An internal combustion engine drives, via a belt drive, a first and a second auxiliary unit embodied respectively as a water pump and an air-conditioning compressor and drives a starter-generator operating as a starter and as a generator depending on the operating mode. The belt drive is connected through a clutch element, which has a freewheel, to the internal combustion engine such that, when the internal combustion engine is at standstill, only the two auxiliary units, but not at the same time the internal combustion engine, are driven by the starter-generator. The clutch element which has a freewheel allows a stepless transition of the drive mode from the internal combustion engine to the starter-generator. In order to start the internal combustion engine, the clutch element is engaged only for a short time, so that the drive power of the starter-generator is transmitted to the internal combustion engine.