Teaching Aids Torque Converter - Module
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Transcript of 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