You must have heard about the torque converters which are used in vehicles with automatic transmission replacing the clutch and pressure plates. It is a marvelous piece of engineering because the incredible design makes it possible to engage and disengage the power from the engine to the gearbox completely automatically and with high efficiency. So, no need to press any clutch pedal and changing of gears. But as the name suggests, how does the torque converter convert the torque to any higher value than the torque produced by the engine?
A fluid coupling consists of an impeller or pump and a turbine. A typical fluid coupling consists of an impeller that moves the fluid, and a turbine which absorbs the energy of that moving fluid and rotates itself. The torque converter is much like a fluid coupling but comes equipped with an extra part called stator.
The stator is installed just in between those two components. Reminding you the basics, the impeller is driven by the engine itself and turbine is connected the output shaft of the torque converter. The output shaft becomes the input to the automatic gearbox. When the impeller is driven by the input shaft, the fluid is sucked in from the center of the impeller and leaves through the perimeter. The turbulently moving fluid after leaving the impeller vanes, is directed towards the turbine blades perimeter due to the shape of the housing.
The turbine starts rotating due to the momentum provided by the fluid. The fluid exits the turbine through its center. The fluid leaving the turbine blades are directed now towards the stator blades. The stator is placed and its blades designed and positioned in such a way that the fluid leaving the stator pushes the impeller blades forward in the direction of its rotation. This extra push makes it possible for the torque multiplication to take place. The impeller then transmits that torque through the turbine to the output shaft. Typically, a torque multiplication of 2 to 3 times is achievable by using a torque converter. Maximum torque conversion takes place when the torque converter is at a stall. This is that condition when the turbine has stopped rotating due to heavy load on the vehicle (as turbine is connected to the output shaft) and the impeller is at its highest rpm to meet that load demand. On the other hand, when both the impeller and turbine start rotating at almost same rotational speed, the torque conversion is at its lowest.
The torque output of the torque converter varies between minimum and maximum between these two conditions. When the impeller is rotating at high RPM but turbine is rotating at low RPM due to heavy loading or it has just started to rotate, a vortex flow of fluid is taking place. A vortex flow is a kind of spiral like flow. More is the vortex flow; more torque is being generated. When the two components start rotating at almost same speeds in the later stage, the vortex flow ceases and rotary flow of fluid starts. The torque conversion also diminishes. During the vortex flow the stator does not rotate because it is fitted with a one-way clutch allowing it to rotate in only one direction. But when the rotary flow commences, the stator also starts rotating in its dictated direction of rotation. The torque conversion ceases when the stator also starts rotating.
Hopefully, it helped you understand how the torque is being converted.
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