The control of hybrid vehicles also requires determining the power distribution strategy between the engine and the electric motor based on information such as engine speed, load, vehicle speed, and the status of related equipment. That is, when the vehicle load is given, the ratio of the output power of the engine and the electric motor must first be determined to ensure that the performance requirements of the vehicle, such as power, economy, and emissions, are met.
Due to the structural differences among various hybrid electric vehicles, different control strategies are required to regulate and control the flow of power between different components, with the aim of achieving the following four main objectives:
1. Optimal fuel economy
2. Minimum emissions
3. Lowest system cost
4. Best driving performance.
To meet the requirements of hybrid electric vehicles, including drivability, a control system and control strategy adapted to the hybrid system need to be designed. The design of the control strategy for hybrid electric vehicles mainly considers the following points:
1. Optimize engine operating point: Determine the optimal operating point based on the engine's torque/speed characteristic curve, based on the best fuel economy, lowest emissions, or one of the two.
2. Optimize the engine's operating curve: If the engine needs to produce different power outputs, the corresponding optimal operating points constitute the engine's optimal operating curve.
3. Optimize the engine's operating range: On the torque/speed characteristic curve, the engine has a preferred operating range in which fuel efficiency is highest.
4. Minimize engine dynamic fluctuations: The engine operating speed should be controlled to avoid fluctuations, thereby minimizing engine dynamic fluctuations.
5. Limit engine speed: When the engine is running at low speed, fuel efficiency is very low. Therefore, the engine should be turned off when the engine speed is below a certain lower limit.
6. Reduce the number of times the engine is turned on and off: Frequent engine start/stop increases fuel consumption and emissions.
7. Proper Battery State of Charge: The battery capacity must be maintained at an appropriate level to provide sufficient power during vehicle acceleration and to recover energy during braking or downhill driving. If the battery capacity is too high, the engine should be turned off or allowed to idle.
8. Safe battery voltage: The battery voltage changes significantly during discharge, generator charging, or regenerative braking charging. It is important to avoid the battery voltage being too low or too high, otherwise the battery will be permanently damaged. Therefore, battery management is crucial.
9. Appropriate division of labor: In the drive cycle, the engine and battery should share the power required by the vehicle in a reasonable manner.
10. In some cities or regions, hybrid electric vehicles are most efficient when operating in pure electric mode, and this transition can be achieved manually or automatically.
