Hybrid Electric Propulsion Systems Report

3.0 SERIES HYBRID

The series hybrid system is the simplest out of the three configurations discussed in this report. This configuration is mainly used for urban vehicles such as buses, taxies, and street-cleaning-vehicles. This section begins by examining the components of the series hybrid system.

3.1 Components and Configuration

The main components of the series configuration are a gasoline engine, a generator, a computer unit, an electric motor, and a battery pack as shown in Fig 3.1.

The series configuration

Figure 3.1 The series configuration
Source: Adapted by H. Hediyeh from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html

The series-hybrid design is unique in that, unlike other hybrid configurations, only the electric motor can directly power the wheels. The motor in turn receives its energy from both the battery pack and a generator, which is powered continuously by a gasoline engine (Jefferson & Barnard, 2002). A computer unit determines how much energy should be supplied by the battery pack to the generator. The battery pack provides almost all the required power to accelerate the vehicle from rest position. If the battery is lower on charge, it can be recharged by regenerative braking, or by the gasoline-engine powered generator (Jefferson & Barnard, 2002).

3.2 Operating Characteristics

This sub-section examines the different modes of operation and the associated behaviour of the series configuration. There are five modes of operation listed below:

3.2.1 Resting Position
When the vehicle is at rest, there is no power flowing through the system. This means that the gasoline engine is off and the electric motor is not drawing any power from either the generator or the battery pack. However, the electric motor is ready to draw power from the battery at any time. In other words, the motor is on standby mode. This is illustrated above in Fig. 3.1.

3.2.2 Startup or Slow Speeds
When the driver first presses on the gas pedal to accelerate the car from the rest position, the electric motor draws the required power from the battery pack. The rate of this drawn power depends on how far the driver presses the gas pedal. After the vehicle reaches a slow, steady speed, the battery remains the only source of power until it runs out of energy. At this point the gasoline engine is started to supply the rest of the energy to keep the car in motion. This is illustrated in Fig. 3.2.

Startup or slow speeds

Figure 3.2 Startup or slow speeds
Source: Adapted by H. Hediyeh from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html

3.2.3 Full Throttle Acceleration or Heavy Load
When more power is need to reach higher speeds or to pull a heavy load, the gasoline engines starts to provide energy to the generator, which in turn assists the battery in powering the electric motor, as shown in Fig. 3.3.

Full throttle acceleration or heavy load

Figure 3.3 Full throttle acceleration or heavy load
Source: Adapted by H. Hediyeh from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html

3.2.4 Normal Driving
At cruising (constant) speed, the electric motor only draws power from the gasoline engine, and there is no power drawn from the battery, as illustrated below in Fig. 3.4.

Normal driving

Figure 3.4 Normal driving
Source: Adapted by H. Hediyeh from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html

3.2.5 Deceleration and Braking
When the driver applies the brakes to decelerate the vehicle, the electric motor turns into a generator and converts some of the kinetic energy of the car into electric potential energy and sends this energy to the battery. This is shown in Fig. 3.5 below.

Deceleration and braking

Figure 3.5 Deceleration and braking
Source: Adapted by H. Hediyeh from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html

3.2.6 Battery Charging
When the battery is low on charge, the control system sends more of the engine power to recharge the batteries, as illustrated in Fig. 3.6.

Battery charging

Figure 3.6 Battery charging
Source: Adapted by H. Hediyeh from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html

3.3 Advantages/Disadvantages

This section examines some of the advantages and disadvantages of the series configuration when compared to the parallel and series-parallel hybrid configurations.

3.3.1 Advantages
The series configuration is most suitable for urban vehicles such as buses that do a lot of stop and go driving. This is because only the electric motor drives the wheels. Therefore, the gasoline engine can power off as long as the battery provides enough energy to the motor to propel the vehicle. Compared to the parallel and series-parallel configurations, the series configuration is simpler and can use a smaller gasoline engine (Jefferson & Barnard, 2002). The latter is because the gasoline engine is not connected directly to wheels, which means the engine is not subjected to the varying power demands of city driving (Hybrid Center, 2005). Therefore the gasoline engine can operate at near peak efficiency at all times.

3.3.2 Disadvantages
The fact that only the electric motor drives the wheels also proves to be a disadvantage for the series configuration. This is because electric motors do not operate very efficiently at high speeds, so therefore the series system is not suited to highway driving. Another disadvantage of this design is that the mechanical energy supplied by the gasoline engine has to be converted to electrical energy and back to mechanical energy at the wheels, which results in a fairly significant amount of energy loss (Jefferson & Barnard, 2002). The series configuration also requires a larger battery pack and a dedicated generator when compared to the parallel configuration, which translates into higher cost (Hybrid Center, 2005).

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