Auto Questions: How Automatic Transmissions Work

The automatic transmission has gained popularity in recent years because of the ease of its use. Information on functionality and maintenance.

The automatic transmission is by far the most common transmission found on the modern automobile, due to its universal ability to be used by all drivers, ease of operation, along with its inherent strength and long-term durability. Unlike a manual transmission, an automatic does away with the clutch pedal allowing drivers to move along seamlessly in heavy traffic without stalling their cars or having a tired left foot (used to operate the clutch pedal in a manual car). So how does an automatic transmission work?

The key to the 'clutch-less' automatic transmission is the torque converter. The torque converter essentially replaces the clutch and multiplies the turning power of the engine. The torque converter has no physical connection to the engine but is turned by means of process known as hydraulic coupling. The transmission shaft that runs through the torque converter fits around the crankshaft coming from the engine. These shafts sit next to each other with the slightest gap in-between. Transmission fluid fills the gap, and as the engine turns the main crankshaft, the transmission shaft (including torque converter) is turned as transmission fluid is not easily compressed. This is known as hydraulic coupling.

The torque converter looks similar to an oversized bagel, and contains three main components: an impeller (pump), stator (guiding wheel) and a turbine. The torque converter is filled with transmission fluid and by similar hydraulic coupling process, the impeller and turbine spin in tandem to each other. As they spin, the transmission fluid is directed towards the outer edges by centrifugal force, where it is re-directed by the stator towards the turbine. In this way, there is a constant flow of fluid which works to multiply the power of the engine. Modern torque converters will also have a lock-up feature that activates at speeds above forty miles an hour to improve gas mileage and the efficiency of the system. This lock-up occurs when a metal pin physically connects the impeller to the turbine. When your speed falls below forty miles an hour, the pin is released again.

The crankshaft and transmission shaft spin in harmony with each other. The transmission shaft runs trough the length of the transmission past a planetary gearset (sometimes multiple gearsets in newer automatics). The planetary gearset will include three main components: a sun gear, ring gears and drums, along with a planet carrier. Similar to our solar system, the planetary gears revolve around the central sun gear using the planet carrier. The planetary gears are of different sizes allowing different gear ratios to be used as the transmission moves through the gears (1st - 4th gears).

When in neutral, only the transmission shaft will turn. As first gear is engaged, the planetary gear corresponding to first, rotates around the sun gear on the planet carrier and locks onto the transmission shaft. This physical 'locking' engages the gear allowing the car to move forward. When the next gear is needed, the planetary gear for first is disengaged and the planetary gear for second is rotated onto the transmission shaft. During this shift into second, the central sun gear is held in place by an intermediate band. This flexible metal band (also call the brake band), fits around the outside of the clutch housing and has friction material around the inside. During shifts it loosens to release gears and tightens back down to 'lock' gears in place on the transmission shaft.

This is in essence how the modern automatic transmission works.

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