It comprises one or more cylinders with pistons therein and mutually opposed power cams connected to respective first and second rotary shafts. The reciprocating pistons act on the power cams to impart a rotating motion to the rotary shafts. An attachment device is provided for connecting the rotary shafts to each other. The attachment device includes a shifting device for changing the relative angular position of the first and second rotary shafts. The result is engine distribution and compression ratio are changed dynamically.