Why does an automatic transmission car's engine speed increase but the speed doesn't when accelerating?

[LeLucia]

When an automatic transmission car's engine speed increases but the speed doesn't when accelerating, it is typically an issue with the automatic transmission, with the most common cause being the torque converter not locking up. Symptoms of a torque converter not locking up: Under normal circumstances, when the engine speed, vehicle speed, and throttle depth reach a certain level, the torque converter should lock up. If the torque converter fails to lock up, it often results in the car revving loudly without accelerating. This phenomenon mainly occurs in vehicles equipped with torque converters, such as all AT (automatic transmission) cars and some CVT (continuously variable transmission) models. Solution: For smooth and powerful acceleration in an automatic transmission car, it is necessary to press the accelerator pedal more firmly. Whether the torque converter locks up depends not only on the vehicle speed but also on the engine speed and throttle depth. If the driver consistently applies light throttle input and maintains low speeds, the engine speed may not rise sufficiently, preventing the torque converter from locking up.