24 they’re mostly on paved roads and they’re speed- and acceleration-limited. “Trucks can weigh more, some need to go off-road, some have huge gradeability needs and some have to go 70 mph on a highway. The transit approach wouldn’t work on the truck. So, we had two choices to make the power and torque required: gears or copper and current – and the latter is a lot more expensive, so we went for a multi-speed gearbox.” The demonstrator was developed to meet two core performance targets: the ability to start on a minimum grade of 20% at full Gross Vehicle Weight Rating (GVWR) and the capability to reach a top speed of 70 mph. That bracketed the amount of torque and power required, giving the engineers two corner points to work from. To meet these requirements, a range of Interior Permanent Magnet (IPM) motors was co-developed with their manufacturing partner. These were specifically designed to marry with the MD4-speed EV transmission unit developed by Eaton – a specialist manufacturer that offered the reputation to be able to go to market with durability and credibility. The GPM-5 – which is the motor used on the demonstration vehicle – is the smallest unit the system can use, and is specifically focused on the Class 6 and Class 7 vehicles. This can be replaced by the medium-sized GPM-10 for up to 55,000 lb single-axle Class 8 or the GPM12 for Class 8 vehicles up to 80,000 lb. All these motors are architecturally the same, being central, remote mount with a multi-speed gearbox, and while BAE Systems does not currently have an in-house e-axle solution, it is open to collaborating with e-axle manufacturers such as ZF, Allison, Accelera, Dana and Linamar on the motor control and overall system control solutions. Those who are not quite ready to commit to a fully electric approach can use the exact same system to convert a diesel truck to run on hybrid-electric power, just with fewer batteries and the use of an Integrated Drive Unit (IDU) between the existing internal combustion engine and transmission instead of the dedicated electric motor. The system is designed to run on a 600–750 V DC architecture, with stateof-the-art power electronics stages that use silicon carbide (SiC) and gallium nitride (GaN) switching technology, variable frequency and advanced motor control to deliver optimum performance with minimum motor and inverter size. To provide the biggest possible range of performance, the use of the fourspeed gearbox allows for high torque at lower current, minimising losses in the components and conductors and lowering the heat. That leads to additional benefits in reduced cooling parasitics, smaller pumps, lower flow rates, and a smaller fan and radiator. On the demonstrator, the original vehicle’s rear axle design and ratio was changed to allow for a lower numeric ratio, reducing the number of the driveshaft turns for each complete rotation. An amboid layout reduces the angle between the ring and pinion gears, decreasing rear axle friction and increasing the regen performance of the axle. The transmission – which has ratios of 4.83:1, 2.82:1, 1.65:1 and 1:1 – also allows the motor to be designed and tuned for performance over a relatively small rotational speed range, between zero and 3500 rpm. This has allowed for a higher percentage of the duty cycle to be spent in high-efficiency operating zones. “The transmission really helped by allowing us to have a pretty small motor that has exceptional torque,” explains Matthews. “A lot of the other electrified vehicles out there have gone with a single motor and direct drive or a single motor with two-speed drive, maybe on an e-axle, but by using four speeds, we have a lot more torque. “In fact, we have an order of two to three times more torque than the other Class 6 and Class 7 trucks on the market – and it’s all usable. You can put it to good use, but at the same time, when we get on the highway, we’re able to pull that motor right down into its sweet spot from an efficiency standpoint. “It allows for full torque in all gears, achieving the maximum use from the available motor performance. The system is fully automated, without the use of any friction type clutches, and the transmission is given authority during the shift event with the motor control coordinating torque and speed based on its commands. “During shifting, the motor is able to speed match to the next selected gear in around 50 ms and the transmission also incorporates an accelerometer, to allow for both grade and load determination. This in turn allows for second-gear starts when the vehicle is at less than GVWR or facing down a grade.” January/February 2026 | E-Mobility Engineering The gearing in the motor ensures there is plenty of torque in the system to cope with all levels of performance requirements
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