Powertrain design optimization for a battery electric heavy-duty truck

In this paper, the energetic benefits of a multi-speed transmission compared to a single speed transmission is analyzed for a long-haul truck. The truck is simulated using a reference long-haul cycle from the Vehicle Energy consumption Calculation Tool (VECTO). A nested optimization routine is used to optimize the battery, electric machine size and the gear ratio value(s) in an outer loop with the particle swarm optimization algorithm and in the inner loop the gear shifting is optimized, as a local minimization problem. It is found that the electric machine size can be reduced (-16%) without compromising the vehicles top speed and acceleration performance, and with a moderate reduction in energy usage (-1.4%). The total number of gears did not significantly influence the energy usage on the long-haul drive cycle studied. The results show the potential of reducing the size of an electric machine and increase in gradeability of the vehicle with the usage of a multi-speed for trucks. Much larger gains in energy usage, without compromising performance, are expected on more dynamic driving conditions and using a dedicated (more compact and integrated) electric machine design for a multi-speed transmission, which is seen as future work.