A unified model for calculating torsional vibration of an engine crankshaft with rubber-damped torsional vibration damper (TVD) with n inertia rings is developed. The unified model can be used to describe torsion vibration of an engine crankshaft system with or without TVD. Torsional vibration of an engine crankshaft is calculated for the crankshaft system without TVD, and for the crankshaft system plus one type of TVD with optimized stiffness and damping parameters. Three types of TVD are included in the model: one inertia ring TVD, two inertia ring TVD arranged in serial way, and two inertia ring TVD arranged in parallel way. The calculated results show that forced torsional vibration analysis of an engine crankshaft system under the generic engine firing excitation is necessary to validate the optimized TVD based on conventional method for optimization of stiffness and damping for a dynamic absorber.