Torque converter
A torque converter is a adapted form of fluid coupling that is utilized to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. Like a common fluid coupling, the torque converter usually takes the place of a mechanical clutch, permitting the load to be separated from the power source. As a more elevated form of fluid coupling, nevertheless, a torque converter is capable to multiply torque when there is a significant difference between input and output rotational speed, thus providing the equivalent of a reduction gear.
Usage
- Automatic transmissions on automobiles, such as cars, buses, and on/off highway trucks.
- Marine propulsion systems.
- Industrial power transmission such as conveyor drives, almost all modern forklifts, winches, drilling rigs, construction equipment, and locomotives.
Torque converter elements
A fluid coupling is a 2 component drive that is unable of multiplying torque, while a torque converter has at least one additional element—the stator—which modifies the drive’s device characteristics during periods of high slippage, creating an increase in output torque.
The Structure of a Torque Converter
In a torque converter there are at least three rotating components: the pump, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the pump and turbine so that it can modify oil flow returning from the turbine to the pump. The standard torque converter design dictates that the stator be prevented from rotating under any circumstance, hence the term stator. In practice, nevertheless, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating with respect to the prime mover but permits for forward rotation.
Alterations to the basic three element design have been periodically integrated, specially in applications where higher than normal torque multiplication is necessitated. Most usually, these have adopted the form of multiple turbines and stators, each set being designed to produce differing amounts of torque multiplication. For instance, the Buick Dynaflow automatic transmission was a non-shifting design and, under normal circumstances, relied exclusively upon the converter to multiply torque. The Dynaflow utilized a five element converter to produce the wide range of torque multiplication required to propel a heavy vehicle.
Although not rigorously a component of standard torque converter design, numerous automotive converters include a lock-up clutch to ameliorate cruising power transmission efficiency. The application of the clutch locks the turbine to the pump,determinating all power transmission to be mechanical, therefore eliminating losses associated with fluid drive.
Operational phases
A torque converter has three stages of operation:
- Stall. The prime mover is applying power to the pump but the turbine cannot rotate. For instance, in an automobile, this stage of operation would take place when the driver has placed the transmission in gear but is preventing the vehicle from moving by continuing to apply the brakes. At stall, the torque converter can develop maximum torque multiplication if decent input power is applied . The stall phase in reality lasts for a short period of time when the load (e.g., vehicle) initially starts to move, as there will be a very large difference between pump and turbine speed.
- Acceleration. The load is accelerating but there still is a comparatively large difference between pump and turbine speed. Under this circumstance, the converter will develop torque multiplication that’s inferior than what could be accomplished under stall circumstances. The amount of multiplication will depend on the actual difference between pump and turbine speed, as well as several other design factors.
- Coupling. The turbine has achieved approximately 90 percent of the speed of the pump. Torque multiplication has in essence ceased and the torque converter is conducting in a manner similar to a plain fluid coupling. In current automotive applications, it’s commonly at this stage of operation where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency.
The key to the torque converter’s ability to multiply torque consists in classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the pump to counterbalance the direction of pump rotation, leading to a significant loss of efficiency and the generation of appreciable waste heat. Under the identical circumstance in a torque converter, the returning fluid will be redirected by the stator so that it assists the rotation of the pump, instead of obstructing it.
The effect is that a lot of the energy in the returning fluid is recuperated and added to the energy being applied to the pump by the prime mover. This process causes a significant increment in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is at the start traveling in a direction contrary to pump rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to alter direction, an effect that is resisted by the one-way stator clutch.
Contrary to the radially straight blades utilized in a plain fluid coupling, a torque converter’s turbine and stator use angled and curved blades. The blade shape of the stator is what modifies the path of the fluid, forcing it to concur with the pump rotation. The corresponding curve of the turbine blades serves to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades is substantial as minor variations can result in significant changes to the converter’s performance.
During the stall and acceleration phases, in which torque multiplication happens, the stator remains stationary due to the action of its one-way clutch. Nonetheless, as the torque converter approaches the coupling phase, the energy and volume of the fluid returning from the turbine will step by step diminish, inducing pressure on the stator to likewise decrease. When in the coupling phase, the returning fluid will reverse direction and now rotate in the direction of the pump and turbine, an effect which will attempt to forward-rotate the stator. At this period, the stator clutch will release and the pump, turbine and stator will all turn as a unit.
Inevitably, some of the fluid’s kinetic energy will be unrecoverable due to friction and turbulence, getting the converter to generate waste heat (dissipated in many applications by water cooling). This issue, often referred to as pumping loss, will be most noticeable at or near stall circumstances. In advanced designs, the blade geometry minimizes oil velocity at low pump speeds, which allows the turbine to be stalled for long periods of time with little risk of overheating.