- Overview of hybrid electric vehicle
With the rapid development of the global automobile industry and the increasingly tight supply of oil resources, countries all over the world actively seek alternative fuels or reduce fuel consumption, and vigorously develop new energy-saving and environment-friendly vehicles. Before solar energy, electric energy and other alternative energy really enter the practical stage, hybrid electric vehicles have attracted more and more attention because of their advantages of low fuel consumption, low emission and high cost performance.
Hybrid electric vehicles will exist for a long historical period and play an important role in the delivery vehicles in the 21st century.
At present, the fuel consumption of hybrid cars (equivalent to ordinary cars and intermediate cars) is required to be about 3L / 100km, and the harmful gases emitted in the exhaust gas meet the emission requirements of “ultra-low pollution”, which is the goal of HEV in the early 21st century.
Although hybrid electric vehicle has not achieved zero emission, its comprehensive indicators such as power, economy and emission can meet the current harsh requirements, which can alleviate the contradiction between vehicle demand, environmental pollution and oil shortage.
- Basic concepts of hybrid electric vehicle
The so-called hybrid electric vehicle refers to a vehicle that carries different power sources and can use different power sources at the same time or separately according to the driving needs of the vehicle. The biggest difference from traditional vehicles is the power transmission system, which generally has at least two power sources and two energy storage systems.
Hybrid electric vehicle is a vehicle between internal combustion engine vehicle and electric vehicle. It is a transition vehicle from internal combustion engine vehicle to ev. At the same time, it is also an “independent” vehicle.
Hybrid electric vehicles can be divided into two categories: hydraulic hybrid vehicle (HHV) and hybrid electric vehicle (HEV). Hydraulic energy storage hybrid electric vehicle is composed of hydraulic drive system and thermal engine drive system.
Hybrid electric vehicles use internal combustion engines and motors as power sources. At present, the vehicles produced are usually started by electric motors and internal combustion engines, and the vehicles are driven by one or more electric motors. The internal combustion engine is responsible for charging the battery or directly providing power when a large amount of thrust is required (such as slope or acceleration). Therefore, in the absence of special instructions, hybrid electric vehicles in this book refer to hybrid electric vehicles HEV.
- Main components of hybrid electric vehicle
Engine is the main power source of hybrid electric motor. Four stroke internal combustion engine (including gasoline engine and diesel engine), two-stroke internal combustion engine (including gasoline engine and diesel engine), rotor engine, gas turbine and stirling engine can be widely used. Generally, the combustion efficiency of rotor engine and gas turbine is relatively high and the emission is relatively clean. Different engines can form different HEV.
2.2. Drive motor
The drive motor is the auxiliary power source of HEV. The drive motors of HEV can be AC induction motor, permanent magnet motor, switched reluctance motor, DC motor and special motor. With the development of HEV, DC motor has been rarely used, most of which are induction motor, permanent magnet motor and switched reluctance motor. The “hybrid” of engine power and driving motor is another form of HEV power “hybrid”. Different hybrid electric vehicles can be composed of different motors.
2.3. Auxiliary power supply
HEV can be equipped with various batteries and super capacitors as “auxiliary power supply”. It can only be used when the hybrid electric vehicle motor starts the engine or motor assisted drive.
- Advantages and disadvantages of hybrid electric vehicle
Compared with pure electric vehicles, hybrid HEV has the following advantages.
(1) Compared with pure electric vehicles, more internal combustion engines provide power, so there are fewer batteries, which reduces the quality of the whole vehicle and contributes to improving the power performance.
(2) Due to the use of auxiliary power drive, the limit of driving range of pure electric vehicles is broken, and its long-distance driving capacity is comparable to that of traditional vehicles.
(3) The highly real-time and dynamic optimization control strategy is adopted in the hybrid electric vehicle. The results of the optimization control try to make the components of the power system work in the best state and the highest efficiency area, which greatly limits the high fuel consumption rate and a large amount of tail gas emission of the internal combustion engine under bad working conditions, and greatly improves the fuel economy of the hybrid electric vehicle. In areas with strict emission restrictions, the auxiliary power can also be turned off and work in a pure electric way to become a zero emission vehicle.
(4) Air conditioning system and other accessories are directly driven by internal combustion engine, which has sufficient energy supply to ensure the ride comfort of the car.
(5) Under the control strategy, the auxiliary power can provide energy to the energy storage device (generally battery pack), so as to ensure that the hybrid electric vehicle does not need to stop and charge. Therefore, the existing gas station can be used without the construction of special charging facilities.
(6) Because the battery pack of hybrid electric vehicle is shallow charging and shallow discharging in the process of use, it can prolong the service life of the battery.
Possible advantages in the future: when the battery technology and cost are more advanced, the internal combustion engine can provide all the energy required for the commuting distance without starting the internal combustion engine. At that time, the vehicle can be in the pure electric mode during commuting (charging back to the garage at night), and the internal combustion engine can be turned on only for long-distance use such as holiday play. The vehicle can even provide power to the Office (or home) during power peak hours; If it can be popularized, the late night charging demand will make the load of the power system more even, which will not only benefit the power industry, but also increase the efficiency of power plants and reduce pollution, and electricity can also be provided by renewable energy.
- Key technologies of hybrid electric vehicle
Hybrid electric vehicle is a high-tech integrated product integrating automobile, electric drive, automatic control, new energy and new materials. Its research involves many fields, and its key technologies mainly include battery and battery management, motor, engine and vehicle energy management.
4.1. Hybrid electric vehicle battery and battery management system
The working condition of the battery on hybrid electric vehicle is different from that on pure electric vehicle, and it is often in non periodic charge discharge cycle, which requires that the battery must have the ability of rapid charge discharge and efficient charge discharge, that is, the battery used in hybrid electric vehicle must have high energy density and, more importantly, high power density, so as to provide large peak power during acceleration and climbing.
The performance and service life of the battery are closely related to the charging and discharging history of the battery, the working temperature of the battery and other factors. Overcharge and over discharge will seriously affect the performance of the battery and even cause battery damage. Therefore, monitoring the working process and working environment of the battery through the battery management system and providing accurate prediction of the remaining power of the battery is of great significance to make full use of the energy efficiency of the battery and prolong the service life of the battery.
4.2. Hybrid electric vehicle motor
Motor is one of the driving units of hybrid electric vehicle. Its selection principles are stable performance, light weight, small size, wide speed range, high efficiency, small electromagnetic radiation and low cost; In addition, the peak power of the motor should have the ability to start the engine, electric drive, vehicle acceleration, maximum regenerative braking, etc. At present, the motors used in hybrid electric vehicles mainly include DC permanent magnet motor, permanent magnet brushless synchronous motor, AC asynchronous motor, switched reluctance motor and so on. Among AC motors, the most representative is AC induction motor, and the structure of this kind of motor determines that the contradiction between power and efficiency is difficult to solve. Advanced motors such as permanent magnet motor and switched reluctance motor with high efficiency, high power density and compact structure should be used as far as possible.
4.3. Hybrid vehicle engine
As the engine of hybrid electric vehicle will start and stop frequently when working, in order to meet the strict emission standards, the design goal of thermal engine has changed from the high power of traditional engine to the pursuit of high efficiency, and the peak shaving task of power is entrusted to the motor. To achieve this goal, the Otto cycle commonly used in internal combustion engines can be used with high efficiency at with large expansion ratio-
Instead of Kinson cycle, or use other high-efficiency heat engines, such as gas turbine and stirling engine, and then use their respective advantages to design hybrid power system. For example, Toyota Prius’s 1.5L gasoline engine adopts Atkinson working cycle with high efficiency and high expansion ratio, compact inclined squeeze combustion chamber and aluminum alloy cylinder block. Its main purpose is to pursue high efficiency rather than high power.
4.4. Hybrid electric vehicle power coupling device
In parallel and hybrid systems, mechanical power coupling device is the key component of coupling engine and motor power. It not only has great mechanical complexity, but also directly affects the vehicle control strategy, so it has become the focus and difficulty of hybrid system development. At present, the dynamic coupling modes adopted include torque combined type (single shaft type and double shaft type), speed combined type and driving force combined type.
4.5. Hybrid electric vehicle drive system control
On series hybrid electric vehicles, electric drive is the only driving mode, so the control system is relatively simple. In the driving system of parallel and hybrid hybrid electric vehicles, there are two power sources: engine and motor. The two power sources have a variety of cooperative working modes, such as pure electric, engine drive, engine drive + motor assist, engine drive + generator charging, etc. According to the needs of vehicle driving, the power system switches between these working modes.
The control strategy of the drive system should be able to determine the working mode of the hybrid electric vehicle and determine the reasonable working point of the engine and motor by analyzing the driving condition of the vehicle, the torque characteristics of the engine and motor and the SOC of the battery in real time. That is, it is necessary to control the dynamic processes such as starting, mode switching and gear shifting of the drive system of hybrid electric vehicle.
The key technologies for the research and development of hybrid electric vehicles can be roughly divided into vehicle system integration and key parts technology. The key technologies of vehicle system integration include: power system parameter matching; Vehicle energy control system; Regenerative braking system; Vehicle data bus; Advanced vehicle control technology.
Key component technologies mainly include: engine for hybrid electric vehicle; Drive motor and its control technology; Power battery and its management system technology; Automatic transmission technology for hybrid electric vehicles.
Main technical problems faced by hybrid power at present:
(1) The specific power and service life of energy storage devices (batteries) should be improved.
(2) Establish more advanced and effective electronic control and detection system.
(3) Power electronic devices must be reduced in size and mass.