Damping of vibrations in a single-cylinder engine test bed
Abstract: A study has been conducted to analyze the vibration behavior of one of AVLs light-duty engine test beds, used to develop and test new combustion engine concepts. It has previously been observed that the test bed exerts large vibrations as the engine speed approaches 3000 rpm. As the test bed is mainly used for testing spark-ignited combustion engine concepts, it is desired to conduct measurements at engine speeds up to 6000 rpm. Therefore the purpose of the study is to find a new design for the test bed which reduces the speed-related vibrations and allow the engine to run at speeds up to 6000 rpm. In addition, the study has sought to find an appropriate method of measuring the engine torque in a more precise way than previous measurement methods. The test bed components such as the engine, driveshaft, flywheel, couplings and the electric motor have been analyzed to determine the influence the mentioned components have on the vibration modes of the test bed. A theoretical model of the test bed components as a system was built in MATLAB, the model was then used to find the optimal dimensions of each component in order to reduce the vibrations. The accuracy of the model was verified using measured engine data; In addition, the vibration modes of the test bed were measured using an accelerometer mounted on the electric motor. The measured data was processed using PULSE. A CAD model of a new test bed concept was created using Autodesk Inventor. The vibration modes of the new test bed were verified using ANSYS. The study concludes that the components generally need to have a higher stiffness as well as a lower moment of inertia in order to reduce the risk of critical resonances being generated at speeds below 6000 rpm. In particular, the inertia of the engine flywheel and the couplings for the driveshaft should be reduced by 50% and 57 % respectively. Furthermore, the counterweight mass of the engine crankshaft should be reduced to 1.32 kg for the balance degree of the engine to lower than 36%, in order to reduce lateral inertial forces in the engine. However, the study concludes that lowering the inertia of these components will cause the fluctuations in speed to increase from the current average value of 2 rpm to 4 rpm. Finally, the study concludes that a digital torque sensor should be used to achieve the desired measurement accuracy.
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