● Speaking before: Why do you need four-wheel drive technology? To understand the complex technical characteristics of four-wheel drive and its performance, we must first have a simple understanding of the development history of four-wheel drive. Four-wheel drive technology is mainly divided into off-road four-wheel drive and highway four-wheel drive. Off-road four-wheel drive was first developed from the war. In order to strengthen the mobility of front-line infantry and commanders in World War II, the US military developed a light four-wheel drive vehicle-Jeep Willis. It adopts the design structure of part-time four-wheel drive. At that time, there was almost no design concept of full-time four-wheel drive. The reason why the rigid transmission shaft with fixed power cannot be distributed to the front and rear wheels is because when the car turns, the trajectories of the four wheels are different and the radius of the turning arc is also different. Therefore, in order to distribute the power rigidly to the front and rear wheels, it is necessary to keep the speed of the front and rear wheels exactly the same, which has no harm when driving in a straight line. On the contrary, it can improve the effective grip of the tire, but the problem comes out when turning. "Each wheel has a different curve." Because each wheel has a different curve when turning, it means that the speed of each wheel cannot be the same. If the power of the engine is rigidly distributed to the front and rear wheels through the transmission shaft, then the rotation speed of each wheel can only be exactly the same, so when steering, the front and rear wheels will interfere with steering. If the road surface is dry, it will produce braking force to stop the car from moving forward, which is what we often call steering braking. Of course, there is not only a speed difference between the front and rear axles, but also a speed difference between the left and right wheels. We know that the drive axle of a car is equipped with a differential. The function of the differential is to adjust the speed difference between the left and right wheels to adapt to different steering trajectories. Because there is no differential limit, the power of the engine is automatically distributed to the wheels with less resistance by the differential, so this differential is called open differential. Almost all two-wheel drive cars are equipped with this differential. Then, if a car is four-wheel drive, it is not hard to imagine that if you want to distribute power to the front and rear drive axles rigidly at 50: 50, you can't stay in the state of four-wheel drive for a long time, and you must switch back to two-wheel drive when turning on the road with high friction coefficient. If we want to ensure that all four wheels can get power all day, then we need a central differential to distribute power to the front and rear drive axles, just like the open differential of a two-wheel drive car. Of course, in addition to the central differential distributing power to the front and rear axles, the front/rear differential is also needed to distribute power to the left and right wheels, which is the prototype of full-time four-wheel drive. The four-wheel drive three-open differential is meaningless for off-road and improving passability. As we know, the function of open differential is to distribute the engine power to the wheels with little resistance. If a car uses three open differentials to adjust the speed difference, then if the resistance of one wheel is the least, the power will be 100% transferred to this wheel. This situation is very common in cross-country and through bad roads. Because when a car passes through a bad road, it is easy for the wheels to slip off the ground or in the mire, if it is equipped with a full-time four-wheel drive with a front, middle and rear three-open differential, it will be impossible to gain traction and move on. So this four-wheel drive is meaningless. As a part-time four-wheel drive like Jeep Willis, because the front and rear axles are rigidly distributed with power, the distribution ratio is 50: 50 in any case, so even if the front wheels slip, the rear wheels can provide power. For the full-time four-wheel drive with three-open differential, this problem can only be solved by other means. "Differential limiter" Differential limiter was born in this way. As we know, since the open differential is characterized by transmitting power to the wheels with small resistance, if we create a resistance to the slipping wheels so that they cannot rotate, then the power can naturally be transmitted to the wheels that do not slip (still have grip), thus getting rid of the dilemma of anchoring; Of course, there is another scheme, such as the part-time 4wd, that is, once a wheel slips, we directly cross the differential and rigidly connect the front and rear drive shafts (locked), then the power will be completely distributed according to the ratio of 50: 50, and then we can get some power without slipping wheels and get rid of the slipping dilemma. These two differential restrictions are the most commonly used four-wheel drive technologies. For full-time four-wheel drive, the key to determine the technical performance of four-wheel drive often depends largely on the design of differential limit. As several well-known automobile manufacturers in the world, they have different understandings of four-wheel drive and have their own unique designs. Let's look at the differences between these methods. ● Mercedes-Benz 4MATIC: Mercedes-Benz named its four-wheel drive technology 4MATIC. This system was first adopted only on the G-class of Mercedes-Benz professional off-road vehicle. Of course, the G-class at that time only considered the four-wheel drive system for passing, and the Mercedes-Benz 4MATIC at that time was also very different from now.
The popular concept of full-time four-wheel drive was not introduced into Mercedes-Benz G-Class in 1980s, but an early part-time four-wheel drive system. But this part-time 4wd does not rely on the driver's operation to switch like Jeep Willis. Instead, the wet multi-disc clutch is used to control the on-off of the front axle power. When the car is running normally, it is actually driven only by the rear wheel, because the central coupler is kept off under the control of the computer at this time, and the power is transmitted to the rear wheel at 100%. When the car is turning, the computer will measure a steering angle through the steering angle sensor, and then calculate the theoretical speed of the front and rear wheels through this steering angle. If the rear wheel speed matches the front wheel speed (the difference is within the allowable error range), it is regarded as normal steering. If the speed difference between the front and rear wheels exceeds the normal range, the computer will judge that the rear wheel has started to slip at this time, and then automatically control the central viscous coupler to open, thus sharing part of the power to the front wheel. At this time, the power obtained by the front wheel is only about 35%, and its purpose is to keep the rear wheel from slipping. If the rear wheel is still slipping at this time, the computer will judge that 35% of the power is not enough to get the car out of the slipping situation, thus automatically locking the multi-disc clutch. At this time, it is equivalent to rigidly connecting the front and rear drive axles and transmitting power at a fixed ratio of 50: 50. From another point of view, it is equivalent to the differential being locked by the differential lock. Of course, this method can only achieve 50: 50 power distribution before and after. If 50% of the power still can't pull the car out of the mud pit, it can only be helpless. However, many years later, with the launch of the second generation of 4MATIC, the performance of Mercedes-Benz's four-wheel drive system has been qualitatively improved. This new generation of 4MATIC four-wheel drive system, which is still in use today, is actually a full-time four-wheel drive system with three open differentials: front, middle and rear. In fact, the design of three differentials is not unusual, but its core lies in differential limiting technology. Mercedes-Benz has introduced a brand-new concept called "4ETS" technology, which is somewhat similar to the PSK technology introduced by Porsche on the 959 model. As we said before, the advantage of the open differential is that it can automatically adjust the power distribution and automatically distribute the power to the wheels with little resistance. But its disadvantage is also obvious, that is, once a wheel loses its grip, the vehicle will be in trouble. 4ETS uses the automatic distribution of braking force (EBD) function of ABS to realize differential limit. There is a simple reason. As we know, the biggest advantage of 4-channel 4-sensor ABS is that it can realize the automatic distribution function of braking force, braking the wheels that need to be braked one by one, instead of braking all the wheels at the same time. The brake on each wheel is controlled by a solenoid valve, which can be in three States under the control of a computer: pressurized state, balanced state and depressurized state. So as to realize the independent braking of the wheels one by one, and all this can be automatically controlled by a computer. Then when one wheel of this full-time four-wheel drive vehicle slips, the computer can control ABS to brake the slipping wheel, thus limiting its idle speed. In this way, the differential will not transmit power to this slipping wheel, but to the other three wheels that are not slipping. If the braking system locks the slipping wheel, the other three wheels can get all the power, which means that each other wheel can get 33% power. If three wheels of a vehicle slip, only one wheel can get grip. Similarly, 4ETS can also generate braking force on these three wheels to limit their slip, so that the power 100% can be transferred to the non-slip wheel, thus freeing the vehicle from difficulties, but the probability of all three wheels slipping is very small. Of course, if all four wheels slip, the gods can't save you. Another advantage of 4MATIC is that it can improve the active safety performance of the car at high speed. We know that the most maddening thing about high-speed driving is that the tires lose grip and the car loses control, which is especially common on wet roads. With the help of 4MATIC, the car can drive better within the safe driving limit. However, this is different from the protection function played by ESP, but the principle is somewhat similar. As we know, the method of ESP to ensure that the car will not get out of control at high speed is to keep the car within the limit range by reducing the throttle opening (reducing the speed) and braking the wheel that may slip once the computer detects the signs of a wheel slipping. But all this is passive, because it takes time to reduce the throttle opening to slow down, which is equivalent to using the engine brake to slow down the car without stepping on the throttle. The braking of ESP will lose power in vain. For 4MATIC, these problems have been solved. The same is true for braking a single wheel that may lose grip, but the situation is different. Because of the adoption of three open differentials, when braking the wheel that will slip, the power is not lost, but transmitted to the other three wheels through the differentials. Because the 4ETS technology of 4MATIC can dynamically adjust the torque transmitted to each wheel within the range of 0- 100%, the reasonable distribution of driving force is greatly optimized, thus ensuring the active safety of the vehicle when driving at high speed, and the speed and limit of cornering can be higher. Of course these are all theoretical conclusions. We know that frequent braking will consume a lot of power and make the braking system hot. But the experiment shows that this kind of fever is not terrible at low speed, but the energy loss can not be underestimated at high speed. Therefore, 4MATIC low-speed off-road is its strong point, and we need to adopt another way to improve highway performance. Therefore, aiming at the weakness of 4MATIC highway performance, BMW's Xdrive came into being.