[YesAuto Technology] On February 17, Audi released a new four-wheel drive system, which it called quattro ultra. In the international market, all Audi models with the word “ultra” at the end of the name mean they have better fuel economy. Therefore, the most fundamental purpose of quattro ultra is not to improve the dynamic performance of the vehicle, but to pursue lower fuel consumption. Let us take a look at how this brand new quattro system does this.
The author of this article is Chen Zhengyi, an overseas part-time editor of Autohome. He is currently studying at TU Braunschweig, Germany, studying for a master's degree in automotive engineering. The previous Honda Civic Type R and BMW's new X1 test drive articles were also Chen Zhengyi's work. He and his classmate, like Guo Xiang, who is also a part-time editor of Auto House Germany, are low-key and didn't put his own photos here. Below, the text continues.
The new quattro ultra system will be deployed on Audi's vertical platform, that is, the MLB platform. The first matching gearbox is the 7-speed wet dual-clutch gearbox code-named DL382 and the 6-speed manual gearbox. We will first see it on the new A4 allroad quattro, which will be officially launched in mid-2016, and then will be promoted to the A5 and the new generation of Q5.
● What is the difference between “new quattro” and “old quattro”?
Unlike all previous quattros used on Audi vertical platforms, this is the first time that Audi has completely abandoned the purely mechanical central differential and instead used electronically controlled multi-plate clutches to distribute the power between the front and rear axles. . Simply put, the previous torque-sensing mechanical central differential is gone.
It should be noted here that although the Torson C-type and crown gear type central differentials have friction linings, their friction linings are used to limit slip. When the power is normally transmitted, there is a gap between the friction linings. There is no friction. In addition, both are purely mechanical structures and do not require electronic systems for control. These two points are the most obvious difference from the electronically controlled multi-disc clutch quattro mentioned below.
-Structural composition of quattro ultra
The quattro ultra system consists of two parts. Its core is a central electronically controlled multi-disc clutch, located on the rear side of the gearbox, with five to seven pairs of clutch plates. There is also a jaw clutch on the rear axle. Their functions are different, let's look at them step by step.
First of all, let's talk about this central multi-disc clutch. Since the quattro ultra system is matched with Audi’s longitudinal S tronic dual-clutch gearbox and 6-speed manual gearbox, this system is equivalent to adding a set to the output shaft of the gearbox on the basis of the front drive. Multi-plate clutch.
Some friends may be able to tell at a glance, this is similar to the Haldex four-wheel drive system structure used on most transverse engine platforms. Indeed, the central multi-disc clutch of the quattro ultra system is the same as that on Haldex. It adjusts the torque delivered to the rear axle by adjusting the compression of the multi-disc clutch. The difference is that quattro ultra uses a motor to adjust the degree of compression and is arranged on a vertical platform.
Next, let's look at the role of this jaw clutch mechanism on the rear axle. Many friends may think that there is a jaw-type differential lock on the rear axle when they first see it, but it is not a differential lock. Because it can be seen from the structure that the jaw differential lock connects the half shaft and the rear differential housing as a whole, so that there is no difference in speed between the half shaft and the housing, forming a hard shaft.
The jaw clutch on the rear axle of the quattro ultra is set on the right half shaft, which separates the right half shaft into two sections, and is not a differential lock arrangement. The role of this clutch is to control the on-off of the power connection between the rear axle and the central drive shaft.
Why is there an extra set of jaw clutches on the rear axle? When the system is in two-wheel drive mode, disengage the central multi-disc clutch and the dog clutch on the rear axle at the same time, which can stop the movement of the central drive shaft and the rear differential housing, and the rear wheels only drag the two half shafts. The rotation of its gears reduces the parts that need to be driven when the vehicle is running, and also reduces the energy loss caused by the moment of inertia and friction, thereby further reducing fuel consumption.
Speaking of this, friends who may be familiar with it will think of the disconnectable Twinster system supplied by GKN on Land Rover Aurora and domestic Freedom. Yes, the working principles of the two are similar, except that Audi puts the dog clutch on the rear axle and the multi-disc clutch behind the gearbox, which has been improved to a system suitable for vertical platform use.
● Advantages and disadvantages of quattro ultra system
What are the benefits of this system? The biggest benefit is naturally fuel saving. Compared with the quattro system of the previous vertical platform, the quattro ultra can achieve a completely continuous and stable 100% front-wheel drive mode. At the same time, the central drive shaft and the rear differential housing can also stop rotating, which was not possible before. According to the test results of Audi in the German headquarters, compared with the previous full-time four-wheel drive quattro system, the new four-wheel drive system can save 0.3L of fuel per 100km.
Secondly, the quattro ultra system no longer uses mechanical torque-sensing control. The on-board computer will calculate and evaluate the vehicle condition every 10 milliseconds based on the information of the ESC control unit and driving mode selection system on the vehicle to determine whether the four-wheel drive needs to be activated. When in a corner, it will predict whether the front wheel will reach the grip limit after 0.5 seconds. If it reaches the limit, the four-wheel drive system will be on standby and intervene at any time. At the same time, it can also predict in advance whether the front is uphill or a curve by accepting navigation data, and pre-select the degree of four-wheel drive intervention.
In addition, compared to the previous Torson and quattro systems with crown gear structures, since both are torque-sensitive limited-slip differentials, if there is no brake assist, as long as one wheel is suspended, the engine power will be affected. Lost from the suspended wheels (note: here is power, not torque), the rest of the wheels are difficult to obtain power to make the vehicle out of trouble. In addition, if it is paired with a 7-speed S tronic dual-clutch gearbox, the braking system cannot perform high-intensity braking on the suspended tires. Therefore, with the torque-sensing limited-slip differential, the off-road performance can be described as very weak. This can be fully reflected in the video of the 2014 A4 allroad quattro driving in the wild.
And this kind of multi-disc clutch type four-wheel drive system can make the central drive shaft form a hard shaft (assuming that the clutch disc is strong enough) when the clutch disc is fully compressed, thus realizing the effect of differential lock. This means that when the wheel on one of the two rear axles is suspended or slips, the torque will be automatically distributed to the gripping tires. In this case, you can get out of trouble without relying on the assistance of the brake system. Therefore, Audi first installed this system on the new Audi A4 allroad quattro. Is it to improve the insufficient off-road performance of the 2014 A4 allroad quattro? We will analyze this point later.
Of course, this brand new four-wheel drive system will certainly have some shortcomings. Because the torque distribution principle of the front and rear axles of the quattro ultra system is the same as that of the Haldex system. Therefore, although this type of system can allocate 100% of the torque to the rear wheel under extreme conditions (the front wheel is completely slipped or suspended), but in actual conditions on a normal paved road, when the four-wheel grip conditions are the same The maximum torque of the rear axle can only reach 50%. The asymmetric torque distribution of the quattro system of the previous Torson and crown gear structure cannot be realized. That is to say, during normal driving, the dynamic performance of the body will be more forward-driving, and lose the characteristics of the previous partial-rear-drive.
In addition, because the multi-plate clutch undertakes the task of power transmission, and because the clutch cannot be compressed frequently (after the clutch is compressed, the speed difference between the front and rear axles in the curve cannot be balanced), in practice, the central clutch plate often There will be a certain slip rate. Since there is sliding, it will generate heat, so although this structure can theoretically provide a stronger ability to get out of trouble, in fact, due to heat and clutch strength, long-term heavy off-roading is not the strong point of this system, so we believe that quattro The ultra system is of limited help to the new A4 allroad quattro model in improving off-road performance.
to sum up:
Audi officials have also stated that, in fact, the quattro ultra is not used to completely replace the traditional mechanical structure of the quattro system. The traditional quattro four-wheel drive system will still be used in high-end models. However, there is also news that Audi may use this The system is applied to the new RS models. Audi’s approach is believed to be a compromise of the manufacturers to the increasingly stringent emission regulations in Europe and even the world. After all, most people don't use the extreme performance of the four-wheel drive system, but more peace of mind when encountering rain and snow. But in the eyes of car fans, due to the change from the full-time four-wheel drive to the timely four-wheel drive, the attractiveness of the Audi quattro gold brand will drop a lot. Whether Audi's decision is right or wrong is waiting for the market to test it.