Drag Torque and Synchronization Modelling in a Dual Clutch Transmission
Abstract: Noise, vibration and harshness (NVH) is a big consideration in the automotive industry. In order to create an as pleasant driving experience as possible for the driver, NVH should be minimized. One of the sources of NVH comes from the transmission due to the synchronization process. When shifting between gears, the speeds of the target gear and the output shaft it is supported by, have to be synchronized. This is achieved through synchronization rings that, through friction, synchronize the speeds of the components before engaging them to each other. However, there is a torque, the gearbox’s drag torque, that can interfere with this process by slowing down the input shaft. This drag torque therefore aids the synchronization process during an up-shift and resists it during a down-shift. Today’s automotive industry lacks a definite method to calculate this drag torque and as a result, values are assumed to simplify the problem. This thesis has provided a model that calculates the drag torque at different operating conditions depending on input speed, input torque, temperature and other variables. The drag torque comes from several different sources in a transmission and can be separated into load dependent and speed dependent drag torque. The sources include viscous shear in the clutch, gears churning in an oil bath, gear windage in an air-oil mist, bearing rolling elements churning in oil, friction in bearings, friction in gear meshes, pocketing (also called squeezing) of oil between surfaces in gear meshes and viscous shear between concentric shafts. The load dependent are those which are generated through friction, i.e. the friction in bearings and gear meshes. Speed dependent are those generated through resistance from a surrounding medium. This thesis has developed multiple models within each source of drag torque and summed them up for a total drag torque. The results have been compared to test data to verify which combination of models from each source of drag torque sums up to a reliable result. The thesis also shows big differences between different models, but manages to acquire a combination of models that lies relatively close to the test results. The thesis has further used the new drag torque model to evaluate the friction lining on the existing synchronizer rings of a particular transmission to see if the design is appropriate. It also analyzes how a different inertia in the gearbox influences the maximum speed the synchronizers are able to synchronize. Here, it is found that only frictional work and the slip time are influenced of the investigated parameters; specific frictional work, slip time, pressure, slip speed and specific frictional power.
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