Torque converter
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Image:Torque-converter-cutbox-model.jpg A torque converter is modified form of a hydrodynamic fluid coupling, and like the fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. As with the fluid coupling, the torque converter takes the place of a mechanical clutch. Unlike a fluid coupling, however, a torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. The most widespread usage of torque converters is in automobile, bus and light truck automatic transmissions. Torque converters are also found in marine propulsion systems and industrial applications.
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[edit] Function
[edit] Torque Converter Elements
A torque converter is a type of hydrodynamic drive whose function is very similar to that of a fluid coupling. The principal difference is that whereas a fluid coupling is a two element drive that is incapable of multiplying torque, a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque. It is suggested to the reader that he or she become familiar with the principles of hydrodynamic drives before continuing by reading the fluid coupling article.
In a torque converter there are at least three rotating elements, namely 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 alter oil flow returning from the turbine to the pump. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on a one way clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation.
Modifications to the basic three element design have been periodically found, especially in applications where higher than normal torque mutiplication is required. Most commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differeing amounts of torque multiplication. For example, the Buick Dynaflow automatic transmission was a non-shifting design and, under normal conditions, relied solely upon the converter to multiply torque. The Dynaflow used a five element converter.
Although not strictly a part of classic torque converter design, many automotive converters include a lock-up clutch to improve cruising power transmission efficiency. The application of the clutch locks the turbine to the pump, causing all power transmission to be mechanical.
[edit] Operational Phases
For the purposes of explanation, a torque converter can be considered to have three stages of operation:
- Stall. The prime mover is applying power to the pump but the turbine cannot rotate. For example, in an automobile, this stage of operation would occur 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 produce maximum torque multiplication if sufficient input power is applied (the resulting multiplication is called the stall ratio). The stall phase actually lasts for a brief period 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 relatively large difference between pump and turbine speed. Under this condition, the converter will produce torque multiplication that is less that what could be achieved under stall conditions. The amount of multiplication will depend upon the actual difference between pump and turbine speed, as well as various other design factors.
- Coupling. The turbine has reached approximately 90 percent of the speed of the pump. Torque multiplication has ceased and the torque converter is behaving in a manner similar to a fluid coupling. In modern automotive applications, it is usually 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 lies in the stator. In the classic fluid coupling design, periods of high slippage cause the oil flow returning from the turbine to the pump to oppose the direction of pump rotation, leading to a significant loss of efficiency (as well as the generation of considerable waste heat). Under the same condition in a torque converter, the returning oil will be redirected by the stator so that it aids the rotation of the pump, instead of impeding it. The result is that much of the energy in the returning oil is recovered and thus can add to the energy being applied by the pump itself. The result is a substantial increase in the mass of oil being directed to the turbine, causing an increase in output torque. Since the return oil is initially traveling in a direction opposite to pump rotation, the stator will likewise attempt to counter-rotate as it forces the oil to change direction, an effect that is resisted by the one-way stator clutch.
Unlike the radially straight blades used in a fluid coupling, a torque converter's turbine, as well as the stator, uses angled and curved blades. The blade shape of the stator is what alters the path of the oil, forcing it to coincide with the pump rotation. The matching curve of the turbine blades helps to correctly direct the return oil to the stator so the latter can do its job.
During the stall and acceleration phases, in which torque multiplication occurs, the stator remains stationary due to the action of its one-way clutch. However, as the torque converter approaches the coupling phase, the energy and volume of returning oil will gradually decrease, causing pressure on the stator to reverse, attempting to rotate the stator in the same direction as the pump and turbine. At this point, the stator clutch will release and the pump, turbine and stator will all (more or less) turn as a unit.
Unavoidably, some of the fluid's kinetic energy will be lost due to friction, causing the converter to waste heat (dissipated in many applications by water cooling). This effect is most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity, which allows the converter to be stalled with the engine at idle speed for long periods with little danger of overheating.
[edit] Stator torque multiplication
A torque converter differs from a simple fluid coupling by the addition of a stator, a disc with fan-like blades connected to the transmission via a fixed shaft with a one-way clutch that allows it to rotate only in the opposite direction of the fluid's radial motion. Without the stator, fluid leaving the turbine would strike the impeller with a radial motion opposite its rotation, causing a braking effect. With the stator, the returning fluid strikes the stator blades, which reverses the radial direction of the fluid's motion so that it is moving the same direction as the impeller when it reenters the impeller chambers. This reversal of direction greatly increases the efficiency of the impeller, and the force of the fluid striking the stator blades also exerts torque on the turbine output shaft, providing additional torque multiplication equivalent to a higher numerical gear ratio.
As engine speed increases, the speed of the impeller and the turbine become nearly the same (reaching their point of minimum slippage). This is called coupling speed or stall speed and is where the converter is generally more efficient. Because the turbine is spinning faster than the fluid can exit its radial chambers, the net angular momentum of the exiting fluid is in the same direction as the turbine's rotation, rather than opposite it. As the impeller approaches this speed, the torque multiplication provided by the stator decreases. At that critical speed (the converter's stall speed) the fluid strikes the back of the stator blades, causing the stator to freewheel so that it will not interfere with the return flow of fluid.
The maximum amount of torque multiplication provided by the stator depends on the angle and design of its blades. Typical torque multiplication ranges from 1.8 to 2.5:1 for most automotive applications, up to 5.0:1 or more for static industrial applications or heavy maritime propulsion systems. The blade angle and shape also affects the stall speed of the stator, although actual stall speed is also a function of the engine's input torque; an engine with less torque will stall the stator at lower rpm.
While stator multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of the torque converter must be matched to the torque curve of the power source and the intended application. Changing the design of the radius and curvature of the toroid will change the torque-stall characteristics. Drag racing transmissions often use converters with high stall speeds to improve off-the-line torque, and to get into the power band of the motor faster. Engineers use lower stall torque converters to limit heat production, and provide a more firm feeling to the car.
Some torque converters, such as certain versions of General Motors' Turbo-Hydramatic, have a variable-pitch stator that can alter the angle of the stator blades between two or more positions depending on engine speed and throttle position, usually by means of a solenoid that moves the blades to a higher angle when engaged. This was marketed in the late 60's as "Switch-Pitch." It was only found in larger cars utilizing the Turbo 400 (TH 400). This enhanced off-the-line performance while keeping similar engine displacement.
Some torque converters use multiple stators and/or multiple turbines to provide a wider range of torque multiplication. Such multiple-element converters are more common in industrial applications than in automotive transmissions, but such automobile systems as Buick's Triple Turbine Dynaflow and Chevrolet's Turboglide dispensed with mechanical gearing entirely except for reverse, relying instead on torque multiplication by the converter to provide the equivalent of a continuously variable transmission. Turboglides are commonly used in non-professional drag racing as less time is lost in shifting, lower weight, and cost are issues. Automakers had largely stopped manufacturing these transmissions by the early 1960s due to market interest. The Turboglide also offered little fuel economy.
[edit] Lock-up torque converters
Because slip within the torque converter reduces efficiency and may generate excessive heat, some converters incorporate a lockup mechanism: a mechanical clutch that engages at cruising speeds to physically link the impeller with the turbine, causing them to rotate at the same speed with no slippage.
The first automotive application of the lock-up principle was Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration. The demand for increased automobile fuel economy brought about a gradual but widespread application of the lock-up converter for automotive transmissions between the late 1970s and mid-1980s.
[edit] Capacity
Torque converters have a rated torque capacity, the maximum input torque that the converter can safely withstand. Torque capacity is a function of the diameter of the converter housing, the volume of hydraulic fluid, available cooling, seal strength, and the materials used for the construction of components such as shafts and bearings.
The torque capacity is proportional to r(N^2)(D^5), where "r" is the mass density of the fluid, "N" is the impeller speed, and "D" is the diameter.
Converters are typically strengthened by means of furnace brazing. This is a process where liquid brass is used to re-enforce the mechanical connection between the blades of the turbine and the concentric ring in the turbine.
