Predictions of rotordynamic performance for electric turbocompound

Abstract

An electric turbocompound (ETC) system for heavy duty diesel engines offers significant system level benefits, such as improved fuel efficiency and reduced NOx emissions with a lower CO2 footprint. Presently, a high speed switched reluctance type motor/generator is integrated into a turbocharger shaft between the turbine and compressor wheels. The motor assists rapid acceleration of the turbocharger shaft, thereby rendering faster transient response. At steady or over-boost operating conditions, the generator provides electric power which can be used directly or stored in an on-board storage device. ETCs operate at high rotational speeds and, if equipped with fluid film bearings, use pressurized engine oil to lubricate the bearings (journal and thrust). This paper presents comprehensive predictions of the linear and nonlinear shaft motions of an ETC supported on floating ring bearings. A rotor structural finite element model integrates the floating ring bearing model for prediction of the rotor-bearing system (RBS) linear and nonlinear forced responses under actual operating conditions. Predictions show a complex rotordynamic behavior of the RBS with large amplitude subsynchronous motions over a wide speed range. However, the subsynchronous whirl motions reach a limit cycle enabling continuous operation without system failure. Most importantly, stiffness of the lamination stack mounted on the shaft has a significant effect on the amplitude and frequency content of the shaft motion. The present analysis effectively aids to accelerate ETC prototype development with increased reliability and product troubleshooting.