45933888-Torque-Converter

How Torque Converters WorkFrom How stuff works

If you've read about manual transmissions , you know that an engine is connected to a transmission by way of a clutch . Without this connection, a car would not be able to come to a complete stop without killing the engine. But cars with an automatic transmission have no clutch that disconnects the transmission from the engine. Instead, they use an amazing device called a torque converter. It may not look like much, but there are some very interesting things going on inside.

In this article, we'll learn why automatic transmission cars need a torque converter, how a torque converter works and what some of its benefits and shortcomings are..Auto Videos »

Torque converters make automatic transmission possible.

The Basics

The torque converter is situated between the engineand the transmission.

Just like manual transmission cars, cars with automatic transmissions need a way to let the engine turn while the wheels and gears in the transmission come to a stop. Manual transmission cars use a clutch , which completely disconnects the engine from the transmission. Automatic transmission cars use a torque converter.A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal.If you were to step on the gas pedal while the car is stopped, you would have to press harder on the brake to keep the car from moving. This is because when you step on the gas, the engine speeds up and pumps more fluid into the torque converter, causing more torque to be transmitted to the wheels.

Inside a Torque Converter

The parts of a torque converter (left to right): turbine, stator, pumpAs shown in the figure below, there are four components inside the very strong housing of the torque converter:

● Pump● Turbine● Stator● Transmission fluid

The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the engine is running at. The fins that make up the pump of the torque converter are attached to the housing, so they also turn at the same speed as the engine. The cutaway below shows how everything is connected inside the torque converter.

How the parts of the torque converter connect to the transmission and engine

The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside, much as the spin cycle of a washing machine flings water and clothes to the outside of

the wash tub. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center.

The pump section of the torque converteris attached to the housing.

The fluid then enters the blades of the turbine, which is connected to the transmission. The turbine causes the transmission to spin, which basically moves your car. You can see in the

graphic below that the blades of the turbine are curved. This means that the fluid, which enters the turbine from the outside, has to change direction before it exits the center of the turbine. It is

this directional change that causes the turbine to spin.

The torque converter turbine: Note the spline in the middle. This is where it connects to the transmission.

In order to change the direction of a moving object, you must apply a force to that object -- it doesn't matter if the object is a car or a drop of fluid. And whatever applies the force that causes

the object to turn must also feel that force, but in the opposite direction. So as the turbine causes the fluid to change direction, the fluid causes the turbine to spin.

The fluid exits the turbine at the center, moving in a different direction than when it entered. If you look at the arrows in the figure above, you can see that the fluid exits the turbine moving

opposite the direction that the pump (and engine) are turning. If the fluid were allowed to hit the pump, it would slow the engine down, wasting power. This is why a torque converter has a

stator.

The Stator

The stator resides in the very center of the torque converter. Its job is to redirect the fluid returning from the turbine before it hits the pump again. This dramatically increases the efficiency of the torque converter.

The stator sends the fluid returning from the turbine to the pump. This improves the efficiency of the torque converter. Note the spline, which is connected to a one-way clutch inside the stator.The stator has a very aggressive blade design that almost completely reverses the direction of the fluid. A one-way clutch (inside the stator) connects the stator to a fixed shaft in the transmission (the direction that the clutch allows the stator to spin is noted in the figure above). Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades.Something a little bit tricky happens when the car gets moving. There is a point, around 40 mph (64 kph), at which both the pump and the turbine are spinning at almost the same speed (the pump always spins slightly faster). At this point, the fluid returns from the turbine, entering the pump already moving in the same direction as the pump, so the stator is not needed.Even though the turbine changes the direction of the fluid and flings it out the back, the fluid still ends up moving in the direction that the turbine is spinning because the turbine is spinning faster in one direction than the fluid is being pumped in the other direction. If you were standing in the back of a pickup moving at 60 mph, and you threw a ball out the back of that pickup at 40 mph, the ball would still be going forward at 20 mph. This is similar to what happens in the turbine: The fluid is being flung out the back in one direction, but not as fast as it was going to start with in the other direction.At these speeds, the fluid actually strikes the back sides of the stator blades, causing the stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving through it.

Benefits and Weak Points

In addition to the very important job of allowing your car come to a complete stop without stalling the engine, the torque converter actually gives your car more torque when you accelerate out of a stop. Modern torque converters can multiply the torque of the engine by two to three times. This effect only happens when the engine is turning much faster than the transmission.At higher speeds, the transmission catches up to the engine, eventually moving at almost the same speed. Ideally, though, the transmission would move at exactly the same speed as the engine, because this difference in speed wastes power. This is part of the reason why cars with automatic transmissions get worse gas mileage than cars with manual transmissions.To counter this effect, some cars have a torque converter with a lockup clutch. When the two halves of the torque converter get up to speed, this clutch locks them together, eliminating the slippage and improving efficiency.

Torque converter

From Wikipedia, the free encyclopedia

ZF torque converter cut-away

A cut-away model of a torque converterA torque converter is a fluid coupling that is used to transfer rotating power from a prime mover , such as an internal combustion engine or electric motor , to a rotating driven load. Like a basic fluid coupling, the torque converter normally takes the place of a mechanical clutch , allowing the load to be separated from the power source. 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 .

Contents[hide]

● 1 Usage ● 2 Function

○ 2.1 Torque converter elements ○ 2.2 Operational phases ○ 2.3 Efficiency and torque multiplication ○ 2.4 Lock - up torque converters ○ 2.5 Capacity and failure modes

● 3 Manufacturers ○ 3.1 Current ○ 3.2 Past

● 4 See also ● 5 References ● 6 External links

[edit] Usage● Automatic transmissions on automobiles , such as cars, buses, and on/off highway

trucks.

● Forwarders and other heavy duty vehicles.● Marine propulsion systems.● Industrial power transmission such as conveyor drives, almost all modern forklifts,

winches, drilling rigs , construction equipment , and railway locomotives .

[edit] Function

[edit] Torque converter elements

A fluid coupling is a two element drive that is incapable of multiplying torque, while 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.In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover ; the turbine, which drives the load ; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. 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 an overrunning clutch , which prevents the stator from counter-rotating with respect to the prime mover but allows forward rotation.Modifications to the basic three element design have been periodically incorporated, especially in applications where higher than normal torque multiplication is required. Most commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differing 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 to produce the wide range of torque multiplication needed to propel a heavy vehicle.Although not strictly a part of classic torque converter design, many automotive converters include a lock - up clutch to improve cruising power transmission efficiency and reduce heat. The application of the clutch locks the turbine to the impeller, causing all power transmission to be mechanical, thus eliminating losses associated with fluid drive.

[edit] Operational phases

A torque converter has three stages of operation:● Stall. The prime mover is applying power to the impeller 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 impeller and turbine speed. Under this condition, the converter will produce

torque multiplication that is less than 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 impeller. Torque multiplication has essentially ceased and the torque converter is behaving in a manner similar to a simple 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 fluid flow returning from the turbine to the impellor to oppose the direction of impeller rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the impeller by the prime mover. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially travelling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the one - way stator clutch .Unlike the radially straight blades used 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 alters the path of the fluid, forcing it to coincide with the impeller rotation. The matching curve of the turbine blades helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades is important as minor variations can result in significant changes to the converter's performance.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 the fluid returning from the turbine will gradually decrease, causing pressure on the stator to likewise decrease. Once in the coupling phase, the returning fluid will reverse direction and now rotate in the direction of the impellor and turbine, an effect which will attempt to forward-rotate the stator. At this point, the stator clutch will release and the impeller, 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 and turbulence, causing the converter to generate waste heat (dissipated in many applications by water cooling). This effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity at low impeller speeds, which allows the turbine to be stalled for long periods with little danger of overheating.

[edit] Efficiency and torque multiplication

A torque converter cannot achieve 100 percent coupling efficiency. The classic three element torque converter has an efficiency curve that resembles ∩: zero efficiency at stall, generally increasing efficiency during the acceleration phase and low efficiency in the coupling phase. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence

and fluid flow interference generated by the stator, and as previously mentioned, is commonly overcome by mounting the stator on a one-way clutch.Even with the benefit of the one-way stator clutch, a converter cannot achieve the same level of efficiency in the coupling phase as an equivalently sized fluid coupling. Some loss is due to the presence of the stator (even though rotating as part of the assembly), as it always generates some power-absorbing turbulence. Most of the loss, however, is caused by the curved and angled turbine blades, which do not absorb kinetic energy from the fluid mass as well as radially straight blades. Since the turbine blade geometry is a crucial factor in the converter's ability to multiply torque, trade-offs between torque multiplication and coupling efficiency are inevitable. In automotive applications, where steady improvements in fuel economy have been mandated by market forces and government edict, the nearly universal use of a lock-up clutch has helped to eliminate the converter from the efficiency equation during cruising operation.The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-element designs as used in the Buick Dynaflow and Chevrolet Turboglide could produce more). Specialized converters designed for industrial, rail, or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inefficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication.While torque 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 carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. For example, drag racing automatic transmissions often use converters modified to produce high stall speeds to improve off-the-line torque, and to get into the power band of the engine more quickly. Highway vehicles generally use lower stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics.A design feature once found in some General Motors automatic transmissions was the variable-pitch stator, in which the blades' angle of attack could be varied in response to changes in engine speed and load. The effect of this was to vary the amount of torque multiplication produced by the converter. At the normal angle of attack, the stator caused the converter to produce a moderate amount of multiplication but with a higher level of efficiency. If the driver abruptly opened the throttle, a valve would switch the stator pitch to a different angle of attack, increasing torque multiplication at the expense of efficiency.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 environments than in automotive transmissions, but automotive applications such as Buick 's Triple Turbine Dynaflow and Chevrolet 's Turboglide also existed. The Buick Dynaflow utilized the torque-multiplying characteristics of its planetary gearset in conjunction with the torque

converter for low gear and bypassed the first turbine, using only the second turbine as vehicle speed increased. The unavoidable trade-off with this arrangement was low efficiency and eventually these transmissions were discontinued in favor of the more efficient three speed units with a conventional three element torque converter.

[edit] Lock-up torque converters

As described above, impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss.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 or as the vehicle slowed down. This feature was also present in some Borg - Warner transmissions produced during the 1950s. It fell out of favor in subsequent years due to its extra complexity and cost. In the late 1970s lock-up clutches started to reappear in response to demands for improved fuel economy, and are now nearly universal in automotive applications.

[edit] Capacity and failure modes

As with a basic fluid coupling the theoretical torque capacity of a converter is proportional to

, where r is the mass density of the fluid (kg/m³), N is the impeller speed (rpm), and D is the diameter(m).[1] In practice, the maximum torque capacity is limited by the mechanical characteristics of the materials used in the converter's components, as well as the ability of the converter to dissipate heat (often through water cooling). As an aid to strength, reliability and economy of production, most automotive converter housings are of welded construction. Industrial units are usually assembled with bolted housings, a design feature that eases the process of inspection and repair, but adds to the cost of producing the converter.In high performance, racing and heavy duty commercial converters, the pump and turbine may be further strengthened by a process called furnace brazing , in which molten brass is drawn into seams and joints to produce a stronger bond between the blades, hubs and annular ring(s). Because the furnace brazing process creates a small radius at the point where a blade meets with a hub or annular ring, a theoretical decrease in turbulence will occur, resulting in a corresponding increase in efficiency.Overloading a converter can result in several failure modes, some of them potentially dangerous in nature:

● Overheating: Continuous high levels of slippage may overwhelm the converter's ability to dissipate heat, resulting in damage to the elastomer seals that retain fluid inside the converter. This will cause the unit to leak and eventually stop functioning due to lack of fluid.

● Stator clutch seizure: The inner and outer elements of the one - way stator clutch become permanently locked together, thus preventing the stator from rotating during the coupling phase. Most often, seizure is precipitated by severe loading and subsequent distortion of the clutch components. Eventually, galling of the mating parts occurs, which

triggers seizure. A converter with a seized stator clutch will exhibit very poor efficiency during the coupling phase, and in a motor vehicle, fuel consumption will drastically increase. Converter overheating under such conditions will usually occur if continued operation is attempted.

● Stator clutch breakage: A very abrupt application of power can cause shock loading to the stator clutch , resulting in breakage. When this occurs, the stator will freely counter-rotate the pump and almost no power transmission will take place. In an automobile, the effect is similar to a severe case of transmission slippage and the vehicle is all but incapable of moving under its own power.

● Blade deformation and fragmentation: Due to abrupt loading or excessive heating of the converter, the pump and/or turbine blades may be deformed, separated from their hubs and/or annular rings, or may break up into fragments. At the least, such a failure will result in a significant loss of efficiency, producing symptoms similar (although less pronounced) to those accompanying stator clutch failure. In extreme cases, catastrophic destruction of the converter will occur.

● Ballooning: Prolonged operation under excessive loading, very abrupt application of load, or operating a torque converter at very high RPM may cause the shape of the converter's housing to be physically distorted due to internal pressure and/or the stress imposed by centrifugal force. Under extreme conditions, ballooning will cause the converter housing to rupture, resulting in the violent dispersal of hot oil and metal fragments over a wide area.

[edit] Manufacturers

[edit] Current

● Allison Transmission , used in bus, refuse, fire, construction, distribution, military and specialty applications

● BorgWarner , used in automobiles ● Subaru , used in automobiles● Twin Disc , used in vehicle, marine and oilfield applications● Voith Turbo - Transmissions , used in many diesel locomotives and diesel multiple units ● ZF Friedrichshafen , used in automobiles● Jatco , used in automobiles

[edit] Past

● Lysholm-Smith, named after its inventor, Alf Lysholm , and used in some British Rail Derby Lightweight diesel multiple units

● Mekydro [2], used in British Rail Class 35 Hymek locomotives.● Voith , used in British Rail Class 52 , and still used in German locomotives,● Packard , used in the Ultramatic automobile transmission system● Rolls - Royce ( Twin Disc ) , used in some British United Traction diesel multiple units

A torque converter is a mechanical device, used mainly in automobiles, that transfers the rotating power generated by an vehicle's engine to the transmission . It is part of the family of mechanisms known as fluid couplings, which use hydraulic fluid to transmit mechanical power. A torque converter is installed in automatic transmissions and does the job a clutch would do in a manual transmission, which is allowing the power created by the engine to be distributed to the wheels.

A torque converter consists of three mechanical parts — a pump, a turbine , and a stator. The pump is attached to directly to the engine, and spins at the same speed as the motor. Inside the pump are many fins, which, as the pump spins, direct hydraulic fluid outward to the turbine. The turbine then spins at close to the same speed as the engine, but in the opposite direction. The spinning of the turbine causes the transmission to rotate and drive the wheels. The hydraulic fluid exits the turbine at its center, moving in the direction opposite to how it was forced in by the pump.At this point, the stator, which is similarly located in the center of the converter, reverses the direction of the fluid a second time. This greatly increases the efficiency of the overall design, but only occurs at relatively low speeds. Depending on the precise specifications of the torque converter, the stator begins to freewheel at a particular speed, because the pump and turbine begin moving at almost exactly the same speed, and the fluid no longer changes direction.One particular advantage a torque converter has over a conventional fluid coupling — and what makes it ideal for use in automatic transmissions — is the fact that it can multiply the amount of torque it generates as the engine provides more power. A real-world example of this is the comparison of the relatively light pressure that must be applied to a brake pedal to keep a car stationary while idling, when compared to the increase in pressure needed to keep it still when gas is also applied. At very low speeds torque can be multiplied two or three times by a torque converter.

One of the major downsides to torque converters, as opposed to normal fluid couplings, is that, given how the pump and turbine never spin at exactly the same speed, some power is always wasted. This, along with its typically heavier weight, is the reason that manual transmission vehicles tend to get better fuel mileage than those with automatic transmission .