Modeling and Torsional Vibration Analysis of Engine Cold-Test Stand Drivelines

Many large-scale engine manufacturers use cold-testing techniques to test engine assemblies for measuring transmission noise and diagnosing cylinder faults, valve-train and oil pump defects, which cannot be detected easily using traditional hot testing. Cold-testing is done by rotating the unfired engines by an external electrical drive and a driveline and analyzing the measured torque signal characteristics at various engine speeds. In this work, two different cold-test stands, which experience large torsional oscillations excited by various engine harmonics, are modeled. The excessive vibratory response of these stands not only makes the engine fault detection process difficult by degrading the measured torque signal, but can also cause structural failure. An appropriate driveline design is required to decrease the torsional vibration and noise levels and preserve the integrity of diagnostic signals. In an effort to prevent undesirable noise and vibration problems, models of engine cold-test stands are developed that involve modeling of driveline components, engine excitation model for cold-testing, and estimation of torsional vibratory response. The developed models are validated by comparing the model predictions with experimental responses. Model parameters that can help suppress the torsional resonances are determined using embedded sensitivity functions. It is shown that using a smaller-sized motor and a softer rubber coupling, the driveline torsional resonant frequencies excited during the speed sweep can be shifted out of the test range and the amplitudes can be decreased. The developed model is, therefore, used to help redesign the cold-test stand drivelines.Copyright © 2007 by ASME