Working principle of hydraulic torque converter

The torque converter (Torque Converter, referred to as TC) is located at the front end of the hydraulic automatic transmission and is installed on the flywheel of the engine. It can automatically and steplessly change the transmission ratio and torque ratio, and has a certain deceleration and torque increase function. At present, a three-element closed-type comprehensive hydraulic torque converter composed of a pump wheel, a turbine wheel and a guide wheel is widely used, as shown in Figure 6.

As shown in Figure 6, the pump wheel is integrated with the torque converter housing, which is the active element; the turbine is connected to the output shaft through splines, and is the driven element; the guide wheel is placed between the pump wheel and the turbine, and is connected by a one-way clutch And the guide wheel bushing is fixed on the transmission case.

After the engine is started, the crankshaft drives the pump wheel to rotate through the flywheel, and the centrifugal force generated by the rotation causes the working fluid between the blades of the pump wheel to be thrown out along the blades from the inner edge to the outer edge; The sub-velocity in the direction, and the subvelocity in the axial direction towards the turbine. These working fluids impact the turbine blades, pushing the turbine and the pump wheel to rotate in the samedirection.

The velocity of the working fluid flowing out from the turbine can be regarded as the synthesis of the tangential velocity of the working fluid flowing out relative to the surface of the turbine blade and the peripheral velocity rotating with the turbine. When the turbine speed is relatively small, the working fluid flowing out from the turbine is backward, and the working fluid impacts the front of the vane of the guide wheel. Because the guide wheel is limited by the one-way clutch and cannot rotate backwards, the vane of the guide wheel guides the working fluid flowing backwards to push the blades of the pump wheel forward to promote the rotation ofthe pump wheel, thereby increasing the torque acting on the turbine.As the turbine speed increases, the peripheral speed becomes larger, and when the resultant speed of the tangential speed and the peripheral speed starts to point to the back of the vane of the stator, the torque converter reaches the critical point. When the turbine speed increases further, the working fluid will impact the back of the vanes of the guide wheel. Because the one-way clutch allows the guide wheel to rotate forward together with the pump wheel, driven by the working fluid, the guide wheel rotates freely along the direction of pump wheel rotation, and the working fluid flows back to the pump wheel smoothly. When the working fluid flowing out of the turbine is exactly in the same direction as the outlet of the vane of the guide wheel, the torque converter does not produce torque increasing effect (the working condition of the hydraulic torque converter at this time is called the hydraulic coupling working condition).

The hydraulic torque converter transmits torque by the working fluid, which is lower than the transmission efficiency of the mechanical transmission. Installing a lock-up clutch in the torque converter can lock the pump wheel and the turbine together under high-speed conditions to realize direct transmission of power and improve the transmission efficiency of the torque converter.

Working principle of planetary gear transmission

Although the hydraulic torque converter can transmit and increase the engine torque, the torque ratio is not large and the speed range is not wide, which is far from meeting the needs of the working conditions of the car. In order to further increase the torque, expand its speed range, and improve the adaptability of the car, an auxiliary transmission is installed behind the torque converter, and a planetary gear mechanism is mostly used.

The planetary gear transmission is composed of a planetary gear mechanism and actuators such as clutches, brakes and one-way clutches. A planetary gear mechanism usually consists of multiple planetary rows. The number of planetary rows is related to the number of gears.

The shift actuators of the planetary gear transmission include shift clutches, shift brakes and one-way clutches.

The shifting clutch is a wet multi-disc clutch. When the hydraulic pressure makes the piston press the active plate and the driven plate, the clutch is engaged; when the working fluid is discharged from the piston cylinder, the return spring makes the piston retreat and the clutch is separated.

There are usually two types of shift brakes: one is a wet multi-disc brake, which is basically the same in structure as a wet multi-disc clutch, except that the brake is used to connect the rotating part and the transmission housing so that the rotating part cannot rotate. Another form of shift brake is the toe-in band brake.

The one-way clutch of the planetary gear transmission has the same structure as the one-way clutch in the hydraulic torque converter.

Automatic Control of Hydromechanical Automatic Transmission

The automatic control system of hydromechanical automatic transmission usually consists of oil supply, manual gear selection, parameter adjustment, shift timing

 control, shift quality control and other parts. The oil supply part adjusts the output oil pressure of the oil pump to the specified value according to the change of the 

throttle opening and the position of the selector lever to form a stable working hydraulic pressure. In the hydraulic-controlled hydraulic automatic transmission, the

 parameter adjustment part mainly includes the throttle pressure regulating valve (throttle valve for short) and the speed control pressure regulating valve (also known

 as the governor). The throttle pressure regulating valve enables the output hydraulic pressure to reflect the throttle opening; the speed control pressure regulating

 valve enables the output hydraulic pressure to reflect the vehicle speed. The shift timing control part is used to switch the oil circuit leading to each shift actuator

 (clutch and brake), so as to realize the shift control. The lock-up signal valve is controlled by the solenoid valve, so that the lock-up clutch in the hydraulic torque

 converter is engaged and disengaged in good time. The role of the shift quality control part is to make the shift process more smooth and soft.

Working principle of mechanical continuously variable automatic transmission

The mechanical continuously variable automatic transmission transmits the engine power through the steel belt and the grooves of the two cone wheels, from the primary wheel to the secondary wheel, and then to the driving wheel through the final reducer to realize the power transmission. Since the center distance between the two pulleys is fixed, but the axial distance between the pulleys on the shaft is variable, so the contact radius between the steel belt and the pulley changes, so the transmission ratio is steplessly adjustable, as shown in the figure 8.

Generally, the range of change that CVT can provide is about 4.69~0.44, so it cannot meet the requirements of the change range of the transmission ratio of the whole vehicle. Therefore, it is necessary to install a final reducer after the CVT for further deceleration and torque increase.

Working principle of electromechanical automatic transmission

In the electromechanical automatic transmission, the driver expresses his intention to the microcontroller through the accelerator pedal and selector (including gear selection range, shift schedule, cruise control, etc.). The best program (best shift schedule, best clutch engagement schedule, engine throttle adaptive adjustment schedule) controls the throttle opening, clutch engagement, and gear shift to achieve optimal matching, thereby obtaining excellent driving performance, The principle of smooth starting performance and rapid gear shifting is shown in Figure 9.

How a dual clutch automatic transmission works

The dual-clutch automatic transmission arranges the transmission on the two input shafts connected to the two clutches respectively in odd and even numbers, and completes the shifting process through the alternate switching of the clutches, realizing uninterrupted power shifting.

When the transmission is in operation, one group of gears is engaged, and when the shift is approaching, the gears of the next group of gears have been pre-selected, but the clutch is still disengaged. When shifting gears, one clutch separates the gears in use while the other clutch is pre-selected to engage, ensuring that at least one group of gears is outputting power during the entire shifting period, so that there will be no power interruption. In order to cooperate with the above operations, the drive shaft of DCT is divided into two parts when it moves, one is a solid drive shaft, and the other is a hollow drive shaft. The solid transmission shaft is connected to 1, 3, 5 and reverse gears, while the hollow transmission shaft is connected to 2, 4 and 6 gears. The two clutches are responsible for the meshing action of one transmission shaft, and the engine power will be driven by one of them. Drive shafts make uninterrupted transmission.

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