How does a torque converter multiply torque in an automatic transmission?

A torque converter multiplies torque in an automatic transmission through a process called hydraulic multiplication, which relies on the interaction of three main components: the impeller, the turbine, and the stator. The impeller, connected to the engine, spins and throws fluid towards the turbine. The turbine, connected to the transmission's input shaft, is driven by the fluid from the impeller. The stator, positioned between the turbine and impeller, redirects the fluid returning from the turbine back to the impeller. When the turbine is stationary or rotating much slower than the impeller (such as during initial acceleration), the fluid returning from the turbine strikes the stator vanes at an angle. The stator's vanes are designed to redirect this fluid in a way that it assists the impeller's rotation. This redirected fluid adds force to the impeller, increasing the torque applied to the turbine. This is torque multiplication. The stator is mounted on a one-way clutch, allowing it to rotate in one direction but preventing it from rotating in the opposite direction. This one-way clutch is essential for the torque multiplication process. As the turbine speed increases and approaches the impeller speed, the angle at which the fluid strikes the stator vanes changes. Eventually, the fluid strikes the vanes in a way that it would try to rotate the stator in the opposite direction. At this point, the one-way clutch allows the stator to freewheel (rotate freely), and torque multiplication ceases. The torque converter then acts as a fluid coupling, transferring torque without multiplication. For example, when a vehicle with an automatic transmission starts from a standstill, the torque converter provides significant torque multiplication to help overcome inertia and accelerate the vehicle. As the vehicle gains speed, the torque converter gradually transitions to fluid coupling mode.