What the 48V starter-generator does in mild hybrids (MHEV), the belt-driven (BSG) and integrated (ISG) designs, and how the unit is built.
Mild hybrid (MHEV) technology combines a 48V starter-generator with a lithium-ion battery. The starter-generator starts the engine, generates electricity, adds torque during hard acceleration and recovers energy when braking. The engine can stop at short halts and restart in a fraction of a second, saving 10–15% fuel and cutting emissions.
- Belt-driven (BSG): integrated into the accessory belt drive and replaces the conventional alternator; more widespread because it fits existing engines easily.
- Integrated (ISG): replaces the flywheel and delivers more power and torque, but requires major engine changes.
A BSG is a three-phase AC motor with a built-in inverter, air- or liquid-cooled depending on output, communicating with the engine ECU over CAN. Its power electronics sit under a single cover, making disassembly labour-intensive.