  This paper presents a nonlinear-model based hybrid optimal control technique
to compute a suboptimal power-split strategy for power/energy management in a
parallel hybrid electric vehicle (PHEV). The power-split strategy is obtained
as model predictive control solution to the power management control problem
(PMCP) of the PHEV, i.e., to decide upon the power distribution among the
internal combustion engine, an electric drive, and other subsystems. A
hierarchical control structure of the hybrid vehicle, i.e., supervisory level
and local or subsystem level is assumed in this study. The PMCP consists of a
dynamical nonlinear model, and a performance index, both of which are
formulated for power flows at the supervisory level. The model is described as
a bi-modal switched system, consistent with the operating mode of the electric
ED. The performance index prescribing the desired behavior penalizes vehicle
tracking errors, fuel consumption, and frictional losses, as well as sustaining
the battery state of charge (SOC). The power-split strategy is obtained by
first creating the embedded optimal control problem (EOCP) from the original
bi-modal switched system model with the performance index. Direct collocation
is applied to transform the problem into a nonlinear programming problem. A
nonlinear predictive control technique (NMPC) in conjunction with a sequential
quadratic programming solver is used to compute suboptimal numerical solutions
to the PMCP. Methods for approximating the numerical solution to the EOCP with
trajectories of the original bi-modal PHEV are also presented in this paper.
The usefulness of the approach is illustrated via simulation results on several
case studies.
