In the case of a Tiptronic transmission, the most important components of an automatic transmission include the torque converter, the planetary gear, and the hydraulic and (or) electronic control device. The planetary gear forms the mechanical part of the automatic transmission and is shifted by means of clutches and brakes without interrupting the power flow. Rotating parts are stopped by a band or plate brake. The torque converter transmits the torque from the engine to the gearbox and serves as a hydraulic clutch when starting off. In a Multitronic gearbox that includes a leaf chain variator (see Technical Dictionary - «The principle of operation of the variator»), there is no need for a torque converter.
It is already impossible to imagine an automatic transmission without the widespread use of electronics today. The automatic transmission has a control unit that constantly communicates with the engine control unit. The latter provides data on engine speed, fuel consumption and throttle position. Other values that are important for regulating the automatic transmission come from the anti-lock braking system control unit (for drive wheel traction control system), gear selector and various sensors that are located in the gearbox.
Torque Converter
Between the planetary gear and the engine is a torque converter that uses torque converter fluid to transmit engine torque to the gearbox. To do this, the motor drives the impeller, which is located in the torque converter. The liquid through the so-called reactor presses on the turbine wheel connected to the gearbox, and the turbine wheel is set in motion.
There is always some difference between the rotational speeds of the pump and turbine wheels. This difference is maximum when starting off, with increasing speed it decreases more and more. Thus, there is always a certain amount of slippage. At high speeds, only about 85 percent of the engine's torque is transmitted, which explains the higher fuel consumption compared to manual transmissions.
To overcome this shortcoming, automatic transmissions are equipped with a torque converter lock-up clutch. This clutch has some similarities with a conventional clutch disc: when the clutch is engaged, the friction lining is pressed against the torque converter cover, on the other hand, the clutch is connected to the input shaft of the gearbox by means of teeth with positive lock. The ATF gear pump is driven by a torque converter.
The section of the torque converter shows the blades of the pump wheel and the reactor. The arrow shows the clutch lining for locking the torque converter, which at high speeds - except in the case of cold liquid - is switched to a rigid through drive to avoid slippage
Sensors
Speed sensor at gearbox input. The control device uses this signal to ensure that the gear shifting process is regulated as gently as possible. If this sensor fails, the transmission operates in accordance with the emergency program.
Gearbox shaft speed sensor. This sensor provides information about the speed of movement. This determines which gear is engaged, as well as the pressure in the control system when changing gears. If the sensor fails, the emergency mode starts to operate.
Accelerator pedal position sensor. If the driver presses the accelerator pedal all the way down, the transmission shifts down one or two steps, and then another step. If the driver holds the pedal to the floor for a long time, then the air conditioner is turned off, this is done in order to increase the available engine power. If this sensor fails, the control device receives the corresponding data from the throttle position sensor.
Stop signal switch. If the brake pedal is pressed during parking, the control device turns off the selector lock. If the pedal is depressed while driving, the computing device assumes downhill travel and downshifts.
Transmission oil temperature sensor. If the oil temperature rises to more than 120'C, the torque converter will lock up sooner.
Signal from the control device of the anti-lock and traction control system. If the traction control is activated, the automatic transmission shifts less, the shift points are shifted.
Signals from the engine controller engine speed, fuel consumption and throttle position. All this information is used to calculate the optimum shift point. If the connection is interrupted, the gearbox starts to work in emergency mode.
Emergency program
In the event of a failure of the most important sensors, the control unit switches to the emergency program. The emergency mode is used in order to ensure the safety of the vehicle in the event of a malfunction without a significant reduction in its driving performance, as well as to protect the gearbox from damage. The car can be used, but it is necessary to contact a workshop as soon as possible. In emergency mode, power is removed from all the solenoid valves of the gearbox, with the help of springs, the valves are transferred to their original position. You can only move forward in fourth gear. The reverse gear continues to operate, but the clutch to lock up the torque converter does not engage. In such a situation, it is necessary to monitor the gear indicator on the speedometer: if all segments are lit, then the control device is still active. If all segments are off, the control unit is not functioning. In this case, the selector lock is not activated.
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