From a facility perspective, engine test cells are rarely evaluated for their vibration levels in their functional configuration. When complicated dynamic systems such as an internal combustion engine and a dynamometer are coupled together using driveshafts and coupling components, the overall system behavior is significantly different from that of the individual sub-systems. This paper details an instance where system level experimental testing and finite element analysis methods were used to mitigate high vibration levels in an engine test cell.
Modal and operational test data were taken to establish baseline vibration levels at a diesel engine test cell during commissioning. Measurements were taken on all major sub-systems such as the engine assembly, dynamometer assembly, intermediate driveshaft bearing pedestal and driveshaft components. Correlation of modal data with order tracked data derived from operational testing revealed an axial mode of the driveshaft bearing pedestal that was getting excited by a higher order vibration of the dynamometer trunnion bearings. Continued operation in this condition would have led to structural failure of the dynamometer bearings and significant wear of the intermediate driveshaft bearings. The sensitivity of the driveshaft bearing mode was quantified using temporary structural changes in the field and subsequent testing. When these changes showed promising results, finite element analysis methods were used to develop a more permanent design solution to achieve vibration mitigation. This led to a revised support structure for mounting the driveshaft bearing on its base pedestal. The final design was installed in the test cell and its performance was validated by repeating the modal and operational tests.
This case aims to emphasize the need for more system level testing in coupled dynamic systems that will eventually enable avoiding damage to test equipment and downtime in engine test facilities.