Electrification is potentially the biggest revolution in truck design for over a century. The first internal combustion engine-powered commercial vehicles were clumsily modified horse wagons, such as the Daimler Lieferungswagen of 1896. However, by 1902, Vagensfabrik and Scania (both of Sodertelge, Sweden) were producing freight vehicles using a layout that we would recognise today: an engine at the front with a gearbox behind it, connected to a rear driven-axle via a propellor-shaft and all mounted in a ladder-frame chassis.
Electrification provides truck manufacturers with an opportunity to change all of this configuration, liberating extra chassis space for batteries and other equipment and, at the same time, remove the parasitic drag and complexity created by turning the drive through 90° at the axle by relocating the motor into the drive axle itself.
So far, the majority of electric trucks running on the roads of the UK have retained a conventional chassis layout: we see an electric motor – or motors – positioned under the cab, with a gearbox bolted to the rear. A prop-shaft then takes the drive to the rear axle, where it turns through 90° via a differential to turn the wheels.
The advantages of this layout are essentially bedded in its familiarity. The chassis does not require major redesign to accommodate the electric powertrain, while existing (and expensive) tooling for the cab can be used for both electric and diesel vehicles. The electric motors are also easily accessible, and experienced diesel technicians dealing with high-voltage systems for the first time can be reassured that pretty much everything is where they expect it to be.
UP TO SPEED
In theory, an electric truck might not need a multi-speed gearbox at all. Most battery cars dispense with one, relying on the power that electric motors can produce at widely varying speeds: typically, an electric motor has a generous powerband (where it operates at over 90% efficiency) stretching from 4,000 to 10,000rpm – beyond the wildest dreams of a diesel-engine designer.
The reality is, though, that the requirement to handle steep grades at high gross weights means that at least two gear ratios are required. A 2022 study by Elif Gözen, M Sedat Çevirgen, and Emre Özgül suggested that while a
two-speed transmission might be adequate on paper, in practice, driveline efficiency could be reduced as the system would ‘hunt’ between the two ratios in many conditions.
A further advantage of a multi-speed transmission is the ability to over-speed the motor. The traction motors used in electric trucks typically have two power outputs: one being the safe continuous output, the other being the maximum output achieved at higher rpm, which can only be maintained for a short time without causing overheating. A multi-speed gearbox allows the motor rpm to be boosted to achieve this figure for brief intervals; not to generate traction, but to increase regenerative braking effort.
It is also clear that for heavy-duty applications, two or more smaller electric motors are more efficient than one large one: one or more motors can simply be shut down when their power is not required.
Some manufacturers have simply retained the multi-speed ratios from their diesel products in their electric vehicles. Volvo’s current electric trucks, for example, use the I-Shift AMT straight from their diesel counterparts. They also boast up to three electric motors.
HORSES FOR COURSES
Scania mounts its gearboxes integrally with the electric motor. The 230kW gets two speeds, motors from 270 to 400kW have four speeds – and the three-motor powerpack, with outputs from 400 to 450kW, gets a six-speed ‘box. The truck handles gear selection in response to driver inputs, but regenerative braking is controlled by the driver in the manner of a retarder on a conventional truck, and the truck will select the correct ratio to maximise retardation where required.
But it looks as though future-generation electric trucks will, like the second-generation of jet fighters, advance to a layout more suited to the new technology.
Gözen, Çevirgen and Özgül’s paper also outlined research used to develop an integrated e-drive axle for Ford Otosan. Daimler Trucks has standardised on electric e-axles almost from the start of its venture into BEVs, using the technology first developed for city buses. And, two years ago, Volvo Trucks showed its own e-axle at the IAA Show in Hannover. Interestingly, the axle appears to have the motors mounted at 90° to the axle – and may still be years away from production.
“We will continue with our versatile battery-electric trucks that are already in production. They can currently cover a wide range of transport assignments,” says Jessica Sandström, senior vice-president of global product management at Volvo Trucks. “In a few years, we will add this new rear e-axle for customers covering longer routes than today,” highlighting the ability of e-axles to liberate space for extra batteries.
The same technology is equally suited for electric fuel cell trucks. Chassis space is also at a premium on these vehicles as they require gas tanks, as well as a battery, to allow the cell itself to operate at a more or less constant output regardless of demand from the vehicle.
There are, however, possible downsides to the e-axle: it is an unsprung component and will therefore be subject to unmitigated roadshocks and vibration – and quite abrupt vertical accelerations.
ALTERNATIVE ARRANGEMENTS
Allison Transmission is a Tier One supplier with a foot in both camps. Best-known for dominating the municipal and fire markets in the UK with fully automatic transmissions, the company also offers electric drive axles in the form of the eGen Power family.
Its conventional fully automatic transmissions offer seamless power through six mechanical ratios, thanks to their hydraulic torque converters. But ‘smart shifting’ electronics and mechanical drive lock-ups also maximise range. The provision of up to three mechanical PTO outlets to the side and top also eases the task of integrating an electric driveline into a vehicle with a complex multi-function body originally designed for diesel engine power, such as a road-sweeper or refuse compactor. Allison is supplying transmissions to specialist companies who are either building all-electric vehicles or converting diesels to electric power.
But the recently introduced Allison eGen Power e-Axle range is now finding applications with OEMs in the bus and utility markets. Examples are the Isuzu Novo VOLT fully electric bus, and McNeilus Volterra ZSL electric RCV (main image, pp18-19).
However, road-sweepers and RCVs both have complex PTO requirements: how easy is it to meet these with an e-Axle? A conventional front motor layout may still have an advantage here. Nathan Wilson, account and area sales manager for the UK and Republic of Ireland, Allison Transmission, says a transmission-driven PTO is lighter and easier to package than an ePTO. “Currently the technology available only allows for an e-PTO fitted near the back of the truck cab. An e-PTO, which is an electro-hydraulic unit that takes its communication from the CANbus and consists of an inverter that integrated into the vehicle’s electrical system, often requires cooling via the vehicle’s existing radiator,” he reasons. “While a conventional PTO weighs around 25kg, an e-PTO will weigh upwards of 120-130kg – factoring in a separate electrical motor, the PTO system connected to it, an inverter (20kg) and cooling.”
BOX: JET ENGINES FAIL TO TAKE OFF
When it comes to potential electric powertrain layouts in trucks, an interesting parallel can be drawn with the aircraft industry. The world’s first two production jet aircraft – WWII’s Gloster Meteor (pictured) and Messerschmitt Me 262 – were both twin-engined aeroplanes which had their engines located halfway along each wing just as they would have done if they were propellor-driven. Their designers were just following convention.
However, there were significant drawbacks, not least the high amount of asymmetric thrust developed when one of the early jet engines failed (which they often did). In the post-war era, 890 Gloster Meteors were lost to accidents, killing 450 pilots. Unsurprisingly, subsequent jet fighters have mostly had their engines embedded as close to the aircraft’s centreline as possible!
