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 CO$_2$ emissions 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 CO$_2$ emissions are very close to the minimum value, i.e., with a deviation of less than 1 for most driving cycles.
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