A composite flywheel (13), comprising a primary flywheel (15) integral with an input shaft (31) and a secondary flywheel (17) coaxial with the primary flywheel (15). The secondary flywheel (17) and the primary flywheel (15) are constrained so that they can rotate freely with respect to each other. A coupling device (61) is further provided, adapted selectively to: transmit power from the input shaft (31) to the secondary flywheel (17)； and disengage the secondary flywheel (17) from the input shaft (31).
