To improve fuel consumption performance by unitedly performing scavenging and noise reduction of an internal combustion engine at the same time, reducing a total energy loss compared to conventional engine operation and sound absorbing, and thereby increasing actual output.

In this internal combustion engine, an expansion chamber 11, an exhaust turbine 12, and a straitening tube 14 are arranged on an exhaust pipe 13 between an exhaust port 3 and a discharge port 15. The exhaust turbine 12 is optimumly controlled for high speed rotation, while pressure inside the expansion chamber 11 is always kept atmospheric pressure or lower. Combustion gas of high temperature and high pressure are instantaneously diffused and expanded, and at the same time scavenging and noise reduction are carried out. The exhaust turbine 12 is driven by power output from an output shaft of an engine directly or through a continuously variable transmission 17. Or, it is driven by a transmission motor. In other case, it is driven by a driving turbine adopting exhaust energy. In any case, an optimum operation condition can be selected through computer controlling etc.