The problem of torsional vibrations of a shafting acted by an internal combustion engine has a very complex character due to the large frequency spectrum of the exciting torques produced by the engine's cylinders.Torsional vibration dampers, which are widely used in conjunction with i.c. engines in order to reduce vibration amplitudes and accompanying stresses, have as a principal drawback the energy dissipation by internal friction. This energy consumption affects the mechanical efficiency of the powerplant and may become important, especially for large engines.Theoretically, the classic dynamic vibration absorber doesn't dissipate mechanical energy; it only exchanges energy with the vibrating system, having as a result the reduction of the vibration amplitude. Such a device -built only with passive mechanical elements - shall be tuned to the vibrating system, being able to control torsional vibration only for a single engine speed. Due to this situation, the use of torsional vibration absorbers in conjunction with i.c. engines is very limited.The increase of electronic control of various engine performances offers the possibility to use the already existing CPU in order to control also torsional vibration of the powertrain. This task may be accomplished with minimum energy consumption, by an active vibration absorber. Such a device is conceived in such a manner as to permit continuous variation of the internal torsional stiffness of the absorber by means of a variable electromagnetic field. This variation is used to control the tuning frequency of the absorber, making him effective for any operating speed of the engine.The paper presents the theory of the active vibration absorber and the construction of a prototype model of such a device.