Torque Converter Operation Explained
The torque converter is one of the least understood components
in an automatic transmission equipped vehicle. I will attempt to
explain what it does and how it does it.
The torque converter has a few different functions.
We first need to understand that there is no direct link between
the crankshaft and the transmission input shaft (except in the
case of a lock up style converter, but we'll talk about that
later). This means that the first function of the converter is
to connect the crankshaft and the input shaft so the engine can
move the vehicle; this is accomplished through the utilization
of a fluidic coupling effect.
The torque converter also replaces the clutch that is required
in a manual transmission; this is how an automatic transmission
vehicle can come to a stop while still being in gear without
stalling the engine.
The torque converter also acts as a torque multiplier, or extra
gear ratio, to help the car get moving from a stop. In modern
day converters this theoretical ratio is anywhere between 2:1
and 3:1.
Torque converters consist of 4 major components that we need to
concern ourselves with for the purpose of explanation.
The first component, which is the driving member, is called the
impeller or "pump". It is connected directly to the inside of
the converter housing and because the converter is bolted to the
flexplate, it is turning anytime that the engine rotates.
The next component, which is the output or driven member, is
called the turbine. The transmission's input shaft is splined to
it. The turbine is not physically connected to the to the
converter housing and can rotate completely independently of it.
The third component is the stator assembly; its function is to
redirect the flow of fluid between the impeller and the turbine,
which gives the torque multiplication effect from a standstill.
The final component is the lock up clutch. At highway speeds
this clutch can be applied and will provide a direct mechanical
link between the crankshaft and input shaft, which will result
in 100% efficiency between the engine and transmission. The
application of this clutch is usually controlled by the
vehicle's computer activating a solenoid in the transmission.
Here's how it all works. For the sake of simplicity, I will use
the common analogy of two fans which represent the impeller and
the turbine. Let's say that we have two fans facing each other
and we turn only one of them on- the other fan will soon begin
to move.
The first fan, which is powered, can be thought of as the
impeller that is connected to the converter housing. The second
fan- the "driven" fan can be likened to the turbine, which has
the input shaft splined to it. If you were to hold the
non-powered fan (the turbine) the powered one (the impeller)
would still be able to move- this explains how you can pull to a
stop without the engine stalling.
Now imagine a third component placed in between the two, which
would serve to alter the airflow and cause the powered fan to be
able to drive the non-powered fan with a reduction of speed- but
also with an increase of force (torque). This is essentially
what the stator does.
At a certain point (usually around 30-40 mph), the same speed
can be reached between impeller and the turbine (our two fans).
The stator, which is attached to a one way clutch, will now
begin to turn in conjunction with the other two components and
around 90% efficiency between the crank and the input shaft can
be achieved.
The remaining 10% slippage between the engine and transmission
can be eliminated by connecting the input shaft to the
crankshaft through the application of the lock up clutch that
was mentioned before. This will tend to lug the engine, so the
computer will only command this in higher gears and at highway
speeds when there is very little engine load present. The main
function of this clutch is to increase fuel efficiency and
reduce the amount of heat that is generated by the torque
converter.
Another term that may be unfamiliar is that of a "high stall"
torque converter. A high stall converter differs from a stock
converter in the sense that the rpm is raised at which the
internal converter components- the impeller, the stator and the
turbine start to turn together, and hence, stop the torque
multiplication phase and begin the coupling phase. The point at
which engine rpm will stop climbing with the drive wheels held
stationary and the throttle fully opened is referred to as
"stall speed".
The idea behind a higher stall torque converter is to allow the
engine to rev more freely up to the point where the powerband
begins, and therefore, enable the vehicle to accelerate from a
stop under more power.
This becomes increasingly important when an engine is modified.
Engine modifications such as ported heads, bigger cams, bigger
turbos (in some cases), bigger intakes, etc. tend to raise the
point where the powerband begins. For best performance, the
stall speed needs to be raised accordingly to work optimally in
conjunction with the given vehicle alterations.
In simple terms, for best performance, the stall speed should be
raised at least to the point where the torque curve is heading
towards its peak. As a rule of thumb, the stall speed should be
set to match the rpm at which the engine is making at least 80%
of its peak torque for a street driven vehicle.
As you can imagine, a vehicle that can accelerate from a stop
with 80% of its peak torque will easily outperform an otherwise
identical vehicle that can only launch at 50% of its available
torque.
For a performance or "high stall" torque converter to produce
maximum gains, it needs to be configured to the specific vehicle
in which it will be installed.
Factors such as engine torque and the rpm at which it is
greatest, differential gear ratio, vehicle weight, camshaft
design, compression ratio, type of induction- forced or
naturally aspirated, and a host of other variables all need to
be taken into consideration.
Be aware that the "off the shelf" type performance torque
converters sold by some manufacturers are very unlikely to be
optimized for all vehicles and their unique requirements.