The development of hybrid systems needs to solve many technical problems, such as the design of control strategies, the optimization of combustion systems for internal combustion engines, the improvement of batteries, the matching design of transmission systems and the application of new materials and new processes.
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1 control system
The control system here refers to the centralized control system of the vehicle powertrain, which is the core unit of the normal driving of the whole vehicle. The control system of a conventional internal combustion vehicle includes an air-fuel ratio (or fuel injection amount) control, an ignition control, and an idle speed control of the engine, and gear shifting and shift feeling control of the transmission. Hybrid vehicle control also needs to determine the power distribution strategy of the engine and the motor according to the information such as the speed, load and vehicle speed and the state of the relevant equipment. That is, when the load of the vehicle is given, the ratio of the output power of the engine to the motor is first determined. Guarantee to meet the requirements of automotive performance, economy, emissions and other performance indicators. In order to meet the requirements of hybrid vehicles including drivability, it is necessary to design a control system and a control strategy that are compatible with the hybrid system.
Hybrid vehicle control strategy
Due to the structural differences of various hybrid electric vehicles, different control strategies are needed to regulate and control the flow of power between different components. The purpose is to achieve the following four main objectives:
- Best fuel economy
- lowest emissions
- Lowest system cost
- The best drive performance
The design of the hybrid electric vehicle control strategy mainly considers the following points:
(1) Optimize the operating point of the engine: based on the optimal fuel economy, minimum emissions or both, determine the optimal operating point based on the torque/speed characteristic curve of the engine;
(2) Optimize the working curve of the engine: If the engine needs to emit different power, the corresponding optimal working point constitutes the optimal working curve of the engine;
(3) Optimize the working area of ​​the engine: On the torque/speed characteristic curve, the engine has a preferred working area, in which the fuel efficiency is the highest;
(4) Minimum engine dynamic fluctuation: The operating speed of the engine should be controlled to avoid fluctuations, so that the dynamic fluctuation of the engine is minimized;
(5) Limit the minimum engine speed: When the engine is running at low speed, the fuel efficiency is very low, so when the engine speed is lower than a certain lower limit, the engine should be shut down;
(6) Reduce the number of engine on/off times: Frequently turn the engine on/off, causing an increase in fuel consumption and emissions;
(7) Appropriate battery state of charge: The capacity of the battery must be maintained at an appropriate level to provide sufficient power when the car accelerates to recover energy when the car is braking or downhill. If the capacity of the battery is too high, turn off the engine or make it idle;
(8) Safe battery voltage: When discharging, charging the generator or braking and recharging, the voltage of the battery will change greatly. The battery voltage should be too low or too high, otherwise the battery will be permanently damaged, so the battery Management is critical;
(9) Appropriate division of labor: In the drive cycle, the engine and battery should reasonably share the power required by the vehicle;
(10) Hybrid electric vehicles in some cities or regions operate most efficiently in pure electric mode, which can be achieved manually or automatically.
2 internal combustion engine
After more than 100 years of development, the internal combustion engine for vehicles has been greatly improved in terms of power, economy and emission control. In recent years, the application of technologies such as electronically controlled fuel injection, exhaust gas recirculation, supercharged intercooling, variable intake turbine, high pressure common rail and catalytic post-treatment has made the performance of automobiles more rapid. Therefore, as a mature Power equipment, the application of internal combustion engines in hybrid electric vehicles is not difficult. Due to its good movability, high specific power and high thermal efficiency, the internal combustion engine is still the key equipment affecting the efficiency and performance of the whole vehicle.
3 battery
The battery is the key technology for the development of hybrid electric vehicles, and is also an important development direction to improve vehicle performance and reduce costs. Since the 1990s, the problems of specific energy, specific power and cycle life of batteries have been the main obstacles to the development of electric vehicles. For hybrid electric vehicles, due to the high proportion of electric motors, they also face batteries. The problem of technical improvement: First, the specific energy is relatively insufficient, so the cost is higher, the higher the specific energy value, the better the economy of the automobile; secondly, the life of the battery is relatively short, and the battery life is generally about 1000 times of charging and discharging. It is much lower than the life of the whole vehicle. If the battery is frequently replaced during the life cycle of the car for more than ten years, the operating cost of the hybrid vehicle will be greatly improved. In addition, the application of the battery also involves problems such as long charging time and battery state of charge (SOC) discrimination, which affect the performance of the vehicle to varying degrees. At present, the batteries used in hybrid electric vehicles are mainly lead-acid batteries, nickel-hydrogen batteries (MH-Ni) and lithium-ion batteries. For example, Chrysler ESX2 uses lead-acid batteries, Toyota Prius and Honda Insight use Ni-MH batteries for Nissan Tino. Lithium Ion Battery.
4 other technologies
Motor technology, torque synthesis technology and new material application technologies also play a pivotal role in hybrid vehicle systems. For example, motor technology involves the working efficiency and energy recovery strategy of the motor; the torque synthesizer couples the engine torque and the motor torque, which has a significant impact on the smoothness and reliability of the system; the application of material technology mainly refers to light The choice of high-strength materials is extremely beneficial for improving vehicle performance.
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