1) Series-parallel hybrid power system
The series-parallel hybrid power system mainly includes a switch-type hybrid power system and a power split hybrid power system. The structure of the power split hybrid power system is shown in Figure 1. The plug-in series-parallel hybrid power system is mainly composed of internal combustion engine, motor 1, motor 2, planetary gear power distribution mechanism, motor controller, transmission, power battery pack and external charging interface. The hybrid power system is characterized by using a single-row planetary gear mechanism to couple the power of the internal combustion engine and two electric motors. The single-row planetary gear structure can realize a continuously variable transmission, making the entire power system more efficient, especially in urban driving cycles. The biggest weakness of the hybrid power system is that its constant torque distribution leads to low power system efficiency when the car is cruising at high speeds.
2) Dual-mode hybrid system
The plug-in dual-mode hybrid power system is composed of a combination of any two of the three basic operating modes: series, parallel and hybrid. The most typical plug-in dual-mode hybrid power system is composed of a series system and a parallel system, and its structure is shown in Figure 2. The plug-in dual-mode hybrid power system is mainly composed of several components such as the main drive motor, internal combustion engine, ISG motor, power coupling mechanism, transmission device, motor controller, power battery pack and external charging interface. Its distinctive feature is the use of dual clutches. Through the opening and closing conditions of the dual clutches, the series and parallel structures can be presented respectively. This hybrid system can make full use of the dual clutches and determine whether the system works in series mode or parallel mode according to the best efficiency of the hybrid system, so that the vehicle can achieve higher fuel economy and reduce exhaust emissions. The disadvantage of this structure is that the system contains two sets of motors and two clutches, which is relatively complicated, and its working mode and control strategy are also relatively complicated. The dual-mode hybrid power system relies on two clutches to make the system’s multiple energy sources more convenient and flexible to combine and control, and has more operating modes than parallel and series.
At the end of 2010, the Chevrolet Volanda extended-range electric vehicle was launched in the United States. The model uses a series plug-in hybrid power system, equipped with a 1.4L low-displacement high-efficiency engine, 70kW drive motor and 53kW generator, and is equipped with a lithium-ion power battery with an energy of 16kW·h. Its power system can provide a maximum power of 110kW and a maximum torque of 370N·m, and the driving range in pure electric mode can reach 64km. In 2015, the second-generation Volanda adopted a new lithium-ion battery pack provided by LG Chem, with a battery capacity of 18.4kW·h. The larger-capacity battery pack and lighter weight enable the second-generation Volanda pure electric vehicle to reach 85km. In addition, the second-generation Volanda adopts a dual-motor structure, with output powers of 87kW and 48kW, and a peak torque of 398N·m. At the same time, it uses a 1.5L naturally aspirated engine with a maximum power of 75kW. Due to the series hybrid mode, this engine will not directly participate in the work under any circumstances, only when the battery power drops to a certain position during long-distance driving, the vehicle will automatically start the engine to charge the battery pack, and then use the electric motor to drive the car. Therefore, under any working conditions, the engine provides power to the battery at the most economical speed, so as to minimize the engine’s fuel consumption. Figure 3 shows the second-generation Volanda plug-in hybrid electric vehicle.
In November 2008, the German government proposed a plan to popularize 1 million plug-in hybrid vehicles and pure electric vehicles in the next 10 years. In 2010, a 420 million euro lithium battery development plan for vehicles was launched. At present, German automobile companies continue to strengthen cooperation with battery companies, which has significantly accelerated the process of power battery technology research and development and industrialization. In addition, the European Union issued 7 billion euros in loans in the first half of 2009 to support automakers in developing new energy vehicles. Currently, major European car manufacturers have launched plug-in hybrid vehicles.
The BMW Vision Efficient Dynamics plug-in hybrid vehicle is equipped with two electric motors and a turbocharged diesel engine; the maximum speed is 250km/h, and the acceleration time from 100 kilometers is 4.8s. The pure electric driving range is 50km, the average fuel consumption per 100 kilometers is 3.76L, the carbon dioxide emission is 99g/km, and the charging time is 2.5h with 220V power supply; charging with 380V power supply can shorten the charging time to 44min, as shown in the figure 7.
The BMW Vision Efficient Dynamics plug-in hybrid vehicle is equipped with two electric motors and a turbocharged diesel engine; the maximum speed is 250km/h, and the acceleration time from 100 kilometers is 4.8s. The pure electric driving range is 50km, the average fuel consumption per 100 kilometers is 3.76L, the carbon dioxide emission is 99g/km, and the charging time is 2.5h with 220V power supply; charging with 380V power supply can shorten the charging time to 44min, as shown in the figure 4.