A coupling system comprising: a dynamoelectric device selectively operable as either a generator or an electric motor; a combustion engine; a main differential operably connected between the dynamoelectric device and the combustion engine; a rotationally powered accessory (30, 32, 34) engaged with the main differential; an epicyclic start differential operatively connected to the main differential; and a 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 combustion 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 (30, 32, 34) and to the dynamoelectric device; characterized in that: 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, wherein the start differential comprises a start differential carrier connected to the brake mechanism and 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; and a main differential carrier rotatably connected to each of the main differential planet gears. a dynamoelectric device selectively operable as either a generator or an electric motor; a combustion engine; a main differential operably connected between the dynamoelectric device and the combustion engine; a rotationally powered accessory (30, 32, 34) engaged with the main differential; an epicyclic start differential operatively connected to the main differential; and a 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 combustion 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 (30, 32, 34) and to the dynamoelectric device; characterized in that: 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, wherein the start differential comprises a start differential carrier connected to the brake mechanism and 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; and a main differential carrier rotatably connected to each of the main differential planet gears. 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, wherein the start differential comprises a start differential carrier connected to the brake mechanism and 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; and a main differential carrier rotatably connected to each of the main differential planet gears.