torque converter

A torque converter is a sealed chamber filled with automatic transmission fluid (ATF) that transfers rotational power from an engine or motor to a load, typically the wheels of a vehicle through an automatic transmission. Unlike a mechanical clutch, it uses fluid motion rather than friction to engage and disengage, and it can multiply torque at the output shaft when the input and output speeds differ significantly. This makes it possible for an engine running at, say, 2000 rpm to accelerate a stationary transmission input from zero rpm without stalling the engine.

The converter contains three main rotating elements: the impeller (driven by the engine), the turbine (driving the transmission input shaft), and the stator (a one-way clutched wheel that redirects fluid flow). ATF is accelerated outward by the spinning impeller and strikes the turbine blades, transferring momentum. The fluid then enters the stator, which redirects it back to the impeller at a more efficient angle when turbine speed is low. This redirection creates the torque multiplication effect. As turbine speed rises and approaches impeller speed, the stator clutch unlocks and the converter loses multiplying efficiency but gains overall efficiency.

The ratio of torque at the output to torque at the input is called the stall ratio or torque multiplication factor. Most automotive torque converters have stall ratios between 1.8 and 2.5, meaning a 200 ft-lb input torque can become 360 to 500 ft-lb at the turbine shaft when the turbine is locked or near-stalled. Heavy-duty industrial converters used on mining equipment or marine vessels may have different stall ratios depending on load profile. As converter speeds synchronize, multiplication decreases; at full coupling (when impeller and turbine turn at nearly the same speed), the torque ratio approaches 1:1 and efficiency approaches that of a solid mechanical shaft.

Common failures and operating constraints

ATF degradation is the primary wear mechanism. Heat, water contamination, and shear damage to the fluid's viscosity modifiers reduce lubrication and cooling, causing turbine blade erosion, impeller cavitation, and bearing failure. Overheating above 200°C (390°F) accelerates fluid breakdown. Metal particles from bearing wear contaminate the fluid further, creating a failure spiral. Converter stall testing (holding the turbine stationary while the impeller spins) is used as a diagnostic check; normal stall speed for passenger vehicles is typically 1600 to 2200 rpm, and deviation signals internal damage. High stall speeds often indicate slipping, low speeds indicate converter lockup or blockage.

In automatic transmissions, the torque converter has largely been supplemented or replaced by lock-up clutches that mechanically couple the impeller and turbine at highway speeds to improve fuel economy. Modern converters may have internal damping springs to reduce drivetrain shock and prolong clutch life in the transmission. Industrial and marine applications continue to rely on larger converters without lock-up devices, since those applications prioritize smooth acceleration and load absorption over fuel efficiency.