A Case Study of an Integrated Starter Generator Failure and Benefits of Time Domain Torsional Vibration Analysis

An Integrated Starter Generator (ISG) was integrated between an opposed piston two stroke engine and a 32 speed binary shift transmission for use in the Advanced Powertrain Demonstrator (APD). The initial design of the ISG integration and accompanying frequency domain torsional vibration analysis was performed considering driveline characteristics within the normal operating speed range of the engine. After a short period of time, the ISG suffered a catastrophic failure. The root cause of this failure is analyzed with special attention to the torsional behavior of the system. Multiple methods are employed to assess the torsional behavior including time domain torsional analysis. The time domain torsional analysis revealed that a significant number of torsional vibration cycles were occurring outside of the normal operating range of the engine as the engine accelerated from engine cranking speed to engine idle speed. The cycle accumulation during these short excursions through resonance ultimately lead to ISG rotor failure. The methods and lessons learned are presented.