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Analysis of the most important design parameters of the crank drive due to the NVH behaviour - analysis of the main bearing forces w.r.t. the acoustic sensitivity
Dr.-Ing. Steffen Nitzschke
The acoustic emissions of the combustion engine are one of the main contributors to the pass by noise of a passenger car, which is a main focus of current NVH activities.
In this context the crank drive has a dominant influence and has to be analyzed in detail to identify the most important design parameters concerning vibrations of the main bearings, which defines one excitation source of the engine's acoustic emission. Thereby, the main focus will be on design parameters, which can easily be changed in an already existing production line for mass production of passenger cars.
The deformation of the crank shaft, which was calculated with the previously defined MBS model under nominal conditions (two rotational speeds) is analyzed concerning the strain energy. The results are averaged for one load cycle to allow for an overall statement, which regions are sensitive concerning the NVH behavior (due to large strain energy). This approach neglects the non-linear interactions between the vibration behaviour of the crank shaft and the bearing parameter, but it allows for a determination of relevant geometrical parameters of the crank shaft. For reasons of further investigations also the contribution of each eigenmode to the strain energy is calculated and weighted with the participation factor.
After a parameter variation, which is performed in time domain, the bearing forces, which are assumed to affect the NVH behaviour dominantly, are transferred into frequency domain including the resulting phase angle with respect to the crank shaft angle. Furthermore, acoustic sensitivities for each bearing (from experimental or computational data) are used to compare different variants. In this context several aspects has to be taken into account: On the one hand vibration with different opposed phases vanish (phase cancel). On the other hand high bearing forces, which result from eigenvalues in the interesting frequency domain, can be shifted towards frequencies with low acoustic sensitivities, leading to a lower sound pressure (eigenvalue dispersion). The last point to mention is a mode control, so that specific modes are tuned to assure that the amplitudes in the bearings tend to zero.

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