thus replace the conventional automotive alternator and may also be used to start the engine and may thus also replace the conventional engine starter motor fig2 shows a modified construction of supercharger and those components which are the same as those in fig1 are provided with the same reference numerals this embodiment differs from that of fig1 in two major respects namely the construction of the epicyclic gearset and thus also the connections of the rotors of the two motor generators and the provision of step up gearing between the output of the gearset and the impeller the epicyclic gearset is of positive type the input shaft 6 is connected to a first toothed sun wheel 30 which is in mesh with a number typically three of toothed first planet wheels 32 which are rotatably mounted on respective planet shafts 14 each first planet wheel 32 is connected to a respective second toothed planet wheel 34 which is mounted on the same planet shaft 14 the planet wheels 34 are in mesh with a second toothed sun wheel 36 which is connected to an intermediate output shaft 5 the planet shafts 14 are mounted on a common carrier 15 the output shaft 5 is linked to the output or impeller.