Ultracapacitor assisted powertrains: Modeling, control, sizing, and the impact on fuel economy

A mild parallel hybrid powertrain is considered in which an electric motor and an ultracapacitor-based energy source assist the combustion engine during periods of high power demand. The ultracapacitor may be recharged by the engine during periods of low demand, and through regenerative braking. A rule-based control strategy is defined, which determines the power split between the engine and motor. Standard city-cycle simulations on a full-order model of the powertrain illustrate an improvement in fuel economy enabled by the ultracapacitor-based mild hybrid configuration and this control strategy. The deterministic dynamic programming is also applied to the reduced-order model of the powertrain to assess fuel economy potential of the hybrid system under pointwise-in-time and terminal constraints on the state of charge and motor power limits.