These automatic transmissions have been continuing to grow in complexity. More gears, different ratios for different versions of the same transmission, and even units that “talk” with four-wheel-drive transfer cases to provide the best torque at the best moment. Yet, from these hundreds of transmissions, there have been some truly amazing transmissions with superb durability, the ability to be remanufactured to fit into all kinds of trucks (the “jack of all trades” transmissions), and one or two that have achieved classic status for just being that damned good.
The most common type of automatic transmission is the hydraulic automatic, which uses a planetary gearset, hydraulic controls, and torque converter. Other types of automatic transmissions include continuously variable transmissions (CVT), automated manual transmissions (AMT), and dual-clutch transmissions (DCT). An electronic automatic transmission (EAT) may also be called an electronically controlled transmission (ECT), or electronic automatic transaxle (EATX).
Most cars use a “P-R-N-D” layout for the gear selector, which consists of the following positions:
- Park ( P ): This position the transmission from the engine (as per the Neutral position) and a parking pawl mechanically locks the output shaft of the transmission. This prevents the driven wheels from rotating (although the non-driven wheels are still free to rotate) which typically prevents the vehicle from moving. The use of the hand brake ( parking brake ) is also recommended when parking on slopes, since this provides greater protection from the vehicle moving. The Park position is omitted on buses/coaches/tractors, which must instead be placed in neutral with the air-operated parking brakes set.
The park position usually includes a lockout function (such as a button on the side of the gear selector or requiring that the brake pedal be pressed) which prevents the transmission from being shifted from Park into other gear selector positions. Many cars also prevent the engine from being started when the selector is in any position other than Park or Neutral (often in combination with requiring the brake pedal to be pressed).
- Reverse ( R ): This position engages reverse gear, so that the vehicle drives in a backwards direction It also operates the reversing lights and on some vehicles can activate other functions including parking sensors, backup cameras and reversing beepers (to warn pedestrians).
Some modern transmissions have a mechanism that will prevent shifting into the Reverse position when the vehicle is moving forward, often using a switch on the brake pedal or electronic transmission controls that monitor the vehicle speed.
- Neutral ( N ): This positionthe transmission from the engine, allowing the vehicle to move regardless of the engine’s speed. Prolonged movement of the vehicle in Neutral with the engine off at significant speeds (“coasting”) can damage some automatic transmissions, since the lubrication pump is often powered by the input side of the transmission and is therefore not running when the transmission is in Neutral.
- Drive ( D ): This position is the normal mode for driving forwards. It allows the transmission to engage the full range of available forward gear ratios.
Some automatic transmissions previously used a layout with reverse as the bottom position (eg P-N-D-L-R) However this layout led to the risk of the driver shifting into Reverse while the vehicle is travelling forwards (especially during maneuvers).