Page 202 - Mechatronics with Experiments

188 MECHATRONICS
from the engine to the torque converter’s impeller shaft, hence the engine and the rest
of the powertrain is effectively disconnected. In a partially engaged state via a propor-
tional pressure control circuit on the neutralizer clutch, partial power is transmitted
between the engine and the torque converter. This proportional control capability of
the neutralizer clutch can be used to limit the torque transmitted to the transmission,
that is to limit the rimpull force (traction torque) for traction control, or to manage
how much power is allowed to go to the transmission in order to allocate the rest of
the available engine power to the other hydraulic systems. This is sometimes referred
as the power management algorithm or power management strategy in vehicle con-
trol terminology. Power management refers to the controlled distribution of available
engine power among two or more subsystems, that is transmission, steering, and
implement hydraulic systems.
Lock-up clutch which is used to mechanically lock the impeller shaft and turbine

shaft without slip, effectively providing a rigid coupling and eliminating the torque
converter’s hydrodynamic coupling function. This also referred to as the direct drive
mode of the torque converter, where the torque converter is effectively non-functional.
This is an ON/OFF type of clutch with a controlled dynamic transition between ON
and OFF states. It is used in heavy equipment applications after the first gear to
eliminate the inefficiency of the torque converter. In such applications, the torque
converter is used (lock-up clutch is not engaged) only on the first gear. When the lock-
up clutch is engaged, the torque converter section is not transmitting power through
the hydrodynamic coupling. In order to minimize the power loss in the circulated
fluid between impeller-turbine-stator, the stator is generally of the free-wheeling type
instead of being fixed to the housing when the lock-up is engaged.

The only components of a torque converter that are subjected to wear, are the friction discs
of the neutralizer and lock-up clutches and the seals. The rest of the components are rather
highly reliable, rigid components that rarely need servicing.
The torque converter behavior is modeled by two steady-state algebraic functions
(Figure 3.29c): torque ratio and primary torque. The torque ratio (also called torque multi-
plication) is higher when there is a larger relative speed difference between the input and
output shafts. When the impeller and turbine shaft speeds are the same, there is no torque
transmitted between the two shafts. The two characteristic functions of the torque converter
are defined as follows, as a function of speed ratio N = w turb ∕w imp
w
1. torque ratio, N (N ), between impeller and turbine shafts as a function of the speed
T w
ratio, and
2. primary torque, T (N ), as a function of the speed ratio.
p w
For a given torque converter, these two characteristic functions can be measured as follows
(Table 3.1):

1. connect the torque converter between an engine and a dynamometer (also called a
dyno),
2. set the engine speed to the rated speed, that is w rated = 1800 rpm, via a closed loop
engine speed controller so that the engine maintains that speed,
3. control the dyno to maintain desired speeds from zero to rated speed at selected
intervals in steady state, that is again using a closed loop dyno speed controller,
w dyno = 0 rpm, 100 rpm, … , 1800 rpm,