Drive train of a motor vehicle having an internal combustion engine and a starter generator

A drive train of a motor vehicle, comprising:

• an internal combustion engine that has a crankshaft with a crankshaft gear;
• a transmission connected downstream of the internal combustion engine, the transmission being drivable by the crankshaft and being operable for driving at least one axle of the motor vehicle;
• a spur gear mechanism connectable to the crankshaft gear;
• a starter generator attached to the spur gear mechanism, the spur gear mechanism having first and second drive trains, the first drive train of the spur gear mechanism having a first transmission ratio and being provided with a first switchable clutch that can be closed for connecting the first drive train to the crankshaft gear, and the second drive train of the spur gear mechanism having a second transmission ratio different from the first transmission ratio and being provided with a second switchable clutch or a freewheel that is active only when the first switchable clutch is open for connecting the second drive train to the crankshaft gear during a starter mode of the starter generator;
• and auxiliary units attached to a belt drive that can be driven selectively by one of the starter generator and the internal combustion engine.