How an Automatic Transmission Shifts Gears Itself
An automatic gearbox uses a fluid-filled torque converter instead of a clutch pedal and planetary gears chosen by hydraulic pressure. Here is how the parts work together.
In this article
A traditional automatic transmission lets a car change gear without a clutch pedal or a gear lever you move yourself. Two ideas make that possible. A fluid-filled torque converter connects the engine to the gearbox and takes the place of the clutch. Inside the gearbox, sets of gears called planetary gearsets produce different ratios, and clutches and brakes worked by fluid pressure choose which ratio is active.
The torque converter replaces the clutch
In a manual car, you press the clutch to separate the engine from the wheels so the car can stop without stalling the engine. Jalopnik's explainer says the torque converter takes the place of that clutch. It is a fluid coupling: an impeller on the engine side spins automatic transmission fluid, and the moving fluid pushes a turbine connected to the gearbox's input shaft. At idle, little force passes through, so the car can sit in Drive at a stop. Press the accelerator and the engine speeds up, the fluid carries more force, and the car moves off.
Between the two sits a part called the stator, mounted on a one-way clutch. It redirects returning fluid so it helps the impeller. The result, Jalopnik says, is that the converter can multiply torque mainly at stall and during acceleration, not at steady highway speed. A lock-up clutch can then join the turbine and impeller mechanically so all engine torque passes straight through. Wikipedia notes that many four-speed automatics gained lock-up converters in the 1980s to improve fuel economy.
Planetary gearsets make the ratios
A planetary gearset has a central sun gear, several planet gears held on a carrier, and an outer ring gear. Because all these parts mesh at once, you can get different ratios by choosing which part is held still, which is driven and which is the output. According to Jalopnik, this lets the transmission change ratio without disconnecting gears. Holding one part gives a low ratio, holding another gives a higher one, locking two parts together gives 1:1, and holding the carrier reverses the direction of rotation for reverse gear.
Jalopnik adds that most modern units use a Ravigneaux compound gearset, which combines two planetary sets with two sun gears, two sets of planets and one shared carrier. Wikipedia credits Chrysler's three-speed TorqueFlite in 1956 as the first to use the Simpson compound planetary gearset, a different compound design.
Hydraulics decide which gear
Clutches and brake bands inside the gearbox hold or release parts of the planetary sets. They are squeezed by fluid pressure. Jalopnik explains that the torque converter drives the fluid pump, and a hydraulic control unit directs pressure to the right clutches and to lock the converter. Wikipedia mentions that sprag clutches, which are one-way locking devices, handle routine shifts, while bands are used for manually selected gears such as low or reverse. Modern gearboxes add an electronic control unit that decides when to shift, but the mechanical principle of holding and releasing parts stays the same.
A short history
- 1904: the Sturtevant gearbox is often called the first automatic transmission, but it was prone to sudden failure, according to Wikipedia.
- 1939: General Motors introduced the Hydra-Matic for the 1940 model year in Oldsmobile and Cadillac models. It used a fluid coupling and three planetary gearsets for four forward speeds plus reverse and was the first mass-produced automatic.
- 1948: the Buick Dynaflow was the first automatic to use a torque converter instead of a fluid coupling.
- 1989 to 2017: gear counts rose, with a five-speed Jatco unit in 1989, ZF's six-speed in the 2002 BMW 7 Series, and Toyota's ten-speed Direct Shift-10A in 2017.
How it differs from CVTs and dual-clutch gearboxes
Not every automatic car uses this design. A continuously variable transmission (CVT) changes ratio smoothly across an unlimited range, most commonly with two pulleys linked by a belt or chain. A dual-clutch transmission uses two clutches, one for odd gears and one for even, so the next gear is ready before the shift. Wikipedia says the first modern dual-clutch unit appeared in the 2003 Volkswagen Golf R32 and that these have increasingly replaced hydraulic automatics since the late 2000s.
Where automatics are common
Wikipedia reports that automatics held over 80% of new US cars in 1957 but only about 10 to 12% of European cars in 1997. For manual gearboxes, it gives 43% of new cars worldwide in 2015, falling to 37% by 2020. These figures vary by region and year.
Summary
- The torque converter is a fluid coupling that replaces the clutch.
- Planetary gearsets create ratios when parts are held, driven or locked together.
- Hydraulic pressure, directed by a control unit, applies clutches and brakes to select gears.
- CVTs and dual-clutch gearboxes are different kinds of automatic.
This article is general information. For maintenance such as fluid changes, follow the car maker's manual.
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