Prediction of automotive turbocharger nonlinear dynamic forced response with engine-induced housing excitations: comparisons to test data

Abstract

The trend in passenger vehicle internal combustion (IC) engines is to produce smaller, more fuel-efficient engines with power outputs comparable to those of large displacement engines. One way to accomplish this goal is through using turbochargers (TCs) supported on semi-floating ring bearings (SFRBs). The thesis presents progress on the nonlinear modeling of rotor-bearing systems (RBSs) by including engine-induced TC housing excitations. Test data collected from an engine-mounted TC unit operating to a top speed of 160 krpm (engine speed = 3,600 rpm) validates the nonlinear predictions of shaft motion. Engine-induced housing excitations are input into the nonlinear time transient rotor model as Fourier coefficients (and corresponding phase angles) derived from measured TC center housing accelerations. Analysis of recorded housing accelerations shows the IC engine induces TC motions with a broad range of subsynchronous frequencies, rich in engine (e) superharmonics. In particular, 2e and 4e vibration frequencies contribute greatly to housing motion. Most importantly, the analysis reveals TC center and compressor housings do not vibrate as a rigid body. Eigenvalue analysis of the TC system evidences four damped natural frequencies within the TC operating speed range. However, only the highest damped natural frequency (first elastic mode, f = 2,025 Hz, ξ = 0.14) is lightly-damped (critical speed = 150 krpm). Predicted linear and nonlinear imbalance response amplitudes increase with TC shaft speed, with linear predictions agreeing with test data at high shaft speeds. The differences between predictions and test data are attributed to an inaccurate knowledge of the actual TC rotor imbalance distribution. For the nonlinear analysis, predicted shaft motions not accounting for housing accelerations show the TC is stable (i.e. no subsynchronous whirl) at all but the lowest shaft speeds (<70 krpm). However, predicted shaft motions accounting for housing accelerations, as well as the test data, reveal TC motions rich in subsynchronous activity. Clearly, engine-induced housing accelerations have a significant impact on TC shaft motions. Predicted total shaft motions show good agreement with test data. Predicted nonlinear subsynchronous amplitudes as well as peak shaft amplitudes also agree well with test data. However, nonlinear predictions only show TC shaft vibrations attributed to engine order frequencies below 6e, whereas test data evidences TC vibrations are due to order frequencies greater than 6e. Overall, nonlinear predictions and test data illustrate the importance of accounting for engine-induced housing vibrations in the design and operation of TC systems. The good agreement between predictions and test data serve to validate the rotor model. The tools developed will aid a TC manufacturer in reducing development time and expenditures.