An automatic transmission uses a hydraulic fluid pushed through a series of clutch plates and servos to drive a vehicle. The fluid is spun through a torque converter to multiply the amount of torque that is produced by the transmission. This torque determines shift points and acceleration, as well as vehicle power output. The torque converter links the transmission to the engine at the flywheel or flex-plate. As engine speeds increase, the torque converter multiplies the amount of power the automatic transmission applies to the drive axle.
The shifting and gear changes in an automatic transmission are accomplished through the flow of transmission fluid through the transmission. Much like the blood flow in the human body, transmission fluid is pumped through a complex maze of passages and channels throughout the transmission. As the fluid flows through the transmission, it applies pressure to the different clutches and drives a specific gear. The gear will change accordingly as the transmission speed is changed either up or down.
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There are many varieties of automatic transmissions, and the main difference between them is the number of gears the transmission has. The traditional transmission was equipped with three forward gears: reverse, park and neutral. Modern transmissions are equipped with at least four forward gears and can have as many as eight. There are also variable torque transmissions available that use a single speed that changes from low to high speed as the engine is brought up to speed. This type of transmission works much like a fluid coupling or hydrostatic drive type of transmission.
Most vehicles that can be equipped with a manual transmission offer an automatic gear box as an option. Typically, base model vehicles are equipped with a manual transmission while the upper-end vehicles are offered with an automatic transmission at an extra charge. Americans typically prefer an automatic-equipped vehicle, while Europeans will often choose a manual-type transmission. Many sports cars and crossover vehicles offer high-performance transmissions complete with paddle shifters. While standard shifters are typically column- or floor-mounted, the paddle shifter places the gear selectors on the steering wheel.
By simply tapping the paddle up or down, the transmission can be shifted through the gears without the driver ever lifting his hands off the steering wheel. This type of performance-shifting selector has made its way from the Formula 1 ranks to street-driven vehicles. Other performance-type automatic transmission shifters offer both manual and automatic shifting through the flip of a switch.