Torque Converter

Fig 1: Identifying Torque Converter

Courtesy of VOLVO CARS CORPORATION

The torque converter is between the engine and the automatic transmission.

When the engine is idling the pump effect is too weak to drive the turbine and the vehicle will not move. As the engine speed rises it successively starts to drive in a gradual way. At higher engine speeds the power transfer to the turbine can be up to 95%. The torque converter functions as an hydraulic clutch. It also amplifies the torque of the engine at lower engine speeds and therefore an automatic transmission does not require as many gears as a manual transmission.

The torque converter consists of a round metal casing which contains two impeller wheels and is filled with oil. One of the impeller wheels, the pump wheel, is fixed to the casing. Both are connected to the engine crankshaft and rotate with it. The other impeller wheel, the turbine wheel, is connected to the input shaft of the transmission and is driven by the oil which is pumped around by the pump wheel. The rotating impeller wheels and the oil in the torque converter "slip" slightly and creates a slight loss of power, which raises fuel consumption slightly.

Thanks to a third, smaller wheel, the stator wheel, the torque converter also reinforces the engine torque at low engine speeds.