Torque converter is a hydraulic fluid coupling that is used to transmit power from one or more engines
or electric 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.
Basic fluid-coupling elements
A torque converter is a fluid coupling. (Commonly, the term "fluid coupling" is restricted to that form of hydrodynamic drive which does not provide torque
multiplication. In other words, it does not have a "stator," the description and function of which will soon follow.) Think of a fluid coupling as a sealed
chamber filled with hydraulic fluid (typically light oil, namely transmission fluid). 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. On some automatic transmissions, you
might be able to hear the turbines make a humming sound before and/or after shifts or when the engine is started. This is caused by the activation of the converter pump.
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.
Since some of the fluid's kinetic energy is lost due to friction, the converter will constantly emit 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 preferable 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 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
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
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.
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.