Page 204 - Mechatronics with Experiments

190 MECHATRONICS
different from those in a passenger car, because these two machines require different torque
converter behavior.
In order to understand the role and dynamics of the torque converter in the vehicle
drive train dynamic simulation, consider the torque converter as a nonlinear coupling device
that is flexible or that can have different slip between input and output speeds. Consider the
beginning of a simulation, where the engine has a speed and the vehicle has a speed.

1. The impeller speed of the torque converter is determined by the engine speed,
w imp = w eng (3.320)
2. Turbine speed is determined by the speed of the vehicle which is reflected to the
turbine by the lower powertrain gear ratio,
1
w = ⋅ w (3.321)
turb tire
N
lp
where w is the vehicle tire speed, and N is the total gear ratio of the lower
tire lp
powertrain (between turbine speed and tire speed).
3. Then, T and T torques are determined from the steady-state characteristics of
imp turb
the torque converter.
The efficiency of torque converter is defined as
P out
= (3.322)
tc
P in
T out ⋅ w out
= (3.323)
T ⋅ w
in in
T turb ⋅ w turb
= (3.324)
T ⋅ w
imp imp
= N ⋅ N (3.325)
w T
The maximum efficiency of a torque converter occurs around 0.7to0.85 speed ratio range
and is around the 85–90% range. When the speed ratio of the torque converter is zero, the
power conversion efficiency is zero because output speed, and hence the output power, is
zero. When the speed ratio is one (both impeller and turbine shaft speeds are the same),
the power conversion efficiency is also zero because the torque output (and torque ratio)
is zero. Between these extremes, the efficiency curve is similar to a parabolic function of
speed ratio, where the maximum efficiency is achieved around the 0.7to0.85 speed ratio
range (Figure 3.29c).
For a specific machine application, the engine and torque converter should be sized
properly. This is called engine–torque converter matching. The basic objective is to match
the power level of the engine and the power transfer capability of the torque converter.
Different machine applications require different engine–torque converter matching in that
the maximum efficiency of the torque converter occurs at the rated speed of the engine
(full-matched case), at above the rated speed of the engine (light-matched case), and at
below the rated spedd of the engine (heavy-matched case). Roading machines typically
have heavy to full match, whereas non-roading machines (such as wheel type loaders)
typically have light to full match.
For a given torque converter, we have the torque gain (N (N )) and primary torque
w
T
(T (N )) functions as functions of the speed ratio. Let us consider a load torque and
w
p
its reflection on the impeller. In other words, the impeller torque is the reflected load
torque acting on the engine output shaft. The intersection of the engine lug curve and this