**2.1. Definitions**

Conventional vehicles are propelled by only igniting fossil fuel in an ICE and converting the ignition energy into mechanical rotation and translation. In contrast, HEVs are characterized by using some combination of a primary propulsion unit (PPU) that can be fuel cell (FC) of an ICE or EM that can be either an electrochemical storage system such as a battery or an electrostatic super capacitor [22]. In addition to the above mentioned components, at least one electric motor is necessary in any HEV to help propel the vehicle either fully or partially. This combination of electric and fossil fuel energy, supervised with a high level controller called energy management strategy (EMS), can improve the performance of the vehicle from a fuel consumption and emission point of view. Comparing HEVs with conventional vehicles shows that the former is more fuel efficient due to the engine operation optimization and the possibility of recovering the kinetic energy during braking [23].

EMs play the role of optimizing the efficiency of the ICE as well as energy recovery during braking. It can also use the excess power of the engine to charge the battery if the power demands on the final drive is lower than the power converted by the engine. Another role can be to assist the ICE in the cases that the ICE alone cannot fulfill the driver demands when the ICE is overloaded in some emergent cases.

There are basically four main advantages in hybridizing a conventional vehicle as follows:


Although HEVs have many advantages, they have some limitations as well. The first issue is the increased cost due to the presence of EMs, energy storage system, electrical converters, and so on. Safety issues due to existence of high voltage electricity and electromagnetic interference due to high frequency switching are also other problems to HEVs.
