Injection Nozzle

Fig. 13 Nozzle and Holder Assembly:

The injection nozzle, Fig. 13, sprays fuel into the combustion chamber. Fuel pressure resulting from each working stroke of the injection pump controls the nozzle. The injection of fuel into the combustion chamber occurs under the high pressure generated by the injection pump. The nozzle consists of nozzle body and needle; these two components are precisely matched to each other and must always be serviced as a pair. The injection nozzle with its holder is built into the cylinder head.
In pre- and turbulence-chamber engines, preparation of the fuel for combustion occurs primarily through air turbulence, reinforced by the appropriate form of the injection spray. Nozzle opening pressure is generally between 110 and 135 bar (1595 - 1960 psi) for this type of injector.
The needle has a specially shaped pintle at one end for a pre-injection of fuel. At first, the needle opens only a very narrow annular orifice, thereby admitting little fuel (a restriction effect). As pressure forces it to open wider, the flow cross-section increases. Not until the needle is almost completely open is the main quantity of fuel injected.
Because it promotes a gradual pressure rise, this action gives "soft" combustion and therefore relatively smooth engine operation. Pintle shape, along with the pressure spring's characteristics and designed-in play in the restriction groove, are parameters that are varied to achieve the desired restriction action.