The design of the automatic transmission includes a torque converter, an oil pump, planetary gears, friction clutches and brakes. The gearbox is controlled by an electronic unit.
The torque converter performs the functions of a clutch. During operation, it is filled with working fluid and, rotating at a sufficient speed, transmits torque from the engine to the transmission, absorbs and smooths out vibrations and jerks from the engine and gearbox. The torque converter consists of three wheels: a pump wheel, a reactor wheel, and a turbine wheel, as well as a clutch for locking it. The pump wheel is combined with the torque converter housing and is fixed to the engine flywheel. The turbine wheel is connected to the input shaft of the gearbox by a splined connection. Upon reaching a certain speed, the torque converter is locked using a clutch, thereby providing a rigid connection between the engine crankshaft and the transmission. This significantly reduces power losses and has a beneficial effect on dynamic qualities and fuel consumption.
The oil pump is driven by the engine and is designed to create the necessary pressure of the working fluid for the operation of the automatic transmission. This is why it is impossible to start the engine of a car with an automatic transmission "by pushing". In the absence of pressure of the working fluid, the torque from the transmission cannot be transmitted to the crankshaft of the engine in any way. Also, the absence of pressure of the working fluid imposes certain restrictions on towing the car (see "Operation and management of the vehicle"), since proper lubrication of the automatic transmission units does not occur.
Planetary gears are installed directly in the automatic transmission housing and are designed to change the gear ratio and engage reverse gear. The planetary gear (transmission) consists of four main elements: a sun gear, a carrier with satellites and a crown gear. The gear ratio and direction of rotation are changed by locking or unlocking two of the three elements (sun gear, carrier, ring gear) in various combinations. Modern automatic transmissions are equipped with several planetary gears, depending on the number of gears and design.
Friction clutches and brakes are designed to block various elements of planetary mechanisms. They are activated by the pressure of the working fluid when the corresponding valve opens. The valves are opened in a certain sequence by the electronic control unit depending on the speed, the degree of opening of the throttle valve, the rotation frequency of the crankshaft, etc.
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