Fuel injector having needle tip and nozzle body surfaces structured for reduced sac volume and fracture resistance

A nozzle assembly for a fuel injector comprising:

• a nozzle body defining a longitudinal axis, and having an outer surface, an inner surface, and a dome-shaped nozzle tip, and having formed therein a nozzle outlet passage;
• the inner surface including a seat surface extending circumferentially around the longitudinal axis and located axially inward of the dome-shaped nozzle tip, and a sac surface located at least predominantly within the dome-shaped nozzle tip;
• the dome-shaped nozzle tip having formed therein a plurality of spray orifices distributed about the longitudinal axis and extending from a plurality of inlet locations within the sac surface to a plurality of outlet locations within the outer surface;
• a needle check positioned for reciprocation within the nozzle body and defining an axis of reciprocation that is collinear with the longitudinal axis, the needle check having a first end that includes a closing hydraulic surface and an opening hydraulic surface, and a second end that includes a needle tip having a sealing surface and a terminal end surface located axially outward of the sealing surface;
• the sealing surface being positioned in contact with the seat surface such that the needle check blocks the nozzle outlet passage from fluid communication with the plurality of spray orifices, and the terminal end surface being positioned in spaced, facing relation to the sac surface, such that a sac cavity extends between the needle check and the nozzle body;
• and
• the terminal end surface is spherically shaped and curves according to a convex radius of curvature, and the sac surface is spherically shaped and curves according to a concave radius of curvature, the outer surface is concave upon the dome-shaped nozzle tip;
• the sac surface comprising two concave surfaces with equivalent radii of curvature mating with a convex sac;
• the sac surface cooperating with the terminal end surface to form a sac volume, wherein the sac surface is inclusive of the plurality of spray orifices;
• and
• each of the terminal end surface and the sac surface is centered about the longitudinal axis, and the convex radius of curvature and the concave radius of curvature are sized such that a clearance between the terminal end surface and the sac surface is nowhere less than a distance along a line segment that extends between the terminal end surface and the sac surface and is collinear with the longitudinal axis.