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.
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
Torque Converter is tuned to provide a stall speed that is optimal
for your power system.
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.
What other ways are there to improve a torque converter? We've
already discussed the use of a tuned stall speed. 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.
The turbine is what connects
to the input shaft of the transmission via a splined turbine
hub. Once the turbine starts to move then the vehicle will move.
The impeller is the outside
half of the converter that is welded to the cover on the
transmission side. The impeller is turned by the engines
flexplate and fluid flow is started by centrifugally generating
fluid flow inside the converter.
The stator resides between
the impeller and turbine. The stators job is to redirect the
fluid back into the impeller after leaving the turbine. The
stator houses a mechanical one way clutch commonly called a
Sprague. This allow the stator to stay stationary while
multiplying torque and will free spin once turbine speed reaches
roughly 40% of impeller speed.
when the fluid enters the
converter it is sent to the outside of the impeller
centrifugally. Once the fluid leaves the impeller it feeds the
outside fins of the turbine. This makes the input shaft move and
therefore the car will drive. When the fluid leaves the turbine
it is redirected back to the impeller via the stator. This is
when torque multiplication occurs hence the name torque
converter. Impeller blade angle and stator blade angle and blade
count all denote how much torque will be multiplied in the
converter.
Converter Stall
Let's start by illustrating how the stall speed works. Even under
light loads, a vehicle with an automatic transmission will start
moving as soon as you take your foot off the brake. The stall speed
comes into play under all load conditions. When we talk about stall
speed, we're referring to engine RPM. If the vehicle isn't moving by
the time the impeller reaches the stall speed, either it will start
to move, or the engine RPM will no longer increase. In other words,
stall speed is the engine RPM at which the torque converter
transfers the power of the engine to the transmission.
In the real world, the torque converter's stall speed roughly
equates to the clutch engagement point on a manual transmission.
Let's say you're driving your stick-shift car around town. Normally,
you'd give the car a little gas and ease off the clutch pedal gently
enough to get a smooth start. Likewise, under most driving
conditions the torque converter will start delivering power to the
transmission at relatively low engine RPM.
Now, let's say you need lots of power, either to make a fast getaway
or to start with a heavy load. You'd rev the engine up to a point
where it delivers more power before letting up on the clutch pedal.
It's under those same circumstances that the stall speed becomes
important. The torque converter will allow the engine to build RPM
without turning the output shaft (the turbine) until the stall speed
is reached.
How would you translate this to a torque converter? With a low stall
speed. Stall between
2,000 and 2,500 RPM so with a heavy load, the torque converter
wont start turning the rear wheels until well beyond the engine's
torque peak. In this case, the stall speed is too high - it is
literally impossible to get the engine's full power to the rear
wheels! In order to access all of the engine's potential power, the
stall speed must be lowered.
Lowering the stall speed has another advantage: It reduces the
transmission's temperature. Let's go inside a high-stall torque
converter under heavy load. The impeller (input side) of the torque
converter is spinning quickly, while the turbine (output side) is
spinning slowly or not at all. The motion energy of the impeller is
being converted into heat energy, most of which is passed on to the
transmission fluid. The higher the stall speed, the more heat will
be generated. Heat is the enemy of a transmission. You want to keep
the fluid temperature as low as possible. With a lower stall speed,
less time elapses before the motion energy of the impeller is
converted to motion energy to drive the turbine, so the transmission
runs cooler and lives longer.
What many people don't know is that the torque converter is a
tunable device. Stall speed is determined by several factors,
including the distance between the impeller and the turbine and the
design of the stator. By properly modifying the converter's internal
components, it's possible to alter the stall speed and create a
torque converter that is tuned for a particular engine.
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