A fuzzy logic controller for hybrid vehicles with parallel configuration was proposed. Using the state-of-charge (SOC) of the energy storage, the driver command, and the motor/generator speed, a set of rules was developed. The fuzzy logic controller can determine the split between the electric motor and the internal combustion engine to achieve better fuel economy and low emission performance without losing vehicle performance.
Hybrid systems use a combination of an internal combustion engine (ICE) and an electric motor (EM). This combination has the potential of improving fuel economy by making use of regenerative braking on deceleration. There are three different types of hybrid systems:
- Series — The internal combustion engine (ICE) is used as a generator, providing electrical power to the electric motor (EM) and the battery.
- Parallel — The electric motor (EM) supplements torque to the internal combustion engine (ICE), both of which are connected to the drive train.
- Series-Parallel — A combination of the two configurations.
In parallel configuration, the ICE is connected to the drive train, and the electric motor by the mechanical torque/speed coupler. The battery is also connected to the electric motor. There are five different ways to operate the system:
- Provide power to the wheels using the EM only.
- Using the ICE only.
- Using both the ICE and the EM simultaneously.
- Charge the battery using the ICE power to drive the EM as a generator.
- Slow down the vehicle by letting the wheels drive the EM as a generator that provides power to the battery (regenerative braking).
In order to manage the flow of energy among all components, a power controller is required to take into account the energy available in the battery.
It is important to optimize the architecture and components of the hybrid vehicle. The energy management strategy used is just as important as the architecture and components. A power controller is used to control the energy flow among all components, and optimizes power generation and conversion in the individual components. The energy in the system should be managed as follows:
- The driver input (from brake and accelerating pedals) is satisfied consistently.
- Battery has full charge at all times.
- All four components — ICE, EM, battery, and transmission — should have an optimized overall system efficiency.
During the operation of the parallel hybrid vehicle, the power controller should determine how much power is needed to drive the wheels based on the driver input and how much is needed to charge the battery. Then, the power controller should split the power between the ICE and the EM. If the battery is low on charge, the controller will assign negative power to the EM. Meanwhile, the ICE will provide the power for both driving the
wheels and charging the battery.
The difference between using the ICE or the EM to drive the wheels is as follows. When the ICE is used, the energy flows directly from the ICE through the transmission to the wheels, meaning that the mechanical power produced will promptly be used to drive the wheels. When the EM is used, energy first flows from the ICE through the transmission to the EM, operated as a generator, to charge the battery. Then, the energy will flow from the battery to the EM, operated as a motor, to the wheels, meaning that the same mechanical power has been converted to electric power, and then back to mechanical.
The power controller optimizes the energy flow among the components of the parallel hybrid vehicle, the energy generation, and conversion in the components (ICE, EM, battery, and transmission). The accelerator and the brake pedal inputs of the driver power command are converted by the power controller. The fuzzy logic controller computes the optimal generated power by using the driver power command, state-of-charge battery, and the EM speed, which are also used to compute the optimal ICE and EM power. The driver inputs (from braking and accelerating) are satisfied consistently by the power controller to ensure the battery is charged all the time.
This work was done by Ali Almufti for the U.S. Army RDECOM-TARDEC. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Mechanics/Machinery category. ARL-0098
This Brief includes a Technical Support Package (TSP).
Parallel Hybrid Vehicles Using Fuzzy Logic Control
(reference ARL-0098) is currently available for download from the TSP library.
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