Butterfly valve assembly for a fuel injection apparatus of an internal combustion engine, comprising:
- a body formed with an admission duct having an axis and a cross-section variable along said axis, for circulation of air in a predetermined direction;
- and
- a disk shaped butterfly valve member of predetermined shape carried by a shaft that extends transversely to the duct, said valve member being mounted by said shaft for rotation in said body between a minimum opening position of said valve member and a maximum opening position in which said valve member lies substantially parallel to the axis of the duct;
- said duct having:
- a cylindrical length along the axis whose cross-section perpendicular to the axis matches with the shape of said butterfly valve member when said valve member is in said minimum opening position, said cylindrical length extending upstream and downstream from said minimum opening position along said axis,
- an upstream zone having a surface merging with said cylindrical length, a surface part of said surface of said upstream zone following adjacent but with increasing separation a path taken by an edge of said valve member during opening thereof from the minimum opening position up to a predetermined degree of opening, and
- a downstream zone having a surface merging with said cylindrical length, a surface part of said surface of said downstream zone following adjacent but with increasing separation a path taken by another edge of said valve member during opening thereof from the minimum opening position up to the predetermined degree of opening,
- said surface parts of said downstream zone and of said upstream zone which follow a respective one of said edges of said valve member each having, at each progressive level from said cylindrical length along said axis, a cross-section transverse to said axis defined by:
- a central circular arc of a circle radially centered about said axis and centered in said surface part, each said circular arc having a radius which is smaller at each progressive level along the axis away from said cylindrical length, and
- lateral arcs of an ellipse on each side of said circular arc, each said lateral arc merging at an outside thereof with a remainder of the associated surface,
- whereby the cross-sectional air flow area increases responsive to the degree of opening of the valve member more slowly than would occur in a cylindrical duct, during an initial portion of the opening movement.