What are rules-based energy management strategies for hybrid electric vehicles?

The actual driving performance (including fuel economy and emission performance) of HEV is closely related to the type of control strategy and the selection of control parameters. On the premise of meeting the driving power requirements of the vehicle, the control strategy should be able to reasonably distribute the driving power between the engine and the motor according to the steady-state characteristics of each assembly and the real-time operating state of the vehicle, so as to obtain the best fuel economy of the vehicle. and emission performance.

The basic idea of ​​rule-based energy management strategy is to determine the energy distribution between the motor and the engine based on engineering experience and the steady-state efficiency characteristics of components. This control method is simple and effective, easy to implement, and has strong practicability, and has been widely used in product engineering. How to distribute the power between the engine and the electric motor is determined based on the steady state efficiency map of the components.

What are rules-based energy management strategies for hybrid vehicles?
Rule-based energy management control strategy

The strategy divides the hybrid electric vehicles control into three modes, namely the normal driving mode, the charging mode and the braking energy feedback mode. At the same time, the efficiency MAP of the engine is divided into ① pure electric (engine off motor drive) area, ② engine drive area and ③ motor auxiliary drive area, as shown in Figure 1. In different modes, the power distribution mode of the engine and the electric motor is determined according to the efficiency map of the engine, and its main functions are realized as follows.

What are rules-based energy management strategies for hybrid vehicles?
Figure 1 Schematic diagram of logic threshold control strategy control principle

1. Normal driving mode
The engine efficiency MAP is divided into 3 regions by using the “minimum engine power” curve, namely the minimum torque line of the engine, and the curve of “minimum motor assist power”, namely the maximum torque line of the engine. The rules of power distribution are: first, if the required driving power is less than the minimum operating power of the engine, the motor will provide all the driving power; second, if the required driving power exceeds the limit, the engine will replace the motor to drive the vehicle forward. : Third, if the required driving power is greater than the minimum power of the electric motor assist, the additional driving power is provided by the electric motor. In the normal driving mode, the engine always operates in the most efficient region between the “engine operating min. power” curve and the “motor assist min. power” curve.

2. Charging mode
The battery energy management adopts the charging maintenance strategy, that is, the state of charge (SOC) value of the battery is always kept between the upper and lower limits of the high-efficiency area (set at 55%~60%). When the SOC value is less than At 55%, it should switch to the charging mode (if and only when the SOC value is greater than 60%, the charging process is completed), and calculate the charging power, which is also used as the target power of the motor, and the target power of the engine is the required driving power and The sum of the charging power. There is an exception in the charging mode: when the target power of the engine is less than the minimum operating power of the engine, in order to avoid the engine working in an extremely low-efficiency area, the electric motor is still used to provide the driving force.

3. Braking energy feedback mode
The driver depresses the brake pedal, indicating the driver’s demand for negative driving power, and should enter the braking energy feedback mode to absorb the energy of the hybrid electric vehicle when braking. However, when the braking energy exceeds the maximum braking energy that can be fed back, the hydraulic braking system will provide the remaining braking energy.

The rule-based energy management strategy mainly relies on engineering experience and experiments to limit the working area and working mode of the engine to achieve the purpose of reducing fuel consumption and emissions. The method is relatively simple and intuitive. However, this method generally only manages energy according to the steady-state efficiency map of the engine, and the effect in the dynamic process is not ideal.

The most common energy management strategy is the logic threshold control strategy: the core idea is to use the electric drive system composed of the motor and the battery as the power buffer of the hybrid electric vehicle powertrain, using its characteristics of good dynamic response and high overall efficiency, Provide the dynamic power required for the vehicle to run, play the role of “cut peak and fill the valley”, and make the engine run in the high-efficiency area to provide the average power required for the vehicle to run. Compared with traditional vehicles, the logic threshold control strategy can ensure that the engine always runs in the high-efficiency range, which can greatly improve the fuel economy of the vehicle.

The specific design method of the logic threshold control strategy is as follows.
(1) According to the static efficiency MAP of the engine and engineering experience, determine the logic threshold of the control variables of the control strategy, and divide the engine working area into driving and non-driving areas. The powertrain has several drive working modes, such as motor-driven alone, engine-driven and power generation, engine-only driven and combined drive.
(2) In order to ensure the power performance of the whole vehicle, the battery power maintenance control strategy is adopted. When the battery SOC deviates from its working range (limited by the maximum and minimum battery SOC limits), the control strategy adjusts the output power of the engine to Return the battery SOC to the normal SOC working range.
(3) When the vehicle is braking, under the premise of ensuring braking safety, the braking energy is recovered according to a certain proportion according to the angle of the brake pedal.