An automatic transmission seems pretty simple; after all, you just put it in Drive and go. But as with most things automotive, a lot of complexity goes into making it look easy.
The engine contains a heavy central crankshaft cylinder that spins, which provides the power to turn the wheels. “Engine speed” is how fast the crankshaft spins, measured in revolutions per minute or “rpm.” Most engines make the majority of their power within a relatively narrow range of speeds, but driving the vehicle requires a wider range. The transmission is the vital link, increasing torque to accelerate away from a stop, or preventing the engine from working too hard at highway speeds.
An automatic transmission uses sensors to determine when it should shift gears, and changes them using internal oil pressure. While there are numerous components stuffed into the transmission, and their actual operation is a bit more complicated than the simplified version presented here, the key components are the torque converter and planetary gearsets.
In order to shift gears, the transmission must be temporarily disconnected from the engine. On a manual transmission, the driver does that by pushing in the clutch pedal, but on an automatic, it’s handled by the torque converter.
There are two fan-shaped components inside the torque converter, which is filled with transmission fluid: an impeller, which is attached to the engine’s crankshaft, and a turbine, attached to the transmission’s input shaft. As the engine turns the impeller, its blades move the fluid, which in turn causes the turbine to turn. The fluid moves in a closed loop. A third fan-shaped component, the stator, sits between the impeller and turbine and helps direct the fluid’s movement. As you push the throttle to speed up, the fluid moves the turbine faster to send more power through the transmission. As you slow down, the fluid’s movement slows, the turbine stops spinning, and the engine can sit and idle without stalling.
The turbine and impeller aren’t permanently attached, and the impeller always spins faster. Most vehicles use a lock-up converter, which has a mechanical clutch that temporarily connects the two components at higher speeds to help improve fuel economy.
Once that power has been transferred to the transmission’s input shaft, it’s time for the planetary gears to do their thing. The name comes from the way they’re arranged. A central gear is called the sun gear, while smaller planet gears revolve around it, held in a ring called a planet carrier. A large-toothed ring gear surrounds them all and is meshed with the planetary gears in their carrier.
Rather than using a separate gear wheel for each gear, the transmission’s various speeds are achieved through combinations of gears. The sun, planetary, and ring gears have meshed in various combinations, such as the outer ring gear turning while the inner sun gear stays stationary. This is achieved with small friction clutches, which engage the gears for turning, and bands, which hold them out of the way so they don’t turn. The clutches and bands are operated by pins and valves that are activated by pressurized transmission fluid.
By creating different gear ratios, the transmission takes the power from the engine and increases or reduces it on its way to the output shaft, which sends power toward the wheels. In first gear, the engine is turning relatively slowly as the driver gradually pushes the throttle, so the transmission uses a low gear to multiply the torque going to the wheels to give them the power needed to accelerate. At highway speeds the transmission uses overdrive, when the transmission’s output speed is faster than what’s coming in from the engine, saving fuel and engine wear-and-tear.
When the transmission is put into Reverse, the small sun gear turns the outer ring gear backward. For Park, a small toothed parking gear is firmly held by a small latch called a parking pawl, which prevents the output shaft from turning the wheels.
The power sent out by the transmission doesn’t go straight to the wheels, which must be able to rotate at different speeds. If they didn’t, you wouldn’t be able to properly turn a corner, and so the vehicle uses a differential to split the power and send the right amount to each wheel. On a front-wheel-drive vehicle, the differential gears are bundled into the transmission housing, and the entire unit is commonly called a transaxle.
Although transmissions don’t require as much maintenance as an engine, they still benefit from a little love. Make sure the transmission fluid level is checked during each oil change, and if your vehicle’s maintenance schedule recommends it, change the transmission fluid when advised. Most transmissions include coolers to regulate the fluid’s temperature, but if you frequently tow with your vehicle, consider adding a heavier-duty cooler if yours didn’t come equipped for heftier workloads.
Has your vehicle checked if the transmission whines or clunks, if it feels like it’s slipping if it hesitates when you accelerate or put it into gear, if you see red fluid leaking under your vehicle, or if you detect a burning smell?