Speaking textbook, the automatic transmission or automatic gearbox – a kind of transmission, which provides automatic (in other words, without the intervention of the driver) selection of the gear ratio corresponding to the prevailing traffic conditions. The main difference between “automatic” from the manual transmission is that the driver shouldn’t frequently use his right hand and left leg. In terms of design and operation of the automatic transmission is characterized its mechanical part – I mean the use of hydro-mechanical actuator and planetary gears. That’s why professionals always say “automatic transmission”, a term more accurately conveys its essence, what the definition of “automatic gearbox”.
The main part of the conventional automatic transmission – a torque converter, planetary gears, freewheel clutches and friction, as well as connecting shafts and drums. In addition, in some cases also apply the brake band, the purpose of which – to brake one of the drums. The only exceptions are Honda made automatic transmissions, instead of the planetary gear using the gear shafts, as in the case of a manual transmission.
The main function, performed by the torque converter – when starting the car it passes the torque with slippage. When the engine is gaining high speed, frictional clutch blocks torque converter and makes it impossible to slip. As for the planetary gear unit, its main goal to indirectly pass the torque to transmission.
Friction clutch, is used to shift gears through connection and disconnection elements of the gearbox.
The main difference between “automatic” and the “manual” is that the manual transmission enables or disables the different gear for the output shaft of the different gear ratios and automatic transmission always uses the same set of gear. This is what allows the automatic transmission the planetary gear.
The meaning of automatic gearbox modes
Since the late 50-s of the last century, almost every automatic gearbox has a standard set of operating modes that are indicated in Latin characters on the gear change:
- N (from the English “Neutral”) – the neutral mode of transmission, which is usually used when towing or when parked for a short time;
- D (from the English “Drive”) – forward mode, when all the steps involved, or all except those that shifting up;
- R (from the English “Reverse”) – the reverse mode, which under no circumstances should use until the moment when the vehicle is fully stopped;
- L (from the English “Low”) – downshift mode to be used for the low speed;
- P (from the English “Park”) – the parking mode locking of the drive wheels (this lock system is not connected to the parking brake and is located directly inside the automatic transmission).
Car manufacturers are often use strict sequence of automatic transmission modes – PRNDL.
Besides the main modes:
- O/D (from the English “Overdrive”) – mode of movement, providing the opportunity to move to overdrive automatically (this is very convenient for highway driving);
- D3 – a mode that uses only the first, second or third gear, or disables upshifts (suitable for driving in the city);
- S (also used the figure “2”) – downshift mode or “winter mode”;
- L (also used the number “1”) – downshift mode, only first gear.