What is an e-axle?

At the recent IAA road transport show in Hanover, Germany, electrification was the strong theme on most of the show stands. All major truck, trailer and axle makers had electrified equipment on display, with ‘e-axles’ being the dominant new product.

First-generation electrified trucks used conventional drive axles and propshafts, but the latest machines have integrated motor and axle units that are known as e-axles.

An e-axle integrates one or more electric motors; two- or three-speed reduction gearing and power-control electronics, into the centre of the axle.

Heavy-duty electric power supply cables from the battery packs replace the propshaft.

Eliminating the diesel engine, transmission and prop shaft makes between-rail space available for batteries and control modules in the case of battery electric vehicles (BEVs), or for fuel cells and a smaller battery pack, in the case of fuel cell electric vehicles (FEVs).

Since both BEV and FEV types have final drives operated by electricity, they can use the same e-axles and don’t need multi-speed transmissions.

Electric motor power and torque are quite different from diesel power and torque characteristics. A typical heavy duty diesel has peak torque at around 900-1200rpm and maximum power at around 1900rpm. In contrast, a truck electric motor develops peak torque at zero revolutions – instant torque – and maximum power at around 4000rpm.

A diesel engine needs a multi-speed gearbox with clutch – manual or automated – or a torque-converter automatic transmission, that allows the engine to build up revs before it has to move the truck off from rest.

Once under way, the transmission shifts when necessary, to maintain a ‘sweet spot’ between the power and torque curves. Remember, torque is for lift-off and hill-climbing and power is for road speed.

In contrast, the electric motor can get away with a two- or three-speed automated transmission, without the need for a clutch or a torque converter, because lift-off torque is available as soon as the driver presses the ‘go-pedal’ that energises the electric motor. Once under way, the electric motor needs only low-speed, mid-speed and cruising ratios to maintain speed and climbing ability.

Incidentally, the reason that battery electric ‘super cars’ have electric motors with ridiculously high horsepower ratings is that the makers want to have only a single-speed reduction gear, not a bulkier, heavier two-speed box. So, they ramp up the torque and power instead.

Advantages for the electric truck e-axle over a mechanical axle include proportionate torque distribution to both half-shafts and the ability to recharge the batteries while also slowing the vehicle on descents.

At the 2022 IAA, all the European heavy truck makers exhibited e-axles of their own design, or those produced for them by specialist suppliers. Because European prime movers are typically 4x2 and heavy rigids are 6x2s, most of the display e-axles were rated at 13 tonnes capacity.

Cummins-Meritor is aiming at some European business in addition to its US and Australian efforts, so the company introduced the Meritor 17Xe ePowertrain, assembled with a Cummins battery system and rated at 44 tonnes GCM, with 13-tonnes axle capacity.

For the US and Australian markets, truck makers are expected to employ tandem-drive e-axle sets and Meritor, Allison and Dana Spicer have been testing these 6x4 configurations in the USA for the past two years.

Related reading:
Volvo Heavy-Duty Electric: Review
Next generation Fuso eCanter: Deep dive
Cummins shows multiple-fuel engines at IAA

Allison Transmission is selling the latest eGen Power 100S and 130D e-axles in the USA, after launching its eGen Power 100D in 2020. This e-axle features two electric motors, each capable of generating greater than 200kW of continuous power, with peak combined power of 648kW.

The eGen Power 130D, a variant of the 100D, is specifically designed for the European and Asia Pacific markets, where many commercial vehicles require a heavier 13 tonne gross axle weight rating.

Dana’s Spicer Electrified e-Powertrain offerings include a family of single and tandem e-axles designed for Class 7 and 8 applications. The new e-axles use Dana's vertically integrated Dana TM4 motors and inverters, Spicer high-efficiency axle gearing and Graziano synchronisers, transmission controllers, system software, shift system and controls.

Other significant e-axle exhibits at the IAA show included the FTP – an IVECO axle subsidiary – medium and heavy models, and Chinese BYD production units, for medium and heavy trucks. Although in the early stages of Australian-market involvement, BYD is by the far the biggest producer of electric vehicles in the world.

BYD has also just sealed a deal to sell 100,000 BEV cars to European rental fleet Sixt.

Mechanical drive axles being replaced by electrified axles isn’t such a radical change – mining dump trucks have been using e-axles for years – but electrifying trailer axles is a new concept.

There are two different types of electric trailer axles: a drive and retarding type, similar to a truck e-axle and the ‘recuperating’ type. The trailer e-axles at the IAA show were all of the recuperating type, because that’s the perceived major European market.

The idea is that the axle’s intelligent electric motor recuperates energy when the truck and trailer combination is cruising with low power demands, or when running down hill. That power can recharge the trailer battery pack or operate ancillary equipment.

At the IAA show, BPW had teamed up with Thermo King and SAF-Holland joined Carrier, to have their respective e-axles – ePower 16kW and TRAKr 20kW – operate trailer chiller units.

BPW claims that an ePower kit eliminates diesel compressor fuel usage – around 2500 litres per year – while also being weight-neutral.

In Europe there’s not much requirement for high-powered trailer e-axles to boost prime mover performance, because the EU standard GCM maximum is only 40 tonnes.

However, a 22kWh lithium-ion battery equipped evaluation e-axle trailer has demonstrated around 18-percent in prime mover fuel consumption, but the kit weighed more than one tonne.

Australia is an entirely different market, and for road trains we can see the application of a drive e-axle fitted to a dolly, to enhance prime mover lift-off power.

Powered dollies have been used in the past on super-sized road trains and it would be far easier to install a battery/e-axle kit than it was to slot a Cummins L10 diesel into a dolly.

Synchronising dolly e-axle input with the prime mover e-axle intput would also be much easier than trying to synch two diesels.

Although the emphasis at the IAA show was on the medium and heavy end of the electrified truck market, there were several small truck and van initiatives, including exhibits from Bosch, Magna – a Mercedes-Benz van and light truck supplier – and little-known British company, Saietta.

Saietta showed a fully-integrated eDrive solution for lightweight commercial vehicles up to two tonnes; an AFT in-wheel motor concept and a new eAxle concept for heavy-duty vehicle manufacturers.

As we said at the outset, the electrified truck revolution has barely begun in Australia, with the Fuso eCanter the only series-production truck available, but more is coming, of that you can be sure. And most will be fitted with an e-axle or two.