How Does an Automatic Transmission Work?
2 Answers
Automatic transmissions are capable of shifting gears automatically because during operation, the position of the accelerator pedal pressed by the driver, the vacuum level in the engine intake manifold, and the vehicle's speed can direct the automatic shifting system to operate. This system controls the engagement and disengagement of clutches within the gear mechanism, altering the power transmission path of the gear mechanism to achieve gear shifts in the transmission. About automatic transmissions: They utilize planetary gear mechanisms for shifting, automatically adjusting gears based on the degree of accelerator pedal depression and changes in vehicle speed. The driver only needs to operate the accelerator pedal to control the vehicle. Hydraulic transmission + mechanical transmission. Hydraulic transmission refers to the torque converter, while mechanical transmission refers to the planetary gear set. Related information: Traditional hydraulic automatic transmissions automatically shift gears based on the vehicle's speed and throttle opening. The gear shift control method mechanically converts signals of vehicle speed and throttle opening into control oil pressure, which is then applied to both ends of the shift valve to control its position, thereby changing the oil circuit of the shift actuators (clutches and brakes). As a result, the working hydraulic oil enters the corresponding actuators, causing the clutches to engage or disengage and the brakes to apply or release, controlling the upshifting or downshifting of the planetary gear transmission to achieve automatic shifting.
Back when I worked at the repair shop, I watched mechanics dismantle automatic transmissions. The heart of it lies in the planetary gearset working in tandem with the torque converter. At low RPMs during startup, the hydraulic fluid in the torque converter drives the turbine wheel. Once you pick up speed, the lock-up clutch engages for direct mechanical connection to minimize power loss. The real marvel is the hydraulic system - computers control solenoid valves to redirect fluid passages, magically shifting gear combinations. Take hill climbing for example: downshifting occurs when high-gear fluid pressure is released, allowing springs to push the lower gear into position. This design makes stop-and-go traffic effortless, though the mechanism is exponentially more complex than manual transmissions.
