|
By
Aaron Gold
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
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 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, 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" 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 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, 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!
|
