Coupling phase

Towards unity speed ratio when the turbine speed has almost caught up with the impeller, the fluid entering the third stator blades S3 will have altered its direction to such an extent that it releases this

Vi - Tangential velocity comportent

Vi - Tangential velocity comportent

Fig. 4.18 Principle of the three stage torque converter

last fixed set of blades. Since there is no more reaction torque, conversion ceases and the input and output elements act solely as a fluid coupling.

4.9 Torque converter with lock-up and gear change friction clutches (Figs 4.21 and 4.22) The two major inherent limitations with the torque converter drive are as follows:

Firstly, the rapid efficiency decline once the relative impeller to turbine speed goes beyond the design point, which implies higher input speeds for a given output speed and increased fuel consumption. Secondly, the degree of fluid drag at idle speed which would prevent gear changing with constant mesh and synchromesh gearboxes.

The disadvantage of the early fall in efficiency with rising speed may be overcome by incorporating a friction disc type clutch between the flywheel and converter which is hydraulically actuated by means of a servo piston (Fig. 4.21). This lock-up clutch is designed to couple the flywheel and impeller assembly directly to the output turbine shaft either manually, at some output speed decided by the driver which would depend upon the vehicle load and the road conditions or automatically, at a definite input to output speed ratio normally in the region of the design point here where efficiency is highest (Fig. 4.22).

To overcome the problem of fluid drag between the input and output members of the torque converter when working in conjunction with either

^4

N\ 3

\ 2 1

W constant input torque

40 it

40 it

Fig. 4.20 Relationship of speed ratio, torque ratio and efficiency for a polyphase stator torque converter

Clutch

lock-up

point

/A

""Coupling

point

v \ /

r^A /

i /

1000 2000 3000 4000 Output speed (rev/mini

5000

1000 2000 3000 4000 Output speed (rev/mini

100 80

5000

Fig. 4.22 Characteristic performance curves of a three element converter with lock-up clutch

Fluid drive Friction drive

Fig. 4.21 Torque converter with lock-up and gear change function clutches

Fluid drive Friction drive

Fig. 4.21 Torque converter with lock-up and gear change function clutches constant mesh or synchromesh gearboxes, a conventional foot operated friction clutch can be utilized between the converter and the gearbox. When the pedal is depressed and the clutch is in its disengaged position, the gearbox input primary shaft and the output main shaft may be unified, thereby enabling the gear ratio selected to be engaged both smoothly and silently.

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