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An auxiliary power unit includes a gas generator that produces a flow of gases to drive a free power turbine that directly drives an electric generator. The electric generator is driven by the free power turbine at a desired speed without the use of step down gearing to provide a lighter weight and

An auxiliary power unit includes a gas generator that produces a flow of gases to drive a free power turbine that directly drives an electric generator. The electric generator is driven by the free power turbine at a desired speed without the use of step down gearing to provide a lighter weight and more cost effective electric power generating system.

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1. An auxiliary power unit comprising: a gas generator including a compressor coupled to a first turbine and a combustor creating a flow of gases driving the first turbine;an electric generator for producing electrical energy; anda power turbine coupled to the electric generator and driven by the fl

1. An auxiliary power unit comprising: a gas generator including a compressor coupled to a first turbine and a combustor creating a flow of gases driving the first turbine;an electric generator for producing electrical energy; anda power turbine coupled to the electric generator and driven by the flow of gases generated by the combustor, the power turbine rotatable independent of the compressor and first turbine. 2. The auxiliary power unit as recited in claim 1, wherein, the first turbine is rotatable at a first speed and the power turbine is rotatable at a second speed different than the first speed. 3. The auxiliary power unit as recited in claim 1, wherein the power turbine is mounted to a shaft driving the electric generator. 4. The auxiliary power unit as recited in claim 1, including a controller controlling operation of the gas generator responsive to an electric load on the electric generator. 5. The auxiliary power unit as recited in claim 1, including a flow controller operable for controlling the flow of gases to the power turbine. 6. The auxiliary power unit as recited in claim 5, wherein the flow controller is operable to bypass at least a portion of the flow of gases that drive the power turbine responsive to an electric load on the electric generator. 7. An auxiliary power unit comprising: a gas generator including a compressor driven by a first turbine and a combustor that produces a flow of gases driving the first turbine;an electric generator generating electrical energy, the electric generator including a shaft rotatable independent of the first turbine; anda power turbine mounted to the electric generator shaft that drives rotation of the generator responsive to the flow of gases produced by the combustor, wherein the power turbine is rotatable independent of the compressor and first turbine. 8. The auxiliary power unit as recited in claim 7, wherein the power turbine rotates at a speed different then the first turbine of the gas generator. 9. The auxiliary power unit as recited in claim 7, wherein the power turbine is mounted to a shaft that drives the electric generator shaft such that a rotational speed of the shaft is the same as the rotational speed of the power turbine. 10. The auxiliary power unit as recited in claim 7, including a controller governing operation of the gas generator responsive to an electrical load on the electric generator, wherein the controller tailors power output of the gas generator to the electric load on the electric generator. 11. The auxiliary power unit as recited in claim 10, including a gas flow control device for controlling the flow of gases directed to drive the power turbine. 12. The auxiliary power unit as recited in claim 11, wherein the gas flow control device bypasses the flow of gases away from the power turbine to reduce a quantity of the gas flow available for driving the power turbine responsive to a determined drop in electric load on the electric generator. 13. The auxiliary power unit as recited in claim 7, wherein the electric generator is disposed on a cold end of the gas generator and through a concentric shaft driven by the power turbine. 14. A method of operating an auxiliary power unit including a gas generator driving an electric generator through a separately rotatable power turbine, the method comprising the steps of: driving a first turbine with gases generated by combustor, wherein the first turbine is coupled through a shaft to drive a compressor,driving the power turbine attached to the electric generator with the gases generated in the combustor such that the power turbine is rotatable independent of the first turbine;sensing an electric load on the electric generator; andchanging power output by the gas generator responsive to the sensed electric load to tailor production of gases driving rotation of the power turbine to the sensed electric load to produce a desired speed of the electric generator. 15. The method as recited in claim 14, including the step of anticipating an electric load on the electric generator and changing power output by the gas generator in anticipation of the change in electric load. 16. The method as recited in claim 15, wherein the desired speed of the power turbine is constant for all electric loads. 17. The method as recited in claim 15, wherein the desired speed of the power turbine varies responsive to an electric load on the electric generator. 18. The method as recited in claim 14, including the step of routing at least a portion of the gas flow produced by the gas generator away from the power turbine to reduce pressure exerted on the power turbine.