Torque converter
From Wikipedia, the free encyclopedia
A torque converter is a hydraulic fluid coupling that is used to transmit power from one or more engines to a driveshaft. In many applications the torque converter can take place of the mechanical clutch. This is because the torque converter will allow the power source to continue to rotate when the output is stationary. Also, since the torque converter doubles as a gear it reduces the need for further gear reduction in the system. The torque converter is also safer as it never is disengaged or engaged. Power is now more quickly available and with no wear and jerk associated with a clutch system. Torque converters are commonly found in automotive automatic transmissions, but are also used in marine propulsion, rail locomotives, and various industrial machine tools.
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[edit] Description
The torque converter consists of the turbine, the pump, and the stator. The efficiency and torque ratio of the torque converter are very non-linear due to forces imparted by the stator (discussed later). The torque converter will have a very high torque multiplication ratio when the driven member is held stationary and this ratio will decay fairly linearly as the speed ratio approaches 1:1. The efficiency of the torque converter is very exponential when the driven member is held and levels off toward the 1:1 ratio. Yet, it does not fall off to 0% at 1:1 as the fluid coupler does.
The turbine has a solid connection to the power source. The impeller will only be connected to the turbine by forces imparted from the oil in the unit. Both parts are made up of blades or fins arranged in a circ,e which are used to propel or redirect the fluid. Additionally, there may be a lock-up clutch, which can create a firm connection between the pump and turbine. The clutch is usually only engaged when a speed ratio of 1:1 has been achieved between turbine and impeller.
The modern torque converter works well in an automotive application because the torque converter will make up for the engine's shortcomings: it will not drag the engine down below idle when the car is stationary or rolling backwards, and will even help multiply torque when engine speed is low. The torque converter cannot be made to run backwards, so the engine will not die even if the driver doesn't make a correct gear selection (possible with manual transmission, where it will stall the car).
The torque converter also runs quietly and smoothly without and of the wear or sticking problems associated with a clutch.
[edit] Workings
When the engine is operating, the 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 flows in at the centre and then is hurled outwards), which causes hydraulic fluid to strike the outer edges of the turbine. The radial fins on the surface of the turbine transmit the angular momentum of the fluid,, reversing its direction (from flowing outwards to inwards) and exerting a twisting force torque on the turbine disc that causes it to rotate in the same direction as the pump. The fluid exits the center of the turbine and returns to the pump, where it strikes the stator blades. These reverse the radial direction of the fluid's motion so that it is moving the same direction as the pump when it re-enters the pump. This reversal of direction greatly increases the mechanical efficiency of the pump, 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 gear ratio.
As engine speed increases, the speed of the pump 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.
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 converters, if the speed of the converter pump is very low - such as at idle speed for a car 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.
[edit] Stator torque multiplication
The fluid coupling will always have a torque ratio of 1:1 with efficiency at 100% when the speed ratio is 1:1. The efficiency is 0% when the driven member is held stationary. The efficiency is very linear with the speed ratio. One thing that must be noted is that the torque transferred between the driven and driving members is zero at the coupling speed. This means that this device strives to always be slightly under the coupling speed.
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. The stator switched from a low angle for crusing to a high angle for performance by as much as 75 degrees.
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 PowerGlide 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. PowerGlide 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 PowerGlide 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 and durability
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.
[edit] Principles
Each member has the same weight but different mean diameters. The driving member is twice the diameter of the driven member. This means it has four times the energy. If the driving member was spinning at 1000 RPM and the driven member was stationary, the stationary member could attain a speed of 2000 RPM if all energy was transferred instantaneously (one time interval). In a clutch system, both members would have to connect and since the new resultant member has a mass of 2X it would never approach 2000 RPM. Much more RPM is lost although it has the same momentum. Despite the same momentum the operating speed of the engine has dropped so it is producing less power over the next time interval, resulting in the advantage of an automatic transmission over a manual for standing starts.
As the input member turns, oil spirals outwards. When it reaches the end of the input member it looks like a rim of spinning oil. The energy that this toriodal shape of oil has is a function of its diameter, how fast it is stopped, and its weight. It does not take very much oil to impart a large amount of energy to a stopped car. This is because since the output member is stationary the oil has no choice but to be stopped very fast as if it hit a wall. When the output member is moving, obviously the oil doesn't hit as hard and less force is exerted. Consider this toriodal of oil moving at 32 feet per second, spinning at 1000 RPM, and with a mean diameter of 12 inches. In one second oil weighing almost 360 pounds will collide with the stalled member and produce a force of 300 foot-pounds if stopped in one second. Actually, it would have to be stopped and returned in one second or all of the oil would be on one side. After entering the output member and imparting energy, the rim of oil will change direction of flow and simply become a straight flow of oil entering the input member at the center.
[edit] Torque converter myths
One of the largest myths out in the rebuilding world is that welding a torque converter will strengthen the product and increase performance. This is done as a cheap substitute for furnace brazing and the results could destroy your driveline. As mentioned earlier, the fluid exits the input member at the edge of the converter pump. This is where welding is usually performed. In reality, no human or currently built machine could possibly tack the fins of the pump small enough to not divert oil flow. While the strength of the fins has been increased it has cost the oil flow momentum and created localized hot spots. If the converter was ran hot enough the fin could loose strength and break off causing extensive damage.
Another converter myth is arguing stall speeds. Unfortunately, the one way to tell a converters true stall speed is with a specially built converter dyno. Testing installed, car or not, the stall speed can not be read accurately enough due to variables in the braking holding the output member and differences in engine power over RPM.
