What is a series-parallel hybrid power system?

As shown in Figure 1, the hybrid hybrid system consists of an engine, a power distribution mechanism, a generator, a motor controller, an electric motor and a power battery. The power of the engine is divided into two parts after passing through the power distributor. One part directly drives the vehicle to form a mechanical transmission channel; the other part drives the generator to generate electricity, and the generated electric energy is supplied to the motor to drive the vehicle through the motor controller to form a power transmission channel. By adjusting the rotational speed of the generator, the power distribution ratio of the mechanical transmission channel and the power transmission channel can be controlled. The system has the dual characteristics of a hybrid hybrid system and a parallel hybrid system. First, the power transmission channel and the power battery are electrically connected to realize the power coupling, and the power flow direction is series; the second is the mechanical coupling between the mechanical transmission channel and the motor, and the power flow is parallel, so it is called a hybrid hybrid system.

Figure 1 - series-parallel hybrid system

Figure 1 - series-parallel hybrid system

The series-parallel hybrid system combines the advantages of series and parallel, so that the advantages of both can be brought into play, and its application prospect is promising. Under NEDC cycle conditions, the fuel-saving rate of vehicles using this method can reach more than 40%.

In a hybrid system, according to the proportion of the output power of the motor in the output power of the whole system, it can be divided into several types such as weak hybrid (also known as micro hybrids), mild hybrid, moderate hybrid, heavy hybrid (also known as full hybrids, strong hybrids) and plug-in hybrid.

(1) Weak hybrid (weak hybrid) power system

The weak hybrid system transforms the starter of the traditional engine to form a belt-driven integrated starter/generator BSG (belt-driven starter generator). The motor is used to control the engine to start and stop quickly, so it can cancel the idling process of the engine and reduce fuel consumption and emissions. The power of the motor carried by the weak hybrid system is relatively small, and the motor alone cannot make the vehicle start, and the engine still needs to be involved in the starting process. It is a primary hybrid system. In the weak hybrid system, there are usually two voltages of the motor, 12V and 42V, of which the 42V motor is mainly used in the diesel hybrid system. The fuel saving rate of the weak hybrid system under the urban cycle conditions is generally 5% to 10%.

(2) Mild Hybrid (Mild Hybrid) Powertrain

The mild hybrid system uses an integrated starter and generator (ISG). Compared with the weak hybrid system, the mild hybrid system can not only control the start and stop of the engine with the motor, but also recover part of the energy under vehicle braking and downhill conditions; during driving, the power of the engine can be adjusted between the driving demand of the wheels and the electricity generation demand of the generator. The hybrid degree of the mild hybrid system is generally below 20%, and the representative model is the hybrid pickup truck of General Motors.

(3) Moderate Hybrid (Medium Hybrid) Power System

The mid-hybrid system also uses the ISG system. The system differs from the mild-hybrid system in that the medium-hybrid system uses a high-voltage electric motor. When the car is accelerating or under heavy load conditions, the electric motor can assist the engine to drive the vehicle, supplement the lack of power output of the engine itself, and improve the power performance of the entire vehicle. The mixing degree of this system is relatively high, which can reach about 30%, and the fuel saving rate can reach 20%~30% under urban cycle conditions. At present, the technology is relatively mature and widely used. Honda Motor Co.'s hybrid vehicles, the Insight, Accord, and Civic, are examples of such systems.

(4) Heavy hybrid (heavy hybrid) power system

The heavy hybrid power system uses a 272~650V high-voltage motor, and the hybrid degree can reach more than 50%, and the fuel saving rate under urban cycle conditions can reach 30%~50%. Its characteristic is that the power system is based on the engine, and the power battery is the auxiliary power source. The power of the motor used is more powerful, which can fully meet the power requirements of the vehicle at start and low speed. Therefore, the heavy hybrid vehicle does not need to start the engine whether it is starting or driving at low speed, and it is fully capable of relying only on the motor, and the driving performance at low speed is closer to that of pure electric vehicles. In the operating conditions where the vehicle requires a large driving force such as rapid acceleration and climbing, the motor and the engine provide power for the vehicle at the same time. With the advancement of motor and battery technology, the severe hybrid system has gradually become the main development direction of hybrid technology. Toyota's hybrid model, the Prius, uses a heavy-duty hybrid system.


Hybrid car

Due to the structural differences of various HEVs, different control strategies are required to regulate and control the flow of power flow between different components. The purpose is to achieve the best fuel economy, lower emissions and better driving performance. The following points should be considered when designing the control strategy of HEV:

①Have the best fuel economy and the best emission index, which is usually achieved by optimizing the design of the engine's working point, working line or working area;

②According to the energy requirements of driving conditions, rationally distribute the energy flow from the engine and the battery;

③ Under complex driving conditions, minimize the fluctuation of engine working speed and the number of engine starts and stops, and avoid the engine running below a certain speed and load (the engine should be turned off when it is lower than a certain speed);

④ Ensure that the battery is always in a suitable state of charge (SOC) and the voltage of the battery is within a safe range so that the battery has a good service life.