Torque Converters
A torque converter connects an engine to a transmission with spinning fluid instead of a mechanical clutch plate. It transmits power smoothly through oil, and — unlike a simple fluid coupling — can actually multiply torque when the engine is spinning much faster than the output shaft.
01Definition
A torque converter connects an engine to a transmission with spinning fluid instead of a mechanical clutch plate. It transmits power smoothly through oil, and — unlike a simple fluid coupling — can actually multiply torque when the engine is spinning much faster than the output shaft.
02Physical Principle
Inside its sealed housing, an engine-driven impeller flings transmission fluid outward into a connected turbine, which drives the output shaft; a third element, the stator, redirects the returning fluid to add rather than fight the impeller's flow. That redirection is what lets a stalled torque converter — output shaft barely turning, as at the start of a dozer push — deliver up to roughly twice the engine's torque to the tracks, tapering back to a 1:1 fluid coupling once both sides spin near the same speed.
03Mathematical Foundation
04Interactive Diagram — Torque Converter Curve
Slide the turbine-to-impeller speed ratio to see torque multiplication fall from its peak near stall to 1:1 once both sides spin together.
Input torque fixed at 500 N·m. Multiplication tapers to 1:1 once impeller and turbine converge — the coupling point.
05Interesting Facts
- A torque converter is why a heavy machine in gear can sit still against the brake with the engine revving and not stall — there is no rigid mechanical link to lock up.
- The fluid coupling inside runs hot enough under sustained heavy load that it needs its own dedicated oil cooler, often larger than the engine's coolant radiator.
- Many modern torque converters include a lock-up clutch that bolts the impeller directly to the turbine once they're spinning together, trading the converter's smoothness for a mechanical connection's efficiency.