Whether automated or manual, the 16-speed (2x4x2) gearboxes that once were standard equipment on most maximum-weight tractor units have now largely disappeared from manufacturers’ specification sheets, even from their last bastion as an option for Special Types prime movers.
The comparatively broader torque ranges of modern diesels made them surplus to requirements on general haulage many years ago, and the costs of homologation plus increased engine torque outputs mean that at the ultra heavy-duty end, the addition of a torque converter will usually allow a 12-speed (2x3x2) gearbox to be specified.
But the trend to shift to fewer ratios will not end there. Electrification of heavy truck drivelines gives further scope for dispensing with multirange transmissions by exploiting the massive rev ranges of electric motors.
Volvo Trucks was one of the first out of the gates with a production heavy electric truck and made a conservative choice in initially installing a conventional I-Shift multi-speed gearbox lifted from the diesel range behind its electric motors.
Other manufacturers made braver choices. Mercedes-Benz eschewed a conventional drive layout for the eActros, going straight to what many see as the ‘second-generation’ solution, a motor integrated into the drive axle (an e-axle) with a four-speed transmission.
Some might query why the manufacturers bother with anything other than a direct drive, single-speed solution: after all the rev range of an electric motor would allow it. Offering a multi-ratio gearbox enables a smaller, cheaper, lighter, and more economical motor installation, plus the ability to overspeed the motor for short periods during regenerative braking at a wider range of road speeds.
Having revealed a prototype e-axle in 2022, Volvo Trucks will bring it into production later this year in a 6x2 tractor that possibly offers the least compromised solution so far for direct replacement of a 44-tonne diesel with a BEV. Volvo is emphasising the extra space created on the chassis by the new layout, which enables the fitment of up to eight batteries giving a total capacity of 780kWh. The number of gear ratios on offer has not yet been revealed, but set to be less than the 12 in the existing I-Shift-equipped Volvo electric trucks.
In North America, manufacturers are increasingly settling on four-speed transmissions for electric trucks, in a market where long-haul trucks boast up to 18 ratios to handle more mountainous terrain and higher road speeds than in Europe.
But things are changing fast with electrification. Paccar subsidiary Kenworth has launched what it claims is the industry’s “first Class 8 vocational battery-electric truck“ (roughly the equivalent of an eight-wheel tipper chassis here in the UK), which has only three speeds. While it has a conventional drive axle, the motor is not located under the bonnet, but rather in a midships location where it is integrated into the transmission.
Power transmission specialist Eaton has also developed a proprietary drop-in electric propulsion system in partnership with BAE Systems. Eaton’s contribution is a four-speed transmission developed exclusively for the application. Eaton’s product director Julian Hopkins says while a single-speed electric transmission was a theoretical possibility, installing a multi-speed transmission meant less compromised startability, gradeability and top speed.
The four-speed transmission will be available in medium and heavy-duty versions, although whether Europe’s vertically integrated manufacturers would be prepared to countenance it is another question.
