Supercharger of an internal combustion engine having Roots pump

A supercharger of an internal combustion engine having a Roots-type pump:

• a pump housing for defining therein a pump chamber having an inner wall and an air inlet and air outlet;
• a pair of rotor shafts mounted in parallel and each rotatably supported in said housing by means of a pair of bearings, said rotor shafts respectively rigidly carrying rotors in such a manner that small clearances are defined between said pair of rotors, and between said rotors and the inner wall of the housing chamber;
• means for rotating said pair of rotors synchronously in opposite directions so as to suck air from the air inlet into the pump chamber and discharge the air from the pump chamber through the air outlet;
• each of said rotors having, at respective axial ends thereof, end faces substantially perpendicular to the rotor shaft;
• said housing defining annular spaces around said rotor shafts and between said axial end faces of the rotors and said bearings;
• and
• air passage means for connecting said annular spaces in the housing to the atmosphere, said passage means having a valve for controlling the air passage means in accordance with the engine driving conditions.