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Optimal Energy Management for a Mild Hybrid Vehicle With Electric and Hybrid Engine Boosting Systems

This paper investigates the utilization of two low-voltage hybrid mechanisms on fuel economy improvement of a vehicle with a downsized engine. The first mild hybrid system, configured with a motor, a planetary gear set, and a supercharger, permits switching between parallel hybrid operation and boosting but not both. This constrained flexibility offers additional degrees of freedom, yet it requires a highly optimized decision of which functionality, torque assist, or boost assist, in addition to how much energy to be used by each. The second mild hybrid system also supports supercharging and torque assist but with two separate motors, hence is more flexible. Both systems enable the use of a downsized engine as a competitive counterpart to widely used turbocharged and full hybrid electric powertrains. The optimal control problem for energy management of both configurations on a vehicle with an automatic transmission is formulated and solved using dynamic programming (DP) over standard drive cycles. The results indicate around 25% fuel consumption improvement compared to a baseline turbo-charged engine over the FTP75 cycle, which is around 80% of the full hybridization benefit. The supercharging versus torque assist share and its sensitivity to the drive cycle, battery size, and compressor efficiency, which influence the availability of electric energy and the losses of electrical and mechanical paths, is also investigated. Few causally implementable rules for selecting the single motor system operating mode are found by analysis of the DP solution, and their effectiveness is shown by incorporating into the DP formulation and comparing the resulting fuel consumption.