Patent ID: 8376070

Claim:
A propulsion system comprising: an electric motor; an energy storage unit configured to supply a primary power to the electric motor; and a plurality of auxiliary power units (APUs) configured to supply a secondary power to at least one of the electric motor and the energy storage unit, each of the plurality of APUs comprising: a free-piston engine configured to generate a mechanical output; a linear generator configured to transform the mechanical output to an electrical power; and a controller configured to: receive a power command from the at least one of the electric motor and the energy storage unit; determine an amount of secondary power needed to meet the power command; and selectively activate a number of the plurality of APUs to generate the needed amount of secondary power, wherein selectively activating a number of the plurality of APUs comprises: individually operating each of the plurality of APUs in one of an on condition or an off condition based on the needed amount of secondary power; and operating each of the plurality of APUs in the on condition above a minimum efficiency threshold, including selectively operating each of the plurality of APUs in the on condition at various speeds at less than a full power-generating capacity and generating an equal fraction of the secondary power.