Modal extraction on a diesel engine in operation

In this paper an output only modal testing and identification of a diesel engine is presented. The only loading on the engine is the unknown loading from the engine itself. Two test cases were considered: engine run-up, and engine Run-Down. The response data were analyzed using two different techniques: a non-parametric technique based on Frequency Domain Decomposition (FDD), and a parametric technique working on the raw data in time domain, a data driven Stochastic Subspace Identification (SSI) algorithm. Validating the results of the two techniques and the results of the two test cases against each other, 6 modes could be identified under 65 Hz. The first three modes were rigid body modes, and the last two modes were clear bending modes. Frequency uncertainty was typically of the order of 0.5 Hz, damping ratios were uncertain, but relatively high (3-7%) for the rigid body modes and around 2% for the bending modes.