Model-based fuel optimal control of hybrid electric vehicle using variable structure control systems

Hybrid electric vehicles provide promising alternatives to conventional engine-powered vehicles with better fuel economy and less emission. Realization of these benefits depends, in part, oil proper control of the vehicle. This paper examines a variable structure control that switches between two operating points. The resulting sliding optimal control provides a better energy management strategy than that obtained conventionally from Pontryagin's minimum principle. Since one of the operating points is at zero engine power, this sliding optimal control is also referred to as engine start-stop strategy. Contrary to the general understanding that the engine should only, stop at low speeds or during decelerations, it is shown that engine start-stop also improves fuel economy during highway cruising. The main contribution of this paper is to theoretically prove that this "duty-cycle" operation mode is indeed optimal using Pontryagins minimum principle.