AUTOMATIC TRANSMISSION
An automatic transmission provides various gear ratios as they are needed. Cars with automatic transmissions have a lever called a selector. The driver can move the selector to positions for park, neutral, drive, low, and reverse. The engine can start only when the selector is in park or neutral.
To put a car into forward motion, the driver moves the selector to the drive position. Initially, the transmission will be in first, or low, gear. This gear provides the highest torque and the lowest speed. As the car picks up speed, the transmission will automatically shift into higher gears. In higher gears, speed is higher and torque is lower.
The driver may move the selector to low when going up or down hills or when driving through snow or mud. With the selector in low, the transmission will remain in low gear, rather than shift automatically into higher gears. Some cars with automatic transmissions have one or more selector positions between low and drive. These positions prevent shifting above the selected gear.
The major parts of an automatic transmission are a torque converter, one or more sets of planetary gears, three kinds of clutches, and a controller.
