Generally, the slope of AT gearbox is too large when starting on the ramp.
Power is transmitted between AT gearbox and engine through hydraulic torque converter, which is a device that transmits power by gearbox oil. There are two rotating bodies similar to fans inside, which are connected to the engine and gearbox respectively. 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. The biggest feature of hydraulic torque converter is that it can realize the flexible connection of power.
For example, the engine drives the pump wheel to rotate and throws the gearbox oil out. After the transmission oil hits the turbine, there are two results. The first is that the turbine end resistance is small, and the gearbox oil can drive the turbine. In fact, the car started when the brakes were released. In another state, the resistance at the turbine end is too great for the gearbox oil to drive. At this time, the turbine remains motionless, and the gearbox oil changes direction and returns to the pump wheel after impacting the turbine. Then the pump wheel continues to throw the gearbox oil out, then impacts the pump wheel and returns. In this process, the pump wheel can keep rotating and constantly apply power to the turbine, while the turbine can keep still. This is what happens when we put D gear on the brake.
However, when we are driving normally, we definitely don't want to use gearbox oil to transmit power, because it will cause great losses. Therefore, when the vehicle speed reaches a certain level, the clutch inside the hydraulic torque converter will be directly combined to lock the turbine and the torque converter housing into a whole, so that the power can be directly transmitted to the gearbox through hard connection. The principle of hydraulic torque converter is very complicated. Here we can intuitively regard the hydraulic torque converter as the clutch of the manual gearbox, but this clutch has only two states: semi-linkage and complete combination.
When we start the engine and the gearbox is in neutral, the internal clutch of the torque converter is in a state of separation, but because the gearbox is in neutral, 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 put the brake in the D position, the turbine is essentially hard connected with the driving wheel through the gearbox. Because the driving wheel is restricted by the brake and cannot rotate, the turbine will stop running 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, the power is continuously transmitted to the driving wheel through the gearbox, and the driving wheel cannot move because of the restriction of the brake. At this time, the driving wheel is ready to go 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. Specifically, it can be started when the brake is loose to a certain extent and the driving force is greater than the driving resistance. And we can also control the speed by lightly stepping on the brakes. This is all because of the hydraulic torque converter. This is the same effect as 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 the ramp starts, if the ramp is large enough, the torque transmitted by the torque converter will be too small to overcome the resistance, leading to slippage. At this time, if you want to avoid slipping, you must step on the accelerator to increase the speed and let the torque converter transmit more torque.
Therefore, the main reason why AT gearbox starts on the ramp is that the engine speed is too low and the torque converter can't 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 keep the car, you need to transfer more power to the driving wheels. The method is simple: step on the accelerator and increase the engine speed.
At this time, you can start with the handbrake. Don't release the brake after shifting gears. Hold the brake first, and then step on the accelerator to speed up. When the speed is high, more power will be transferred to the driving wheels. At this time, you can release the handbrake and start. Of course, if the handbrake is not pulled so tightly and the accelerator is pressed hard, it is also possible to run forward without releasing the brake.
The main drive of wet dual clutch gearbox is gearbox control logic.
Power transmission between wet dual clutch gearbox and engine depends on wet clutch. 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 hydraulic torque converter, there is one more state: complete separation! It is this state of "complete separation" that causes auto start to slip away.
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 be closed. On a flat road, it is easy to say that if you encounter a little slope, you will start to slide. Because at this time, although the gearbox has been engaged, the power of the engine has not been transmitted. At this time, you must step on the accelerator and tell the gearbox, "I'm going to start, let's move." Then the clutch began to enter the semi-linkage, and the car began to move.
Therefore, it can be summarized as follows: the main reason for the start of automatic gear ramp with hydraulic torque converter is that the slope is too large and the power transmitted by hydraulic torque converter is not enough. The control logic of the gearbox itself is the main reason for the automatic car to start slipping on the ramp with wet clutch.
The handling method of this situation is also very simple, mainly including the following methods:
1. With the help of the mechanical hand brake, directly apply the hand brake and let the hand brake replace the brake to restrict the car. Then step on the accelerator, and the torque transmitted by the hydraulic torque converter can be increased for automatic transmission vehicles using the hydraulic torque converter. For the automatic gearbox with 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 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 automatic release function. When starting and stopping on the ramp, first pull up the electronic handbrake, and then fasten the seat belt. At this time, as long as you lightly step on the accelerator, the handbrake will automatically release, and it is not easy to slip the car.
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.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.