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

Peter Donaldson reports on the steps EV engineers take to optimise the dynamic behaviour of their designs Moving targets H ow a car moves in response to the forces acting on it is a fundamental determinant of safety, comfort, energy efficiency and driver engagement, and electrification of the powertrain profoundly affects that dynamic behaviour. This influence results from differences in the vehicle’s overall mass and its distribution, when compared with IC-engined vehicles, along with the speed and precision of response inherent in electrical machines. That presents engineers with problems as well as opportunities that differ from those associated with IC-engined vehicles and vary between different categories of EVs. Depending on the type of powertrain, there are architectural differences between those powered by hybrid systems, by batteries alone, and fuel cells. Even among pure battery EVs, there are differences in mass distribution between those derived from IC-engined vehicles or that share platforms in a manufacturer’s product range, and clean-sheet designs. Like any object that moves, a car will pivot in the x, y and z axes, in movements known as roll, pitch and yaw, and surge, sway and heave. In addition to the movement of the vehicle as a whole, the wheels and tyres move in relation to the body, around and along their own x, y and z axes under the influence of forces from the road below, the body above and the powertrain, brakes, steering and suspension. Key parameters Weight, and weight distribution, affects all of these things and therefore the amount of grip the tyres can generate and the predictability of that grip, the key parameters being the centre of gravity (CoG), the roll centre and the polar moment of inertia. The CoG is the point at which the weight appears to act, so an object supported at that point would be in balance. The roll centre is determined by suspension geometry, and is the point about which the vehicle rolls under cornering forces. The front and rear suspension both have roll centres, and a vehicle effectively rolls about the axis defined by the line that joins them. If the CoG is a long way above the roll centre, the car will have a greater tendency to roll in corners, because the distance between the two points is effectively a moment arm, or lever. Finally, the polar moment of inertia determines how quickly the car will accelerate in yaw, and it can be visualised with the aid of a dumbbell held in one hand. With the weights out at the ends of the bar, the polar moment of inertia is high and rotating the dumb- bell by turning the wrist is difficult, but with the weights moved in towards the 42 Autumn 2021 | E-Mobility Engineering