Torque-Assist Hybrid Electric Powertrain Sizing: From Optimal Control Towards a Sizing Law

In this study a novel method is proposed with which the optimal hybridization ratio of a torque-assist hybrid electric powertrain can be found with very little computational effort. The objective is to minimize the total CO2emissions of the vehicle, while maintaining its drivability at a constant level. The starting point is an analysis in which the optimal driving strategy is found for eight typical driving cycles using dynamic programming. Analyzing these results, a simple yet powerful rule-based method is proposed that allows to choose the sizes of the combustion engine and of the electric motor such that the CO2emissions are very close to the minimum value, i.e., with a deviation of less than 1% for most driving cycles.