PURPOSE: To collect excessive output by driving a roots compressor using a partial flow-in exhaust gas turbine and connecting this drive shaft to the crank shaft of an interal combustion engine.

CONSTITUTION: An engine has a piston 2, intake valve 4, and exhaust valve 5, and a roots compressor for supercharging has a rotor 6, synchronous gears 7, air intake port 9, and compressed air exhaust port 10. An exhaust gas turbine is provided with a number of partial flow-in nozzles 15, 15', and 15" and automatically switches changeover valves 21, 21', and 21". It makes the engine have the maximum efficiency independently of the speed and load of the exhaust gas turbine. Since this permits the exhaust gas turbine to be directly or mechanically connected to a crank shaft 12, excessive output can be collected.