I. Introduction
Today's hybrid electric vehicles (HEVs) have much better fuel economy than conventional nonhybrid vehicles. The improved fuel economy is mainly due to the use of extra battery energy storage and one or more electric machines that assist the combustion engine by providing additional power, therefore allowing the use of a smaller combustion engine operating under more fuel-efficient conditions. Battery storage also provides a buffer that enables capturing the braking energy that is normally wasted as heat. The extra degree of freedom provided by the auxiliary power source enables substantial improvements in fuel efficiency, as demonstrated by commercially available HEVs, yet the added efficiency of any HEV is dependent on the power-management strategy (PMS), which determines the split of power request between the combustion engine and the electric drive [22].