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A torque converter is a hydraulic fluid coupling that is used to transmit power from one or more engines or motors to a driveshaft or other output shaft. It takes the place of a mechanical clutch, and, within certain operating speed ranges, multiplies input torque, providing the equivalent of a reduction gear. Torque converters are commonly found in automotive automatic transmissions, but are also used in marine propulsion systems and various industrial machine tools.
Function
Basic fluid-coupling elements
A torque converter is a fluid coupling. Think of a fluid coupling as a sealed chamber filled with hydraulic fluid (typically light oil). The driving side, connected to the engine, applies force to the driven side, connected to the input of the transmission. The driving member is known as the converter pump. The pump has a direct, constant mechanical connection to the engine at all times. The driven member is known as the converter turbine. The turbine has a direct connection to the engine only when the converter lock-up clutch is activated for fuel-savings. However, the connection between the two turbines within the converter is more of a "fluid connection." This fluid connection between the engine and transmission is what allows automatic transmission automobiles to sit in an idle state without placing the transmission into neutral.When the engine is operating the converter pump turns. This rotation of the radial chambers on the inner surface of the pump imparts a centrifugal radial flow to the fluid in the converter, which causes hydraulic fluid to strike the outer edges of the turbine. The radial chambers on the surface of the turbine transmit the angular momentum of the fluid centripetally, reversing its direction and exerting a twisting force torque on the turbine disc that causes it to rotate in the same direction as the impeller. The fluid exits the center of the turbine and returns to the impeller, to begin the cycle again.
Because some of the fluid's kinetic energy is lost due to friction, the converter will constantly put off heat as a byproduct. With low torque-stall ratio (factory type) converters, if the speed of the converter pump is very low -- such as at idle speed for an automobile engine -- little torque will be transmitted to the driven side. The fluid will have little to no contact with the turbine fins due to their angles and the redirection provided by the converter's stator.
Despite the efficiency loss designed into converters, moderate slippage of the coupling provides a smoother, more usable flow of power to the wheels of the automobile. As a further benefit, torque multiplication from high torque-stall ratio converters is perferable to using a manual transmission when towing.
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 efficent. 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.
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.
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.Converters are typically strengthened by means of furance brazing. This is a process where liquid brass is used to re-enforce the mechanical connection between the blades of the turbine and the cocentric ring in the turbine.
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