Torque Converter: Description and Operation

Torque Converter

The torque converter serves two main functions: it acts as a fluid coupling between engine and transmission gear train and it provides multiplication of engine torque when required.

The torque converter consists of an impeller, a turbine and a stator which is mounted on a one-way clutch. The converter cover is fixed to the impeller to seal the three elements within a common housing through which oil flows when the transmission is in operation. The impeller is therefore connected to the engine and rotates at engine speed when the engine is operating.

Torque multiplication is at its greatest when the vehicle is driven away from standstill. The stator bears against the housing through its one-way clutch and deflects the oil so that the flow is accelerated. As road speed builds up, the impeller and turbine wheels turn at almost the same speed, so that the stator ceases to accelerate the oil flow and instead is disengaged from the housing and turns freely in the flow of oil. At this stage, the torque converter acts as a fluid coupling

The torque converter acts in all four forward gears and in reverse. The converter lock-up clutch closes in 4th gear at a point depending on road speed and accelerator position; engine power is then transmitted by purely mechanical means.

A low-loss oil pump in the front of the transmission housing supplies the converter, the epicyclic gear train, the hydraulic control system and the lubrication circuit; oil is drawn from a sump in the bottom of the gear train housing through a filter. The pump drive gear is keyed to the converter impeller hub and therefore turns when the engine is running, thereby pressurizing the oil.