Turbochargers (TCs) improve performance in internal combustion engines. Due to low production costs, TC assemblies are supported on floating ring bearings (FRBs). TCs show subsynchronous motions of significant amplitudes over a wide speed range. However, the subsynchronous whirl motions generally reach a limit cycle enabling continuous operation. The paper advances progress on the validation against measurements of linear and nonlinear rotordynamic models for predicting shaft motions of automotive TCs. A comprehensive thermohydrodynamic model predicts the floating ring speeds, inner and outer film temperatures and lubricant viscosity changes, clearances thermal growth, operating eccentricities for the floating ring and journal, and linearized force coefficients. A nonlinear rotordynamics program integrates the FRB lubrication model for prediction of system time responses under actual operating conditions. Measurements of shaft motion in a TC unit driven by pressurized air demonstrate typical oil-whirl induced instabilities and, due to poor lubricant conditions, locking of the floating rings at high shaft speeds. Nonlinear predictions are in good agreement with the measured total amplitude and subsynchronous frequencies when implementing the measured ring speeds into the computational model. The computational tools aid to accelerate TC prototype development and product troubleshooting.

Current trends for advanced automotive engines focusing on downsizing, better fuel efficiency, and lower emissions have led to several changes in turbocharger bearing system design and technology. Automotive turbochargers run faster and use engine oils with very low viscosity under high oil inlet temperature and low feed pressure. The development of high performing bearing systems, marrying innovation with reliability, is a persistent challenge. This paper shows progress on the nonlinear dynamic behavior modeling of the rotor-radial bearing system (RBS) incorporating two oil films in series: a hydrodynamic one with a squeeze film damper commonly used in turbochargers. The developed fluid bearing code predicts bearing rotational speed (in the case of fully floating design), operating inner and outer bearing film clearances, effective oil viscosity, taking into account its shear effect, and hydrostatic load. A rotordynamics code uses this input to predict the nonlinear lateral dynamic response of the rotor-bearing system. The model predictions are validated with test data acquired on a high speed turbocharger RBS of a 6.0 mm journal diameter running up to 250,000 rpm (maximum speed), 5W30 oil type, 150°C oil inlet temperature, and 4 bar oil feed pressure. The tests are conducted at a rotordynamics technology laboratory using a high performance data acquisition system. Turbochargers with four combinations of inner and outer RBS clearances are tested. Prediction and measured synchronous response and total motion are in good agreement. Both demonstrate the nonlinear character of the RBS behavior, including several subsynchronous frequency components across the operating speed range. The nonlinear predictive model aids the development of high performance and optimized turbocharger RBS with faster development cycle times and increased reliability.

No abstract

Passenger vehicle turbochargers (TCs) offer increased engine power and efficiency in an ever-competitive marketplace. Turbochargers operate at high rotational speeds and use engine oil to lubricate fluid film bearing supports (radial and axial). However, TCs are prone to large amplitudes of sub-synchronous shaft motion over wide ranges of their operating speed. Linear rotordynamic tools cannot predict the amplitudes and multiple frequency shaft motions. A comprehensive nonlinear rotordynamics model coupled to a complete fluid-film-bearing model solves in real time the dynamics of automotive turbochargers. The computational design tool predicts the limit cycle response for several inner and outer film clearances and operating conditions including rotor speed and lubricant feed pressure. Substantial savings in product development and prototype testing are the benefits of the present development. The paper presents predictions of the linear and nonlinear shaft motion of an automotive turbocharger supported on a semi-floating ring bearing. The shaft motion predictions are compared to measurements of shaft motion at the compressor nose for speeds up to 240 krpm, and for lubricant inlet pressure of 4 bar at 150°C. Linear and nonlinear rotordynamic models reproduce very well the test data for synchronous response to imbalance. The nonlinear results show two sub-synchronous whirl frequencies whose large magnitudes agree well with the measurements. A large side load predicted for this turbocharger must be considered for accurate prediction of the rotordynamic response.

Advances on the modeling of nonlinear rotor-bearing models for prediction of the dynamic shaft response of automotive turbochargers (TCs) supported on floating ring bearings (FRBs) are presented. Comprehensive test data for a TC unit operating to a top speed of 65 krpm serves to validate the model predictions. The static forced performance of the support FRBs considers lubricant thermal effects, thermal expansion of the shaft and bearings, and entrance pressure losses due to centrifugal flow effects. The bearing analysis also yields linearized rotordynamic force coefficients for the inner and outer lubricant films. These coefficients are used with the rotor model to predict the synchronous response to imbalance and the system natural frequencies and stability. A method renders an accurate estimation of the test rotor imbalance by using the actual vibration measurements and influence coefficients derived from predictions using linearized bearing force coefficients. Predicted ring rotational speeds, operating radial clearances and lubricant viscosities for the inner and outer films are the main input to the nonlinear time transient analysis. The nonlinear response model predicts the total shaft motion, with FFTs showing the synchronous response, and amplitudes and whirl frequencies of subsynchronous motions. The predicted synchronous amplitudes are in good agreement with the measurements, in particular at high shaft speeds. The nonlinear analysis predicts multiple frequency subsynchronous motions for speeds ranging from 10 krpm to 55 krpm (maximum speed 70 krpm), with amplitudes and frequencies that correlate well with the test data. The comparisons validate the comprehensive rotor-bearings model whose ultimate aim is to save TC design time and accelerate product development.