Energy Management Strategy and Optimal Hybridization Level for a Diesel HEV 2012-01-1019

The design and the supervision of hybrid electric vehicles (HEV) are strongly coupled. The mutual influence between the optimal components sizing and the optimal operating points choice makes the problem complex. This was previously exposed in literature for spark ignition (SI) HEV. In this paper, we address the same issue for diesel HEV. In this case, the energy management strategy must take nitrogen oxides (NO_x) emissions into account in addition to fuel consumption. This paper presents an optimal supervision strategy and its impact on the electric components sizing. The energy management strategy is based on the equivalent consumption minimization strategy (ECMS) using Pontryagin's minimum principle. It allows an adjustable trade-off between NO_x and fuel consumption to be minimized. It was validated experimentally with a hardware-in-the-loop test bed. By moving the engine operating points into optimal zones, this strategy allows important NO_xemissions reduction while keeping interesting fuel consumption. In comparison to a fuel consumption reduction-oriented strategy, the integration of NO_x emissions deeply changes the energy management strategy choices and, as a consequence, the optimal electric components size. Indeed, to limit NO_x emissions engine high-load-operating points are avoided and a smaller hybridization level is sufficient. Compared to the SI engine, the diesel has a better efficiency at part load. As a consequence, high NO_x reduction is possible without important fuel consumption increase. The interactions between electric components sizing and energy management strategy design are studied in this paper. Depending on the chosen value of the adjustable trade-off of the strategy, the optimal hybridization level changes because the engine optimal operation areas move. The driving cycle influence is also studied here to draw global conclusions.