Electronic knowledge of the electronic differential lock

Introduction

Electronic differential lock English called ElectronicDifferentialSystem, it is an extended function of ABS, used to identify whether the car's wheels lose their friction, so as to control the car's skid wheels.

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The working principle of EDS is relatively easy to understand. Because the differential allows the wheels on both sides of the drive shaft to rotate at different speeds, if the wheel on one side of the drive shaft slips or floats, it will cause the other side wheel to completely lose power when the EDS electronic differential lock passes the ABS system. When the sensor detects the phenomenon of different speeds of the wheels on both sides due to wheel slippage or suspension, the ABS system brakes the wheel on the side of the skid so that the driving force effectively acts on the non-skidding side. Wheels ensure that the car starts steadily. When the vehicle's driving conditions return to normal, the electronic differential lock stops.

When the two wheels of the drive shaft of the vehicle are respectively started on roads with different adhesion coefficients, for example, one driving wheel is on a dry asphalt road and the other driving wheel is on the ice, the EDS electronic differential lock passes through the sensors of the ABS system. Automatically detects the rotation speeds of the left and right wheels. When the rotation speeds of the wheels on both sides are different due to wheel slip, the EDS system brakes the wheels on the side of the skid through the ABS system, so that the driving force effectively acts on the non-skid wheels. The side wheels ensure that the car starts steadily.

In the domestic golf GTI we heard a new term: XDS electronic differential lock. On the official website, manufacturers advertise their products in this way: "GTI's excellent dynamic balance in corners also benefits from another magic weapon - XDS vehicle dynamic electronic differential lock, XDS built into the ESP system can avoid the inside drive The skidding of the wheel effectively improves the understeer of the front-wheel drive; the large-size brake disc provides extremely excellent braking performance, providing a safer protection for the driver's ultimate speed.” The XDS system seems to be very powerful. Of course, the propaganda of the manufacturers needs to be viewed in a dialectical manner. Moreover, there are probably many people who do not understand why the lack of understeering can be avoided by avoiding the skidding of the inner drive wheel.

Measure the merits and demerits of a car, in addition to see the linear acceleration capability, the key is still in the performance of the curve, high-performance models equipped with ordinary differential, high-speed cornering will produce a lot of problems. In daily driving, we believe that the four wheels are always close to the ground, and the difference in grip between the left and right wheels is almost negligible, and the differential distributes the power evenly to the left and right wheels. However, the situation becomes more complicated when driving hard.

Note: The “inner wheel” and “outer wheel” mentioned below refer to the drive wheels on both sides, excluding the driven wheels.

● Problem 1: Loss of power

Careful drivers will have the feeling that an important factor affecting the dynamic performance of vehicles is the so-called weight transfer. For example, why is the front brake disc of the car larger than the rear brake disc? Because the inertia of the vehicle causes the body to lean forward during strong braking, most of the weight of the body moves to the front axle, so the braking force of the front wheel must be large, and the rear axle actually only shares a small part of the braking work.

In the same way, when a car turns at a high speed, it will generate a lot of centrifugal force, and the faster the turning speed, the greater the centrifugal force will be. The centrifugal force will cause the weight of the car body to be transferred to the outside of the bend. The members of the car can clearly understand the outward. The force of the fling, and we see from the outside of the body performance is that the outer side of the suspension is compressed, and the inner side of the wheel can be almost off the ground, grip is also a sharp decline.

At this time, the shortcomings of ordinary open differentials began to be exposed, that is, the torque is always distributed evenly to the left and right axles and tends to the side with less resistance. Specifically for high-speed cornering vehicles, because the resistance of the inner drive wheel is so small that it is almost floating, the torque acting on the side axle is greatly reduced compared with the straight travel. The insufficient grip may even cause the wheels to start to slip. On the other hand, a wheel with a large grip on the other side has a similarly low torque, which is equal to the loss of power for the driver. This is a bit similar to the situation where we left a wheel off the ground during a four-wheel-drive system test: the vacant wheels spin madly and the vehicle just stops there, barely creeping. The difference is that the driving wheels may not be completely separated during high-speed cornering. The ground and the duration is very short. Some people may think that this phenomenon will only last for a few seconds, will not have any effect on the control, but in the race against the seconds, each corner of the difference of even 0.1 seconds may determine the victory or defeat.

● Problem 2: Insufficient forward steering

Now we talk about oversteer and understeer. The current general view is that the front-wheel drive tends to understeer and the rear-wheel drive tends to over-steer, which is mainly related to the weight distribution of the front and rear axles. Most of the front-wheel drive has both the front and rear axles and the front and rear axles are in front of the front axle. Allocation is already "top-heavy," and the forward movement of the weight in the curve will increase the load on the front axle. This may cause the front wheel to break through the limit of the grip and lose the effect of steering. The body no longer turns in the desired direction but The tangent direction of the turning arc is what we usually call “pushing head”.

Pushing the head is obviously unfavorable for raising the cornering speed. Can we minimize the effect of understeering? For the front-wheel drive, it may be more difficult to change the inherent deficiency of the weight distribution. From another perspective, it is possible to create a yaw moment.

What is yaw moment? To give a simple example, everyone has a scull boat, what will we do when we turn? If it is turning to the left, it will force the right pulp, which will produce a yaw moment to the left, and the ship will turn left. The same principle can be used for turning a vehicle. Have you ever observed how the tank turned? With the differential on both sides of the track, the tank can even rotate in place.

Returning to the car, there is now a system that enhances manoeuvrability by applying a yaw moment, the most typical of which is the SH-AWD system of Acura, the fourth-generation four-wheel-drive system of Jude, and the four-wheel drive models of Audi and BMW. Shaft equipped with active torque distribution devices and more. They all use the same principle, that is, when the vehicle is turning, it actively distributes the torque to the outside wheels and generates a yaw moment inside the bend to help the vehicle corner.

● The role of XDS electronic differential lock:

XDS electronic differential lock is to solve the above two problems and appear. To put it plainly, a limited-slip differential that is simulated by the brakes of an electronic system. Its working principle is to apply braking to the inner drive wheel with very low grip when the vehicle is in the limit state. According to manufacturers, XDS will apply brake force of 5-15 bar to the brake disc, 1 bar is 0.1 N per square millimeter, and the equivalent square centimeter is 10 N, that is, 1 kg per square centimeter. Its principle is similar to that of the electronic braking of some off-road vehicles.

The act of giving a braking wheel brake has two effects:

1. The increase of the resistance of the inner skid wheel allows the engine to transmit more torque, which means that the wheel with better grip on the outside has obtained more torque and improves the cornering performance of the vehicle;

Second, because the grip on the inside wheel is small and the outside wheel grip is large, even though the torque is still evenly distributed, more torque is applied to the ground through the outside wheel for the vehicle, resulting in a yaw that points into the bend. Torque helps the vehicle turn to some extent, inhibiting understeer.

● Is XDS really amazing?

Objectively speaking, XDS can really improve the handling of vehicles, but if you use "magic" to describe it, it is obviously exaggerated.

First of all, we can see from the description of the official website, XDS is based on the extended function of ESP, today's mainstream ESP system already has the ability to independently brake four wheels, that is to say in the hardware With the XDS condition, the key lies in the software upgrade.