Torque converter

A torque converter is a hydraulic device prevalent in automatic transmissions. It consists of a toroidal chamber containing two elements, a centrifugal pump (forming the front part of the housing and attached to the motor shaft with a flex plate), and an similar hydraulic motor, contained within (but not bound to) the after part of the housing and attached to the remaining portion of the transmission with an output shaft. The remaining portion of the transmission will usually contain gears in the form of planitary gearsets for changing the output ratio and for reverse operation.

The hydraulic fluid (usually a light oil) is drawn from the inner portion of the pump and coerced to move with the pump by radial impeller segments within the pump. This action imparts both a radial flow (by centrifugal force) and an angular momentum to the fluid. The fluid exits the pump at the outer portion and impinges upon similar impeller segments in the hydraulic motor part of the converter. The fluid then returns toward the axis of the device and is recirculated into the inner portion of the pump, completing the fluid cycle.

Even with the hydraulic motor portion motionless, the redirection of the fluid mass from its spiral motion in the pump to the radial motion in the (converter) motor, will cause a torque (twisting force) to be applied to the output shaft. This torque can exceed the torque applied to the input, hence the name of the device. A lower torque at high rotational speed is converted to a higher torque at lower speed. This conversion is the essential function of any vehicle transmission, manual or automatic.

With the input shaft at low speed, the converter becomes quite inefficient (it is said to be "stalled"), and so only a small torque is applied to the output shaft. This property enables the elimination of the clutch, an essential part of drivelines using manual transmissions, but requires the addition of a parking pawl to lock the output shaft.

When a large displacement motor capable of high torque is used it is possible to build an effective transmission with only two forward gear ratios. In the interest of higher efficiencies and better performance, transmissions are now designed with four to six forward ratios, so the torque converter will operate only within a narrow range of input to output speed ratios.

Torque converters, by their nature, exhibit "slip", which tends to waste energy by heating the fluid. To obtain higher efficiencies modern torque converters contain an additional clutch-like mechanism to mechanically lock the pump and motor portions together. This is called a "locking" torque converter. The segments are locked when the transmission's computer elements determine that the vehicle is cruising at near constant speed.