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A coupling system includes a dynamoelectric device selectively operable as either a generator or an electric motor, a gas turbine engine, a main differential operably connected between the dynamoelectric device and the gas turbine engine, a rotationally powered accessory engaged with the main differ

A coupling system includes a dynamoelectric device selectively operable as either a generator or an electric motor, a gas turbine engine, a main differential operably connected between the dynamoelectric device and the gas turbine engine, a rotationally powered accessory engaged with the main differential, an epicyclic start differential operatively connected to the main differential, and a brake mechanism. The brake mechanism is configured for selectively braking the epicyclic start differential when the dynamoelectric device operates as an electric motor to direct torque transmission to the gas turbine engine, and is further configured to cease braking the epicyclic start differential when the dynamoelectric device operates as a generator such that the main differential transmits torque to power the rotationally powered accessory and to the dynamoelectric device.

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1. A coupling system comprising: a dynamoelectric device selectively operable as either a generator or an electric motor;a gas turbine engine;a main differential operably connected between the dynamoelectric device and the gas turbine engine, wherein the main differential is a sunless epicyclic gear

1. A coupling system comprising: a dynamoelectric device selectively operable as either a generator or an electric motor;a gas turbine engine;a main differential operably connected between the dynamoelectric device and the gas turbine engine, wherein the main differential is a sunless epicyclic gear assembly comprising: a plurality of main differential planet gears arranged to mesh with each other;a first main differential ring gear that meshes with a first of the plurality of main differential planet gears;a second main differential ring gear that meshes with a second of the plurality of main differential planet gears; anda main differential carrier rotatably connected to each of the main differential planet gears;a rotationally powered accessory engaged with the main differential;an epicyclic start differential operatively connected to the main differential; anda brake mechanism configured for selectively braking the epicyclic start differential when the dynamoelectric device operates as an electric motor to direct torque transmission to the gas turbine engine, wherein the brake mechanism is configured to cease braking the epicyclic start differential when the dynamoelectric device operates as a generator such that the main differential transmits torque to power the rotationally powered accessory and to the dynamoelectric device, and wherein the start differential is configured to transmit torque between the first main differential ring gear and the second main differential ring gear when the brake mechanism brakes a start differential carrier. 2. The system of claim 1, wherein the epicyclic start differential comprises: a start differential sun gear operatively connected to the main differential;a plurality of start differential planet gears that each mesh with the start differential sun gear, wherein the start differential carrier is rotatably connected to each of the start differential planet gears, and wherein the brake mechanism is connected to the start differential carrier; anda start differential ring gear arranged to mesh with each of the start differential planet gears. 3. The system of claim 2, wherein the first main differential ring gear meshes with a rotor gear of the dynamoelectric device, wherein the second main differential ring gear meshes with the start differential ring gear, the second main differential ring gear configured to mesh with an input gear of the rotationally powered accessory, and wherein the main differential carrier is operatively engaged with the gas turbine engine. 4. The system of claim 1, wherein the rotationally powered accessory is a fixed displacement hydraulic unit engaged with the second main differential ring gear. 5. The system of claim 1 and further comprising: a variable displacement hydraulic unit engaged with the main differential carrier. 6. The system of claim 1 and further comprising: an accessory pump engaged with the first main differential ring gear. 7. The system of claim 6, the brake mechanism further comprising: an actuator for producing a relative axial movement between the first clutch member and the second clutch member. 8. The system of claim 1 and further comprising: a fluid pump engaged with the main differential. 9. The system of claim 8, wherein the fluid pump comprises a variable displacement hydraulic unit engaged with the main differential carrier. 10. The system of claim 1, wherein the brake mechanism comprises: a first clutch member fixed to a non-rotating structure; anda second clutch member fixed to the start differential carrier, wherein the clutch is configured to selectively engage the first clutch member and the second clutch member. 11. The system of claim 10, wherein the brake mechanism comprises a mechanical face clutch that engages the first clutch member and the second clutch member through mechanical interference between mating teeth. 12. The system of claim 10, the brake mechanism further comprising: an actuator for producing a relative axial movement between the first clutch member and the second clutch member. 13. A coupling system comprising: a dynamoelectric device comprising: a rotor; anda stator, wherein the dynamoelectric device is selectively operable as either a generator or an electric motor;a combustion engine;a main differential operatively connected between the rotor of the dynamoelectric device and the combustion engine, wherein the main differential is a sunless epicyclic gear assembly comprising: a plurality of main differential planet gears;a first main differential ring gear that meshes with a first of the plurality of main differential planet gears; anda second main differential ring gear that meshes with a second of the plurality of main differential planet gears;an epicyclic start differential operatively connected to the main differential and the rotor of the dynamoelectric device, wherein the epicyclic start differential comprises: a start differential sun gear operatively connected to the main differential;a plurality of start differential planet gears that each mesh with the start differential sun gear;a start differential carrier connected to each of the start differential planet gears; anda start differential ring gear arranged to mesh with each of the start differential planet gears;an accessory device operatively engaged to the main differential; anda brake mechanism connected to the start differential carrier for selectively braking the start differential carrier,wherein the brake mechanism can brake the start differential carrier when the dynamoelectric device is operated as an electric motor to transmit torque from the dynamoelectric device to the combustion engine through the epicyclic start differential and the main differential, with the start differential adjusting torque transmission through the main differential,wherein the start differential is configured to transmit torque between the first main differential ring gear and the second main differential ring gear when the brake mechanism brakes the start differential carrier, andwherein the brake mechanism is configured to be released when the combustion engine transmits torque through the main differential to power the dynamoelectric device as a generator, with the main differential also transmitting torque to power the accessory device. 14. The system of claim 13, wherein the combustion engine comprises a gas turbine engine. 15. The system of claim 13, wherein the start differential ring gear engages the second main differential ring gear, and wherein the start differential sun gear engages the first main differential ring gear. 16. The system of claim 13, wherein the brake mechanism comprises: a first clutch member fixed to a non-rotating structure; anda second clutch member fixed to the start differential carrier, wherein the clutch is configured to selectively engage the first clutch member and the second clutch member. 17. The system of claim 15, wherein the brake mechanism comprises a mechanical face clutch that engages the first clutch member and the second clutch member through mechanical interference between mating teeth. 18. A method for operating a dynamoelectric device in conjunction with a gas turbine engine, the method comprising: depriming the hydraulic unit prior to operating the dynamoelectric device as an electric motor;braking a carrier of an epicyclic start differential;operating the dynamoelectric device as an electric motor to produce a starter torque output to the gas turbine engine;ceasing braking of the carrier of the epicyclic start differential;priming the hydraulic unit at or after ceasing braking of the carrier of the epicyclic start differential;operating the dynamoelectric device as a generator with an engine torque output from the gas turbine engine; andpowering an accessory unit with the engine torque output from the gas turbine engine. 19. The method of claim 18, wherein torque is transmitted between the dynamoelectric device and the gas turbine engine via a sunless epicyclic main differential.