What is the principle of an automatic transmission?
2 Answers
Automatic transmission works by the computer opening or closing certain oil passages in the valve body. The transmission oil pump then pressurizes the transmission fluid, which flows through the passages in the valve body to actuate the physical gear-shifting mechanisms, thereby engaging or shifting gears. The functions of the transmission are: 1. Changing the gear ratio to meet the varying traction needs under different driving conditions, allowing the engine to operate under favorable conditions; 2. Enabling the vehicle to move in reverse; 3. Interrupting power transmission—when the engine starts and idles, or when the vehicle shifts gears or stops, power transmission to the drive wheels is interrupted; 4. Engaging neutral gear—when the clutch is engaged, the transmission stops outputting power.
After driving for so many years, I find the working principle of automatic transmissions quite fascinating. It doesn't require manual shifting; the core relies on the torque converter and planetary gears. The torque converter uses fluid to connect with the engine, cushioning vibrations during startup to prevent stalling. The planetary gear set includes the sun gear, planet gears, and ring gear, providing multiple gears by combining different positions. Vehicle speed and throttle signals are sent to the control computer, which then directs the hydraulic system to adjust oil pressure and automatically shift gears. It smoothly upshifts during acceleration and downshifts during deceleration, making operation very simple—just shift to D (Drive). This design makes city driving in traffic jams less tiring, though maintenance requires regular transmission fluid changes, as degraded fluid can affect shifting precision. In summary, it automates the entire process through mechanical and hydraulic systems, making driving more convenient and safer.
