Early Evaluation of Battery Electric Powertrain Architectures with Focus on Axial Flux Machines

Powertrains commonly used in Battery Electric Vehicles (BEV) are mostly based on electric axles with either permanently excited synchronous machines (PSM) or externally excited synchronous machines (EESM). These are usually designed as radial flux machines. The torque is transmitted to the wheels via a single-stage spur gear reducer with differential and output shafts on both sides. With high demands for drivability, efficiency, and feature integration for better packaging, new drive architectures, and novel components are being developed. This paper determines specifications for the driveline and electric machines based on longitudinal dynamic drivability requirements for a B-segment vehicle. The research was conducted with an example of axial-flux machines. The methods however can be applied to various other electric machine (EM) types. The drivetrain consists of an EM and a planetary gear reducer. An existing map of an axial flux machine was scaled along with the gear ratio to ensure the driving requirements and, drivetrain efficiency over the “Worldwide harmonized Light vehicles Test Cycle” (WLTC). To accomplish this a multi-criteria optimization approach is used. The efficiency analysis also considers corresponding gearbox maps for different gear ratios and designs. Based on the results, different performance and design variants for EMs were created. A parameter study and local optimization using an inverse vehicle model determined the drivetrains with the highest efficiencies. The results of this work provide a key decision factor which powertrain architecture and EM will be developed further with a higher level of detail.