I. Introduction
The petroleum crisis of the 1970's along with rapidly increasing environmental concerns and emission standards have led to the development of alternative means of transportation. The plug-in hybrid electric vehicle (PHEV) is one such form of transportation that gained popularity in the last decade due to its high fuel efficiency. It has two on board energy sources, namely the gasoline and the battery, which are both used to meet the varying vehicular power demand. Unlike conventional vehicles, this optimal power distribution reduces the fossil fuel consumption and the emission of harmful green house gases [1].