Performance Evaluation of an Electric Vehicle with Multiple Electric Machines for Increased Overall Drive Train Efficiency 2019-24-0247

Proposed solutions for electric vehicles range from the simple single-motor drive coupled to one axle through a mechanical differential, to more complex solutions, such as four in-wheel motors, which ask for electronic torque vectoring. Main reasons for having more than one electric machine are: reduction of the rated power of each motor, which most likely leads to simplification and cost reduction of all the electric drive components; increased reliability of the overall traction system, enhancing fault tolerance ability; increase of the degrees of freedom which allows for control strategy optimization and efficiency improvement. In particular, electrical machines efficiency generally peaks at around 75% of load and this usually leads to machine downsizing to avoid operation in low efficiency regions. The same output performance can be achieved by using two or more electrical machines, rather than only one, of smaller size and running them at unequal load - one of the machines at higher load and the other(s) at lower load.

In this paper, the performance of an electric vehicle with multiple electric machines is analyzed to assess the potential of overall drive train efficiency increase. In particular, the powertrain drive system comprises of multiple novel compact and high-efficiency high-speed electrical machines and a three-stage constant ratio high-speed gearbox, which can be adapted to different configurations of the powertrain. In particular, the gearbox has a modular design which allows connecting it to one single electrical machine and the axle or to two electrical machines and the axle. Therefore, two different integrated drivetrain modules can be realized: a module composed by two motors connected to one single gearbox and a drive module composed by one motor and one gearbox. Different combinations of these modules within the powertrain will be compared in terms of overall drivetrain efficiency in order to assess the potential of the proposed solutions against the simplest single-motor-drive powertrain.