Automatic transmissions — working medium in a torque converter In a hydrodynamic torque converter used with automatic transmissions, which medium actually transfers torque between pump and turbine elements?

Introduction / Context:
A torque converter is a fluid coupling with torque multiplication capability. Understanding the working medium clarifies maintenance needs and the role of fluid properties such as viscosity and oxidation stability.

Given Data / Assumptions:

• Pump (impeller), turbine, and stator are present.
• Converter is filled with the specified ATF to the correct level.
• Automatic transmission with appropriate cooling circuit.

Concept / Approach:
Torque transfer occurs by momentum exchange of the working fluid. The pump accelerates the ATF, which then impinges on the turbine blades, imparting torque. The stator redirects returning flow to enhance multiplication at high slip.

Step-by-Step Solution:

Verification / Alternative check:
Loss of ATF or contamination leads to poor coupling, overheating, shudder, and converter clutch issues, proving the fluid’s central role.

Why Other Options Are Wrong:

• Air: would cavitate and cannot transmit torque effectively in this design.
• Gears and steel belt: belong to mechanical transmissions (gearboxes, CVTs), not torque converters.
• Silicone grease: unsuitable and not used as a hydrodynamic working medium.

Common Pitfalls:
Using incorrect ATF type or running low on fluid; confusing fluid coupling with mechanical gear reduction.

Final Answer: