The essential role that a car's transmission system performs is to transform the power put out by the engine into the drive shaft in such a way that, despite the engine's narrow range of speeds (revolutions per minute), a wide range of output speeds can be achieved by the car. It does this by changing the arrangement of the set of gears that are the central component of a car's transmission. In a manual transmission system, the driver has to manually shift these gears. An automatic transmission system, however, can rearrange these sets of gears with little intervention from the person operating the vehicle.
The clearest indication that a car has an automatic transmission system is the absence of the clutch pedal. This is no longer needed, as the car's transmission is now run by hydraulics and does not have to be manually engaged. The stick shift that used to manually control the gears has also been replaced by a gear shift lever that selects various modes and gear ranges. This lever can be located on the steering column or on the floor next to the driver. It is marked by indicators that symbolize the various modes. These are commonly P for Park, R for Reverse, N for Neutral, D for Drive and L for First or Low Gear. These symbols are usually arranged in this sequence from top to bottom, left to right or in clockwise fashion.
To select a mode, the driver must first push a button known as the shift lock button and then move the gear shift lever to the appropriate mode. In cars where the lever is positioned in the steering column, the driver must first pull out the handle.
What the various modes do:
Park--This stops the vehicle from moving by mechanically locking its transmission. This mode is usually selected when the car comes to a complete stop, and is one of two modes where the car's engine can be started.
Reverse--Just like in a manual transmission car, this enables the vehicle to move backwards. In most car models this mode can only be selected when the car comes to a complete stop.
Neutral--In this mode, no gear is actually selected and the transmission is disconnected from the vehicle's wheels. This is the other mode where the car's engine can be started.
Drive--This enables the vehicle to move forward and accelerate through various speeds. The transmission automatically adjusts its range of gears according to how fast the car is moving. Depending on the model, this range is usually from third to fifth gear.
First or Low Gear--There is usually an additional mode that comes before this, denoted as S or L2, and indicates the second gear. These modes usually limit the transmission to the first two low gears, and are used when the car is on a slippery or steeply inclined road or any other driving conditions that require low gear.
The heart of a car's automatic transmission system is the planetary gearset. This mechanism responds to the output of the engine, and adjusts accordingly based on the amount of hydraulic pressure exerted by a special fluid (automatic transmission fluid, or ATF) within the transmission's assembly. The various configurations that the planetary gearset can manage provide the possible gear ranges of the transmission.
1904--The Sturtevant brothers of Boston used flyweights that engaged a low and then a high gear as the engine sped up. This was the first self-shifting transmission.
1934--Reo built the Reo Self-Shifter, which was two transmission units joined in a series. One unit shifted up its gears based on the car's speed, while the other was manually controlled to shift to lower gears.
1937--Buick and Oldsmobile came out with the Automatic Safety Transmission. This still made use of a clutch for forward or reverse motion, but once in forward the gears shifted automatically through hydraulic control.
1938--General Motors used a fluid coupling instead of a clutch system to connect the engine and transmission, which was composed of three hydraulically controlled planetary gearsets. This was called the Hydra-Matic, and was the first mass-produced automatic transmission.
1948--Buick upgraded the fluid coupling technology into a hydraulic torque converter. The 1948 Buick Dynaflow was the first car to use this mechanism, and became the model for today's automatic transmission systems.
A distinct and modern variety of the automatic transmission system is the continuously variable transmission, or CVT. Where regular automatic transmissions make use of shifting gearsets that allow only particular ratios (1st gear, 2nd gear), a CVT makes use of pulleys shaped like opposing cones that can change diameters. This new design enables a smooth and constant adjustment of the engine's output power based on the car's speed. This type of automatic transmission is thus more fuel-efficient, and the driver never feels or hears the transmission shift.
