The HEV is an intermediate step between conventional IC engine power train and full electric vehicle. It has the following main components:

• A  prime mover:  IC engines are most commonly used, gas turbine or Stirling engine can also be used
• Electric motor in parallel or series
• Electric generator
• Battery to store electricity and run motor
• Regenerative braking system using devices like flywheel, ultra-capacitor etc.
• Power transmission system

The high performance nickel-metal hydride (Ni-MH) batteries are more commonly used as the main energy storage device. Recently, Lithium-ion (Li- ion) batteries, which have higher energy storage capacity (≈2.5 kWh/kg) compared to the Ni-MH batteries (≈1.4 kWh/kg) are also being used on some hybrid vehicles.  To supplement the batteries, other energy storage systems such as flywheel and ultra or super-capacitors are also used. The flywheel stores energy during vehicle braking/deceleration and the super-capacitors store energy when the power drawn from the batteries is low and the excess engine power is available. The super capacitors are capable of generating short bursts of very high power

Types of HEVs

Two basic types of HEV are

      (i) Series and

     (ii) Parallel type.

Other variants of the two basic types have also been developed. The HEV configurations are shown in Fig 7.14

Series Hybrid

 In the series hybrid, electric motor is the only propulsion unit. It is connected with the drive wheels. The engine is coupled with an alternator/ generator that runs the motor as well as charges batteries. All the energy from the engine to the wheel passes through electric machine and hence, it is called as series HEV. The electric power is always generated on board. The engine is not required to follow the transient operation needs of the vehicle.