ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

“That said, airflow is still a major consideration in our design work because it also affects how we cool the PV cells.” Cooling is critical to maintaining solar yield. As with any conductive material, excess heat is tied inextricably to losses, and Squad Mobility intends for its cars to be fully useable in hot cities where sunlight is most plentiful, from southern Europe to the Arabian Gulf and beyond. “An aluminium backing is installed underneath our PV roof. That’s done primarily for mechanical reinforcement, but it also comes with the secondary benefit of acting as a very effective heat sink,” Hoevers says. “We’ve designed the roof shape not just to have airflow rushing over the top, but also the interior of the car guides a wash of cooling air underneath the roof to cool the aluminium. “That means we maintain that 22% yield almost all the time, without needing any cooling fans or other power-parasitic active thermal management systems.” Passenger safety has also been a major factor in determining the shape of the Squad. As Hoevers explains, the structure has been designed to protect against crashes from both front and rear, although he adds that they were not required by certification to do so, and to his knowledge none of their competitors in their weight classes have done so. “We also have a full roll cage, or rollover structure if you prefer, which is also not a requirement but fits well with the size and shape of the frame,” he says. “It helps protect passengers from shock or injury if there’s an accident, and further boosts the ruggedness and longevity of the Squad. “Customers in the ride-sharing market tend not to be always careful with their vehicles. After all, they only have them for the afternoon in many cases, so we want our EVs to last even if customers are quite rough with them.” To make the most efficient drivetrain possible, Hoevers and his team looked at a wide range of configurations, including reductions, differentials and e-motor arrangements. The best solution overall for cost and energy efficiency was found to be a direct drive powertrain with two permanent magnet in-wheel motors at the rear and two inverters on the chassis. Having the motors sitting in the wheels has also helped with packaging, as the batteries and other electrical and electronic systems could be moved around more freely, with no motors, gearing or axles to block them. The inverters are kept separate from the in-wheel motors to make it easier to mount and replace them if better systems come along in the future. “We use field-oriented control inverters, which are a little more expensive than trapezoidal control but they’re much better in terms of performance, quietness and efficiency,” Hoevers adds. “They also give a smooth, controllable power output, which is great for rider comfort as well as regenerative braking. “We use a single pedal drive system. Getting the regeneration right is vital for that, because if it’s too harsh you can give customers whiplash, yet if it’s too soft or doesn’t respond you won’t brake quickly enough.” He adds that the company also looked into induction motors, but found that within the L6-L7 size class they came with a much smaller range of inverters to choose from. Their cost and controllability relative to those of small- form PM motor-inverter arrangements were problematic as well. Future plans While the current Squad car is classed as an L6e vehicle, Hoevers aims to produce a four-seater, class L7 version in the future, which will be faster and better at tackling inclines. “We’re capped at 4 kW now – 2 kW per e-motor – but an L7 EV can output 7 kW or more,” he says. “That will give us more flexibility to design such a vehicle according to how the users want them.” As that also brings opportunities to trial and adopt more efficient EV technologies, Hoevers and his colleagues are keeping a close eye out for new kinds of motors, inverters, battery cells and all the other critical points of an electric powertrain, whether it be new COTS systems or whether they decide to try their hand at designing such parts in-house. Being charged by sunlight means the Squad cars can forgo plug-in charging - battery swap stations could be made available for quick top-ups when needed (Courtesy of Squad Mobility) Autumn 2021 | E-Mobility Engineering 19 InConversation | Robert Hoevers