When the gearbox starts on the ramp, the rolling is usually too steep.
The transmission between AT gearbox and engine is powered by hydraulic torque converter, which is a device that transmits power through gearbox oil. There are two rotating bodies like fans inside, called pump impeller and turbine, which are connected to the engine. The engine drives the pump wheel to rotate, the pump wheel pushes the gearbox oil, and the gearbox oil pushes the turbine, and the power is transmitted to the gearbox. One of the biggest characteristics of hydraulic torque converter is that it can realize the flexible connection of power.
For example, when the engine drives the pump wheel to rotate, the gearbox oil will be thrown out. After the transmission oil hits the turbine, there will be two results. First of all, the turbine end resistance is small, and the gearbox oil can drive the turbine. In fact, the car will move when the brakes are released. Another state is that the turbine end resistance is too great for transmission oil to drive. At this point, the turbine remains stationary. Transmission oil impacts the turbine, then changes direction and returns to the pump impeller. Then the pump wheel continues to throw out the transmission oil, then hits the pump wheel and returns. In this process, the pump impeller can keep rotating and continue to provide power to the turbine, while the turbine can remain stationary. This is what happens when we step on the brakes.
However, when we are driving normally, we definitely don't want to use gearbox oil to transmit power. Due to the large loss, when the vehicle speed reaches a certain level, the clutch inside the torque converter will be directly combined to lock the turbine and the torque converter housing into a whole, so that the power will be directly transmitted to the gearbox through hard connection. The principle of torque converter is very complicated. Here we can intuitively regard the torque converter as the clutch of manual transmission, but this clutch has only two states: semi-linkage and complete combination.
When we start the engine and the gearbox is in neutral position, the internal clutch of the torque converter is in a disengaged state. But because the gearbox is in neutral position, the turbine resistance is very small, so the pump wheel can drive the turbine to rotate, which is equivalent to the engine rotating with the input shaft of the gearbox.
When we step on the brake and put in D gear, the turbine is essentially hard connected to the driving wheel through the gearbox. Because the driving wheel is restricted by the brake and cannot rotate, the turbine will stop immediately. At this time, the engine rotates with the pump wheel and throws the gearbox oil to the turbine. The turbine has been receiving the power transmitted by the pump wheel, and at the same time, it continuously transmits the power to the driving wheel through the gearbox, and the driving wheel cannot move because of the limited braking. At this time, the driving wheel is ready like an arrow. As long as the brake is released, its power will be released and the vehicle can start.
Therefore, after the brake is released, the AT gearbox will start immediately. More specifically, when the brake is released to a certain extent and the driving force is greater than the driving resistance, you can start. In addition, we can control the speed by lightly stepping on the brakes. It's all because of the torque converter. This is similar to the semi-linkage start of manual gearbox.
However, the torque transmitted by the torque converter is proportional to the engine speed, and the higher the speed, the greater the power transmitted. When we release the brake when we start on the ramp, if the ramp is large enough and the torque transmitted by the torque converter is too small to overcome the resistance, the car will slip. At this time, if you want to avoid slipping, you have to step on the accelerator to accelerate and let the torque converter transmit more torque.
Therefore, the at gearbox starts on the ramp, mainly because the engine speed is too low, which makes the torque converter unable to transmit enough power to the driving wheels. Or it can be said that the slope is too big and the power transmitted by the torque converter is not enough to overcome the starting resistance. In short, there is too little motivation. If you want to leave the car, you must transfer more power to the driving wheels. The method is simple: step on the accelerator and increase the engine speed.
At this point, you can start with the handbrake. Don't release the brake after you put it in gear. Hold down the brake first, then step on the accelerator to accelerate. When the speed is high, more power will be transferred to the driving wheels. At this time, you can release the handbrake first. Of course, if the handbrake is not pulled so tightly and the accelerator is not pressed too hard, it is also possible to run forward without releasing the brake.
Wet dual clutch gearbox starts with gearbox control logic.
Wet dual clutch gearbox and engine rely on wet clutch to transmit power. The clutch characteristics of wet clutch and manual gearbox are the same, and there are three States: complete separation, semi-linkage and complete combination. Compared with the torque converter, there is one more state: complete separation! It is this "completely separated" state that leads to the slip of automatic start.
Because many people will not start immediately even if they are in gear, many manufacturers are also very smart: when we step on the brake gear, the clutch plate of the gearbox will not immediately enter semi-linkage, but will remain disconnected. This can reduce clutch wear. When we step on the accelerator, the gearbox computer will control the clutch to enter the semi-linkage state.
This leads to an embarrassing situation: even if the brake clutch is released after gear shifting, it will not engage. It is easy to say that on a flat road, it will start to slip when it encounters a little slope. Because at this time, although the gearbox has been put into gear, the power of the engine has not been transmitted. At this time, you have to step on the accelerator and tell the gearbox, "I'm going to start. Let's go." Then the clutch began to enter the semi-linkage, and the car began to move.
Therefore, it can be roughly summarized as follows: the main reason why an automatic car with a torque converter starts on a ramp is that the slope is too large and the torque converter transmits insufficient power. The ramp start of wet clutch automatic transmission vehicle is mainly caused by the control logic of gearbox itself.
There are some simple ways to deal with this situation, mainly including the following methods:
1. With the help of the mechanical hand brake, directly pull the hand brake, and let the hand brake replace the brake to limit rolling. Then the accelerator pedal is depressed, which can increase the torque transmitted by the torque converter of the automatic transmission vehicle using the torque converter. For automatic transmission using wet clutch, this can control the clutch to enter semi-linkage. After stepping on the accelerator, we slowly release the handbrake and the vehicle can start normally. This kind of operation is very similar to manual half-slope start, but it is much simpler, because you don't need to control the clutch, just step on the accelerator.
2. With the help of electronic handbrake, electronic handbrake generally has the function of automatic release. When the car begins to slide on the ramp, first pull up the electronic handbrake and then fasten the seat belt. At this time, gently step on the accelerator, the handbrake will automatically release, so it is not easy to slip.
3. Use the uphill auxiliary function
Now many cars have uphill assist function. When this function is turned on, the system will brake the wheels, and we will automatically release them after stepping on the brakes. This can also prevent the car from slipping.
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