Teaching Aids Torque Converter - Module

7/29/2019 Teaching Aids Torque Converter - Module

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1) ADVANTAGES OF TORQUE CONVERTER

- Allows the engine to continue running while the vehicle is stopped and if vehicle is in

gear, it eliminating the clutch pedal.

2) THE CONSTRUCTION OF TORQUE CONVERTER

- To understand the basic design of torque converter, pretend that you have a hollow steel

doughnut. Cut the hollow doughnut down the center into 2 halves.

- Vanes or fins are placed in the halves of the torque converter. The vanes are straight and

equally spaced. In actual torque converter, a greater number of vanes are used.

- One half is attached to the engine crankshaft through the flywheel. The half attached to

the flywheel is called the impeller or pump.

- The other half is splinted to the transmission input shaft. The housing that holds the

impeller also extends around the half that is attached to the transmission input shaft. This

half, called the turbine, can turn inside of the housing.



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- They are placed face to face with a slight clearance between them.

- The housing is filled with fluid, so there is no mechanical connection between the

impeller and turbine.



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- Since the impeller is continuing to turn, the fluid would again be thrown outward and

upward against the vanes of the turbine. This circular motion of the fluid is termed vortex

flow.

- Engine power is transferred by the fluid from the impeller to the turbine. The impeller

moves the fluid, which strikes the turbine and causes it to move.

Power transfer = Impeller Fluid Turbine

- To assist the fluid in maintaining a smooth vortex flow, a hollow ring is placed in each

member. The fluid is guided around, reducing the tendency for the fluid to work against

itself in the center area.

- At idle, the movement of the fluid striking the turbine cannot overcome the vehicle

brakes. There is no power transfer, even though the engine continues to run.

- When the brakes are released and the engine is accelerated, power flows smoothly from

the impeller to the turbine and out through the transmission. This hydraulic coupling

provides a smooth power transfer and reduces drive train wear. When coasting, the

turbine tries to drive the impeller and allows the engine to act as a brake.



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4) THE HYDRAULIC COUPLING EFFICIENCY

- At normal vehicle speed, the coupling is very efficient.

- Slippage (the difference between the speeds of the impeller and turbine) is often less than

1%.

- At low engine speeds or during heavy acceleration, the fluid coupling is very inefficient.

When the fluid strikes the turbine blades, it transfers its energy to the turbine. When the

fluid reenters the impeller, it is travelling at turbine speed.

- At low vehicle speeds, turbine speed is much slower than the impeller. The fluid from the

turbine is actually turning in the opposite direction of impeller rotation.

- The impeller must use the engine power to reverse the fluid flow before returning it to the

turbine. This wastes energy and makes torque multiplication impossible.

- This constant slippage in the coupling creates friction between the blades and fluid,

which can overheat the fluid.



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5) STATOR

- Stator is added to make the torque converter more efficient.

- The stator is a small assembly made of curved blades. It is placed between the impeller

and the turbine.

- The job of the stator is to intercept the fluid thrown off by the turbine and redirect the

fluid’s path so it will enter the impeller in the same direction as impeller rotation.

- The impeller and turbine blades are curved to work more efficiently with the stator

blades.

- As the impeller begins to spin, fluid is thrown outward into the curved vanes of the

turbine. The fluid then circulates around through the turbine vanes. Instead of being

discharged back into the impeller vanes, as in the simple hydraulic coupling, the fluid

strikes the stator.

- The stator blades are curved to intercept the fluid leaving the turbine blades. The stator

blades change the direction of flow fluid so that it enters the impeller in the same

direction as impeller rotation.

- Instead of wasting engine power to reverse the fluid, the impeller is actually helped to

turn. This reduces power loss and allows the engine torque to be multiplied.

- The stator is mounted on a stationary shaft and can only spin in the direction of impeller

rotation. At low vehicle speeds, the fluid coming from the turbine will attempt to move it

in the opposite direction. To solve this, the stator will lock to the shaft at low vehicle

speeds.



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Rotary and vortex flow with stators

- When the vehicle reaches cruising speed, the turbine will be rotating at almost the same

speed as the impeller and the stator will begin to impede the flow of fluid. However, fluid

will strike the stator in the same direction as impeller rotation. This will cause it to unlock

and the converter will function as a simple hydraulic coupling.

- Stator locking and unlocking action is produced by using a one-way or overrunning

clutch. Overrunning clutches can be of the sprag or roller type.

One-way roller clutch stator



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6) TORQUE CONVERTER OPERATION

6.1 Accelerating From a Stop

- With the transmission in drive range and engine idling (rest), there is very little transfer

of torque from the impeller to the turbine when the vehicle is standing still.

- As the engine is accelerated from a stop, impeller speed increases rapidly. As impeller

speed increases, fluid is thrown into the turbine with increasing force. Leaving the

turbine, the fluid strikes the stator.

- As the stator is forced backward by the fluid, the one-way clutch will lock it up. The fluid

flow is then diverted before it reaches the impeller.

- As this vortex flow increases in speed, more and more torque is applied to the turbine.

The maximum torque multiplication is delivered when the impeller has reached its

highest velocity, and the turbine is standing still or at stall .



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6.2 Cruising Speed

- As the turbine begins to turn, torque multiplication tapers off. When turbine speed

increases to about 90% of impeller speed, the fluid leaving the trailing edges of the

turbine will change its angle so that it begins to strike the rear face of the stator.

- This change of angle is caused by the fact that even though the fluid is being thrown

toward the stator, the turbine is moving by the stator faster than the fluid is moving

toward the stator.

- As the turbine speed surpasses the fluid speed, the fluid strikes the back of the stator

blades, causing the stator overrunning clutch to unlock and freewheel with the turbine.

Since there is no torque multiplication, the converter performs much like a fluid coupling.

This condition is known as the coupling point .



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7) TORQUE MULTIPLICATION CURVE

- Torque multiplication drops off as turbine speed increases. The torque converter provides

a variable drive ratio as opposed to the manual transmission’s three or four fixed ratios.

- This provides a smooth flow of power that automatically adjusts to varying load

conditions within the limit of the unit.

A torque curve chart



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8) LOCKUP TORQUE CONVERTERS

- The normal converter allows some slippage, even at cruising speeds. This is due to the

only connection between the pump and the turbine is the transmission fluid.

- To prevent this slippage and improve fuel economy, most modern converters are

equipped with a lockup clutch or torque converter clutch.

- Before lockup, the turbine and impeller are mechanically free of each other and drive is

through the transmission fluid. There is no contact between the turbine and clutch friction

surface. The lockup converter operates like a conventional torque converter.

- When lockup conditions are present, fluid will move through a passage in the

transmission input shaft. It will flow into the space between the turbine and clutch apply

piston. The clutch apply piston will engage the clutch friction surface to lock the

converter housing and turbine together.

- When the lockup clutch is activated, the engine and transmission input shaft are

mechanically locked together. Slippage is reduced to zero and the vehicle gets better

mileage. The elimination of slippage between the converter blades and the fluid also

eliminates the major source of transmission fluid heating, extending fluid lifespan.



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8.1 Lockup Converter Control System

- The lockup clutch is never applied until the vehicle is at a certain speed, or the

transmission is in at least second gear.

- If the converter was locked at idle, it would stall the engine. To allow converter lockup,

most computer control systems are designed so that the transmission must be in third or

fourth gear, with vehicle speed above 35 to 40 mph (56-64 kmh) and without an

excessive engine load (such as from steep hills or heavy acceleration).



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9) SUMMARY

Position of Torque Converter

Torque Converter Components Assembly



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10) REFERENCES

1. Auto Fundamentals

Martin W. Stockel, Martin T. Stockel, Chris Johanson

The Goodheart-Willcox Company, Inc.

2005

2. www.autoshop101.com/forms/AT02.pdf

Teaching Aids Torque Converter - Module

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