4.0 PARALLEL HYBRID
The parallel hybrid configuration is the system found in many production hybrid vehicles, such as the Honda Civic, Accord and Insight hybrids. Unlike the series hybrid configuration, the parallel configuration uses both the engine and electric motor to drive the wheels. Like the previous section, this section begins by analyzing the basic components and configuration of the system.
4.1 Components and Configuration
The main components of the parallel hybrid configuration are a gasoline engine, a battery, an electric motor/generator, a transmission, and an electronic control system (Hybrid Center, 2005).
The gasoline engine in a parallel system is connected directly to the wheels just like a traditional vehicle (Jefferson & Barnard, 2002). However, the engine is usually smaller and uses technologies such as cylinder idling systems and friction reducing methods to maximize efficiency. Like a traditional vehicle, the hybrid parallel system uses a conventional (fixed ratio) transmission that changes the gear ratio between the engine and wheels.
In addition to these traditional parts, a parallel hybrid system includes an electric motor/generator and a battery (Westbrook, 2001). In this system, the electric motor/generator is used to boost the engine power during acceleration, start the engine, absorb brake energy (for regenerative braking), and power vehicle auxiliaries (Jefferson & Barnard, 2002). An electronic control system is in place to balance the load between the motor and the gasoline engine to meet driver demands while obtaining the best efficiency (Westbrook, 2001). The parallel system is displayed below in Figure 4.1.
Figure 4.1 The parallel configuration
Source: Adapted by S. L. from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html
4.2 Operating Characteristics
Like the other hybrid electric configurations, the parallel system behaves differently depending on operating conditions. Below are the six different modes of operation of the system.
4.2.1 Resting Position
When the vehicle is at rest, there is no power flowing through the system However, the electric motor is ready to draw power from the battery at any time to start the gasoline engine (Hybrid Center, 2005)
4.2.2 Startup
When starting up, the electric motor draws power from the batteries to start the gasoline engine. This process is illustrated in following Figure 4.2.
Figure 4.2 Startup
Source: Adapted by S. L. from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html
4.2.3 Normal Driving
At cruising speeds, the wheels are driven mainly by the gasoline engine (Hybrid Center, 2005). The electric motor/generator is free to perform other functions in this mode of operation including charging the battery as shown in Figure 4.3.
Figure 4.3 Normal driving
Source: Adapted by S. L. from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html
4.2.4 Full Throttle Acceleration or Heavy Load
Under heavy acceleration, the electric motor draws power from the battery to assist the gasoline engine in driving the wheels. This process is illustrated in Figure 4.4 below.
Figure 4.4 Full throttle acceleration or heavy load
Source: Adapted by S. L. from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html
4.2.5 Deceleration and Braking
During the braking phase, the main electric motor acts as a generator and recharges the battery. By reversing the direction of the flow of power, the motor translates the kinetic energy at the wheels into electric energy (Kawahashi, 2004). This energy is then sent to the battery for storage.
4.2.6 Battery Charging
When the battery is low on charge, the control system sends some of the power from the gasoline engine to the electric motor in order to charge the battery as illustrated in Figure 4.5 below.
Figure 4.5 Battery charging
Source: Adapted by S. L. from http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood-2.html
4.3 Advantages/Disadvantages
Some advantages and disadvantages of the parallel hybrid propulsion system are discussed below.
4.3.1 Advantages
Out of the three configurations discussed in this report, the parallel configuration is least expensive and uses the fewest number of components. Because of its design, the parallel configuration can use a smaller electric motor and battery than either the series or the series-parallel configurations. The small amount of parts also means the system can be made to be very compact (Jefferson & Barnard, 2002). In addition, the parallel hybrid configuration is compatible with conventional fixed ratio transmissions. These facts added together make the parallel configuration ideal for integrating into an existing traditional vehicle design.
Compared to the series configuration, the parallel configuration has only one conversion between electrical and mechanical power, as opposed to two conversions in the series system (Westbrook, 2001). This means the parallel configuration loses less energy during conversion processes. The fact that the gasoline engine drives the wheels makes the parallel engine more suitable for highway driving than the series configuration (Hybrid Center, 2005).
4.3.2 Disadvantages
The main disadvantage of the parallel configuration is that the gasoline engine must be turned on in order to move the vehicle. This means the parallel system is not as suited to stop-and-go driving as the series or the series-parallel configuration since the gasoline engine does not operate very efficiently in those situations.