Torque Converter Operation At Low Speeds

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At low engine speeds there is a large difference in rotational speed between the impeller and the turbine
Fluid flow is directed from the impeller to the turbine. Fluid strikes the vanes of the turbine. The turbine is driven forward in the direction of engine rotation.
Fluid flow is then directed back towards the impeller.
Before the fluid reaches the impeller, the fluid strikes the vanes of the stator.
When the fluid strikes the stator, the one way clutch prevents the stator from rotating.
The fluid is then re-directed by the curved vanes of the stator. The fluid is now flowing in the same direction as the impeller.
The fluid that is acting on the impeller increases the force on the impeller which multiplies torque. See Fig 1.

Fig 1: Torque Converter Operation At Low Speed
Courtesy of BMW OF NORTH AMERICA, INC.