What are the coupling types of hybrid electric vehicles?

A power coupling device is a mechanism in a hybrid vehicle that produces a certain influence relationship between the output of multiple power sources and the power output of the entire vehicle. For a common oil-electric hybrid vehicle, it refers to a device that can couple the power of the engine and the motor. According to the different power coupling modes, hybrid electric vehicles can be divided into series, parallel, hybrid and traction combined. The series structure is the simplest, the parallel is the second, and the combined and traction combined is the most complex, and it is also a hybrid. The direction of automobile development.

What are the coupling types of hybrid electric vehicles?
power coupling

1. Function of power coupling device
Although there are big differences in the power coupling modes of hybrid electric vehicles, their functions are basically the same. In summary, there are the following items.
1). Power coupling function
Realize the combination of speed, torque and power of multiple power sources to form the power to drive the vehicle. The output power of each power source cannot interfere with each other, and each power source can drive the vehicle alone or can be driven by several power sources together, without affecting the transmission efficiency. When necessary, it can also decompose the power output by a power source. The driving power generation mode is to divide the power of the engine into two parts, one part drives the vehicle, and the other part drives the motor to generate electricity.

2). Energy feedback function
Regenerative braking is one of the four energy-saving ways for hybrid electric vehicles. It uses the kinetic energy of the car to drive the motor to generate electricity. This process needs to maintain the mechanical connection between the drive wheel and the motor and disconnect the connection with the engine. The power coupling device should realize this connection during regenerative braking.

3). Mode switching function
The power coupling device should be compact in structure, closely matched with other parts of the power transmission system, convenient and reliable in control, capable of easily implementing multiple driving modes and ensuring smooth and impact-free mode switching.

4). Accessibility
The power coupling device should be able to meet the low-speed and high-torque requirements of hybrid electric vehicles when starting, and avoid the energy loss consumed by traditional vehicles on the starting clutch: In addition, the device can also use the reverse characteristics of the motor or change the direction of the engine torque. The reverse function, and then cancel the reverse gear mechanism of the transmission.
The first two of the power coupling device are basic functions, and the latter two are advanced functions that can only be achieved when the hybrid electric vehicle develops to a certain stage. At present, the power coupling device of hybrid electric vehicles basically realizes the first two functions, but the latter two functions have not yet been realized.

2. Classification of power coupling devices
Hybrid electric vehicles use the following four types of coupling methods to synthesize and output the power of multiple power sources:
1). Torque coupling
The torque output by each power source is independent, the speed conforms to a certain proportional relationship, and the torque output by the power coupler is equal to the linear sum of the torque of each power source. This kind of paste method can be further subdivided into three types: ①Gear coupling type, as shown in Figure 1, the hybrid city buses developed by FAW Group and Second Automobile Group adopt this structure: ②The magnetic field coupling type, as shown in Figure 2, arranges the motor rotor and the engine crankshaft on the same axis, and couples the motor torque and the engine torque together through the excitation control of the motor. Honda’s IMA system and China Changan The group’s ISG system adopts this coupling method; ③Chain or belt coupling type, which synthesizes the power output by the power source through a chain or belt. This coupling method has a simple structure, small impact but low efficiency. The BSG coupling scheme adopted by FAW Pentium hybrid car is this structure.

What are the coupling types of hybrid electric vehicles?
Figure 1 Fixed shaft gear type power coupling device
What are the coupling types of hybrid electric vehicles?
Figure 2 Magnetic field coupling type power coupling device

2). Speed ​​coupling
The speed of each power source is independent, the torque is in a certain proportion, and the speed output by the power coupler is equal to the linear sum of the speed of each power source. This coupling method can be further subdivided into two types: ① Planetary gear coupling. Figure 3 shows the hybrid vehicle planetary gear coupling method developed by Warsaw University of Technology. This coupling method uses a set of planetary gears to couple the power of the engine and the motor. Through the engagement/disengagement of a set of clutches and two sets of brakes, the working mode of the whole vehicle and the switching process of mode 1 are controlled. ②Differential coupling type. Figure 4 The differential coupling hybrid car developed by Hunan University based on the diamond car. The driving wheel of the diamond car is located in the middle, and the two steering wheels are respectively located at the front and rear of the car. The car is cleverly “Reverse” uses the automobile differential as the power coupling device, and uses two clutches/brakes to control the working mode of the car and its switching process.

What are the coupling types of hybrid electric vehicles?
Figure 3 Planetary gear coupling type power coupling device
What are the coupling types of hybrid electric vehicles?
Figure 4 Differential coupling type power coupling device

3). Traction coupling type
The front and rear axles are driven by independent power sources, and the coupling of the power sources is realized through the driving force of the front and rear axles. This coupling method has good independence between the front and rear axles and can divide the driving power of the entire vehicle into several levels. A 4-wheel drive hybrid off-road vehicle developed by Changfeng Automobile Company in 2004 uses this coupling method. As shown in Figure 5, the front wheels are driven by electric motors and the rear wheels are driven by engines.

What are the coupling types of hybrid electric vehicles?
Figure 5 Traction-coupled hybrid 4WD off-road vehicle

4). Hybrid coupling
In recent years, there have been designs that use two or more power coupling methods on the same hybrid electric vehicle, which is called hybrid coupling. Toyota’s Pruis, Camry, Ford’s Escape, and many newly developed hybrid vehicles use hybrid coupling. Figure 6 is a schematic diagram of Camry’s power coupling. The engine and the MG1 motor form a speed coupling through the planetary gear at the speed synthesis end, and the power is output from the ring gear. Because the MGI motor has a speed adjustment function, the engine speed and the vehicle speed are independent, that is, ECVT is realized. The coupled power then forms a torque coupling with the MG2 motor, and the power is superimposed and output on the ring gear. This coupling method can bring together the advantages of multiple coupling methods, avoid their shortcomings, realize a variety of working modes, and work closely with the transmission system, so that the advantages of energy-saving and emission reduction of hybrid electric vehicles can be fully utilized, but it is also the most complicated structure. The most difficult power coupling method to control. This coupling method has become the development trend of hybrid electric vehicles.

What are the coupling types of hybrid electric vehicles?
Figure 6 Camry’s dynamic coupling schematic

3. Development trend of power coupling device
From the above analysis, it can be seen that the power coupling device of hybrid electric vehicles is developing in the direction of more complete functions, more complex structure, and more advanced control. Some foreign automobile companies rely on their rich product development experience, exquisite manufacturing technology and advanced control technology to continuously develop new power coupling methods, and the hybrid vehicles introduced mainly adopt hybrid coupling methods.
China is still in its infancy in this regard, and it can only meet the basic requirements in terms of function, and the coupling methods are limited to the first three categories, which is a big gap compared with foreign countries. China has made significant progress in the technology of key units of hybrid electric vehicles. How to use more advanced coupling methods to form hybrid electric vehicles with more complete functions is a top priority.