The torque converter consists of three parts - converter impeller, turbine and
stator. Power is input by the engine to the torque converter housing through its
connection to the flywheel. The converter impeller portion consists of vanes
mounted on the rear inside of the housing. Thus when the housing is rotating,
hydraulic fluid is driven by these vanes through the stator vanes to the turbine
vanes.
The torque converter is also connected to the transmission pressure pump and,
through the drive sprocket, chain and driven sprocket, to the transmission gears
and clutches. Thus power is transmitted to the final drive and axles.
The stator provides a means by which engine torque can be "multiplied"
between the normal shift points of the transmission gears.
Torque Multiplication
Torque multiplication occurs when oil from the impeller is forced against the
turbine blades and the turbine blades are rotating at a slower speed such that
the force of the oil does not generate a complete reaction at the turbine. Since
the stator rotation is limited to the same direction as the impeller, this
causes the oil to be forced back through the stator vanes to the impeller vanes
in the same direction as the impeller is rotating.
This increased pressure makes the impeller pump turn faster in relation to the
turbine providing more force on the turbine. This condition maintains until the
turbine speed, which is directly proportional to vehicle speed for a particular
gear setting, matches impeller or engine speed. When this happens, the oil flow
starts striking the back side of the stator vanes and the stator rotates freely
at the same speed as the impeller and turbine.
Torque multiplication is commonly referred to as "slippage".
Effect of Slippage
Inasmuch as there is a less than maximum transmission of energy from the
impeller to the turbine (or engine to final drive) during "slippage",
the "lost" energy is transformed into increased heat in the torque
converter. Thus, under ideal conditions the period of torque multiplication or
"slippage" should be kept to the minimum to reduce excess heating in
the torque converter.
Minimizing Slippage
Obviously slippage may be minimized by downshifting so that the gears provide
the necessary torque multiplication. This may be accomplished manually or
"kicking down" so that the down shift solenoid shifts gears. Then
engine torque (impeller speed) will more closely match driving speed {turbine
speed).
A properly operating automatic transmission should provide this matching through
the functions of the Vacuum Modulator System and the Governor.
Vacuum Modulator System
The Vacuum Modulator and Modulator Valve control downshifting or up shifting
so that the gears of the transmission are automatically set, through internal
valving of hydraulic pressure, to the approximate range where engine torque
(impeller speed) most nearly matches over-the-ground torque {turbine speed).
This is accomplished by using manifold vacuum as a control medium. When the
engine is pulling hard (maximum torque), either when starting or on a grade,
manifold vacuum drops.
This is obvious considering the effect on carburetor performance by noting that
stepping down on the accelerator and reducing manifold vacuum permits the power
valve to open and feed more fuel to the carburetor.
Vacuum Modulator Operation
The Vacuum Modulator, to the left in the figure below, consists of an evacuated
(sealed) bellows, diaphragm and spring arranged so that the bellows and spring
tend to force the Modulator Valve into a position where modulator pressure will
be increased. This action is further augmented by the Governor where pressure
from the governor acting on the modulator valve spool causes the modulator
pressure to the 1-2 Detent Valve and 1-2 Shift Valve to be increased.
As engine speed increases, flow from the Governor is more restricted and there
is less pressure on the modulator valve spool tending to hold it closed against
line pressure. Thus, with the increased manifold vacuum from more efficient
engine operation, the Vacuum Modulator is free to permit the Modulator Valve to
react to the line pressure and retract. This, in turn, allows more pressure from
the Modulator Valve to be directed to the shift valves permitting an up shift.
In the figure, the Vacuum Modulator is in the normally extended position with
the spring loaded bellows expanded. This places the Modulator Valve in a
position where modulator pressure is directed to the 1-2 Shift Detent Valve to
either hold the 1-2 Shift Valve downshifted or permit a 2-1 downshift.
The effect of manifold vacuum may be offset mechanically by the screw adjustment
on the Vacuum Modulator. Tightening this minimizes the effect of manifold vacuum
causing the Modulator Valve to remain in the extended or detent position and the
1-2 Shift Valve to hold in the down-shifted position. Thus the shift points may
be adjusted slightly by this adjustment.
Conversely too loose a setting may permit manifold vacuum to override the
bellows and spring and Governor action to allow the Modulator Valve to slip out
of the detent position and delay down shifting. If downshifting does not occur
at the points intended, slippage may occur more readily.
The system functions in a similar manner for 2-3 shifting.
A handy device for controlling Torque Converter lockup is http://www.dieseltrans.com/tcsaver.htm
This is a work in progress so please bear with us while
we do the research and keep the site up to date.
We ask that you double check the information with other
sources to insure accuracy.
you can reach us at
emeryk@eksco.net for comments/suggestions
