The present invention relates to a supercharging system for an internal combustion engine, comprising: a charging device (6, 6a) with a turbine (7, 7a) and a compressor (8, 8a), said compressor having a high speed shaft (30); a planetary gear (25) coupled to said high speed shaft (30) and an electric motor, or generator, (20) via a low speed drive shaft (31); a clutch unit (18a); and a power transmission (16) for connecting a crank shaft (4) of the combustion engine (1) to said drive shaft (31) via the clutch unit (18a), wherein a system control unit (23) is configured to control an in-cylinder mean temperature of said internal combustion engine by controlling the electric motor, or generator, (20) speed.