Parallel Hybrid System:

The parallel HEV (Figure 4b) allows both ICE and electric motor (EM) to deliver power to drive the wheels. Since both the ICE and EM are coupled to the drive shaft of the wheels via two clutches, the propulsion power may be supplied by ICE alone, by EM only or by both ICE and EM. The EM can be used as a generator to charge the battery by regenerative braking or absorbing power from the ICE when its output is greater than that required to drive the wheels. The advantages of the parallel hybrid drivetrain are:

• •  both engine and electric motor directly supply torques to the driven wheels and no energy form conversion occurs, hence energy loss is less

•  compactness due to no need of the generator and smaller traction motor.

The drawbacks of parallel hybrid drivetrains are:

• •  mechanical coupling between the engines and the driven wheels, thus the engine operating points cannot be fixed in a narrow speed region.

•  The mechanical configuration and the control strategy are complex compared to series hybrid drivetrain.

Due to its compact characteristics, small vehicles use parallel configuration. Most passenger cars employ this configuration.

 

Series-Parallel System

In the series-parallel hybrid (Figure 4c), the configuration incorporates the features of both the series and parallel HEVs. However, this configuration needs an additional electric machine and a planetary gear unit making the control complex.

Complex Hybrid System

The complex hybrid system (Figure 4d) involves a complex configuration which cannot be classified into the above three kinds. The complex hybrid is similar to the series-parallel hybrid since the generator and electric motor is both electric machines. However, the key difference is due to the bi-directional power flow of the electric motor in complex hybrid and the unidirectional power flow of the generator in the series-parallel hybrid. The major disadvantage of complex hybrid is higher complexity.

References:

[1] M. Ehsani, Modern Electric, Hybrid Electric and Fuel Cell Vehicles: Fundamentals, Theory and Design , CRC Press, 2005

Suggested Reading:

[1] I. Husain, Electric and Hybrid Electric Vehicles , CRC Press, 2003