Auto Questions: How Do Automatic Transmissions Work?

Planetary gears sets connect engine with transmission in input/output combinations of rotational speeds and direction, through a torque converter, clutches and hydraulics.

The role of transmission, whether manual or automatic, in a vehicle, is to enable outputs of different speeds even as the engine works within a narrow band of rotation per minute. A car without transmission would work on only one gear ratio, making it difficult to drive and wearing out the engine in a short time. Automatic transmission adds to driver comfort, especially in traffic.

Gears are used to vary the torque or rotational power of the engine to turn the crankshaft and thereby the wheels. A fluid coupling between the engine and the crankshaft, known as a torque converter, is used to engage and disengage the rotation of the engine from that of the crankshaft, in automatic transmission. This allows the engine to idle when the vehicle is not in motion. (A torque converter in automatic transmission performs the role of a clutch in manual transmission). An automatic transmission uses a planetary system of gears. A planetary gear system has a central gear known as a sun gear that is surrounded by gears called planetary gears. This arrangement resembles planets around the sun, and hence the name "planetary system". An outer gear surrounds this planetary configuration. The planetary gears are connected with the outer rim as their teeth face each other.

Any one of the gears in the planetary system can provide the input and any other gear can provide output to a shaft. Outputs to the crankshaft may be higher, lower or in reverse direction to the input from the engine by engaging and disengaging various combinations of the sun, planetary and ring gears. Automatic transmission has 2 sets of sun and planetary gears in a single housing with one outer gear. The 2 sets of sun and planetary gears differ in size. This produces 4 forward gears and one reverse gear. Automatic transmission has a series of clutches and steel bands to engage and disengage various combinations of the 2 sets of planetary gears. A hydraulic system moves fluid through channels to control and to direct the motion in the transmission.

The first forward gear works when the torque converter turns the smaller of the 2 sun gears clockwise. The planetary gears are held stationery and the outer ring gear provides output to the transmission. Since there are 2 planetary gear sets in the same housing, the outer ring gear turns in the same direction as the smaller of the 2 sun gears. The outer ring gear is held stationery while the torque converter continues to drive the small sun gear, to produce the second forward gear. Output is through the planetary carrier in the second gear. The third forward gear is engaged when the torque converter turns both sun gears. Both sets of planetary gears turn in opposite directions, so the outer ring provides output in the same ratio and direction as the 2 sun gears. Automatic transmission also has a facility in which the torque converter is disengaged. Engine rotation is connected directly to the crankshaft and the output is faster than the input from the engine.

This enables the vehicle to move at relatively high and steady speed while the engine rotates at the lower end of its range of revolutions per minute. This system is called overdrive and improves fuel efficiency. Reverse gear operates like the first forward gear except that the torque converter drives the larger sun gear rather than the smaller one. The latter freewheels in the opposite direction. Ingenious design and layout of gears is at the heart of an automatic transmission system. A vehicle with automatic transmission can move both forwards and in reverse at varying speeds and rates of rotation, with minimal physical effort by the driver.

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