In this era of rapid development of new energy vehicles, various new terms emerge in an endless stream. BYD Tang's "Fast Electric Four-wheel drive", Lexus's "E-FOUR", Mitsubishi's "Double Motor 4WD", Tesla's "Double Motor All Wheel Drive" and so on. In fact, these are the aliases of electric four-wheel drive. The core performance of the four-wheel drive technology is to increase the grip of the car and to walk freely on any road surface. The electric four-wheel drive, on the physical level, is different from the traditional four-wheel drive, but it can achieve the same effect. Today, the protagonist of this article is the electric four-wheel drive technology, while Xiao Bian also recommended a four-wheel drive electric car about Tesla, let's take a look.

The traditional four-wheel drive system is actually the ordinary fuel vehicle to achieve four-wheel drive through the main traditional components such as the transfer case, the drive shaft and the differential. The engine is the only source of power. In the traditional four-wheel drive system, the parts of the transfer case, the drive shaft and the differential must exist. They mainly realize the transmission and distribution of power, and these parts can be partially or completely canceled in the electric four-wheel drive system. .

In an electric four-wheel drive system, multiple power sources are allowed, either a conventional engine or a motor, but there is at least one motor that can drive the vehicle in an "electric four-wheel drive" system. Existence eliminates the need for power distribution and transmission of parts. Among them, the type of differential (differential lock) determines the handling performance, off-road performance, and off-hook performance of the traditional four-wheel drive system. Simply put, the hardware determines the performance. The electric four-wheel drive system does not rely solely on the strengths and weaknesses of the components to determine its performance. The macro world mainly controls the torque distribution between the front and rear axles and wheels through the ECU. In fact, the software determines the performance. The electronic auxiliary system popularized in most traditional SUVs can also be applied in new energy vehicles. Considering that the performance of electronic auxiliary systems is further strengthened, the utilization rate in new energy vehicles should be high in the future.

1, single motor with transmission

The basic structure of the shaft-only electric vehicle is roughly equivalent to the four-wheel drive we played in childhood, and the four-wheel drive can realize a simple "full-time four-wheel drive" through a single drive shaft (Fig. 1). This method can also be applied to pure On the electric car.

Take the BMW X5 Xdrive40e as an example (although not pure electric vehicles), it has only one motor, which is placed between the engine and the transmission, and relies on the traditional four-wheel drive system structure to realize the four-wheel drive. The biggest advantage of this structure is that the structure is simple. Because the motor is placed in front of the transmission, the matching work of the vehicle is mainly concentrated on the smoothness of the engine and the motor. The torque distribution between the front and rear bridges only needs the ordinary X5. The logic can be. Both the R&D cost and the manufacturing cost can be greatly controlled, which is a small profit. However, it is followed by a larger vehicle weight and lower transmission efficiency, which will affect the vehicle's pure electric range to a certain extent.

At present, this technology is mostly used in hybrid vehicles of European car companies, especially the luxury car group (ABB, etc.). The main reason is that the technology is relatively simple and easy to implement, and European manufacturers generally agree that hybrid technology is only a short transition technology. Its R&D orientation is more to pure electric, hydrogen or bio-energy development. It can even be considered that most of the hybrid technology and electric four-wheel drive technology of European car companies are only dealing with the EU's super-strict emission reduction policies.

2, dual motor all-wheel drive

A motor is arranged in each of the front and rear drive axles, and a conventional differential is used to realize the four-wheel drive. Currently only Tesla's cars and SUVs use this structure. The Tesla Model S P85D is the first mass-produced pure electric vehicle to use this type of arrangement. Tesla engineers are able to add a motor like the rear axle to the front axle of the ordinary Model S, but it is quite equivalent. Excellent sports performance, especially acceleration performance, the official 100 km acceleration of the Model S four-wheel drive version is 2.8s, the Tesla dual motor drive system is shown in Figure 2.

In theory, as long as Tesla is willing to adjust the control logic, this structure can achieve any kind of four-wheel drive (full time, time-sharing, timely), not to mention simple predecessors and stamina.

The Tesla Model X is designed with the Tesla Model S platform, so its performance and off-road performance is mainly determined by the electronic aid system, which is basically used by mainstream urban SUVs. Of course, we are not sure if the Tesla Model X will be equipped with a conventional differential lock, and if possible, the off-road performance of the electric car is quite worth looking forward to.

3, hub motor or wheel motor (four motors)

The difference between a hub motor or a wheel motor is that the former hub is the rotor of the motor and the bearing seat (shofar) acts as the stator. The latter is that the ordinary motor is placed beside the wheel, and the drive shaft is required to drive the vehicle. The explosion diagram of the hub motor is shown in Figure 3.

This structure can also be constructed in the same way as a two-motor. In principle, any type of drive can be realized. However, due to the high cost, there is no manufacturer to introduce a production car, and more as a test vehicle or a modified vehicle. For example, Babs installed four wheel motors for the Mercedes E class.

The four-wheel motor can save a lot of space, and each wheel is an independent power unit, which is faster and more efficient, which is not possible with the traditional four-wheel drive technology. If the reliability of the electrical system is comparable to the traditional mechanical four-wheel drive system, the SUV with hub motor or wheel motor can theoretically completely surpass all current hard-core SUVs.

Overall, the electric four-wheel drive system can cancel part of the transmission parts to improve space utilization efficiency and transmission efficiency. The most intuitive is the degree of protrusion and fuel consumption (power consumption) of the rear platform.

According to the vehicle positioning property, the driving method of the vehicle (precursor, rear drive) can be easily changed. Taking an electric vehicle with front and rear motors as an example, in the same vehicle, the two different requirements can be met theoretically by adjusting the control logic. At present, the conventional four-wheel drive system cannot be easily realized.

The electric four-wheel drive system relies entirely on electrical components, and its reliability and stability performance are worrisome in a variety of harsh field locations. In particular, when the battery is at a low temperature, it will cause a loss of power, which will undoubtedly have a great impact on vehicles that are often used in cold regions. Therefore, the electric four-wheel drive system is only suitable for cars and urban SUVs for the time being.

With the advancement of technology, especially the development of new energy, the four-wheel drive technology of automobiles began to evolve from the traditional mechanical four-wheel drive to the electronically controlled four-wheel drive. Although they belong to the same electric four-wheel drive, there are different types. The following are three different four-wheel drive solutions in practical applications.

1. Lexus NX 300h:

This car has two versions of the front and four wheels. Its four-wheel drive version is not equipped with a set of four-wheel drive system. The power of the front axle is distributed to the rear axle. Instead, the front axle is driven by the engine and the electric motor, and the rear axle is driven by a separate motor. A four-wheel drive. But even in this form, the NX 300h is still a timely four-wheel drive model, usually only driven by the front wheel, the rear axle motor will only intervene when the rear wheel is slipping. Plug-in hybrid vehicle with front wheels powered by engine and motor drive

2, Volvo V60 Plug-in Hybrid:

The car is moving and the rear wheel is also driven by a separate motor. Unlike the NX 300h, the V60 is a full-time four-wheel drive that will always output power.

3. Tesla Model S P85D:

Because it is a pure electric car, its four-wheel drive mode is simpler, that is, the front and rear axles are each equipped with an electric motor. Compared with the traditional design of the engine + mechanical four-wheel drive, the electric drive's four-wheel drive achieves complete electronic control, and the torque adjustment is more precise.

As an emerging technology, electric four-wheel drive is far from being developed into its final form, and there are many aspects that can be improved. The "in-wheel motor" distributed in four wheels at the same time may be the final form of electric four-wheel drive.

Taking a traditional fuel car as an analogy, the engine is like a "teacher", and the four wheels are like "four children." It is difficult for a "teacher" to take care of "four children" at the same time, but it is much better for two "teachers" to manage two "children". So why didn't the four "teachers" do this thing in the end? As the name implies, the wheel motor is mounted on the hub, and the four hub motors are responsible for powering the four tires.

This form can already meet the needs of various road conditions. The hub motor can even get 100% power for a single tire, while other tires can even reach 0%. In fact, the hub motor was proposed and implemented by Ferdinand Porsche in 1900, but it was not affected by the technology of the battery at the time. Nowadays, the hub motor seems to be the ultimate form of electric four-wheel drive.

For the body structure of the electric car, the Tesla electric car is not applied to the traditional gearbox as a medium for transmitting power in the transmission system, but is simplified as a torque converter, thereby Deliver power for efficient and direct delivery. Moreover, as an electric four-wheel drive system, the vehicle is not equipped with a main drive shaft connected to the front and rear axles, but two sets of electric motors of different specifications are used to drive the front and rear wheels respectively, thereby reducing the power transmission process. Unnecessary loss in the process, thereby improving the utilization of electricity, and ultimately improving the cruising range of the vehicle. 　　

Currently, Tesla includes three electric vehicles, the Model S, Model 3 and Model X, which cover the coupe, sedan and SUV models. In the future, Tesla is expected to launch a version with a full-time four-wheel drive system for the above three models. Since the electric vehicle model is simpler in the design of the power configuration, it is only necessary to add two half shafts on both sides of the front axle and an additional torque converter, thereby only slightly increasing the quality of the vehicle body. At the cost, the goal of improving transmission efficiency was achieved, and the cruising range of the three models exceeded 320 kilometers.

The Tesla model with a full-time four-wheel drive system will be officially released in 2018, and it is expected to introduce a new electric car model, Model C. The new electric car is expected to have a cruising range of 150 kilometers and is priced at only 15,000 euros, compared to 109,524 yuan. At the same time, after the release of Model C, Tesla is also expected to launch its new electric roadster Model R to further improve its product line.

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