A fuel injector for injecting fuel into the combustion chamber of an engine comprising: (a) an injector housing comprising a bottom, a top and a circular end cap which includes an inner screw threaded portion for attachment to a screw threaded portion of the top of the housing; the housing defining an internal cylindrical chamber having a bottom portion in which a hemi-spherical inner nozzle surface is provided; (b) an inlet nozzle attached to said housing for receiving pressurized fuel; (c) an outlet nozzle positioned at the bottom portion of said injector housing and extending from said hemi-spherical inner nozzle surface to provide an egress into the combustion chamber; (d) a piezoelectric stack positioned inside said injector housing, said piezoelectric stack having a displacement stroke; (e) drive electronics connected to said piezoelectric stack for providing power to expand and contract said piezoelectric stack throughout its displacement stroke in real-time; and (f) a flow control member having a lower cylindrical body portion with a hemi-spherical nose, said nose having a first radius of curvature and said hemi-spherical inner nozzle surface having a second radius of curvature, smaller than the first radius of curvature, wherein said flow control member is in direct contact with said piezoelectric stack within said injector housing, said piezoelectric stack providing for direct actuation of said flow control member, said flow control member being directly moveable by expansion and contraction of said piezoelectric stack between a closed state in which fuel flow from said inlet nozzle through said outlet nozzle into the combustion chamber is blocked and a plurality of intervening open positions wherein fuel may flow through said outlet nozzle in relationship to expansion and contraction of said piezoelectric stack; wherein an annular flow area is created between said nose of said flow control member and said inner nozzle surface by a movement of said flow control member, said annular flow area being a function of said first radius of curvature, said second radius of curvature and the movement of said flow control member within said cylindrical chamber in direct relationship to the expansion and contraction of said piezoelectric stack; and wherein said movement of said flow control member between a fully open state and a fully closed state corresponds to the displacement stroke of said piezoelectric stack which is one percent or less of a height of said piezoelectric stack in a prestressed state wherein said fuel injector further comprises means (50, 26) for applying a prestress load to said piezoelectric stack; said prestress load being applied to said piezoelectric stack by screwing the end cap onto the threaded portion of the top of the housing to seal the injector. (a) an injector housing comprising a bottom, a top and a circular end cap which includes an inner screw threaded portion for attachment to a screw threaded portion of the top of the housing; the housing defining an internal cylindrical chamber having a bottom portion in which a hemi-spherical inner nozzle surface is provided; (b) an inlet nozzle attached to said housing for receiving pressurized fuel; (c) an outlet nozzle positioned at the bottom portion of said injector housing and extending from said hemi-spherical inner nozzle surface to provide an egress into the combustion chamber; (d) a piezoelectric stack positioned inside said injector housing, said piezoelectric stack having a displacement stroke; (e) drive electronics connected to said piezoelectric stack for providing power to expand and contract said piezoelectric stack throughout its displacement stroke in real-time; and (f) a flow control member having a lower cylindrical body portion with a hemi-spherical nose, said nose having a first radius of curvature and said hemi-spherical inner nozzle surface having a second radius of curvature, smaller than the first radius of curvature, wherein said flow control member is in direct contact with said piezoelectric stack within said injector housing, said piezoelectric stack providing for direct actuation of said flow control member, said flow control member being directly moveable by expansion and contraction of said piezoelectric stack between a closed state in which fuel flow from said inlet nozzle through said outlet nozzle into the combustion chamber is blocked and a plurality of intervening open positions wherein fuel may flow through said outlet nozzle in relationship to expansion and contraction of said piezoelectric stack;