Parallel (Mechanically Coupled) Hybrid Electric Drivetrain Design

There are many possibilities of configurations in a parallel hybrid drivetrain, as mentioned in Chapter 5. The design methodology for one configuration may not be applicable to others. Each particular configuration may be only applicable to the specified operation environment and mission requirement. This chapter will focus on the design methodology of parallel drivetrains with torque coupling, which operates with the electrically peaking principle. That is, the engine supplies its power to meet the base load (operating at a given constant speed on flat and mild grade roads, or the average of the load of a stop-and-go driving pattern), and the electric motor supplies the power to meet the peak load requirement. This chapter focuses on the design methodology of parallel drivetrains with torque coupling, which operates with the electrically peaking principle. That is, the engine supplies its power to meet the base load, and the electric motor supplies the power to meet the peak load requirement. Unlike the series hybrid drivetrain, the parallel or mechanically coupled hybrid drivetrain has features that allow the engine and the traction motor to apply their mechanical power in parallel directly to the drive wheels. In the drivetrain design, the important factors are the power of the engine; motor, peaking power source (PPS), and its energy capacity; transmission; and, more importantly, the control strategy of the drivetrain. The design objectives are as follows: satisfying the performance requirements, achieving high overall efficiency whenever possible, maintaining the SOC-of-PPS at reasonable levels while driving on highways and in urban areas without the need of charging the PPS from outside the vehicle, and recovering as much brake energy as possible.