PURPOSE: To increase the torque in the low speed range without imparing the torque performance in a high speed range, by allowing two suction passages to converge in the neighborhood of the intake, where one of the suction passages shall have a smaller sectional area than the other.

CONSTITUTION: When a control valve 27 is opened fully, a shaft torque curve Te is obtained as marked with C1. When the valve is closed fully, a shaft torque curve Te marked with C2 is obtained. Here the suctional area A1 of the suction passage 2a shall be smaller than the one A2 of the suction passage 2b, so that the equivalent pipe length when the valve 27 is closed will be longer (inverse proportional to the passage area of suction pipe). Accordingly the equivalent pipe length for suction gas becomes longer to provide a shaft torque curve Te marked with C3.