Fuel injector with pressure regulated trapped volume nozzle assembly

A fuel injector comprises an injector body 11 defining a low pressure space 15, a trapped volume 21 and a fuel pressurization chamber 20. A needle valve member 30 is positioned in the injector body and movable between an injection position in which the fuel pressurization chamber 20 is open to a nozzle outlet 14 and a closed position in which the nozzle outlet 14 is blocked to the fuel pressurization chamber 20. At least one of the needle valve member 30 and the injector body define a pressure release passage 53 extending between the trapped volume 21 and a low pressure space 15. A pressure regulating valve 50 is positioned in the pressure release passage 53 and has a regulating valve member, ball 51, movable to an open position when fluid pressure in the trapped volume 21 is above a pop-off pressure. Figure 2 shows an alternative embodiment with a plate valve. At idle operating conditions there is time for pressure that has built up in the trapped volume 21 to decay before the next injection event. Therefore the next event opening pressure is low. At higher speeds, for example rated, there is insufficient time for the pressure to decay so a greater opening pressure is required.