Basic knowledge of hybrid electric vehicles

1. Principles of Hybrid Electric Vehicles
The basic principle of hybrid vehicles is to use appropriate fuel conversion devices (such as internal combustion engines, fuel cells), energy storage devices, and electric motors as hybrid power sources. According to certain control strategies, fuel conversion devices, energy storage devices, and motors are used to drive the work. Work as far as possible in a high-efficiency and low-emission area under vehicle conditions. In the case of vehicle braking, part of the vehicle’s braking energy is recovered through the adjustment of the motor’s working quadrant, so as to improve the vehicle’s fuel economy and exhaust under various driving conditions. Emissions and other performance properties.

Basic knowledge of hybrid electric vehicles
Principles of Hybrid Electric Vehicles

Because of the organic combination of traditional internal combustion engine power and electric assist, hybrid electric vehicles have the dual advantages of convenient refueling, long driving range, and low pollution and high efficiency of pure electric vehicles. It not only solves the problems of traditional large horse-drawn vehicles. The problem is that there is no need to worry about the long charging time, short life and high cost of the battery. A small increase in cost is exchanged for considerable fuel economy and emission performance benefits, which is accepted by the market and has become the current focus of the automotive industry. .

2. Energy-saving mechanism of hybrid electric vehicles
In order to meet the power requirements of acceleration and gradeability, traditional cars often design some parameters of the car, such as maximum power, to be very large, resulting in a very low load rate of the car, and what we call a “big horse-drawn car” phenomenon , And hybrid power is a drive system invented to overcome this defect. It consists of two power sources-an engine and an electric motor. You can choose a smaller engine to meet the power requirements of the vehicle in most cases. When there are fewer extreme drive requirements, it is assisted by the motor. Due to the low selected power of the engine, on the one hand, it can reduce its energy consumption and improve its emission performance. On the other hand, it can greatly increase the work load rate of the engine itself, thereby improving the efficiency and emission performance of the entire vehicle. At the same time, due to the addition of the motor, it becomes feasible to eliminate idle speed (the engine can be automatically controlled by the motor to start and stop instantly), and the operating points between the motor and the engine can complement each other, that is, the motor can be used to adjust the engine in a highly efficient working area. In addition, the motor can also recover braking energy to improve the economic performance of the vehicle. In summary, hybrid electric vehicles can achieve energy-saving goals from the following four aspects:
(1) Choose a smaller engine (downsize) to increase the engine load rate.
(2) Improve the control strategy to make the engine work in the high-efficiency area to improve the fuel consumption of the vehicle.
(3) Eliminate engine idle speed to reduce fuel consumption.
(4) Recover braking energy.

Through the research on the energy-saving mechanism of hybrid electric vehicles, the following conclusions are drawn: Under typical urban cycle conditions, hybrid electric vehicles increase their load rate by choosing a smaller engine, so that the efficiency of the vehicle is improved, thereby improving fuel economy 13 %-20%: The contribution of engine working area control to fuel economy improvement is 1%~3%; regenerative braking energy recovery can save energy by 3.5%~7%; and eliminating idle speed can save fuel by 6%~10%. Comprehensive analysis shows that the total energy saving potential of hybrid technology under specific working conditions may reach 30% to 60%, as shown in Figure 1.

Basic knowledge of hybrid electric vehicles
Figure 1 Statistical analysis of the effects of HEV fuel-saving measures

3. Characteristics of Hybrid Electric Vehicles
The vehicle control system of a hybrid electric vehicle controls the working quadrant of the generator or motor according to the different driving conditions of the vehicle to ensure that the energy in the energy storage device is always maintained at a certain level. Generally, there is no need to stop for charging or frequently replace the battery. At the same time, when driving in certain areas with strict restrictions on vehicle emissions, hybrid vehicles can turn off the internal combustion engine and turn it into a pure electric drive to achieve zero exhaust emissions.

(1) Hybrid electric vehicles have the advantage of fuel saving
Because the hybrid vehicle can control the engine, battery, and motor to work as much as possible in their respective high-efficiency areas under different working conditions through the vehicle control strategy. At the same time, it can also control the start and stop of the engine, that is, eliminate the engine idling, so that Hybrid electric vehicles have obvious fuel-saving advantages. Compared with traditional diesel vehicles and CNG vehicles, the fuel economy of hybrid vehicles has increased by 30% to 40% on average. For the Toyota Prius hybrid sedan, fuel consumption can be reduced by 4.54L/100km under NEDC conditions.

(2) Hybrid electric vehicles have good emission performance
Since the hybrid vehicle can control the engine to work in the high-efficiency area, and can cancel the engine idling condition, its pollutant emissions can be significantly reduced, thereby ensuring that the hybrid vehicle has a good emission performance. As fuel consumption is reduced, vehicle combustion pollutant emissions are also reduced at the same time, as shown in Figure 2.

Basic knowledge of hybrid electric vehicles
Figure 2 Comparison table of measurement results of exhaust pollutants from hybrid vehicles and traditional vehicles

Figures 3 to 5 show the comparison of emissions from hybrid vehicles, conventional diesel vehicles and CNG vehicles. It can be seen from the figure that the emissions of particulate matter (PM) are reduced by 50%, NOx and volatile organic compounds. Reduced emissions by 30%-40%, reduced CO emissions by 70%, and reduced greenhouse gas emissions significantly.

Basic knowledge of hybrid electric vehicles
Figure 3 Comparison of PM emissions between the New York bus cycle and the central commercial bus cycle
Basic knowledge of hybrid electric vehicles
Figure 4 Comparison of NO and volatile organic compound emissions between the New York bus cycle and the bus cycle in the commercial center area 1mile-1.609 344km
Basic knowledge of hybrid electric vehicles
Figure 5 Comparison of CO emissions from the New York bus cycle and the bus cycle in the commercial center area 1mile=1.609 344km

(3) Hybrid electric vehicles have complex structure and high technical content
Compared with traditional internal combustion engine vehicles, hybrid vehicles have at least one more drive system (motor + battery), so the structure is more complex than traditional vehicles. This also makes hybrid electric vehicles require strict control logic, and need to use advanced technologies such as automatic control, computers, and microelectronics to achieve effective control of the system. Therefore, the technical content is high and the cost is also increased. Moreover, the power motor and power battery also increase the cost of the vehicle. Toyota’s Prius hybrid sedan is priced at about US$20,000 in the United States. Although users can get a tax rebate of US$3,000, the price is still much higher than that of the same-level internal combustion engine sedan. Therefore, how to include power motors and battery suppliers The original parts system of the automobile industry effectively reduces the production cost of hybrid vehicles, which is a common problem faced by automobile manufacturers in various countries.

4. Development Trend of Hybrid Electric Vehicles
With the continuous improvement of market maturity and public awareness of hybrid technology, the development trend of its technology presents the characteristics of functional modularity and multiple energy and power.

1) Trend of functional modularity
According to the layout characteristics of hybrid vehicles, their powertrains are mostly developed in modules, which reduces development costs and improves versatility. Typically, the engine and the motor are used as the main modules. From a structure with only the engine as the main body, it has developed to a structure with a highly integrated engine transmission-motor, as shown in Figure 6.

Basic knowledge of hybrid electric vehicles
Figure 6 The development trend of modularity Note: The data before “/” in the figure is electric power; the data after “/” is vehicle power

The main modules are classified as follows, and their functions are shown in Figure 7.
(1) Micro-hybrid structure module: Through the belt pulley starter motor technology, the automatic start-stop and power generation functions of the engine are realized in the parallel hybrid configuration, which has the characteristics of low cost and simple structure.
(2) Light mixing structure module: adopts integrated starter motor technology to increase the mixing effect.
(3) Deep-mix structure module: Based on the transmission with a higher power motor in parallel, the auxiliary drive and pure electric driving function are added, which can realize stronger braking energy recovery.
(4) Full-hybrid structure module: Continue to increase the proportion of motor power on the series platform to achieve all pure electric drive capabilities.

Basic knowledge of hybrid electric vehicles
Figure 7 Main module types and functions of hybrid electric vehicles

2) The trend of multiple energy and power
The ultimate development direction of hybrid electric vehicles will form a hybrid system platform represented by series configuration and deep-mix configuration. On the system platform, the realization of the trend of energy multiplexing is as follows.

(1) Internal combustion engine hybridization: to achieve the main fuel supply of diesel and gasoline, various alternative fuels including alcohol ether and diesel/gasoline blending as supplementary fuel supply; by controlling the combustion method, introducing homogeneous compression ignition, low-temperature combustion, etc. The new combustion method improves the economy and emission performance of the engine.
(2) Power transmission system hybridization: The electric transmission system is used to partially replace the original transmission shafts and gears and other mechanical transmission devices to realize the flexible distribution of power and the function of continuously variable transmission.
(3) Fuel cell system hybridization: The fuel cell and battery/supercapacitor together form a power device, which has excellent instantaneous charging and discharging capabilities and improves the dynamic response characteristics of the vehicle.