Fuel injector having a trapped volume nozzle assembly with a pressure relief valve

A fuel injector comprises an injector body 11 that defines a pressure relief passage 47 with one end opening into a trapped volume 21, and a fuel pressurization chamber 20 in fluid communication with a nozzle outlet 14. A needle valve member 30 is positioned in the injector body 11 and movable between an injection position in which the fuel pressurization chamber 20 is open to the outlet 14 and a closed position in which the nozzle outlet 14 is blocked. A pressure relief valve 40 is placed in the pressure relief passage 47. The pressure relief valve 40 has a valve member 41 with an opening surface 43 exposed to pressure in the trapped volume 21 and a closing surface 42 exposed to pressure in the fuel pressurization chamber 20. The valve member may be a plate or a ball (figure 2). The trapped volume 21 may contain a spring 39 that biases the needle valve member 30 toward the closed position.