TORQUE CONVERTER
A 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. As a more advanced form of fluid
coupling, 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.
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 pump, which is mechanically driven by the prime mover;
the turbine, which drives the load;
and the stator, which is interposed between the pump and turbine so that it can
alter oil flow returning from the turbine to the pump. 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 pump,
causing all power transmission to be mechanical, thus eliminating losses
associated with fluid drive
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OPERATIONAL PHASES
A torque converter has three stages of operation:
- Stall
the prime mover is applying power to the pump 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 pump 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 pump.
Torque multiplication has essentially ceased and the torque converter is
behaving in a manner similar to a plain 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
pump to oppose the direction of pump 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 pump, 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 pump 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
traveling in a direction opposite to pump 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 pump 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 pump and turbine,
an effect which will attempt to forward-rotate the stator. At this point, the
stator clutch will release and the pump, 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
pump 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 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 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.
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, n is the impeller speed (rpm), and d is
the diameter. 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 add’s 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.
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