Inside the Cat MEUI Injector : Injector Nozzle Assembly
This part of the MEUI is the area from which the fuel is injected into the cylinder of the engine. It consists of the nozzle and the needle. The needle (which fits inside of the nozzle) lifts to admit fuel when the fuel pressure in the nozzle rises to a level high enough to lo overcome the pressure applied by the spring which is keeping the nozzle closed. The pressure at which the nozzle opens is called the 'Nozzle Opening Pressure' or NOP for short.
The shim, spring and control rod fit inside the upper section of the nozzle assembly (the large piece next to the spring in the photo).
The shim is placed in first, then the spring. The control rod fits down the middle of the spring. The upper end of the injector needle (the blunt end) protrudes through a hole in the bottom part of the spring assembly and presses against the shim, compressing the spring against the fuel delivery block (the piece at the top of the picture) when the nozzle assembly is fitted to the MEUI and secured by the cap nut. The NOP can be adjusted by changing the shim for a thicker (to raise the NOP) or thinner (to lower the NOP) shim.
The purpose of the control rod is to limit how far the needle can lift when the nozzle opens.
View of the nozzle assembly fitted to the MEUI prior to clamping with the cap nut
The fuel delivery block has two functions: First it acts as a sold part for the injector spring to act against and second, it carried fuel from the pumping element through to the nozzle. The fuel is delivered through small drillings in the fuel delivery block and the upper part of the nozzle assembly. Dowel pins are used to ensure that the holes through the various parts line up.