An epicyclic gear system includes an input sun gear configured to receive rotational input from a turbine of a turbo charger. A first set of planet gears is distributed around and meshes with the input sun gear. A carrier holds the first set of planet gears and defines a rotational axis about which the carrier rotates. A second set of planet gears is mounted to the carrier. An output sun gear is included, wherein the second set of planet gears are distributed around and mesh with the output sun. The output sun gear is configured to deliver rotational power to an internal combustion engine. The carrier is configured to selectively be driven by a variable speed drive to regulate output to the output sun over a range of input rotational speeds of the input sun gear.