Abstract

The torque converter and torque converter clutch are

critical devices governing overall power transfer

efficiency in automatic transmission powertrains. With

calibrations becoming more aggressive to meet

increasing fuel economy standards, the torque

converter clutch is being applied over a wider range of

driving conditions. At low engine speed and high

engine torque, noise and vibration concerns

originating from the driveline, powertrain or vehicle

structure can supersede aggressive torque converter

clutch scheduling. Understanding the torsional

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characteristics of the torque converter clutch and its

interaction with the drivetrain can lead to a more

robust design, operation in regions otherwise

restricted by noise and vibration, and potential fuel

economy improvement. The objective of this paper is

to present a generalized integration summary for

torque converter clutches (TCC) to enable aggressive

apply at high engine torques and engine speeds below

1500 rpm without adversely affecting noise and

vibration (N&V) performance. The focus is on

optimizing existing damper technologies found in a

large percentage of current production automatic

transmissions. The overall goal is to demonstrate that

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through proper hardware selection and TCC

calibration, a balance between N&V and fuel economy

can be achieved. A validated lumped parameter

modeling technique for optimizing torque converter

clutch hardware that comprehends the complete

powertrain and driveline system is used to investigate

required TCC hardware. A torsional isolation metric

across the powertrain is presented for comparing the

performance of various damper designs. For operating

points with inadequate isolation a controlled amount

of slip across the torque converter clutch is required

to increase isolation. An analytical methodology for

minimizing the amount of slip required to satisfy noise

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and vibration targets and maximize fuel economy is

reported. The effect of gear state, torque converter

slip and power delivered to the driveline on fuel

economy are also discussed based upon powertrain

dynamometer measurements.A 100- to175-word

summary of your term paper.

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Introduction

A torque converter uses fluid to smoothly transfer engine torque to the transmission. The torque converter ia a doughnut-shaped unit, located between the engine and the transmission and filled with ATF (Figure 1). Internally, the torque converter has three main parts: the impeller, turbine, and stator (Figure 2). Each of these has blades, which are curved to increase torque converter efficiency.

The impeller is driven by the engine and directs fluid flow against the turbine blades, causing them to rotate and drive the turbine shaft, which is the transmission’s input shaft. The stator is located between the impeller and the turbine and returns fluid from the turbine to the impeller, so that the cycle can be repeated.

During certain operating conditions, the torque converter multiplies torque. It provides extra reduction to meet the driveline needs while under a heavy load.

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Figure 1. A typical torque converter

When the vehicle is operating at cruising speeds, the torque converter operates as a fluid coupling and transfers engine torque to the transmission. It also absorbs the shock from gear changing in the transmission. Not all of the engine’s power is transferred through the fluid to the transmission, some is lost. To reduce the amount of power lost through the converter, especially at cruising transmissions with a torque converter clutch.

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The engagement of the converter clutch is based on both engine and vehicle speeds and the clutch are controlled by transmission hydraulics and on-board computer electronic controls. When the clutch engages, a mechanical connection exists between the engine and the drive wheels. This improves overall efficiency and fuel economy.

Figure 2. A torque converter’s major internal parts are its impeller, turbine and stator.

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Method

CVT UNITS.

All automatic transmissions, except some designs of

the continuously variable Transmission (CVT), use a

torque converter to transfer power from the engine to

the transmission. The CVT is a compact transmission

that uses belts and pulleys to match the transmission

torque multiplication with the needs of the vehicle.

These transmissions do not have fixed gear ratios;

rather, the size of the drive and driven pulleys changes

in accordance to engine speed and load. This provides

for constantly changing torque multiplication factors.

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The same basic design of the CVT transmission is used

by different manufacturers. The primary design

difference among the manufacturers relates to the

method used to transmit engine torque to the

transmission. Some manufacturers use a torque

converter; however, some do not.

When the electronic torque is energized, engine

torque is transferred to the transmission’s input shaft

and the drive pulley. At the end of the transmission’s

input shaft is the transmission’s oil pump; therefore,

the clutch control unit has indirect control of the oil

pressure in the transmission. Pressure from the pump

is used to control the sheaves of both the drive and

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driven pulleys. Line pressure is controlled by a three-

way solenoid in the transmission valve body.

Torque converter principles

A single stage torque converter has three elements: the

impeller, the turbine and the stator.

The purpose of a light vehicle transmission is to transmit engine torque to the driving wheels. It also has to provide for this torque to be increased or

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decreased to suit differing road and operating conditions.

In a manual transmission this is done by the driver, manually selecting gear ratios to suit.

In an automatic transmission, it is done by both the transmission control system, automatically selecting the gearing according to load and speed, and by the torque converter.

The torque converter is mounted on the engine in the same place as a manual clutch, and does the same job, transmitting engine torque to the input shaft of the transmission. It can also multiply torque according to driving conditions.

It’s a fluid coupling, with the fluid acting as the driving medium, since none of the converter components are physically connected to the others. It acts as an automatic clutch. At engine idle speeds, it allows the engine to operate while the vehicle is stationary and the transmission is in a drive range. In its simplest form, a single-stage torque converter has three elements: The Impeller, the Turbine and Stator.

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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

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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 impeller 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

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turbine, producing an increase in output torque. Since the returning fluid is initially traveling 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

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reverse direction and now rotate in the direction of the impeller 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.

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

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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

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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 Dyna flow and Chevrolet Turbo glide 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

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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

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more common in industrial environments than in automotive transmissions, but automotive applications such as Buick's Triple Turbine Dyna flow and Chevrolet's Turbo glide also existed. The Buick Dyna flow utilized the torque-multiplying characteristics of its planetary gear set 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

All three have angled or curved vanes and are contained in a single casing. They are separated from each other by thrust bearings, but maintain a close relationship for efficient torque transmission.

The impeller has a large number of vanes attached to the converter case to form the driving member. The vanes rotate with the casing as the engine rotates. Each one has a slight curvature and is set radially in the case.

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The turbine is similar in construction to the impeller, but with more vanes, and with a greater curvature. The direction of curvature of the turbine vanes is opposite to that of the impeller vanes. The turbine is free to rotate in the casing, and a central spline mates with a spline on the input shaft. When the fluid coming from the impeller rotates the turbine, the transmission input shaft also rotates.

Both the impeller and the turbine are fitted with a torus or guide ring. This helps to secure the vanes in position. It also reduces turbulence at the center to improve efficiency.

The stator has a small set of curved blades attached to a central hub and is positioned between the impeller and the turbine. The central hub is mounted on a one-way clutch, splined to a stator support shaft, and fixed to the transmission case. The one-way clutch allows the stator to rotate only in the same direction as the impeller. Trying to rotate the stator in the opposite direction locks the stator on the support shaft and holds it stationary.

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LITERATURE

The torque converter is one of the most misunderstood – or, perhaps, non-understood – parts of the powertrain. Torque converters are sealed units; their innards rarely see the light of day, and when they do, they're still pretty hard to figure out! This article will take you on a guided tour of the torque converter from front to back (well, technically, we'll go back to front), and help you to understand how the parts work together.

Let's start with a little theory. The torque converter in an automatic transmission serves the same purpose as the clutch in a manual transmission. The engine needs to be connected to the rear wheels so the vehicle will move, and disconnected so the engine can continue to run when the vehicle is stopped. One

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way to do this is to use a device that physically connects and disconnects the engine and the transmission – a clutch. Another method is to use some type of fluid coupling, such as a torque converter.

Imagine you have two fans facing each other. Turn one fan on, and it will blow air over the blades of the second fan, causing it to spin. But if you hold the second fan still, the first fan will keep right on spinning.

That's exactly how a torque converter works. One "fan," called the impeller, is connected to the engine (together with the front cover, it forms the outer shell of the converter). The other fan, the turbine, is connected to the transmission input shaft. Unless the transmission is in neutral or park, any motion of the turbine will move the vehicle.

Instead of using air, the torque converter uses a liquid medium, which cannot be compressed – oil, otherwise known as transmission fluid. The spinning impeller pushes the oil against the turbine, causing it to spin. But if the turbine is held still (the car is stopped with the brakes applied) the impeller can

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keep right on spinning. Release the brakes, and the turbine is free to turn. Step on the accelerator and the impeller will spin faster, pushing more oil against the blades of the turbine and making it spin faster.

Once the oil has been pushed against the turbine blades, it needs to get back to the impeller so it can be used again. (Unlike our fan analogy, where we have a room full of air, the transmission is a sealed vessel that only holds so much oil.) That's where the stator comes in.

The stator is a small finned wheel that sits between the impeller and the turbine. The stator is not attached to either the turbine or the impeller – it freewheels, but only in the same direction as the other parts of the converter (a one-way clutch ensures that it can only spin in one direction). When the impeller spins, the moving oil pushes against the fins of the stator. The one-way clutch keeps the stator still, and the fins redirect the oil back to the impeller. As the turbine speeds up, oil begins to flow back to the impeller on its own (a combination of the turbine's design and centrifugal force). The oil now pushes on the back side of the stator's fins, and the one-way clutch allows it to spin. It's job now done,

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the stator spins freely and doesn't affect oil flow.

Because there is no direct connection in the torque converter, the impeller will always spin faster than the turbine – a factor known as "slippage." Slippage needs to be controlled, otherwise the vehicle might never move. That's where the stall speed comes in. Let's say a torque converter has a stall speed of 2,500 RPM. If the vehicle isn't moving by the time the engine (and therefore the impeller) reaches 2,500 RPM, one of two things will happen: either the vehicle will start to move, or the engine RPM will stop increasing. (If the vehicle won't move by the time the converter reaches the stall speed, either it's overloaded or the driver is holding it with the brakes.)

The stall speed is a key factor, because it determines how and when power will be delivered to the transmission under all conditions. Drag racing engines produce power at high RPM, so drag racers will often use a converter with a high stall speed, which will slip until the engine is producing maximum power. Diesel trucks put out most of their power at low RPM, so a torque converter with a low stall speed is the best way to get moving with a heavy load. (For more information, see "Understanding Stall Speed"

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elsewhere on this site.)

And now we get to one of the best-kept performance secrets: by altering the design of the torque converter, it is possible to tune the stall speed to match an engine's power curve. The Banks Billet Torque Converter is tuned to provide a stall speed that is optimal for Banks Power systems.

Torque converter slippage is important during acceleration, but it becomes a liability once the vehicle reaches cruising speed. That's why virtually all modern torque converters use a lock-up clutch.

The purpose of the lockup clutch is to directly connect the engine and the transmission once slippage is no longer needed. When the lockup clutch is engaged, a plate attached to the turbine is hydraulically pushed up against the front cover (which, you will recall, is connected to the impeller), creating a solid connection between the engine and transmission. Having the engine and transmission directly connected lowers the engine speed for a given vehicle speed, which increases fuel economy.

If a vehicle has a heavy enough load, its possible for

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the lockup clutch to slip, which can cause excessive heat and wear. How can the clutch be prevented from slipping? Since the converter clutch is held in place by oil pressure, its possible to increase the pressure for a firmer lock, though too much pressure can damage the transmission's oil seals. Another way is to use a multi-element clutch, which sandwiches an additional layer of friction material between the clutch plate and the front cover. A third method is to use better material on the clutch face a fourth is to increase the clutch surface. The Banks Billet Torque Converter uses the last two methods, where applicable. Its clutch surface is lined with a carbon-ceramic material, which is finely etched to allow oil to drain away during lockup. This improves the lockup clutch holding power. On Dodge applications, the total clutch area is increased by 33% too.

What other ways are there to improve a torque converter? We've already discussed the use of a tuned stall speed and a more durable lockup clutch. Another area that can be improved is the front cover, which is the side of the converter that faces (and is attached to) the engine's flywheel or flexplate.

Since the front cover connects directly to the engine,

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it is subject to incredible amounts of stress. Many stock torque converters use a stamped steel front cover because they cost less, but under high power loads they can bend or deform. The solution is to use a billet front cover.

Technically speaking, a billet part is something that is machined from a solid chunk of material. Some torque converter manufacturers use a solid disc and weld it to the sidewall, while others simply weld a reinforcement ring into the stock stamped-steel cover. This compromises the cover's strength and can cause it to warp under load. The strongest covers are precision-machined from a single piece of forged steel, which is then welded to the impeller to form the outer shell.

So as you can see, the torque converter isn't just a "little black box." It's a complex device that, if properly tuned, can have a tremendous impact on your vehicle's performance, economy and durability, and turn your automatic from a "slushbox" into a powerhouse.

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Torque Converters

Torque converters are categorised by stall speed and diameter size. The stall speed (RPM) is the speed the engine can get to before the converter starts holding it back. When the engine can’t increase it’s RPM, the RPM will stall (or stop going up!) The stall speed is related to the engines ability to produce power and the torque converter holding the RPM back.

So if we change the power output OR the change the converter this will change the stall speed. A high stall converter won’t stall at the same stall speed when changed from one engine to another if the torque produced by the engines is different.

Factory torque converters are 12-13 inch in diameter with a stall speed around 1600-1800 RPM. They are designed for fuel economy and drive-ability and certainly achieve this. However, when modifications are done to the fuel system, camshaft and other engine control systems problems can start to appear. It’s important to note that if you are planning on making modifications to your engine, especially performance modifications where you’ll be aiming to increase the torque curve to a higher RPM range so you’ll need to install a high stall converter.

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Choosing a high stall converter.

As the diameter of a torque converter decreases the stall speed goes up To allow engines with a lot of camshaft to idle better smaller diameter converters are needed to provide more slip. As the torque converter and horsepower range is moved higher through the RPM range, the smaller torque converters with their higher stall speeds provide maximum torque multiplication where needed. You’ll need to make your converter selection with the help of a qualified professional because if your car’s set-up is mismatched your motor will stall easily. That’s where Baileys Automatics comes in! We have many converter combination's to make your car or truck perform at its utmost potential.

Working principle of a torque converter.

A torque converter uses fluid coupling to multiply torque.

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Some factory torque converters fall short in terms of specification when you are using your car for heavier duties than just street driving, for example towing or drag racing. There are 2 types of torque converters- 1. Standard- single disc lock-up Or 2. Heavy duty converter which have 2 discs inside. The heavy duty torque converter can handle tremendous torque without warping or deforming and will extend the life of the automatic transmission. Your automatic transmission will also be cheaper to operate with a heavy duty torque converter over the years. A highly modified engine will need a double disc clutch converter.

Lock up Torque converters

Lock up Torque converters have a clutch inside- the clutch grips together (controlled electronically) and allows better fuel economy by dropping the rpms down. When the clutch engages the converter locks the engine to the transmission input shaft so there is a direct connection between the motor and the

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transmission. It works like a fluid coupler, with an outside drum spinning, then the inside spins on the shaft of the transmission. This can produce tremendous heat and heat within the torque converter is bad for fuel economy! It can also transfer to the transmission or clutch and damage them by causing separation. The lock-up torque converter reduces the slippage which reduces heat which means better fuel economy.

Torque Converter shuddering.

This is often described as a problem when a car is going through gears 1- 4 and a shudder is noticed when you hit fourth gear. So that we can diagnose if this is a torque converter shudder we’ll need to know if the shudder goes away as you accelerate more and more. If the shudder does go away as you increase your acceleration, the lock up lining on your torque converter is likely to be deteriorating and possibly on it’s way out.

How do Bailey Automatics test my Torque Converter?

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You will need to bring the car in for a road test. We’ll check the oil level and look at your car’s history to see when you first noticed the shudder and how often the shudder is occurring.

From there we will run a scan tool on the vehicle. The on-board computer ecu will register a code that tells us if there’s been a torque converter slip THEN we’ll take a drive to see if we feel the shudder. When testing the oil level the smell of the oil will tell us if the lock up lining has gone on the torque converter which could be causing the torque converter to shudder. If we catch it early a torque converter shudder could be an easily fixed problem such as needing to fix oil levels or repair the solenoid. The torque converter solenoid is in charge of controlling the fluid pressure to the torque converter. If the oil is too low it could be causing the shudder. Alternatively we may have to pull the transmission out of the car to diagnose and fix, in minor cases a reconditioning of the torque converter will help. In this case we service the transmission- change the oil

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and filters and put the car back together! But if material from the torque converter has gone through the transmission we may need to complete a full recondition to fix the torque converter shudder.

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

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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 Dyna flow and Chevrolet Turbo glide 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

34

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

35

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 Dyna flow and Chevrolet's Turbo glide also existed. The Buick Dyna flow utilized the torque-multiplying characteristics of its planetary gear set 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

36

favor of the more efficient three speed units with a conventional three element torque converter.

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.

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Capacity and failure modes

As with a basic fluid coupling the theoretical torque capacity of a converter is proportional to , where is the mass density of the fluid (kg/m³), is the impeller speed (rpm), and 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

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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

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such conditions will usually occur if continued operation is attempted.

Stator clutch breakage: A very abrupt application of power can cause shock loading of the stator clutch, resulting in breakage. If this occurs, the stator will freely counter-rotate in the direction opposite to that of 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: If subjected to abrupt loading or excessive heating of the converter, 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

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to be physically distorted due to internal pressure and/or the stress imposed by inertia (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.

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