Prototyping of HEV powertrains is cumbersome, expensive, and time-consuming. Thus, design optimization is the most feasible alternative process for accomplishing a good design. This project will identify key system components and develop a high-fidelity model of the hybrid drive train capturing key dynamics. Modes such as engine mode, motor mode, hybrid-parallel mode, regenerative braking mode, and driving modes, such as crawling, take-off, cruise, traffic mode, heavy and frequent braking mode, will be investigated.