Linear model for the identification of critical speeds in an MCIA turbocharger

Abstract

Linear models to identify critical points of operation depend largely on linear coefficients representing film lubrication, in which the shaft is supported, and the exciting forces of the shaft. However, the magnitudes of the coefficients of the oil film determined in rotational dynamics studies of small turbochargers (TC) still have great uncertainty. In present paper the linear coefficients of the film hydrodynamic lubrication in automotive turbocharger with semi-floating bearings have been estimated using different methodologies. These methodologies are based on the resolution of the film lubrication Reynolds equation using different approaches. These approaches are short bearing and long infinite bearing. Different tests have been made with various magnitudes with a synchronous frequency to generate excitation forces on the compressor and turbine wheels. At the end, the input parameters of the model (linear coefficients of the lubrication film and synchronous forces) are fitted in order to predict the measured orbit described by the shaft tip on the compressor side. The main critical operation points have been identified by a Campbell diagram. These points are related with the synchronous frequencies of the shaft. The obtained results have been compared with experimental measurements.