7 In-Situ Source Characterization for NVH Analysis of the Engine-Transmission Unit 87 Fig. 7.8 On-board validation of the TPA results on the soft rubber mount configuration. Obtained from 1000 to 4000 rpm (left →right) no engine load, excitation through chassis dynamometer. Validation responses (orange) and in-situ TPA predicted responses (blue) 7.3.3 Discussion The identified blocked forces with the two types of rubber mounts compared in Figs. 7.4, 7.5, 7.6, and 7.7 show clear discrepancies. One explanation for the offset between the identified blocked forces might be that the requirement to use the in-situ TPA method of f assembly,soft 1 = f assembly,stiff 1 is not fulfilled. As a result, a transfer of the identified blocked forces from one structure to another is not possible. Through the on-board validation, a correct execution of the experiment could be shown for the discrete speed case with both rubber bearings, soft and stiff. The on-board validation for the run-up and run-down respectively shifts the attention to a very interesting aspect. Figure 7.12 on the left shows a clear deviation between the predicted and expected response. Interestingly, this is just the case for the run-up of the soft rubber mount configuration. Looking at the run-down case, instead, an almost perfect match can be observed. Considering the fact that the combustion engine fires during run-up, an additional preload on the rubber mounts could be generated. Note that this is not the case for the run-down, since the foot is taken off the accelerator pedal and the vehicle rolls out. This leads to the second hypothesis for why the two identified forces do not coincide. The other reason could be a significant non-linear behavior of the soft rubber mounts. Since the excitation took place on the source’s side of the rubber mounts, the damping characteristics of the rubber influence the result. Looking at the on-board validation for the stiff rubber mounts in Fig. 7.13, one can see a mostly continuous match between predicted and expected response. This is true for both the run-up and the run-down case.
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