A Real Time Energy Management Strategy for Plug-in Hybrid Electric Vehicles based on Optimal Control Theory

Abstract

Plug-in hybrid electric vehicles are commonly designed to work in Charge Depleting/Charge Sustaining (CD/CS) mode, depleting the battery by driving in only-electrical mode until the SoC reaches its minimum acceptable threshold, and then sustaining the state of charge till the end of the mission, operating as a traditional hybrid vehicle. Nonetheless, a simple application of an optimal control framework suggests a blended discharge strategy, in which the powertrain is operated as to gradually deplete the SoC and reach the lower threshold only at the end of the trip. Such an algorithm has the drawback that the optimal solution can only be reached offline, depending on the a-priori knowledge of the driving event, making it unsuitable to be implemented online, as it is. The paper presents a methodology to design a heuristic controller, to be used online, based on rules extracted from the analysis of the powertrain behavior under the optimal control solution. The application is a parallel plug-in vehicle, derived from a re- engineered engine-only driven powertrain, and the optimal problem is solved with the Pontryagin's Minimum Principle. Results are also compared to the same vehicle in its standard internal combustion engine version, as well as the commonly implemented Charge Depleting/Charge Sustaining strategy.