Tools and Strategies for Hybrid-Electric Drivesystem Optimization 961660
While hybrid-electric drivesystems have the potential to substantially improve fuel economy without sacrificing performance, the extent of the fuel savings is highly dependent on component optimization within the system. Effectively optimized components require the drivesystem designer to understand component trade-offs and to supply the component developers with sufficient information to optimize their designs. Without a systems approach to all levels of drivesystem design-and, clearly, of vehicle design as well-components will tend to either be substantially oversized and inefficient or provide compromised vehicle performance.
Component specifications typically include power requirements, such as would be needed to provide adequate starting torque, gradability, acceleration, and braking. Fundamental specifications should also include time-sensitive power and energy requirements based on anticipated duration of maximum starting torque and peak continuous power for gradability, and of transient peaks for acceleration and braking.
Component designers must also have a means of optimizing distribution of highest efficiency to match anticipated use patterns. Designers of powerplants, load-leveling devices, and power electronics need to know the typical distribution of cumulative energy throughput at various power levels over a realistic driving cycle. Motor designers need to know the distribution of cumulative energy throughput on a map of torque and speed so that motor efficiency can be optimized to correspond. The design of reduction gears and choice of fixed-ratio or multi-speed transmission must also be an integral part of this optimization.
Following fundamentals of drivesystem design and component selection, time-sensitive power and energy requirements and their derivation are discussed. Cumulative energy throughput or work for an illustrative driving cycle is then presented in forms directly applicable to component design optimization-first as a function of tractive power alone, then on a map of motor speed and torque. A brief discussion of drivesystem control strategies addresses drivability issues as required by component choices. Conclusions are drawn throughout the paper as to the application methodology, design implications, and usefulness of the presented tools and strategies for hybrid-electric drivesystem optimization.