5.0 SERIES-PARALLEL HYBRID
The series-parallel hybrid system is a combination of both series and parallel hybrid configurations (Hybrid Center, 2005). It is the most complex of the three configurations discussed in this report. The series-parallel system is used by Toyota hybrid vehicles such as the Prius and Camry Hybrid (Miller, 2004). Like the two previous sections, this section begins by examining the components and configuration of the system.
5.1 Components and Configuration
The main components of the series-parallel hybrid system are electric motors, a battery, a gasoline engine, a generator, a control system, a power control unit (inverter/converter), and a power splitting mechanism (Hybrid Synergy Drive, 2007). Figure 5.1 provides a simple block diagram of this system. Note that the control system is not shown in the diagrams.
Figure 5.1 The series-parallel configuration
Source: Adapted by X. T. Jiang from Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.159.
The power splitting mechanism is unique to the series-parallel system, and it is implemented using a planetary gear system as shown in Figure 5.2.The main electric motor is connected to the outer ring gear, which is in turn coupled to the wheel axle through a reduction gear (Westbrook, 2001). The output of the gasoline engine is connected to the planetary gear carrier, and the generator is connected to the inner sun gear (Jefferson & Barnard, 2002). This system effectively splits the power produced by the gasoline engine between the wheel axle and the generator (Miller, 2004). The electricity produced by the generator can then be used to power the electric motor or charge the battery (Westbrook, 2001). The power splitting mechanism also acts as a continuously variable transmission (CVT) between the engine and the wheel axle (Jefferson & Barnard, 2002).
Figure 5.2 Power-splitting mechanism
Source: Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.158.
5.2 Operating Characteristics
Like the other two hybrid electric configurations, the series-parallel system behaves differently depending on operating conditions. Below are the different modes of operation of the system, accompanied by descriptions and/or figures.
5.2.1 Resting Position
When the vehicle is at rest, no energy flows through the system, but the control electronics are ready to send power from the battery to the electric motor (Hybrid Center, 2005).
5.2.2 Startup or Slow Speeds
When starting from rest, moving at slow speeds, or going down a gentle slope, the gasoline engine is shut off because the engine does not operate very efficiently under these conditions (Kawahashi, 2004). Only the electric motor drives the wheels by drawing power from the battery, as illustrated in Figure 5.3.
Figure 5.3 Startup or slow speeds
Source: Adapted by X. T. Jiang from Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.159.
5.2.3 Normal Driving
Under normal driving conditions, the power from the gasoline engine is split between the wheels and the generator, which is in turn used to power the electric motor, as illustrated in Figure 5.4 (Hodkinson & Fenton, 2001).
Figure 5.4 Normal driving
Source: Adapted by X. T. Jiang from Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.159.
5.2.4 Full Throttle Acceleration or Heavy Load
When under full throttle acceleration or heavy load, the electric motor draws power from the battery and assists the gasoline engine in driving the wheels, as illustrated in Figure 5.5 (Hodkinson & Fenton, 2001).
Figure 5.5 Full throttle acceleration or heavy load
Source: Adapted by X. T. Jiang from Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.159.
5.2.5 Deceleration and Braking
Like the other hybrid configurations, the main electric motor acts as a generator during the deceleration and braking process and converts the kinetic energy at the wheels to electric energy, which is sent to the battery. This is illustrated in Figure 5.6 (Kawahashi, 2004).
Figure 5.6 Deceleration and braking
Source: Adapted by X. T. Jiang from Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.159.
5.2.6 Battery Charging
When the battery is low on charge, the control system sends more of the engine power to the generator to charge the batteries. This is illustrated in Figure 5.7 (Westbrook, 2001).
Figure 5.7 Battery charging
Source: Adapted by X. T. Jiang from Michael H. Westbrook, The Electric and Hybrid Electric Car (London, United Kingdom: The Institution of Engineers 2001). P.159.
5.3 Advantages/Disadvantages
Here are some of the advantages and disadvantages of the series-parallel hybrid system.
5.3.1 Advantages
The arrangement of components in a series-parallel hybrid allows the vehicle to be driven by only the electric motor, or use both the engine and electric motor together, depending on driving conditions (Hybrid Synergy Drive, 2007). This means the vehicle is able to choose the most efficient combination of the two power sources at a given speed, which leads to higher fuel efficiency and lower emissions in both city and highway driving (Hybrid Synergy Drive, 2007). In other words, the series-parallel system is able to maintain both the advantage of the series configuration in city driving and the advantage of the parallel configuration in highway driving (Hybrid Center, 2005). These facts make the series-parallel system the most versatile and potentially the best performing hybrid configuration (Hybrid Center, 2005).
5.3.2 Disadvantages
The series-parallel system is more complex than either series or parallel configurations, and therefore it incurs a higher cost. The system requires a larger battery, a more advanced control logic, and an extra power splitting mechanism when compared to the other two systems (Hybrid Center, 2005). Compared to the parallel configuration, the series-parallel system also requires an extra generator (Hybrid Center, 2005). The extra parts also make it more difficult to fit the system into a reasonably-sized package (Jefferson & Barnard, 2002).