What is AT? Definition, types, characteristics, and how it functions

What is AT? Definition, types, characteristics, and how it functions

AT is an automatic transmission. It is a system which changes gears on its own, due to different parameters, the main of which is the car driving speed. One of the main advantages of AT use is comfort of car handling. The automatic transmission allows to smoothly change gears which is reached due to the torque converter. Function of the latter is in transmission of the driving torque from the power unit to the manual gearbox which is one of AT elements.

 The most popular automatic transmission are: 
  • ZF 5HP 24/30 (BMW 5 E39, 7 E38, Z8; Aston Martin DB7; Land Rover Range Rover; Jaguar XJ8; Rolls-Royce Silver Seraph);
  • GM 5L40-E (Cadillac CTS, STS; BMW 3 E46, 5 E39, X5 E53, Z3);
  • Jeep 545RFE (Jeep Grand Cherokee, Commander Wrangler; Dodge Dakota, Durango);
  • Toyota A340 ( Lexus LS, GS; Toyota 4Runner, Supra);
  • Toyota A750 ( Lexus LX; Toyota Land Cruiser);
  • Mercedes 722.4 (Mercedes 190, 200-300 W124);
  • Jeep A904 (Jeep Cherokee XJ, Wrangler TJ, Y);
  • Mazda / Ford FN4A-EL / 4F27E (Ford Transit Connect, Focus; Mazda 3, Mazda 6).

Feature of AT with the classic automatic gearbox is that the driving torque is transmitted from the engine to driving wheels via working fluid. One of the main disadvantages of such transmission is high fuel consumption.

Planetary gearsets
 Functional element of transmission of this type is planetary gearsets which includes: 
  • sun gear;
  • epicycle;
  • 2 or more planet gears;
  • carrier.

All planet gears are rotating on their axles. It is reached due to gears are connected by the carrier.

Use of 2 or more planet sets in different combinations in AT system allows to increase the number of speeds. Their shifting in modern vehicles is performed by the computer. Thus, optimal conditions for engine operation are provided, which minimizes possibility of its overload.

Modern trim levels with automatic transmission are not equipped with the clutch pedal. The selector lever of classic automatic gearbox, as a rule, has the following positions: driving forward, neutral speed, reverse driving and parking.


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