The emergence of new technology of fuel efficient dual clutch, electronically shifted, automatic transmissions employs conventional manual transmission architecture where synchronizers play a major role of precise gear shifting. Although shift quality is not directly affected by the synchronizers, smooth and quick transition of gears, including upshift, downshift, and skip shift is logically a significant factor in accomplishing the fuel economy objective.
DCT design with dry clutches is feasible in low torque applications with manageable thermal capacity and clutch wear, whereas wet clutches are used in high torque applications that can manage heat with cooling. The synchronizer design is critical in both applications substantially affected by system efficiency including clutch drag, bearing frictional losses, and fluid churning losses, plus energy losses due to friction and deflection in shift and clutch actuation mechanism. Hence synchronizer design must satisfy the following two major demands:
- 1
Synchronization torque essentially must be greater than index torque at every point during synchronization event to prevent clash or grating noise.
- 2
Index torque must be high and provide ability to shift in cold conditions to overcome the total transmission losses to insure smooth gear shiftability.
The above requirements establish the boundary limits for index torque to be smaller than synchronization torque and greater than total transmission drag.
Increasing index torque by reducing the pointing angle to compensate for clutch drag and total transmission losses could conceivably require increasing synchronization torque by increasing cone coefficient of friction to safeguard (gear shift) against clash.
The relationship of above significant parameters for smooth gear transition is mathematically established and experimentally verified. Synchronization torque measurement procedure is known, but index torque is usually not measured. For verification purposes a measurement method was formulated and a fixture was built accordingly. Calculated and measured drag torque values were observed to have good correlation with coefficient of friction between sleeve and ring chamfer surfaces as variable.
The test results manifest that in a dual clutch transmission precise and quick shifting is elemental to the function of the transmission, and system efficiency is fundamental to both fuel efficiency and synchronizer function.