Which component acts as a fluid-coupling device in an automatic transmission?

The torque converter is the component in an automatic transmission that serves as a fluid-coupling device. It allows the engine to continue running while the vehicle is stationary and provides smooth acceleration when the vehicle is in motion. The torque converter uses hydraulic fluid to transfer power from the engine to the transmission by creating a coupling effect.

Inside the torque converter, there are typically three main components: the impeller, turbine, and stator. The impeller, connected to the engine, forces the transmission fluid into the turbine, which is connected to the transmission. This creates a coupling between the engine and transmission allowing for a seamless transition of power. When the vehicle is stationary, the fluid recirculation feature of the stator helps to multiply torque, which enhances engine performance during acceleration.

The clutch pack is responsible for engaging and disengaging different gears within the transmission using friction but does not serve the same fluid-coupling function as a torque converter. The planetary gear set provides gear reduction required for different speeds and torque magnification but is not involved in the fluid coupling process. Similarly, the shift solenoid is an electrical component that controls fluid flow to operate the shifting mechanisms in the transmission, but it does not perform the function of a fluid coupling device.