Reduction of engine torsional vibrations via hydrodynamic dampers

Abstract

Within the scope of this master's thesis, the vibration damper used to dampen the torsional vibrations of internal combustion engines has been examined. In this manner, the hydrodynamic damper, which is a special type of vibration damper, is investigated. The calculation methods of the stiffness coefficient and damping coefficient, which are the two main characteristics of the hydrodynamic damper, have been studied. CATIA and Hypermesh software programs are used in order to perform the needed 3D model creation and analysis. Values obtained from the calculation by analytical and numerical methods are compared with test results performed on measurement systems. After the determination of these characteristic parameters, a crank train model is built over the AVL EXCITE program to examine the effect of the hydrodynamic damper in an 8-cylinder diesel engine. This established model is considered as two different sub-models, with and without vibration damper, and the differences between both cases are determined. Angular displacement and stress values in different sections of the crankshaft are used as the output of the analysis. Thus, the effect of the hydrodynamic damper used on the crank train and the engine has been clearly demonstrated. Currently, efficiency of the engine is increased by increasing power density and reducing engine volumes in internal combustion engines. On the other hand, this trend increases the loads on the components and affect the strength limits. In an 8-cylinder diesel engine with a high power density, the vibrations observed with increasing cylinder pressures and loads also increase. These vibrations seen in the crank mechanism directly affect the operation of the engine and the lifetime of the parts. For this reason, vibration dampers are used for the damping of these vibrations and for the safe operation of the engine by reducing their amplitudes. In this thesis, a hydrodynamic vibration damper coupled to the free end of the crankshaft is used. The hydrodynamic damper is a vibration damper that consists of leaf springs arranged in an inner part and that creates damping by providing oil flow between these spring packages. Firstly, the stiffness coefficient of this damper is determined. For this purpose, finite element analysis is applied to the damper, which is 3D modeled, through the Hypermesh program. As a result of the analysis, the angular displacement values against the acting torque are obtained and the related stiffness coefficient is determined. Also, a test system is designed to measure the stiffness coefficient. The stiffness coefficient value is obtained by the measurement made on this test system. The stiffness coefficient calculated by the numerical method is compared with the stiffness coefficient measured from the test system. As a result of the comparison, it is seen that both values coincided with each other within a certain margin of error. Analytical calculations have also been made to determine the damping coefficient, which is another specific feature of the hydrodynamic damper. The passage of the oil in the damper between the chambers next to each other creates a damping effect. In this context, the damping coefficient is tried to be calculated with two different methods. The first of these methods is the control oriented transient method, which is used in a similar study before. With this method, the damping coefficient is calculated over different coefficients based on the oil flow. However, since the geometry is very small and there are many parameters affecting the flow, the damping coefficient calculated with this method is found to be quite different from the damper coefficient in the catalog information from the manufacturer. The second method is to reduce the damper to an equivalent dashpot system. With this reduction, the damping coefficient is calculated from the dashpot. With this method, the damping coefficient differs from the value in the catalog, depending on the assumptions and reductions used. In addition to the calculations, a test system is designed for a static damping coefficient and it is measured on this system. The damping coefficient is obtained from the measurement result using the logarithmic decrement method. Since the engine operating conditions cannot be reflected in the test system and the damper is tested statically, not dynamically, the value obtained from the measurement does not represent the dynamic damping coefficient. For this reason, the values obtained from the calculations and the values in the catalog have not been compared. Finally, after determining the characteristics, a crank train model is created using the AVL EXCITE program to examine the effect of the hydrodynamic damper on the engine and to reveal the damping effect of the hidrodynamic damper. In this model, the crankshaft, connecting rod, piston and flywheel are modeled under the forces after combustion pressure and inertia forces. In order to see the effect of the damper on the system, two models, with and without the damper, are created. From these two models, the angular displacement and the resulting stress values in different parts of the crankshaft are investigated. The results from the model with the damper obviously stated that the hydrodynamic damper effectively reduces the torsional vibrations in the engine compared to the model without the damper. It can be said that the lifetime of the components and therefore the engine is extended since the stress values on the parts are reduced at the same time. It is claimed that adding torsional vibration damper into the crank train has no negative effect on the engine in terms of torsional vibrations. According to the results of the torsional vibration analysis, almost every part of the crank train elements has minimized amplitude level of torsional vibration and angular displacements with hydrodynamic damper. Additionally, 7 different damper case studies are revealed in order to determine the possible optimized stiffness and damping coefficients of the current hydrodynamic damper. In order to compare the different cases, the existing stiffness and damping coefficients of the current damper are changed by 25%. Updated stiffness and damping coefficients of the damper are applied into the torsional vibration analysis and corresponding results are obtained, respectively. These results are compared with the results of the current hydrodynamic damper. In some cases, the angular displacements and the shear stresses are increased due to the change in the stiffness and damping coefficients. On the other hand, some cases have improved results due to the higher stiffness and damping coefficients compared with the current hydrodynamic damper. These improved cases are investigated in order to determine the possible optimized version of the existing damper. It is not hundred percent possible to claimed that the revised damper has better results in the all speed intervals, but it can be said that the updated dampers with high stiffness and damping coefficients have better torsional vibration results in the most of the engine operating speeds. Regarding this aspect, Case 5 and Case 7 have better results than the other cases and the current hydrodynamic damper. Moreover, Case 5 has less angular displacements and shear stresses in the most speed ranges than the Case 7. Although Case 7 has better results in some speeds than the Case 5, there are some speed intervals that the Case 7 has higher angular displacements and shear stresses than the current hydrodynamic damper. Also, the reduction of the results in Case 7 is less than the increase coming from the Case 7 for specific speeds. Thus, the hydrodynamic damper in Case 5 has better results than the Case 7. It can be concluded that the revised hydrodynamic damper in Case 5 can be the optimized version of the current hydrodynamic damper in terms of torsional vibrations. Lastly, the needed structural changes in order to obtaine the stiffness and damping coefficients of the hydrodynamic damper in Case 5 are determined by using the formulations derived in previous sections.