Dynamic gear shifting of an automated manual transmission
Abstract: To be able to change gears in a transmission it is necessary to remove the torque when disengaging and engaging gears in order not to damage the teeth of the cogs. If the driving torque from the engine to the wheels is interrupted for a longer period of time it can be perceived as very uncomfortable for both the driver and passengers in the vehicle. A gear change can be divided into three main stages; disengaging the current gear, synchronizing the countershaft and finally engaging the new gear. Engaging and disengaging a gear is a very fast process as it is and no attempts are made to improve upon this further. However, synchronising the countershaft is rather slow in conventional hybrid vehicles thus the potential for improving this stage is vast. One way of speeding up the synchronisation process is to actively steer the speed of the countershaft with the aid of an electrical machine. A long and slender machine is preferable since the acceleration is proportional to the inertia of the machine, and therefore increases with the radius. This thesis studies the potential of such an electric machine aided dynamic gear change. The investigation is carried out as a series of empirical tests in a testing rig comprised of a transmission, an electric machine, a flywheel, power electronics and a control system based on both hardware and software from National Instruments. A key element to achieving a successful gear change is a quick yet stable motor control since this determines the synchronisation time. In a conventional vehicle the countershaft brake used for synchronisation can achieve a deceleration of 2000 rpm/s. Using the electrical machine instead a deceleration of 3347 rpm/s is recorded which corresponds to a 67 % time improvement. It is also shown that a full gear change from 3rd to 5th gear can be completed in 160 ms. These are very positive results and the main objectives of the thesis have been met.
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