Series hybrid drivetrain

Series hybrid drivetrain – In this article, we will discuss the drivetrain. For some articles on hybrid car drivetrains, you can read the following article.

Read more: Concept of Hybrid Vehicle Drivetrain

Series hybrid drivetrain

A series hybrid drivetrain is a drivetrain in which two power sources feed a single powerplant (electric motor) that propels the vehicle, as shown in Figure 1. The typical primary power source is IC engine coupled to an electric generator. The output of the electric generator is connected to an electric power bus through an electronic converter (rectifier). The secondary power source is a battery pack, which is connected to the bus by means of a power electronics converter (DC/DC converter). The electric power bus is connected to the controller of the electric traction motor. The traction motor can be controlled either as a motor or a generator, and either in forward or reverse motion. This drivetrain may need a battery charger to charge the batteries by wall plug-in from the power network. 

Series hybrid drivetrain
Figure 1 Series hybrid-electric drivetrain.

Series hybrid electric drivetrain potentially has the following operation modes:

  1. Pure electric mode: The engine is turned off and the vehicle is propelled only from the batteries.
  2. Pure engine mode: The vehicle traction power only comes from the engine generator, while the batteries neither supply nor draw any power from the drivetrain. The electric machines serve as an electric transmission from the engine to the driven wheels.
  3. Hybrid mode: The traction power is drawn from both the engine generator and the batteries.
  4. Engine traction and battery charging mode: The engine-generator supplies power to charge the batteries and to propel the vehicle.
  5. Regenerative braking mode: The engine-generator is turned off and the traction motor is operated as a generator. The power generated is used to charge the batteries.
  6. Battery charging mode: The traction motor receives no power and the engine generator charges the batteries.
  7. Hybrid battery charging mode: Both the engine-generator and the traction motor operate as generators to charge the batteries.
Series hybrid drivetrain
Figure 2 Torque-coupling device.

Series hybrid drivetrains offer several advantages;

  1. The engine is fully mechanical decoupled from the driven wheels. Thus, it can be operated at any point on its speed-torque characteristic map. Therefore, it can be potentially operated solely within its maximum efficiency region as shown in Figure 2. The efficiency and emissions of the engine can be further improved by optimal design and control in this narrow region, which is much easier and allows greater improvements than an optimization across the entire range. Furthermore, the mechanical decoupling of the engine from the driven wheels allows the use of a high-speed engine. This makes it difficult to directly power the wheels through a mechanical link, such as gas turbines or powerplants with slow dynamics like the Stirling engine.
  2. Because electric motors have near-ideal torque-speed characteristics, they do not need multi-gear transmissions. Therefore, the construction is greatly simplified and the cost is reduced. Furthermore, instead of using one motor and differential gear, two motors may be used, each powering a single wheel. This provides the speed decoupling between the two wheels like a differential but also acts like a limited-slip differential for traction control purposes. The ultimate refinement would use four motors, thus making the vehicle an all-wheel drive without the expense and complexity of differentials and drive shafts running through the frame.
  3. Simple control strategies may be used as a result of the mechanical decoupling provided by the electrical transmission.

However, series hybrid electric drivetrains suffer some disadvantages;

  1. The energy from the engine is converted twice (mechanical to electrical in the generator and electrical to mechanical in the traction motor). The inefficiencies of the generator and traction motor add up and the losses may be significant.
  2. The generator adds additional weight and cost.
  3. The traction motor must be sized for maximum requirement since it is the only powerplant propelling the vehicle.

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