Torque Converter With Lock-Up

Fig 1: Identifying Cross Section Of Torque Converter With Lock-Up

Courtesy of VOLVO CARS CORPORATION

The torque converter is between the engine and the automatic transmission. It consists of a round metal casing which contains three impellers. The torque converter is filled with oil. The first impeller, pump rotor, is secured in the torque converter casing, which is connected to the engines crankshaft via a carrier plate. The pump rotor rotates with the crankshaft.

The second impeller, the turbine rotor, is connected to the transmission's input shaft and is driven by the oil that is circulated by the pump rotor.

The third impeller, the stator, is located between the pump rotor and the turbine rotor.

There is a hydraulically controlled lock-up clutch with a friction plate in the torque converter. The turbine rotor can connect with the torque converter housing via the lock-up clutch. This reduces fuel consumption.

The torque converter functions like a hydraulic automatic clutch. At idle, the pump action is too weak to drive the turbine rotor and thus the vehicle is stationary. When the engine speed is increased the turbine rotor starts to drive in a smooth way. Thanks to the stator rotor, the torque converter can also reinforce the engines torque when starting and low driving speed.

The rotating impellers and the oil in the torque converter "slip" slightly. This gives a certain power loss, which slightly increases fuel consumption.

When the vehicle is moving the torque converter's reinforcement of the engines torque is not needed. In this situation the lock-up function is activated and, at a certain speed, mechanically connects the transmission's input shaft with the engine. This is automatic and cannot be affected by the driver. The engine speed drops and, since the torque converter's slipping disappears, fuel consumption is also reduced.