehicle is heavy, it’ll never really start in gears one to five,” Collins says. “For, say, general haulage, you might use six, seven or even eight.” It is important to remember, however, that Volvo is a Swedish brand, and a lot of the Scandinavians run at 65 tonnes. “Using I-Shift means we’ve already enabled a gross combination weight of up to 50 tonnes,” he reminds us. “So rest assured the electric vehicles will definitely be capable of expansion at the heavy end.”
For Volvo’s lighter product, meanwhile, there’s a brand new two-speed medium-duty gearbox, built specifically for the task (pictured p19). Again, the motors connect to the front face of the gearbox. “The FL, which is only available as a 4x2, has one motor, while the FE, which can be a 4x2 or 6x2, has two motors,” explains Collins. “It’s an automated gear-shifting system but, in effect, the first gear is there for when you’re heavy or starting on a steep gradient,” he continues. “The majority of the driving will all be done in second gear, including pulling off.”
There are no specific reverse gears with this unit, however. “To reverse, the rotational direction of the electric motors switches, by changing the polarity,” Collins says. “So that’s the key difference between the medium and heavy-duty products.” The other difference is that there is no option for a gearbox PTO with the two-speed transmission, although there are other solutions should one be needed.
DRIVE TO SURVIVE
MAN has opted for what it calls a “central drive unit”, which sits slightly behind where the gearbox would be in a diesel. A self-contained motor and gearbox in one, it then connects to the rear axle via a prop shaft (pictured above). “We’ve got three motor power types and two gearbox types: a two-speed or a four-speed gearbox, both Tipmatic,” says Ashlea Awbery, product manager.
The two-speed is for lightweight distribution vehicles and is coupled to the smallest (245kW) motor – it has a maximum load weight of 28 tonnes. The more common variant is the four-speed, which couples to the 330 and 400kW motors and can be used at gross train weights up to 50 tonnes. “Top gear is the same ratio in both gearboxes, because you still want to maintain a reasonable top speed,” Awbery tells us, “while the lower speeds have different ratios.”
This central drive unit solution has been chosen as it can be suspended in the chassis, keeping it safe from vibration. It also means operators can have mechanical PTOs on their electric gearbox as well, in the same way as with a diesel gearbox. Unlike most, however, MAN only uses a single motor for its CDU, mounted directly to the gearbox.
“Electric motors have a huge amount of torque from zero,” says Awbery. “But, in reality, to get one motor to give us enough torque to both pull away and get us completely moving, you’d need quite a big motor.” It works out to be slightly more efficient to use a smaller motor and a set of gears to achieve the same thing – having a bigger motor would, of course, consume a lot more power.
Again, one familiar component missing from this set up is a clutch. “When you pull up to a standstill with an electric motor, unlike an engine it just stops turning, which makes gear selection really easy,” Awbery explains. “When the vehicle is driving along, it’s necessary to synchronise the motor speed with the output gear, in order to be able to switch across. That is all done through the gearbox software, which can interrupt the power to the motor without actually disconnecting it, to allow it to slow down or stop.”
MERCEDES MACHINERY
Always fond of being first over the line when it comes to technology, as seen with MirrorCam, Mercedes has gone straight to what most see as being the ultimate solution – an eAxle.
The most recent version was built specifically for its long-haul model, eActros 600. An 800V eAxle, this has two Bosch 400kW electric motors, each controlled by a separate HV inverter. These sit in front of the axle, followed by an epicyclic gear system and differential (pictured below).
“The epicyclic gear system shifts four forward gears and two reverse gears; the gear ratios are automatically selected depending on the vehicle's load and inclination,” says David Simm, product manager at Mercedes-Benz Trucks UK. To do this, it uses an electric transmission control module, or eTCM. “This has similar sensors and data that today’s diesel trucks house, triggering the power train inverters to control each motor to change through those speeds.” In overrun mode, kinetic energy is converted into electrical energy. “Because there are no connecting parts – unlike other current competitor vehicles, we’ve got no prop shaft – those transitions go pretty much unnoticed by the driver,” Simms adds.
This does beg one final question – why is it that truck marketeers, including those connected to Mercedes, continue to use talk about the “gearbox” when describing their product? Is it purely about keeping things familiar while the technology beds in? “We’ve tried to drive the language away from ’gearbox’ to ’transmission’, although even that still isn’t really right,” Simms says. “I think it’s about those newer to the industry, or early adopters, challenging the status quo and creating a new glossary. If we speak again in five years’ time, the approach will be completely different.”
